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Physical Characterisation of Latex Film Formation and Film Properties

by

Tecla G Weerakkody

Submitted for the degree of Doctor of Philosophy

Department o f Physics Faculty o f Engineering and Physical Sciences

University o f Surrey

August 2009

Abstract

In this work, physical characterisation of film formation and film properties of coating and adhesive formulations was performed. Organic/inorganic nanocomposite coatings draw remarkable academic and industrial interest, due to their expected enhancement of combined properties. Film formation and film properties, such as drying, transparency, and final film quality of coating systems containing Laponite clay were studied, in particular the influence of excess surfactant on the drying process. It was found that excess surfactant in the system does affect the physical characteristics of the film formation process. Experiments using magnetic resonance profiling and photographs found lateral flow of liquid from the edges to the centre. Reducing the amount of surfactant in the system improves film quality by reducing lateral flow during film formation.

In pressure sensitive adhesive (PSA) applications, such as labels or tapes, it is vital that the films remain optically transparent, regardless of the exposure to high humidity or water. Core/shell PSAs were investigated to determine their drying, water whitening, moisture absorption from high humidity, and adhesive properties. The dependence of these properties on pH and poly (acrylic acid), PAA, was studied. It was found that the hydrophilic pathways created by PAA shells do not contribute to faster drying. In addition, this investigation disproves the idea that a “hairy layer” of PAA keeps the film more open and leads to faster drying. It was found that films with hydrophilic boundaries absorbed more water. The water is evenly distributed along the boundaries, so that films still retain their transparency. It was proved that transparency or water whitening is not necessarily a reliable measurement of water uptake, at least for this system.

The findings from this study define guiding principles for organic/inorganic nanocomposite coatings and core/shell PSAs, to have better film formation characteristics and film properties.

Acknowledgements

It has been a great privilege to be a postgraduate student at the Department of Physics, where I have spent memorable few years and met so many wonderful people. I would like to thank many of whom encouraged me and were involved in the preparation of this thesis.

First and foremost I would like to express my sincere gratitude to my supervisor, Professor Joseph Keddie for his support, guidance and understanding. I am grateful to Joe for introducing me to the materials science and polymer physics, and that I was given this opportunity to work in this project. His passionate interest in science with enthusiasm and hard working always influenced me. The constant encouragement, even from the very first email I received from Joe, genuinely motivated me. I am profoundly thankful for his valuable discussions, stimulating advice and patience throughout my PhD.

I would also like to thank my second supervisor, Professor Peter McDonald, for his energetic advice, dedication and invaluable assistance (particularly when we needed practical help with the GARField magnet) throughout this research. At many stages, this work has benefltted from his expertise, which would otherwise have been impossible to obtain. My special thanks go for Dr Jamie Cleaver for his strong support, insightful discussions and especially, for the water sorption experiments which are presented in Chapter Five.

I would like to extend my appreciation to my NAPOLEON collaborators throughout this research. Prof. Jose M Asua, Prof. Maria J Barandiaran, Dr Maria Paulis (POLYMAT The University of the Basque Counfry, Spain) for their insightful discussions and comments on my work and especially the immense support and guidance I received during Emulsion Polymerisation Course also during my experiments in their laboratories. I am grateful to Ms Aitziber Lopez, Ms Mihaela Manea and the rest of the POLYMAT group for their support in the university, as well as out in the city, to make that short visit so memorable. I would like to extend my appreciation to Dr Elodie Bourgeat-Lami (CNRS-LCPP, France) for her fascinating insight into the work which is presented in Chapter Four and Dr Veronique Mellon (from the same laboratory) for preparing the samples and helpful discussions. My special thanks go to Prof. Diethelm Johannsmann (Clausthal University of Technology, Germany) and his team, especially Mr Alexander IConig for developing the software programme to process the NMR

data and training us, at University of Surrey, to use the programme, and for their invaluable, energetic discussions and thoughtful ideas. My special thanks also go to Dr Alex Routh (Cambridge University, UK) and his team for ongoing insightful discussions, comments and guidance which I received throughout my PhD. I would like to express my gratitude to Prof. Yves Holl (CNRS-ICS, France) and Dr Celine Arnold for their professional comments, suggestions and discussions and preparing some of the dialysed latex sample for this work.

In addition, I would like to express profound gratitude to all the partners of Designed Nanoscale Heterogeneities for Controlling Water-Borne Pressure-Sensitive Adhesive Performance (NsHAPe) Project, especially Prof. Peter Lovell (University of Manchester, UK) for his invaluable discussions, stimulating advice and thoughtful ideas during the six months of my work to the project and providing the samples for the work presented in Chapter Five. Also, many thanks for Dr Chunhong Lei for water whitening measurements presented in Chapter Five.

Within the Department of Physics, special thanks go to all the technical staff present and past, especially, Mrs Violeta Doulcova, Mr John William-Brown, Mr David Munro and Mr Jeff Dahllce, Dr David Hemsley, Mr Tom Gibbons, Mr Bob Derham, Mrs Liz Griffiths, Mi' Keith Proctor. All of whom, at all times, provided essential support for equipment and facilities used throughout my studies. My special thanks also go to Ms Shirley Hankers, Mrs Cristobel Soares- Smith, Mrs Noelle Hartley, Dr Annika Lohstroh and Miss Clare Harvey for spending their valuable time on proof reading my thesis. Thanks also go to present and past members of Soft Condensed Matter (SCM) Group, especially Dr Piyasiri Elcanayake, Dr Peter Doughty and Mr Simon Pitts for introducing me to NMR and immense practical help during experiments. Dr Tao Wang, Dr Carolina de Las Heras Alarcon, Ms Dan Liu, Dr Victor Rodin, Dr Eisabetta Canetta, Dr Ibraheem Bushnak, Dr Fiona Frehill for their help, advice, feedback, good company and invaluable friendship during my PhD experience. I also like to thank my office mates Piyapong Asanithi, Patnarin Woijittiphon, Akarin Intaniwet, Alice King, Eric Brunner for all the help, advice and support. Special thanks goes for Dr Izabela Jurewicz for being there for me from day one, Ms Ann Henderson for all her help and advice and all the other friends on and off the university, for all their support.

I also acknowledge the financial support for my studies from EPSRC and European Commission under NAPOLEON and NsHAPe Projects. Thanks for UK Polymer Colloid Forum for best poster award for 2007 and 2008.

I am profoundly thankful to my parents for their unconditional love and support, without whom this work would not have been possible. Special thanks also go to my brother and his family, my mother-in-law, late father-in-law and all the in-laws. Most importantly, to my husband, whose encouragement, understanding, patience and emotional support throughout the duration of my PhD, especially the last very stressful months has my undying gratitude and love.

And finally, I would like to dedicate this thesis to my lovely children Thilini and Akila for their love and the happiness they bring to my life. I love you both very much!

v

Glossary

AA acrylic acid

ACPA 4’-azobis (4-cyaiiopentanoic acid)

AFM atomic force microscopy

ABBA 2 ,2 -azobis (2 -methylpropionamidine) hydrochloride

BuA butyl acrylate

BylA behenyl acrylate

CCC critical coagulation concentration

CEC cation exchange capacity

CMC critical micelle concentration

CTAB cetyltrimethylammonimn bromide

DDAB didodecyldimethylammonium bromide

EA ethyl acrylate

FCC face-centred cubic

GARField gradient at right-angles to the field

MFFT minimum film-forming temperature

MMA methyl methacrylate

MMT montmorillonite

MRI magnetic resonance imaging

MRP magnetic resonance spectroscopy

NMR nuclear magnetic resonance

OHP Helmholtz plane

OTAB octadecyltrimethylammonium bromide

PAA poly acrylic acid

PI DA poly isodecyl acrylate

PSA pressure-sensitive adhesive

RF radio frequency

SEM scanning electron microscopy

TEM transmission electron microscopy

VOCs volatile organic compounds

y-MPTMS y~ methacryloxypropyl-trimethoxysilane

Table of nomenclature

A surface area of a sphere (nm2)

Bo external magnetic field (T)

Bi field due to RF coil (T)

D distance between two particles (nm)

Do diffusion coefficient (m2/s)

e electronic charge (C)

E evaporation rate (m/s)

Ea energy dissipated during the debonding process (J/pm2)

F force (N)

G t shear modulus (Pa)

G y magnetic field gradient (T/m)

H film thickness (pm)

h Plank’s constant (Js)

I spin quantum number

I n value of the distribution at Xn

k Boltzmann constant (J/K)

m0 integral, zeroth moment

M0 initial mass (mg)

M o o mass at the equilibrium or plateau value (mg)

MFFT minimum film formation temperature (K)

Mp mass of the pan (mg)

Mt total mass at a given time t (mg)

n number of echos

N number of the points of a distribution

no ionic concentration (mol/L)

NS number of scans

P c a p reduced capillary pressure (Pa)

Pe Peclet number

R radius (nm)

RD repetition delay (s)

SF spectrometer frequency (MHz)

SI number of points

T absolute temperature (K)

Ti spin-lattice relaxation time (s)

t 2 spin-spin relaxation time (s)

Tg glass transition temperature (°C)

V volume of a sphere (nm3)

w interaction energy (J)

WA the sum of the attractive energies (J)

W r the sum of the repulsive energies (J)

W t total interaction energy (J)

Xn the value of the independent variable

z valency of the ions in solution

AX pixel spacing (pm)

Az field of view (pm)

AQ bandwidth of the pulse (Hz)

ft zero shear rate viscosity of the polymer (Pa s)

¥ Potential (mV)

CO Lannor resonant frequency (MHz)

6 stress (MPa)

h reduced Plank’s constant (Js)

CD volume fraction

1C1 Debye screening length (nm)

8 dielectric constant or permittivity of the continuous phase (F/m)

eo dielectric constant or permittivity of vacuum (F/m)

Y charge on the particle (C)

X I'def/ T'dry

[X viscosity (Pa.s)

poo ionic concentration of ions in the bulk (mol/L)

T pulse gap (ps)

Tdef characteristic time for particle deformation (s)

T'dry characteristic time for drying (s)

? Zeta potential (mV)

Contents

List of Figures .................................................................................................................... xiii

List of Tables .................................................................................................................. xxvi

Chapter 1 - Overview...................................................... 1

1.1. References. .................................................................................................. 5

Chapter 2 - Introduction.................................................... 6

2.1. Introduction to colloidal dispersions............................................................ 6

2.2. Classification of colloidal dispersions................................................ 9

2.3. Synthesis of latex........................................................................................... 11

2.3.1. Emulsion polymerisation............................................................. 1 1

2.3.2. Miniemulsion polymerisation...........................................................

2.4. Charges in the colloidal dispersions lg .

2.4.1. Helmholtz m odel........................................................................... 18

2.4.2. Gouy - Chapman m odel.............................................................. 19

2.4.3. Stern m o d e l.................................................................. 20

2-5. Stability in the colloidal dispersions........................................................... 23

2.5.1. The DLVO Theory...................................................................... 23

2.5.2. The factors affecting the stability in the colloidal dispersions. . 26

2.5.2.1. Particle s ize ................................................................... 27

2.5.2.2. Zeta - potential............................................................... 27

2.6. Fihn formation of colloidal dispersions.................................................. 29

2.6.1. Stages of film formation................................................................ 29

2 .6 .1.1. Stage I - Evaporation of water and particle ordering 30

2.6.1.2. Stage II - Particle deformation.................................... 31Stage III - Coalescence/Interdiffusion

2.6 .1.3. 32across particle - particle boundaries.........................

2.6.2. Particle deformation and coalescence.............................. 33

2.6.2.1. Wet sintering................................................................. 33

ix

2.62.2. Dry sintering............................................... 33

2.6.2.3. Capillary deformation................................ 35

2.6.2.4. Receding water front.................................. 35

2.6 .2.5. Sheetz deformation..................................... 36

2.6.3. Recent models for drying dispersions......................................... 36

2.6.3.1. Peclet number (P e).................................... 36

2.6 .3.2. The Routh and Russel model for normal drying . . . . 38

2.6.3.3. The Routh and Russel model for lateral drying . . . . . 40

2.7. References....................................................................................................... 42

Chapter 3 - Magnetic Resonance Profiling......................................... 46

3.1. Historical Background of MRP..................................................................... 46

3.2. The Principles of NMR.................................................................................. 48

3.3. MR Profiling using GARField magnet........................................................ 52

3.4. a Typical GARField Profile......................................................................... 5 7

3.5. References.................................................................................................... 61

Chapter 4 - Physical Characterisation of Film Formation and FilmProperties of Organic/Inorganic Nanocomposite Coatings................... 64

4.1. Introduction....................................................................................................... 64

4.1.1. Surface tension, surfactant concentration and CMC.................... 85

4.1.2. Marangoni Flow............................................................................. 71

4.1.3. Overview of Nanocomposite Materials....................................... 724.2. Materials......................................................................................................... 77

4.2.1. Latexes by Route 1......................................................................... 77

4.2.2. Latexes by Route I I ...................................................... 78

4.3. Techniques..................................................................................................... 80

4.3.1. Magnetic Resonance Profiling (MRP)........................................... gO

4.3.2. Optical Transmission Measurements.............................. 904.3.3. Profilometry..................................................................................... 814.3.4. Methods for obtaining photos of the films..................................... 81

4.4. Results and Discussion........................................................................... 83

x

4.4.1. Original Route I and Route I I ....................... .............................. 83

4.4.1.1. MR Profiling and Images from sid e ......................... 83

4.4.1.2, Optical Transmission................................................... 90

4.4.2. Effect of the DDAB content on Route II latexes.......................... 9 4

2 Comparison of drying properties of ‘original’ and newpolymer/Laponite latexes............................................................... 98

MRP of ‘original’ and new polymer/Laponitelatexes........................................................................ 98

4 4 2 2 Visual appearance of the ‘original’ and newpolymer/Laponite latexes........................................... 103

4 4 3 3 Optical transmission of ‘original’ and newpolymer/Laponite latexes...................................... 103

, „ „ , Thickness variations of the films of ‘original’ and4.4.3.4. new formulation polymer/Laponite latexes............. 105

4 • 5. Conclusions..................................................................................................... 110

4.6. References....................................................................................................... 112

Chapter 5 - The Effects of Acrylic Acid and pH on the PhysicalCharacteristics of Pressure Sensitive Adhesives....................................

5.1. Introduction........................ 117

5.1.1. Water uptake.................................................... 120

5.1.2. pH and PAA dependence................................................ 125

5.1.3. Properties.................................................................................... 128

5.2. Aims of present research w ork .................................................................... 132

5.3. M aterials......................................................................................................... 133

5.4. Methods............................................................................................................ 135

5.4.1. Magnetic Resonance Profiling (M RP)........................................ 1355 4 2 Measurements of Water Whitening by Optical ^

Transmission.................................................................................5.4.3. Probe-Tack Adhesion Measurements.......................................... 136

5.4.4. Moisture Soiption Measurements.................................................. 142

5.5. Results and Discussion ................................................................... 146

5.5.1. Drying measurements..................................................................... 146

5.5.1.1. Effect of PAA shell at a lower pH (pH of 3 ) ........... 147

5 5 12 Effect of PAA shell at a higher pH adjusted with NaOH (pH of 8) ...............................................

xi

5.5.1.3. Effect of pH on latex with PAA shell........................ 149

5.5.2. Water whitening measurements.................................................... 150

5.5.3. Moisture Sorption Measurements................................ 154

5.5.3.1. Effect o f PAA at a lower pH (pH of 3 ) .................... 154

Effect of PAA at a higher pH adjusted with5 5 3 2

NaOH (pH of 8) ......................................................... 155

5.5.3.3. Effect o f pH on latex with PAA shell........................ 155

Correlation of Drying, Water Whitening and Moisture5.5.4.

Sorption Measurements................................................................ 158

5.5.4.1. Correlation of Drying and Water Whitening............

5.5.4.2. Correlation of Drying and Moisture Sorption ^

Correlation of Moisture Sorption and Water5.5.4.3.

Whitening.................................................................... 163

5.5.5. Probe-Tack Adhesion Measurements........................................... - , ~

5.6. Conclusions,

5.7. References. ,

Chapter 6 - Conclusions and Future W ork............................. 175

6.1. Conclusions.................................................. .................................................. 1 7 5

^ ̂ j Conclusions about Organic/Inorganic NanocompositeCoating Formulations............................................................. 175

f\ 19 Conclusions about Pressure Sensitive Adhesive Formulations . 1776.2. Future Work.................................................................................. I 7 9

6 2 1 Future work for Organic/Inorganic NanocompositeCoating Formulations............................................................. 179

f\ 0 0 Future work for Pressure Sensitive Adhesive Formulations. . . 1796.3. References •, Qn

Figure 2.1.

Figure 2.2.

Figure 2.3.

Figure 2.4.

Figure 2.5.

Figure 2.6.

Figure 2,7.

List of Figures

Schematic diagram illustrating the three intervals of an emulsion

polymerisation process [11]. Interval I: Particle formation stage,

represented by the increase in both the number of particles and the

polymerisation rate. Interval II: Particle growth: during which, both the

number of particles and the polymerisation rate remains relatively

constant. Interval III: The number of particles remains unchanged, while

polymerisation rate and the monomer concentration decrease[ll],. . . 13

Polymerisation rate as a function of time for three intervals of emulsion

polymerisation process [8]. During interval I, the polymerisation rate

increases, during Interval II it stays relatively unchanged and finally, it

decreases during interval III.....................................................................14

Principle of miniemulsion polymerisation. In the first step of

miniemulsion polymerisation, submicron size monomer droplets are

produced by shearing and in the second step, these droplets act as

individual batch reactors that undergo the polymerisation reaction [5,

14, 15].......................................................................................................... 16

Schematic of the Helmholtz model, which failed to explain the

experimental results [4]............................................................................ 19

Schematic of the Gouy - Chapman model [4]..........................................20

Schematic of the Stem model which consist of Stem layer, where

potential drops linearly and the diffuse layer, where the potential drops

exponentially with the distance [4]......................................................... .21

Schematic of the energy versus the distance between particles profiles of

DLVO interactions. This figure is taken from Jacob Israelachvili,

Intermolecular and Surface Forces 1992: Academic Press Limited,

London; page 248 [23]. Highly charged surfaces repel strongly and

remain stable which illustrated in lower inset ‘a’. ‘b ’ where surfaces

come into stable equilibrium at secondary minimum and colloidal

particles remain kinetically stable, ‘c’ when surfaces come into

secondary minimum and particles coagulate slowly, ‘d’ Surfaces may

remain in secondary minimum or adhere and colloids coagulate rapidly,

at the ‘critical coagulation concentration’, ‘e’ Surfaces and colloidal

particles coalesce rapidly [23]................................................................... 25

Figure 2.8. The effect of £ potential on the shape of the total interaction potential

curve for polystyrene latex. From top to bottom the £ potentials are -80

mV, -50 mV, -25 mV and -20 mV [4].................................................... 27

Figure 2.9. Illustration of the stages of film formation process [6]. In stage one,

water evaporates from the polymer film and particles re-order to a close

packed array. In stage two, particles deform, when the drying

temperature is greater than the MFFT. In the final stage, particles

coalesce and molecules diffuse, when the drying temperature is higher

than the Tg of the latex, and a mechanically coherent homogenous film is

formed..........................................................................................................29

Figure 2.10. Distance between different size particles as a function of volume

fraction of polym er.................................................................................... 31

Figure 2.11. Schematic illustrations of particle deformation theories for film

formation. (A) Wet sintering - when particles are deformed before water

has evaporated and the reduction of the polymer water interfacial energy

is the driving force. (B) Dry sintering - when water recedes before

particles are deformed and the reduction of the polymer-air interfacial

energy is the driving force. (C) Capillary deformation - when the air-

water interfacial energy is the driving force for particle deformation. (X)

concave meniscus of air-water interface. (D) Sheetz deformation -

“skin” formation by wet sintering and capillary forces due to rapid

evaporation, before the dispersion reaches close packing below . . . . 34

Figure 2.12. Schematic illustrations of P e « l and P e » l . Initially stable polymer

colloid dispersion (on left) could show vertical water uniformity (right

top) if diffusion is stronger than evaporation (A) during drying and

P e « l . Alternatively, if evaporation is stronger than diffusion (B)

during drying the result is non-uniform water distribution (right bottom)

and P e » l .................................................................................................... 37

xiv

Figure 2.13.

Figure 2.14

Figure 3.1.

Figure 3.2.

Figure 3.3.

Figure 3.4.

Figure 3.5.

Figure 3.6.

Schematic diagram of the drying regimes according to the Routh and

Russel model for normal drying and the influencing factors [37]..........40

Schematic diagram shows a lateral drying in a colloidal film with a

central wet region and dry edges. Between these regions, water fills the

void space between packed particles. The inner boundary is the particle

packing front, and the outer boundary is the drying front [39]..............41

Schematic illustration of randomly oriented magnetic dipole moments

where no external magnetic field (A), and when external magnetic field,

Bo, is applied, the orientations align with the field (B)...........................49

Schematic illustration of the splitting of the Zeeman energy levels due to

the application of a static magnetic field Bo. The lower energy level has

m = + Y z where magnetic moment is parallel to the external magnetic

field, Bo and higher energy level has m = - Y z where magnetic moment is

anti-parallel to B0 [25]................................................................................50

Cross section through the centre of the magnet which shows the shape of

the pole pieces and the sample location in relation to the magnets and

the pole pieces. The RF coil is located directly below the sample

location. Image taken from [27]...............................................................53

Schematic diagram of the GARField magnet. The magnetic field

gradient, Gy, is perpendicular to the direction of the magnetic field, Bo

[1 1 ]. Bj is the magnetic field generated by the planar radio frequency

(RF) coil. The profiles o f the sample are a measure of the intensity of

the magnetisation signal as a function of the height of the sample, in the

direction of the gradient which is shown on the right hand side 54

Typical GARField profiles of a model acrylic latex (coating), where

each profile was taken in 5 minute intervals. The arrow shows the

increase of the drying time. On right is the film-air surface (top of the

film) and on left is the film-substrate surface (bottom of the film). . . . 57

Typical GARField profiling of a latex with low Tg (adhesive) using 3D

waterfall type of plotting............................................................................58

xv

Figure 4.1.

Figure 4.2.

Figure 4.3.

Figure 4.4.

Figure 4.5.

Figure 4.6.

Figure 4.7.

(a) The structure of a single tetrahedral shaped unit of a central four-

coordinated silicon atom surrounded by four oxygen atoms and (b) the

resulting tetrahedral sheet, (c) is the structure of an octahedral unit with

a central six-coordinated magnesium atom surrounded by six oxygen

groups and (d) the resulting octahedral sheet. Image taken from [14]. 6 6

A schematic illustration of the ‘T-O-T’ stacking pattern, where one

octahedral sheet is sandwiched between two tetrahedral sheets (on the

left). The same stacking pattern can be found in Laponite structure. On

the right is the crystalline structure of a Laponite clay disk. Image taken

from [7, 9]....................................................................................................67

Acido-basic equilibrium equations: The top equation is for acidic media,

where positive species are predominant and the equilibrium shifts to the

right, and as a result, clay particles are positively charged. In the basic

media, negatively charged ions are predominant and according to the

second equation, the resulting clay particles will be negatively charged.

68

TEM image of polystyrene latex particles containing Laponite clay.

Image taken from [22]................................................................................69

TEM images of polystyrene saponite composite films; on left with

unmodified saponite and on right with saponite modified with OTAB.

Image taken from [23]................................................................................70

Schematic illustration of surface tension as a function of the logarithm

of the surfactant concentration. Surface tension of pure water or at 0 log

concentration is 72 mN/m and the inflection point where surface tension

no longer decreases with the increase of surfactant concentration is the

CMC of a given surfactant........................................................................ 71

Schematic illustration of Route I and II. In Route I (on the left),

Laponite plates were functionalised with the cationic initiator, AIBA

and dispersed into the water phase. In Route II (on the right) double

functionalised Laponite by MPTMS and DDAB, was dispersed in the

monomer phase. After the miniemulsion polymerisation in Route I it is

xvi

Figure 4.8.

Figure 4.9.

Figure 4.10.

Figure 4.11.

Figure 4.12.

Figure 4.13.

Figure 4.14.

expected that the clay will be at the surface of the latex particles,

whereas in Route II, the clay will be encapsulated within the latex

particles. Image taken from [41]...............................................................74

(a) the chemical structure of y-MPTMS molecule, (b) the chemical

structure of the DDAB molecule and (c) a schematic diagram of a

double functionalised Laponite disc with y-MPTMS molecules grafted

on its edges and DDAB molecules on its surface.................................... 75

Cryo-TEM images of polystyrene/Laponite nanocomposite particles by

Route I (a) and Route II (b). Images were taken from [43]. (c) and (d)

are the schematic representations of the polymer particles by Route I

and Route II, respectively...........................................................................76

The Easy Drop Standard setup, which was used to take the photos from

the side of a drying film. The image was taken and modified from [47] .

82

MR profiles of (a) 0 wt.% Laponite latex, synthesised by Route I and (b)

3 wt.% Laponite latex, synthesised by Route I over time. The profiles

were obtained every five minutes. In both cases the thickness decreases

at a constant rate, and non-uniformities in water concentration in the

vertical direction do not develop until the later stages of drying 83

Series of photographs taken from the side of a drying film of 0 wt.%

Laponite - Route II latex. The drying times are indicated on the figure.

................................................................................................................... 84

The GARField profiles of the latex with 3 wt.% Laponite by Route II.

The Film thickness initially decreases with no non-uniformities or

gradient in water concentration in the vertical direction. At a later stage,

the film thickness increases over time and simultaneously develops

gradient in water concentration................................................................. 85

Series of photographs taken from a side view of the drying film of 3

wt.% Laponite - Route II latex on a 2 cm x 2 cm glass substrate. The

drying times are indicated on the figure. Only the left-half of the film is

sh o w n ......................................................................................................... 86

Figure 4.15.

Figure 4.16.

Figure 4.17.

Figure 4.18.

Figure 4.19.

Figure 4.20.

Figure 4.21.

A series of MR profiles obtained at five minute intervals, from 3 wt.%

Laponite sample by Route II, showing the four stages of drying; Stage 1

- film thickness constantly decreases, no water concentration gradient,

Stage 2 - film thickness decrease slows down to a constant thickness,

Stage 3 - film thickness increases in the centre and water concentration

gradient develops, Stage 4 - film thickness decreases in the central

region. Arrows show the direction of increasing drying time................ 87

Comparison of normalised film thickness as a function of normalised

drying time for latexes with various Laponite concentrations by Route

II. The thickening effect is stronger with 3 wt.% Laponite, but it is still

noticeable with 5 wt.% and 7 wt.% Laponite concentrations................ 88

Comparison of (a) GARField profiles for the as-received latex of 3 wt.%

Laponite by Route II to profiles for (b) the same latex when its solids

content was raised to 30 wt.%, by evaporating water............................. 89

The normalized thickness as a function of the normalized drying time of

the latex with 3 wt.% Laponite by Route II. Results are shown for as-

received latex (black), latex with a higher solids content of 30 wt.%

(red), as received latex with added Servoxyl (blue), and for latex with a

30 wt.% solids content plus added Servoxyl (green).............................. 90

Optical transmissions (at a wavelength of 600 mn) as a function of

Laponite content for the films by Route I (red) and Route II (blue). . .92

Optical transmission (at a wavelength of 600 nm) as a function of

Laponite concentration for Route II latex as-received (red) and after

raising the solids to 30 wt.% and adding Servoxyl wetting agent (blue).

....................................................................................................................93

Schematic illustration of double functionalisation of a Laponite disc by

200% CEC of DDAB and MPTMS molecules, (a) A formation of a

double layer of DDAB molecules on the surface of Laponite disc, (b)

migration of excess DDAB molecules to the monomer droplet/water

interface and (c) migrated DDAB molecules diffuse to the water phase

xviii

and form surfactant aggregates when water is evaporated, 95

Figure 4.22.

Figure 4.23.

Figure 4.24.

Figure 4.25.

Figure 4.26.

Explanation of drying behaviour of the latexes by Route II. (a) Initial

polymer/Laponite particle, where excess DDAB molecules stick out of

the particle, (b) Due to water evaporation polymer particles are closely

packed near the edges of the drying film, (c) Desoiption of excess

DDAB molecules to the surrounding serum, (d) Development of a

DDAB concentration gradient, which causes a surface tension gradient

between the edges and the centre of the film, (e) Marangoni flow of

liquid from low to high surface tension regions. This process could

continue as long as the drying continues................................................ 9 7

Drying profiles of: (a) new formulation with less DDAB and low solids

content (19.2 wt.%), the film thickness decreases steadily over time and

the water distribution was non-uniform towards the end of drying, (b)

New formulation with less DDAB and high solids content (29.9 wt.%),

the film thickness decreases steadily over time and the water distribution

is uniform in the depth of the film...........................................................1 0 0

Normalised film thickness as a function of Normalised Drying Time for

original Route II (blue), new formulation with less DDAB and low

solids (red) and less DDAB with high solids (black). All three samples

contained around 3 wt.% Laponite.........................................................101

(a) Normalised zeroth moment and (b) Skewness of original Route II

nanocomposite (blue triangles), new formulation low solids (red circles)

and high solids (black squares). All three samples contained around 3

wt.% Laponite...........................................................................................102

Comparison of the photographs of the films made by (a) original Route

II, (b) new formulation with a low solids and (c) new formulation with a

high solids dispersions. The original Route II sample illustrates the most

irregularities on the film/air interface compared with the other two

xix

samples 103

Figure 4.27.

Figure 4.28.

Figure 4.29.

Figure 4.30.

Figure 4.31.

Figure 5.1.

Figure 5.2.

Comparison of optical transmission of the films obtained from original

Route II (red), new formulation with low solids (green) and new

formulation with high solids (blue) nanocomposites at a wavelength of

600 n m ..................................................................................................... 104

Thickness variations (film thickness as a function of normalised

distance, i.e. distance divided by total lateral distance) of the films by (a)

original Route II, (b) new formulation with low solids and (c) new

formulation with high solids using profilometry. It is significant that the

original Route II sample (a) has a ‘Mexican Hat’ shape, in comparison

to the other two film s..............................................................................106

Schematic illustration of a (a) concave wet film, (b) low surface tension

at the centre and high surface tension at the edges of the drying film, (c)

Marangoni flow from centre to the edges, (d) dry film with relatively

thin centre................................................................................................. 107

Illustration of the Perspex mould with the 20 mm diameter and 0.2 mm

deep cylindrical reservoir to obtain a concave wet film ..................... 107

Surface variations (vertical distance as a function of normalised, i.e.

distance divided by total lateral distance) of the dry films obtained by

initially concave wet films of (a) original Route II, (b) new formulation

with low solids and (c) new formulation with high solids using

profilometry............................................................................................. 108

Schematic illustration of (a) optically transparent film and (b) loss of

transparency due to larger pockets of water between particles 118

Schematic illustration of ‘skin formation’ or particle coalescence near

the air interface during drying of a latex film...................................... 119

xx

Figure 5,3.

Figure 5.4.

Figure 5.5.

Figure 5.6.

Figure 5.7.

Figure 5.8.

Figure 5.9.

SEM images of freeze-fractured surfaces of blend films of (a) 70% and

(b) 50% soft particles. Images taken from Reference [16]....................120

Three different types of absorption curves of relative mass uptake as a

function of square root if time; (a) typical sorption curve, (b) the ‘Two-

stage’ sorption curve and (c) the ‘Sigmoidal’ sorption curve, where the

curve is ‘S’ shaped. Drawn after [17].................................................... 122

Visible transmission spectra for films: (1) a newly formed film, (2) a

recently formed film exposed to 100 % RH for two days, (3) a film

heated in low humidity at 60 °C for two hours, (4) the well-heated film

exposed once again to 100 % RH for two days. Figure taken from [19].

.............................................................................................................124

Schematic representation of polymer particle with ‘hairy layer’, (a) At

high pH the charge repulsion by PAA chain increases and the chains are

widely spread; (b) as pH decreases, the hairy layer collapses Drawn after

[26] 126

Comparison of the water uptake of films by latex dispersions of

core/shell particles (poly styrene and poly butyl acrylate hydrophobic

core surrounded by a thin layer of hydrophilic poly acrylic acid and poly

butyl acrylate shell): % weight gain as a function of time. Films from

latex kept in the acidic form at pH 2 (filled circles), films from latex

partly neutralised at pH7 by NaOH (unfilled diamonds), films from latex

fully neutralised at pH 10 by NaOH (filled triangles), films from latex

neutralised by Ba(OH)2 at pH 7 (+). Figure taken from [2 1 ]............... 128

Predicted optical transmission of 100 pm thick films of a continuous

medium with n = 1.5 and containing spherical air voids (nv =1.0). The

volume fraction of voids (fv) is taken to be 0.25 (—); 0.025 (—) and

0.0025 (••■). The figure is taken from [25].............................................. 131

A schematic diagram of a latex particle with PIDA core and a PAA

xxi

shell, 133

Figure 5.10. (a) Photograph of the probe-tack analyser, (b) The appearance of fibrils

between the PSA and the probe during the debonding of the probe from

the PSA. Image (b) taken from [34].................................................137

Figure 5.11. Schematic illustration of cavitation and fibrillation development in PSA

debonding process, (a) Initial stress is imposed on the bulk of the film,

(b) at critical stress cavities will form in the bulk or at the interface of

the probe and the film, (c) formation of new cavities and expansion of

existing cavities, (d) the inter-cavity distance reach the initial film

thickness, (e) fibrils starting to appear before detaching. Image taken

from [7]...............................................................................................138

Figure 5.12. Typical force ( ------------ ) and displacement ( ----------- ) curves as a

function of time. Image was taken from [31] and modified. Details of

the process is given in the main text................................................ 139

Figure 5.13. The stress as a function of strain for PIDA without PAA at a pH of 3.

hi this probe-tack curve (Figure 5.13), the point when most cavities are

initiated is indicated by the maximum stress, a max. The plateau stress, ctp,

is related to the stress required to draw the fibrils. The maximum strain,

emaxor failure strain ef is the end of the deformation............................. 141

Figure 5.14. Stress-strain curves corresponding to different adhesion energies of

PSAs. Curve I is for low, Curve II is for intermediate and Curve III is

for high work of adhesion. Image taken from [7 ] .................................142

Figure 5.15. Water sorption kinetics for PIDA with 0 wt% PAA at a pH of 8 adjusted

by NaOH (blue), undergoing a humidity step change from 0% to 70%

RH at 25°C, (% mass change of the sample as a function of time in

minutes). The red line show the data fitted using the Equation 5.8 .. 144

Figure 5.16. (a) MR profiles of PIDA , acquired every five minute intervals. When

xxii

Figure 5.17.

Figure 5.18.

Figure 5.19.

Figure 5.20.

Figure 5.21.

the film reached its final thickness, the intensity of the NMR signal is

from the mobile polymer, (b) the zeroth moment (the area under each

profile), which is proportional to the water content of the sample at that

time, as a function of drying time, (c) the film thickness as a function of

time. Both the zeroth moment and the film thickness have a final value

due to the signal from the mobile polymer............................................. 146

(a) Solids fraction as a function of drying time and (b) as a function of

drying time normalised by the initial film thickness, for as receive,

therefore pH = 3, for PIDA with 3 wt.% PAA (filled squares) and for

PIDA with no PAA (empty squares). Pure PIDA reach the maximum

solids fraction earlier then PIDA with PAA sample, therefore, pure

PIDA completed the drying before PIDA with PAA............................ 147

Solids fraction as a function of drying time normalised by the initial film

thickness for pure PIDA and PIDA with PAA at a pH of 8 adjusted by

NaOH. The effect of PAA shell at a higher pH adjusted by NaOH is

insignificant............................................................................................. 149

Solids fraction as a function of drying time normalised by the initial film

thickness for PIDA with PAA (pH = 3), a pH of 8 adjusted by NaOH

and NH4OH..............................................................................................150

Photograph to compare the change in the optical transparency over time:

(a) PET substrate with the latex film as soon as it was submerged in

water, (b) after 1 hour, (c) after 7 hours and (d) after 24 hours. Image

from C.-H. Lei, University of Surrey...................................................151

Optical transmission at a wavelength of 600 nm as a function of the time

the film immersion in water. Open symbols are for the PSA films by

pure PIDA latexes and the filled symbols are for the PIDA with PAA

samples. The squares for low pH latexes (pH of 3), diamonds are for a

pH of 8 adjusted with NH4OH and the circles are for a pH of 8 adjusted

with NaOH. PIDA with PAA (pH of 3; filled squares) least lost its

xxiii

transparency and pure PIDA with a pH of 8 adjusted with NaOH the

fastest to lose its transparency................................................................. 152

Figure 5.22.

Figure 5.23.

Figure 5.24.

Figure 5.25.

Figure 5.26.

Figure 5.27.

% mass change as a function of square root of time for 0 wt.% PAA

sample (empty squares) and 3 wt.% PAA sample (filled squares).

Sample with PAA in its shell shows higher % mass change, compared to

no PAA sample........................................................................................ 154

% mass change as a function of square root of time for 0 wt.% PAA

sample (empty squares) and 3 wt.% PAA sample (filled squares) at a pH

of 8 adjusted by NaOH. Sample with PAA in its shell shows higher %

mass change, compared to no PAA sample, in a basic form................ 155

% mass change as a function of square root of time for the samples with

PAA in their shells. Red squares for a pH of 3, blue circles for a pH of 8

adjusted by NaOH and black diamond for a pH of 8 adjusted by

NH4OH...................................................................................................... 156

M m as a function of calculated characteristic time, t, for all six samples.

The samples without PAA showed lower compared with the

samples which contained PAA in their shells.......................................157

Optical transmissions of the films after 200 minutes in water as a

function of normalised time to lose all the mobile water within the films.

The drying properties get desirable towards the direction of arrow 1 and

the water resistance properties get desirable towards the direction of

arrow 2 ....................................................................................................... 160

(a) M n as a function of normalised time to lose all mobile water with in

the films. The drying properties get better towards the arrow 1 and the

dry films adsorb less moisture from high humidity towards the arrow

head two. (b) t as a function of normalised time to lose all the mobile

water with in the films. Towards the direction of arrow head two, the

xxiv

dry films adsorb moisture faster from high humidity. 162

Figure 5.28.

Figure 5.29.

(a) and (b) x as a function of optical transmissions (at 600 mn) of

the films after 200 minutes in water........................................................164

(a) The adhesion properties of PSA film with 3 wt.% PAA in the shell,

at a pH of 3, performed at 90 minutes, 24 hours and 6 days after it was

cast, (b) The adhesion properties of PSA film with 3 wt.% PAA, at a pH

of 8 by NH4OH for different drying times and (c) summarises the

adhesion energy of PSA film with 3 wt.% PAA in shell at a pH of 3 and

at a pH of 8 NH4OH for different drying times..................................166

xxv

List of Tables

Table. 2.1.

Table. 2.2.

Table. 3.1.

Table. 4.1.

Table. 4.2.

Table. 4.3.

Table. 5.1.

Table. 5.2.

Table. 5.3.

Colloids as products and processes in everyday life [4 ] ............................. 8

Classification of colloids depending on the type of dispersed and continuous

media, with some common examples [2 ] ...........................................................9

The meanings of the first five moments [16].............................................59

Characteristics of Polymer/Laponite nanocomposite latexes synthesised

through miniemulsion polymerisation with AIBA functionalised Laponite

for Route I ..................................................................................................... 77

Characteristics of Polymer/Laponite nanocomposite latexes synthesised

through miniemulsion polymerisation with MPTMS/DDAB

functionalised Laponite for Route II............................................................78

Components and characteristics of the ‘original Route II and new

formulations (less DDAB/low solids and less DDAB/high solids) 98

Summary of the compositions and characteristics of the latex samples

....................................................................................................................... 134

Calculated and characteristic time, t for different samples 157

A summary of the results for all six samples of PSAs............................. 169

xxvi

Chapter 1

Overview

Film formation of latex has been studied since the early 1950s, due to the enormous

practical interest in these types of products. [1-3]. Application of modem

instrumental methods and the fundamental understanding of the process were greatly

developed during the last couple of decades. Latexes are being used in a broad range

of fields from adhesives, inks, paints, coatings, drug delivery systems, medical assay

kits, gloves, paper coatings, floor polish, films, carpet backing, foam mattresses,

cosmetics and many more [4]. The increased use of latexes as key replacement

materials for many solvent-based systems has largely been driven by tightening

legislation worldwide [5].

Contributions by scientists to this field during recent years have revolutionised the

development of latex synthesis [6 ]. Additionally, the interest in film formation

mechanisms was renewed thanks to the introduction of new techniques such as

atomic force microscopy (AFM), fluorescent labelling, environmental scanning

electron microscopy, direct nonradiative energy transfer, small angle neutron

scattering and other optical techniques [2 ]. hi the literature, leading scientists have

Chapter 1

developed theoretical models to understand the mechanisms of film formation [7].

Even so, due to the complexity of the film formation process, there is still room for

development.

This thesis is written in the frame of NAPOLEON - NAnostmctured waterborne

POLymeEr films with OutstaNding properties - project [8] which began in June

2005. The main objective of this project is to develop a technology platform to

produce waterborne nanocomposite colloidal dispersions to overcome the

environmental issues associated with coatings and adhesives dissolved in organic

solvents. These dispersions are to be of high solids, which offer energy savings in

production and transport. The breakthrough idea is to use waterborne nanocomposite

nanoparticles with well controlled structure as building blocks to produce films with

outstanding properties. Among these properties are good adhesion to substrates,

increased hardness, strength and wear-resistance with low dirt pick-up, higher

impermeability to liquids and gases, and greater fire resistance.

To achieve these objectives, the NAPOLEON Project integrated 9 EC companies and

12 academic research centres. Nanoparticle production and study of their process

and properties were mainly expected from the academic partners, while industries

ensured the proper scale up and product development. All 21 partners of this project

were divided into different key scientific aspects, called ‘work packages’,

considering their expertise, and assigned one or several tasks. The University of

Surrey group is part of work package four, which focuses on the film formation

process of the waterborne nanocomposite colloidal dispersions developed by some of

the other NAPOLEON partners.

The overall task of this thesis was to investigate the film formation process of the

latexes developed by the NAPOLEON project through the physical characterisation

of the process and film properties. A series of organic/inorganic waterborne

nanocomposites for coating application were investigated. In addition, the influence

of pH and acrylic acid (AA) on the physical characteristics of film formation and

film properties of waterborne pressure sensitive adhesive films were studied.

2

Chapter 1

The contents of the thesis are organised as follow.

Chapter Two provides an introduction, which is to present the different scientific

fields used throughout this work. It starts with a general introduction to polymer

colloids and their classification, followed by an introduction to latex dispersions and

their synthesis by different polymerisation methods. Then, the charges in the

colloidal dispersions, their stability and the factors affecting their stability are

discussed. This is followed by a detailed insight into film formation of colloidal

dispersions, stages of film formation, driving forces of particle deformation and the

supporting models for drying dispersions.

Chapter Three is devoted to magnetic resonance profiling (MRP). Out of all the

techniques used in this study, such as UV/visible spectroscopy for optical

transmission measurements, gravimetric analysis for moisture sorption

measurements, probe tack tests for adhesion measurements, profilometry for surface

variation analysis and the methods used for obtaining photos, MRP was used to

collect the majority of data. For that reason, Chapter Three is dedicated to historical

background, the fundamental principles of MRP followed by a detailed description

o f MRP using the GARField magnet, together with MRP measurements and further

calculations on these measurements.

Chapter Four explores the physical characterisation of the film formation process and

the film properties of organic/inorganic nanocomposite coating formulations. Methyl

methacrylate (MMA) and butyl acrylate (BuA) were used as the monomers to obtain

the organic phase, and clay (Laponite) was used as the inorganic phase. These

coating formulations were synthesised by miniemulsion polymerisation. Two

systems referred to as “Route I” and “Route II”, were developed. The main

difference between the two approaches lies in where the clay - Laponite - was at the

beginning of the polymerisation process. In Route I, Laponite was in the water phase

and in Route II, it was dispersed in the monomer phase. In Route I, the clay plates

are expected to be encapsulated in the latex particles. Whereas, in Route II, the latex

particles are expected to be surrounded by the clay plates. At the beginning of this

3

Chapter 1

chapter, an introduction is given to Laponite clay particles and synthesis of

nanocomposite formulations by Route I and II. The film formation and film

properties of Route I and II latexes were investigated. Greater insights into latexes by

Route II are given, as they were significantly different to that of Route I, followed by

discussions on the modifications which were made to Route II latexes to improve the

drying properties.

Chapter Five presents studies on the influence of pH and acrylic acid (AA) on drying

and water whitening of latex pressure-sensitive adhesive (PSA) films. “Water

whitening” is the loss of optical transparency when colloidal films are exposed to

high humidity or soaked in water, which is a common problem. To minimize the

water whitening, a film should achieve complete particle coalescence during drying.

However, early stage good coalescence could lead to “skin formation” and slow film

drying - which is not wanted in most applications. The coalescence of particles near

the air interface during drying is referred to as “skin formation”. The skin can lead to

slower transport of water and hence retard water loss during drying and cause water

entrapment within the film. Trapped water is also associated with inhibited

interdiffusion and hence weaker films. Preventing coalescence could avoid skin

formation, but the resulting film could be subject to water whitening and poor barrier

properties. In this chapter, the effects of AA and pH on drying, “water whitening”,

water sorption from high humidity, and adhesion properties o f waterborne adhesive

films and correlation between them are reported.

Chapter Six presents the final reviews of this research work and proposes potential

future work.

4

Chapter 1

1.1. References:

1. Guigner, D., Fischer, C., and Holl, Y., Film formation from concentrated

reactive silicone emulsions. 1. Drying mechanism. Langmuir, 2001.17(12):

p. 3598-3606.

2. Keddie, J.L., Film formation o f latex. Materials Science & Engineering R~

Reports, 1997. 21(3): p. 101-170.

3. Brown, G.L., Formation o f Films from Polymer Dispersions. Journal of

Polymer Science, 1956. 22(102): p. 423-434.

4. Hunter, R.J., Introduction to Modern Colloid Science. 1993: Oxford

University Press.

5. Jotischlcy, H., Coatings, regulations and the environment reviewed. Surface

Coatings International Part B-Coatings Transactions, 2001. 84(1): p. 11-20.

6 . Qiu, J., Charleux, B., and Matyjaszewski, K., Controlled/living radical

polymerization in aqueous media: homogeneous and heterogeneous systems.

Progress in Polymer Science, 2001. 26(10): p. 2083-2134.

7. Routh, A.F. and Russel, W.B., Deformation mechanisms during latex film

formation: Experimental evidence. Industrial & Engineering Chemistry

Research, 2001. 40(20): p. 4302-4308.

8 . http ://www. ehu. es/nanoleon/.

5

Chapter 2

Introduction

2.1. Introduction to colloidal dispersions

Evidence of humans’ use o f colloids dates back to the earliest records of civilisation.

Written records o f Egyptian pharaohs and Stone Age paintings in the Lascaux caves

of France were found to be produced by colloidal pigments. Colloidal systems were

used in many of our earliest technological processes, such as papermaking, pottery

making and cosmetic and soap fabrication. In the literature one can trace that

Francesco Selmi, in 1845 described the first examples of colloidal particles and

defined their common properties. In the 1850s Michael Faraday made extensive

studies of solid colloidal gold particles in water. But, it was Thomas Graham, in

1861, who coined the term colloid, meaning “glue” in Greek. Over the years, the

fundamentals have developed in parallel to industrial usage of colloidal systems [1 ].

Colloidal dispersions are defined as at least a two phase system, where one phase,

usually sub micrometer particles (the dispersed phase) is dispersed in the second

phase (continuous phase). The dimension of the dispersed phase is within the range

Chapter 2

from 1 nm to 1 jam. The continuous phase may be gas, liquid or solid whilst the

dispersed phase also can be gas, liquid or solid [2 ].

The large surface area and high surface area-to-volume ratios, due to their very small

dimensions, forms an extremely important and a basic characteristic of all colloidal

systems that involves directly with its ability to interact with its environment. Larger

particle surface area allows more functionalised groups to be on its surface. As

reactions take place on the surface, this leads to faster reactions between colloidal

particles and their environment.

As an example, the surface area of a sphere (A) given by 4nR2, and its volume (V) by

(4/3) ttR where R is the radius. The surface area-to-volume ratio for a spherical

particle is therefore given by:

Furthermore, 1 litre of a dispersion of 50% solid content with particles of 100 nm

surface area.

In addition, colloidal particles are in constant motion in the dispersion, and

demonstrate Brownian motion, which results from a random number of impacts of

random strength from random directions in any short period of time. Brownian

motion was first recorded by the botanist Brown while studying a suspension of

pollen grains using an optical microscope [3].

The understanding of colloidal phenomena has advanced in recent years at a greater

speed due to their importance as nanomaterials and nanotechnology in everyday life.

The products and processes listed in Table 2.1 show some cases where colloid

technology is directly involved [4].

A 4 ttR1 3(2 .1)

radius would have a particle surface area of 5 xlO4 m2. In comparison, the area of a

football pitch is around 1 0 4 m2, which shows that colloidal particles have a large

7

Chapter 2

Table 2.1. Colloids as products and processes in everyday life [4]

ProductsH Surface coatings : paints, photographic films, video tapes

■ Cosmetics and personal care : creams, toothpaste, hair shampoo

■ Household products : liquid detergents, polishes, fabric conditioners

H Agrochemicals : pesticides, insecticides, fungicides

■ Pharmaceuticals : drug delivery systems, aerosol sprays

■ Food : butter, chocolate ice cream, mayonnaise

■ Pigmented plastics

B Fire-fighting foams

Processes■ Clarification of liquids : water, wine beer

■ Mineral processing : flotation, selective flocculation

■ Detergency : ‘soil’ detachment, solubilisation

H Oil recovery : drilling fluids, oil slick dispersal

■ Engine and lube oils : dispersion of carbon particles

H Silting of river estuaries

H Ceramic processing

■ Road surfacing : bitumen surfacing

In natural systems■ Biological cells

■ Mists and fogs

8

Chapter 2

2.2. Classification of colloidal dispersions

Colloids can be classified depending 011 the type of dispersed and continuous phase.

Table 2.2 gives a summary of various possible types of dispersions, their technical

name with some common examples.

Only the gas/gas dispersion is not listed in Table 2.2, as those systems could last only

for a very short time [2 ].

Table 2.2. Classification of colloids depending on the type of dispersed and continuous media, with some common examples [2 ].

Dispersed phase Continuous phase Technical name Examples

Solid Gas Aerosol Smoke, dust

Liquid Gas Aerosol Fog, mist

Solid Liquid Sol or colloidal solLatex paints, adhesives, pigmented ink

Liquid Liquid Emulsion Milk, mayonnaise, hand cream

Gas Liquid Foam Foam, froth, whipped cream

Solid Solid Solid sol Ruby glass, cranberry glass

Liquid Solid Solid emulsion [5] Cheese, gelatine

Gas Solid Solid foam Aerogel,Styrofoam

9

Chapter 2

Colloidal dispersions may be classified as lyophilic (‘solvent-loving’) or lyophobic,

(‘solvent hating’), depending on the ease with which the system can be re-dispersed

once it is allowed to dry [2], Furthermore, when the particles of the dispersed media

are nearly of the same size (only vary by one or two percent), the colloidal system is

called monodisperse, and in the opposite cases they are heterodisperse or

polydisperse.

When solid particles are dispersed in a liquid, it is called a sol. If these solid particles

are polymer particles and the liquid is water, then the resulting dispersion is latex [6].

Increased interests in latexes were initiated in the beginning of the 1980s. The

underpinning reasons were a combination of increased public awareness on

environment and increased knowledge of the negative environmental and health

effects of solvent-based products. These products emit volatile organic compounds

(VOCs) to the atmosphere. The introduction of legislation reducing and regulating

the emission of VOCs has driven industry to convert from traditional solvent-based

products to more environmentally friendly products [7]. hi response, the demand for

waterborne systems was drastically increased. Currently latexes are undergoing

extensive research and development as key replacement materials for many solvent-

based products. One should appreciate that there is still much to understand and

develop, so it is still a developing science.

10

Chapter 2

2.3. Synthesis of latex

Latexes are commonly produced by emulsion and miniemulsion polymerisation

processes. Both processes are performed in water and produce stable latex

dispersions.

2.3.1. Emulsion polymerisation

Emulsion polymerisation process allows the production of submicron size polymer

particles, finely dispersed in the aqueous medium, thus forming latexes [8], that have

many advantages. One of them is the low viscosity of the latex it produces. Due to

the low viscosity, heat removal is easy and good temperature control can be obtained.

This allows a higher polymerisation rate (i.e. shorter production time). Low viscosity

of the system allows an easy removal of unreacted monomers and VOCs. Emulsion

polymerisation is environmentally friendly as water is used as the reaction medium.

There is no need for further treatment of the latex obtained by emulsion

polymerisation before their applications (Post-polymerisation additives, e.g. surface-

active agents, antioxidants, fungicides etc. can be added to the latex if needed). More

importantly, new products can be produced by emulsion polymerisation to meet

today’s market needs. The noticeable disadvantage is that there could be some

impurities in the polymer, like emulsifier and the rest of the initiator, which gives

water sensitivity to the polymer. In modern days up to a certain degree this is tackled

by using reactive surfactants [9]. Surface active molecules with an active vinyl group

are known as reactive surfactants. They are used to bind the surfactant chemically to

the surface of the particles. The advantage of using reactive surfactants is to reduce

desoiption of water during film formation and also to reduce the water sensitivity of

the latex film [1 0 ].

Emulsion polymerisation process itself is a complex heterogeneous free-radical-

initiated chain polymerisation process. A typical emulsion polymerisation starts with

11

Chapter 2

water, hydrophobic monomer (or mixture of monomers), surfactant and a water-

soluble initiator. The most important aspect of a surfactant (also referred to as

emulsifier) is its structure, which gives it its amphiphilic nature. It consists of at least

two parts: a polar group (hydrophilic, usually a water solubility enhancing functional

group) and a nonpolar group (hydrophobic or lyophilic, usually a long alkyl chain).

The surfactant molecules perform the dual function of providing sites for particle

nucleation, as well as providing colloidal stability to the growing particles [9].

In principle the reaction is initiated using a water-soluble initiator(s), and the most

commonly used is the inorganic salt of persulfuric acid (e.g. potassium persulfate).

The initiator dissociates into two sulphate radical anions which can initiate the

polymerisation reaction [10]. The stability of the reaction mixture is normally

ensured by the utilization of stabilizer(s). Other ingredients such as buffers and chain

transfer agents are also commonly present in a reaction mixture [9]. Usually, the

monomer(s) is dispersed in an aqueous surfactant solution with a concentration

exceeding the critical micelle concentration (CMC). At or above CMC, surfactants

form aggregates. The aggregates formed in an emulsion polymerisation are often

spherical and refer to as micelles [9], the core of which is swollen by the monomer.

Emulsion polymerisation is usually characterised by three distinctive intervals,

named interval I, interval II and interval III. Figure 2.1 illustrates these three

intervals.

12

Chapter 2

interval I *T

monomer T droplet &C

S

R\free radical surfactant initiator latex particle micelle

Interval If. j /-f-x

y . 9 * < $ ■

Interval Iff.

ft

ET

,

Figure 2.1. Schematic diagram illustrating the three intervals of an emulsion polymerisation process [11]. Interval I: Particle formation stage, represented by the increase in both the number of particles and the polymerisation rate. Interval II: Particle growth: during which, both the number of particles and the polymerisation rate remains relatively constant. Interval III: The number of particles remains unchanged, while polymerisation rate and the monomer concentration decrease[l 1 ].

Upon mixing the monomer(s), water and surfactant(s), the latter molecules cluster

into micelles with their hydrophobic cores being swollen with the monomer. This

stage is illustrated in the interval I - Particle formation stage - in Figure 2.1. The

interval I represented by an increase in the number of particles. The polymerisation

rate also increases, as shown in Figure 2.2. However, the bulk of the monomer exists

in the form of large-size droplets with surfactant molecules absorbed on to their

surfaces. When the initiator is added radicals are formed in the aqueous phase and as

these radicals are too hydrophilic to enter into the organic phase of the system, they

react with the monomer dissolved in the aqueous phase, forming oligoradicals. After

13

Chapter 2

adding a few monomer units, the oligoradicals become hydrophobic enough to be

able to enter into the micelles. Due to the high concentration of monomer in the

micelle, the oligoradicals that entered into the micelle grow fast and form a polymer

chain (or particle) and this is known as nucleation [1 0 ] .

As the nucleation progresses, the number of micelles decreases, because they become

polymer particles, and after some time all the micelles disappear. This corresponds to

the end of interval I. The duration of interval I vary within the range of 2 - 10 %

conversion. This depends on a few factors: the type and the concentration of

surfactants, the initiation rate, the degree of water solubility of monomers, etc.

Interval II extends from 5 - 1 0 % to 3 0 -7 0 % conversion. At the end of interval II,

monomer droplets disappear [8 ]. hi interval III the monomer, only present in the

particles, continues to polymerise, resulting in both decreasing monomer

concentration and polymerisation rate, as shown in the Figure 2.2. The number of

particles remains the same [8 , 1 2 ].

Figure 2.2. Polymerisation rate as a function of time for three intervals of emulsion polymerisation process [8]. During interval I, the polymerisation rate increases, during Interval II it stays relatively unchanged and finally, it decreases during interval III.

In emulsion polymerisation three types of processes are commonly used, they are:

batch, semi-continuous (or semi-batch) and continuous, hi batch polymerisation all

14

Chapter 2

the ingredients are added at the beginning of the processes and the polymerisation

starts as soon as the initiator is added to the system. The temperature rises with the

simultaneous formation and growth of the particles. Removal of heat generated by

polymerisation is a daunting task in batch polymerisation [9]. hi a semi-continuous

polymerisation process, one or more ingredients are added continuously or in

batches. The advantage of this process compared to batch polymerisation is the

ability to have more control over the process. The rate of polymerisation and the rate

o f generation and removal of heat can be controlled more easily. Greater control over

the particle number, colloidal stability, copolymer consumption and particle

morphology are other aspects of the semi-continuous polymerisation process [10]. In

the continuous process, the ingredients are fed continuously into a stirred tank (or

more than one connected in series). The latex product is also simultaneously

removed at the same rate. The main advantages are high production rate, more

controlled heat removal and higher quality of latexes [1 0 ].

2.3.2. Miniemulsion polymerisation

Miniemulsion polymerisation has been identified as a special case of conventional

emulsion polymerisation. It is a heterogeneous process. Miniemulsions are aqueous

dispersions of stable nanodroplets of a diameter within a range o f 50 - 500 mn with a

narrow size distribution. Monomer droplet size and the size distribution are the most

important parameters of the miniemulsion polymerisation as they affect both

miniemulsion stability and droplet nucleation directly [13]. The main differences

between miniemulsion polymerisation and emulsion polymerisation are the addition

of a hydrophobic agent (also called a co-stabilizer or an osmotic pressure agent) and

high shearing of the system such as with ultrasonication [12]. These so-called ‘co-

surfactant’ hydrophobic agents enhance the stabilisation of the submicron size

monomer droplets produced by the high shearing [1 2 ].

The first step of the miniemulsion polymerisation process is to form submicron size

stable monomer droplets through the shearing of the system containing the dispersed

15

Chapter 2

phase, the continuous phase, a surfactant and a hydrophobic agent. This step is

shown in Figure 2.3, left and the middle panels. In the second step, these droplets

undergo the polymerisation reaction (Figure 2.3, middle and right panels). The

absence of a diffusion process makes this process hugely useful for industrial scale

for continuous reaction processes [14].

r r

ultrasoundm Phase I • •

• • • 3 ^ 3 ^ reaction

- v i 1© - o

* N s

^ o ^^ Phase II

stable nanodroplets I : I copyas small compartments (nanoreactor)

Figure 2.3. Principle of miniemulsion polymerisation. In the first step of miniemulsion polymerisation, submicron size monomer droplets are produced by shearing and in the second step, these droplets act as individual batch reactors that undergo the polymerisation reaction [5, 14, 15].

In emulsion polymerisation the particle size is determined by the kinetic parameters

like temperature and initial initiator concentration. Where as, in miniemulsion

polymerisation it is essentially the amounts of surfactant and the monomer present

and the intensity of the shearing used to prepare the droplets that determine their size.

In the miniemulsion polymerisation, all or most of the stable submicron size droplets

which were formed are polymerised into polymer particles. The key issue for a

successful miniemulsion polymerisation is for these droplets to remain stable for the

entire reaction time. Therefore, the choice of a suitable hydrophobic agent is vital

[8].

There are some unique advantages of miniemulsion polymerisation over an emulsion

polymerisation. Undoubtedly, the monomer droplets are the main focus of the

16

Chapter 2

process. Therefore, the nucleation step is not as complex as in emulsion

polymerisation. Furthermore, the system is simplified to two phases, a water phase

and monomer/polymer particles throughout the process. Miniemulsion

polymerisation is a far better process for the very water-insoluble monomers, as they

would transport slowly through the aqueous phase in an emulsion polymerisation

[16]. It is also very useful process for conducting polymerisations of monomers with

significantly different water solubilities and for various encapsulating systems [17,

18].

17

Chapter 2

2.4. Charges in the colloidal dispersions

According to fundamental thermodynamics, all systems strive to decrease their total

free energy in a system kept at a constant temperature [19]. hi addition, between

neutral particles in any suspension, due to the induced dipoles, there is an attractive

van der Waals interaction. Due to this attractive force, the particles tend to aggregate.

To prepare stable colloidal dispersions it is necessary to introduce interactions

between particles that are opposite to the van der Waals attractions. One way of

achieving this is to have a charge on the particles. This surface charge would result in

a repulsive inter-particle force [2 0 ].

During synthesis, many polymer particles gain a surface charge, due to the ionisable

groups at the end of the polymer chains. In addition, the surface charge can be further

modified by changing the environment of continuous phase, as an example by

changing the pH, or by adding a differently charge surfactant [4].

In an electrolyte, the charged particles are surrounded by ions that shield their

surface charge. The boundary layer of ions is defined as the electrical double layer.

In the literature, there are a few models to describe how the potential changes as a

function of the distance from the centre of the particle, into the surrounding medium.

During 1860’s Helmholtz presented the first model [21], Then Gouy and Chapman

worked independently to develop their models. Later Stern combined these models

and further developed them [4].

2.4.1. Helmholtz model

Helmholtz suggested a simple model of a charged metal interface immersed in

electrolyte. In his model, he suggested that the surface charge is counterbalanced by

a layer of oppositely charged ions. Figure 2.4 shows, in the proposed model the

potential is dropped across the single layer of ions [4].

18

Chapter 2

Figure 2.4. Schematic of the Helmholtz model, which failed to explain the experimental results [4].

2.4.2. Gouy - Chapman model

In early 1910 Gouy and Chapman independently developed a model, where the

surface charge of the solid is balanced by a diffuse layer of ions. This diffuse layer

contains an excess of ions of opposite charge on the charge of the surface as shown

in Figure 2.5 [4].

19

Chapter 2

© © ©© © ©

© ° ©© © © © ©

Diffuse layer

© ©©

©

©

©©©

Figure 2.5. Schematic of the Gouy - Chapman model [4].

2.4.3. Stern model

Later, Stem combined both the Helmholtz and Gouy-Chapman models and further

developed them. In his model, the counter charge exists in two distinctive regions:

( 1 ) the inner layer of ions of opposite charge to the surface, ‘stuck’ to the surface and

(2) a diffuse layer, as shown in Figure 2.6. The plane between the electrode surface

and the outer Helmholtz plane (OHP) is defined as the Stem layer, which is the

closest approach of the centre of a hydrated ion to the electrode. At the electrode

surface the potential is \\is and at the OHP the potential is mu. The potential drop

across the Stem layer (\jjs - MU ) is assumed to be linear. However, in the diffuse

layer the potential drops exponentially [4].

20

Chapter 2

Outer Helmholtz plane

Stem layerY*̂ Diffuse layer

Figure 2.6. Schematic of the Stem model which consist of Stem layer, where potential drops linearly and the diffuse layer, where the potential drops exponentially with the distance [4].

The electrostatic potential can be positive or negative and depends on the chemical

nature of the surface group and on the surface charge density. Boltzmann’s

distribution law says that if the energy associated with some state or condition of a

system is s then the frequency with which that state or condition occurs, or the

~ s/ k T

probability of its occurrence, is proportional to e where T is the system’s

absolute temperature and where k is the Boltzmann Constant = 1.38 x 10'23 J/K. For a

plane surface, if the ions are distributed according to the Boltzmann’s distribution

law in the diffuse layer, the potential energy ( ®x) at a distance x from the surface

can be determined by [2 2 ]:

0 , = <J>se (-fa) (2.2)

21

Chapter 2

For a spherical particle with a radius of R, the potential energy (O,.) at a distance rx

from the particle is given by:

the double-layer “thickness” [22]. The Debye screening length is a characteristic

length for a given aqueous solution. The magnitude of the Debye screening length

depends solely on the properties of the liquid and not on any other property of the

surface, e.g. its charge or potential [23].

K can be calculated by the following equation.

where e, electronic charge = 1.6 x 10' 19 C; no, ionic concentration; Z , valency of the

ions in solution; s ; the dielectric constant of the continuous phase; So, the dielectric

constant of vacuiun. At 298 K the Debye length of 1:1 electrolytes such as NaCl

298 K, for NaCl solution, the Debye length, UK is equal to 30.4 nm for 10'4 M

solution, 9.6 nm at 1 mM solution, 0.96 nm at 0.1M solution and 0.3 nm at 1 M

solution. The Debye length for totally pure water at pH 7 is 960 nm [23].

(2.3)

The parameter K is important, as K 1 is the Debye screening length or more usually

(2.4)

aqueous solutions is equal to 0.304 / in mn< For 2:1 and 2:1 electrolytes/ sjlNaCl]

(e.g., CaCL and Na2S0 4 ) it is 0.176 / in nm and for 2 :2 electrolytes (e.g.,/ *v \CctCl2 J

M gS04) the Debye length is equal to 0.152 / ----------- in nm. As an example, at/ f [ M g S Q 4]

22

Chapter 2

2.5. Stability in the colloidal dispersions

One of the important aspects of colloidal dispersions is the stability of the colloidal

state. This phenomena, which fascinated Faraday 140 years ago, still remains

interesting to today’s scientists, although with more sophisticated implications. The

competition between attractive van der Waals and repulsive double-layer forces

determines the stability or instability of many colloidal dispersions [24]. The stability

in colloidal dispersions is defined either in terms of their tendency to aggregate or

their tendency to sediment under the action of gravity [4]. Reversible aggregation is

often called flocculation while irreversible aggregation is called coagulation [24].

2.5.1. The DLVO Theory

During the 1940s a major advance in colloidal science occurred when two groups of

scientists - Boris Deijaguin and Lev Landau from the Soviet Union and Evert

Verwey and Theo Overbeek from the Netherlands - independently published their

analysis of the stability of colloidal systems. It is now called the DLVO theory in

honour of them [24].

The DLVO theory assumes that the interparticle interactions control the stability of

colloidal system. It considers two types of forces: the attractive van der Waals force

and the repulsive double layer force. The van der Waals forces operate irrespective

of the chemical nature of the particles or the medium, and if the particles are similar,

this force is attractive. On the other hand, the double layer forces are acquired due to

a charge on the particles; either from surface charge groups or by specific ions

adsorbed to the particles from the solution and are sensitive to variations in

electrolyte concentration and pH. The double layer forces are repulsive and may be

considered as fixed in a first approximation, hi addition, the van der Waals attraction

must always be greater than the double layer repulsion at small distances, D, between

particles because it is a power-law interaction, that is interaction energy,

23

Chapter 2

W oc ~ y'j^n , whereas the double-layer interaction energy remains finite or rises

much more slowly as D decreases to 0 [23].

The total potential energy of interaction ( W t) can be expressed as the sum of the

attractive (W A) and repulsive (Wr) energies [23]:

WT = W A + W R (2.5)

The attractive energy can be written in more detail as:

WA = -AR/ 6 D (2.6)

where A is the Hamalcer constant and typical values for A of condensed phases,

whether solid or liquid, are about 10' 19 J for interactions across vacuum [23].

The repulsive energy in more detail given by:

WR ={647tlcBTR pa>r 2 I K 2)e~KD (2.7)

where is the ionic concentration of ions in the bulk, and y is the charge on the

particle [23].

24

Chapter 2

Figure 2.7. Schematic of the energy versus the distance between particles profilesof DLVO interactions. This figure is taken from Jacob Israelachvili, Intermolecular and Surface Forces 1992: Academic Press Limited, London; page 248 [23]. Highly charged surfaces repel strongly and remain stable which is illustrated in lower inset ‘a’, ‘b* where surfaces come into stable equilibrium at secondary minimum and colloidal particles remain lcinetically stable. *c’ when surfaces come into secondary minimum and particles coagulate slowly, ‘d’ Surfaces may remain in secondary minimum or adhere and colloids coagulate rapidly, at the ‘critical coagulation concentration’, ‘e’ Surfaces and colloidal particles coalesce rapidly [23].

The total potential energy of interaction (Wj) curve has a number of important

features: The particles with highly charged surfaces in a dilute electrolyte have a

relatively long Debye length; hence they have a long-range repulsion which is quite

strong. This repulsion would peak usually around 1-4 nm at the energy barrier. This

situation is illustrated in Figure 2.7 (a). When the concentration of the electrolyte

25

Chapter 2

solution is slightly higher, the Debye length is not as long as in the previous

situation. As the distance decreases, the total potential curve goes through a

significant secondary minimum, usually beyond 3 nm, before the energy barrier,

which is illustrated in Figure 2.7 (b). The primary minimum is defined as the

potential energy minimum when the particles are at contact. If the primary minimum

is deep and the energy barrier is too high to be overcome, the particles could stay in

the secondary minimum or they could be totally dispersed in the solution. The

particles with a very low charge density have a relatively short Debye length; their

energy barrier would be much lower leading to a slower aggregation called

flocculation (Figure 2.7 (c)). When the energy barrier falls below the interaction

energy, W = 0 (Figure 2.7 (d)), the concentration of the electrolyte is called the

critical coagulation concentration (CCC). Then the particles coagulate rapidly, and

this colloidal system is referred to as being unstable. For the particles with very low

or zero surface charge, the Debye length is very small or nonexistent, hi this case, the

interaction curve approaches the pure van der Waals attraction curve and particles

attract each other strongly, which is illustrated in Figure 2.7 (e) [23].

There are two types of aggregation: Coagulation is the rapid aggregation that occurs

in the absence of an energy barrier or a primary maximum (Figure 2.7 (e)). This

leads to a strong irreversible structure. Flocculation is the reversible aggregation that

occurs in a secondary minimum (Figure 2.7 (b), (c), (d)). Flocculation is reversible

on the addition of energy to the system usually by applying shaking, stiring or other

mechanical process [4].

2.5.2. The factors affecting the stability in the colloidal

dispersions

In the literature, it has been reported that various factors could affect the stability of

colloidal system. The ion type and their concentration in the system, the size of the

particles and the value of the £ - potential of the particles are considered to be most

important [4].

26

Chapter 2

2.5.2.1. Particle size

The total potential (Vt) is directly proportional to the particle size, at small particle

sizes (<100 nm radius). However, at larger sizes, the variation of the total potential is

not that straight forward. If all other factors remain constant, a larger particle radius

leads to a higher energy barrier therefore increased electrostatic stability. For small

particle sizes the energy barrier is directly proportional to the radius [4].

2.5.2.2. Zeta - potential

The Zeta (£,) - potential is the potential at the shear plane, the effective location of

the solid/liquid interface. It is possible to measure Zeta (C,) - potential

experimentally using electrokinetic methods. [4], [25].

D (nm)

Figure 2.8. The effect of ̂ potential on the shape of the total interaction potential curve for polystyrene latex. From top to bottom the £ potentials are -80 mV, -50 mV, -25 mV and -20 mV [4].

Figure 2.8 show an example for polystyrene particles in 1 mM NaOH solution.

Since, V r is proportional to the square of surface potential; a doubling of C, potential

27

Chapter 2

should lead to a quadrupling of the value of Vmax. From the VT as a function of

distance plot in Figure 2.8, when the £ potential is -25 mV the value of Vmax is

around 40 kB. When the (^potential is doubled to -50 mV, Vmax is around 160 kB.

This shows that V r is proportional to the square of surface potential.

28

Chapter 2

2.6. Film formation of colloidal dispersions

When a latex dispersion is applied on a substrate and evaporation is allowed, a

formation of a continuous, homogeneous film under appropriate conditions is called

film formation [6 ]. Formation of a latex film arises with the coalescence of the latex

particles. Normally, in a latex dispersion, these individual latex particles are held

apart by stabilising forces resulting from the charges on polymer particles or by the

addition of emulsifiers and stabilisers.

2.6.1. Stages of film formation

In the most general sense, film formation involves three stages [6 ]: Stage I -

Evaporation of water and particle ordering, Stage II - Particle deformation and Stage

III - Coalescence/Interdiffusion across particle-particle boundaries. These three

stages are schematically illustrated in Figure 2.9.

Polymer dispersionPolymerparticles

Aqueous phase

Mechanically coherent homogenous film

Waterevaporation

> = >

T>Tg

< = >

Particles in close packing

Coalescence/Interdiffusion/

aging Honey-comb structure of deformed particles

Figure 2.9. Illustration of the stages of film formation process [6]. In stage one, water evaporates from the polymer film and particles re-order to a close packed array. In stage two, particles deform, when the drying temperature is greater than the Minimum Film Formation Temperature, MFFT. In the final stage, particles coalesce and molecules diffuse, when the drying temperature is higher than the Tg of the latex, and a mechanically coherent homogenous film is formed.

29

Chapter 2

2.6.1.1. Stage I - Evaporation of water and particle

ordering

At the beginning of the Stage I of the film formation process the particles move with

their characteristic Brownian motion. The water concentration in the polymer

dispersion is uniform. During this stage, water evaporates at a constant rate across

the water/vapour interface and water evaporation rate is close to that of pure water

[26]. According to the conventional thermodynamic theory, the main parameters

affecting the drying performances at this stage are the temperature, the relative

humidity and the vapour pressures of the water surface and the air. Differences in

size of the polymer particles and their composition have no noticeable effect on the

drying rate. Additionally, film thickness also has no effect on the drying rate [27].

The mass transport o f water vapour from the water surface to air occurs by diffusion

and convection. The evaporation predominantly occurs across the water/gas

interface, rather than across the polymer/gas interface [6]. Furthermore, within the

dispersion, the transport of water between the particles is faster than that through the

particles.

As evaporation of water proceeds the concentration of the polymer particles in the

dispersion increases and the particles come into close proximity. In addition, the

colloidal stability is affected as well. At this stage, the particle ordering in the

dispersion is a function of volume fraction of particles and ionic strength. Both of

these factors change as a result of evaporation of water.

If we consider particles are arranged in a face-centred cubic (FCC) structure, Figure

2 . 1 0 illustrates the dependency of the average distance between particles on the

volume fraction of polymer dispersion (® po/), for different sized particles. For a

given value of ® po/ ( less than 0.74), the distance between larger particles is greater

than for smaller particles. As an example, at 40wt% solids, 200 nm radius particles

are 100 nm apart compared with 90 nm radius particles that are only around 50 mn

apart.

30

Chapter 2

Volume fraction of poljmer [+]

Figure 2.10. Distance between different size particles as a function of volume fraction of polymer.

2.6.1.2. Stage II - Particle deformation

The transition from Stage I to Stage II of film formation occurs with a significant

decrease in the rate of evaporation. This is presumed to be due to decrease in

diffusivity due to tighter packing of the particles. It has been generally accepted, at

the transition point all the particles have reached close packing but not yet start to

deform. Furthermore, Stage II is associated with the development of optical clarity.

For a mixture of two phases with two different refractive indexes to be optically

clear, the size of each phase must be far below the wavelength of light. Therefore,

optically clear films could still contain nm size voids of water [6]. As evaporation

31

Chapter 2

continues further, the interfacial and capillary forces exceed the mechanical

resistance of the particles, resulting in particle deformation. A void-free,

mechanically weak honey-comb structure is formed. For this to occur, the application

temperature must be higher than the minimum film-forming temperature (MFFT) of

the system. In the literature, the MFFT is defined as the minimum temperature at

which a latex cast film becomes continuous and clear. If a latex dries below this

critical temperature, it is expected to be opaque and powdery [28]. At this stage

interfaces still exist between the particles.

2.6.I.3. Stage III - Coalescence/Interdiffusion across

particle - particle boundaries

Coalescence, which is dissolution of the cell membranes, brings the polymer particle

cores into close contact, and permits interdiffusion of the polymer molecules

between particles [29]. During this stage the film properties change significantly, as

polymer inter-diffusion across the particle boundaries imparts mechanical strength

and resistance character to the film. The mechanical strength of a given film

increases with the depth of polymer interdiffusion. The diffusion rate increases with

increasing temperature and decreasing molecular weight.

This final stage is characterised by a very slow rate of water loss until it is complete.

In this final stage, it has been suggested that water could move from the bulk of the

film to the polymer/air interface through the channels between the particles or by

diffusion through coalesced polymer particles, which is considerably slower. As

coalescence progresses, the water transport eventually slows to approach that of

diffusion alone. Now, the evaporation rate-limiting step is the transport of water to

the polymer/air surface. The diffusion rate increases with increasing temperature and

decreasing molecular weight.

32

Chapter 2

2.6.2. Particle deformation and coalescence

Particle deformation and coalescence is favourable from the thermodynamic point of

view, as they correspond to a decrease in the surface area of the particle-water or

particle-air interfaces. In the literature, there are a number of theories and models that

have been presented, each considering different driving forces for particle

deformation.

2.6.2.I. Wet sintering

For wet sintering, the driving force for the particle deformation and subsequent

coalescence is provided by the interfacial tension between the particles and water

[30]. In wet sintering, which is illustrated in Figure 2.11 A, the particles should

deform before water has evaporated. Later, it was concluded by Dobler et al. that

wet sintering can contribute to particle deformation, but the process is to be too slow,

when compared with evaporation, to be the primary driver under normal conditions

[31].

2.6.2.2. Dry sintering

When the polymer-air surface tension provides the driving force for particle

deformation and subsequent coalescence it is referred to as dry sintering (Figure 2.11

B). During dry sintering the water recedes before particles are deformed and the

temperature is above the glass transition temperature (Tg) of the polymer. Sperry and

co-workers [32] reported experimental evidence for dry sintering. They dried a film

well below the polymer Tg to prevent the possibility of particle deformation, and then

raised the temperature. The appearance of this film was compared with another film

cast wet at a higher temperature. They reported that both films reached the cloudy to

clear transition at the same temperature after a similar time. It was proved that the

33

Chapter 2

presence of water is not important at lower temperatures. Later, Lin and Meier [33]

argued, that atmospheric humidity preserves the residual water at the particle contact

and the particle deformation is caused by the capillary pressure associated with this

residual water at the particle contact, which was later named as moist sintering.

Figure 2.11. Schematic illustrations of particle deformation theories for film formation. (A) Wet sintering - when particles are deformed before water has evaporated and the reduction of the polymer water interfacial energy is the driving force. (B) Dry sintering - when water recedes before particles are deformed and the reduction of the polymer-air interfacial energy is the driving force. (C) Capillary deformation - when the air-water interfacial energy is the driving force for particle deformation. (X) concave meniscus of air-water interface. (D) Sheetz deformation - “skin” formation by wet sintering and capillary forces due to rapid evaporation, before the dispersion reaches close packing below.

34

Chapter 2

2.6.2.3. Capillary deformation

Brown [34] suggested that the air-water interfacial tension dominates the particle

deformation. As evaporation proceeds, because of the presence of particles, in the

neck region, the water creates a meniscus. There is a negative pressure on the convex

side of the air-water interface. This capillary pressure increases as the water

evaporates from the close packed spheres. According to Brown’s hypothesis, the

atmospheric pressure, pressing on the exposed particles at the surface compresses the

film [34].

According to Brown’s criterion for film formation,

when the film formation of latex occurs, where Gt is the polymer’s shear modulus

determined on the observation of the strain resulting from the application of the

stress for the time of evaporation, y is the surface tension of the air-water interface

[34]. Capillary deformation is illustrated in Figure 2.11 (C), and concave meniscus of

air-water interface is marked by X.

In 1995 Keddie and co-workers [35] first put forward this inhomogeneous regime.

According to them, the deformation initially occurs by capillary mechanisms. By the

time the capillary pressure reaches its maximum, the deformation is not yet

completed. As the evaporation continues, the water recedes through the drying film,

leaving behind the dry particles. At this stage the deformation system switches to

either a dry or moist sintering mechanism.

(2 .8)

2.6.2.4. Receding water front

35

Chapter 2

2.6.2.5. Sheetz deformation

Sheetz [36] proposed that half-way through the film formation process, wet sintering

and capillary forces create a thin membrane or “skin” layer at the air-dispersion

interface, before the dispersion reaches close packing below. Formation of the skin

layer is illustrated in Figure 2.11 (D). This skin could be permeable to water vapour

but hinder evaporation. Sheetz believed that measured evaporation rates were

certainly fast to be only by evaporation from the air-water meniscus. In addition, he

observed that a continuous latex film placed in contact with wet latex did not prevent

its film formation. One of the principles on which Sheetz’s model is based that the

energy for film formation is supplied as heat from the surroundings. This heat is

change into useful work by evaporation of water from the drying film. This water

diffuses through the ‘skin’ at the film surface. As a result it generates a compressive

force normal to the film. Sheetz compared this to a piston, which is permeable to

water vapour, compressing water. As water evaporates from the top of this piston,

creates an osmotic pressure in the dispersion below. Despite this, there is not enough

direct evidence for ‘skin’ at a drying latex surface. Therefore, the theory proposed by

Sheetz not proven valid or invalid [6 ].

2.6.3. Recent models for drying dispersions

Further to the several studies mentioned above, new ideas were further developed by

several scientists during the last few years. Out of them, Routh and Russel’s model

for normal drying is the latest and the most complete theoretical model to date.

According to the Routh and Russel model [37], different drying and particle

deformation regimes can be predicted for normal drying dispersions by determining

two dimensionless numbers Pe (Peclet number) and X[37]. X is the ratio of the

characteristic time for particle deformation to the characteristic time for drying, for

normal drying dispersions.

36

Chapter 2

2.6.3.I. Peclet number (Pe)

According to Routh and Russel, the Peclet number is a measure of the relative rate of

the recession of the water by evaporation over the rate of particle transport by

Brownian diffusion [38]. The Peclet number can be used to predict the vertical water

uniformity of a film during drying. The Peclet number, Pe, is defined as

P e = — (2.9)D 0

where, H is the film thickness; E is the evaporation rate; Do is the diffusion

coefficient of the particles.

If diffusion is stronger than evaporation, we get P e « 1, then a more uniform drying

is observed. If evaporation is stronger than diffusion, we get P e » 1, which

corresponds to non-uniformity in drying, which schematically illustrated in Figure

2 . 12.

Figure 2.12. Schematic illustrations of P e « l and P e » l . Initially stable polymer colloid dispersion (on left) could show vertical water uniformity (right top) if diffusion is stronger than evaporation (A) during drying and P e « l . Alternatively, if evaporation is stronger than diffusion (B) during drying the result is non-uniform water distribution (right bottom) and P e » l .

37

Chapter 2

For colloidal particles, the Stokes-Einstein Diffusion coefficient, Do is given by

d - k TD ° ~ 6 ^ R (210)

where, /u is the solvent viscosity and kT is the thermal energy. Substituting for Do into

Eq. 2.9, leads to

GmiRHE

P e ’ — t r ~ <2“ »

Therefore, we can conclude the solvent viscosity, the particle radius, the film

thickness, the evaporation rate and the drying temperature affect the uniformity of a

film during drying. Except the temperature, as all the other parameters increase, the

Peclet number increases, therefore non-uniform drying is predicted. According to Eq.

2.11, the temperature is inversely proportional to Peclet number and we can predict

more uniform drying with higher temperature. However, the temperature also affects

the evaporation rate, E. Therefore, Peclet number’s dependency on the temperature is

complicated.

2.6.3.2. The Routh and Russel model for normal

drying

As stated earlier, the Routh and Russel model predicts different drying and particle

deformation regimes for normal drying dispersions by determining two

dimensionless numbers Pe and X [37]. X is the ratio of the characteristic time for

particle deformation to the characteristic time for drying, for normal drying

dispersions. It can be written

38

Chapter 2

o _ T<1efA ~ — (2 .1 2 )

d i y

where, the characteristic time for particle deformation is given by

Tdef ~ (2.13)r

where, ?]0 is zero shear rate viscosity of the polymer and y is the interfacial energy

for the polymer-water or polymer-air interfaces. The characteristic time for drying

can be written as:

= Y (2.14)

Therefore, by substituting for zdcf and rdiy into Eq. 2.12,

x = 1]sf f (2-15)yH

The number X determines the type of deformation valid during the drying process

and separated into four regimes as shown in Figure 2.13.

1. X<1 -W et sintering occurs when the deformation is slow and is

driven by the surface tension between the particles and the solvent (or

water).2 * • •2. 1< X <10 - Capillary deformation occurs when the capillary pressure

is the driving force for film formation. The particles are close-packed

without undergoing deformation.

3. 1 0 2 < X < 104- The receding water front is an inhomogeneous regime

between the capillary and dry sintering mechanisms.

4. X > 104 - Dry sintering occurs when the driving force is the surface

tension between the polymer and the air.

39

Chapter 2

Softer particles are more likely to show wet sintering. A higher evaporation rate or

thicker film leads to a higher Peclet number and non-uniformity in drying. Softer

particles with a higher Peclet number in the wet sintering regime leads to skin

formation during drying [37].

S o f t n e s s o f t h e p a r t i c l e s

X

10000

100

Dry/moist sintering

F ilm

Receding waterfront \r a te

Capillary ^ ............deformation j Partial skinning

i

Wet sintering j Skinning

1 Pe

Figure 2.13. Schematic diagram of the drying regimes according to the Routh and Russel model for normal drying and the influencing factors [37].

2.6.3.3. The Routh and Russel model for lateral drying

A process referred to as lateral drying occurs when the solids fraction nearer to the

centre remains close to the initial value and a high solids region develops at the edge.

As the drying continues, a drying front or the air/water interface moves from edge to

the centre. On the other hand, when the solids fraction increases relatively uniformly

or to only a limited extent in the lateral direction, this is called vertical drying. In this

case, drying does not occur strongly from the edges, unlike lateral drying [39].

The various theories that attempt to explain the origin to lateral flow do not allow

predicting when an eventual drying front will occur. In 1998, Routh and Russel

40

Chapter 2

developed a model using a dimensionless number, the reduced capillary pressure

(Pcap), to predict the onset for the propagation of a drying front in dispersion. That

is, PCap can predict how long the edges of the drying dispersion remain wet [40]. The

reduced capillary pressure is a function of several parameters and can be written as

follows:

p 20 j 3 cap 751 E

(2.16)

where $ tl is the volume fraction of solids at close-packing of the particles, and rjod is

the low shear viscosity of the dispersion [40]. This model predicts that when Pcap <

1, water recedes from the edges of the film. Pcap is often larger than 1 at the initial

stages of drying and is enough to support regions with more densely packed

particles, usually at the edges and maintain a wet surface. However, Pcap decreases

with the time as particles become close-packed at the edges and advance towards the

centre of the film [40].

Figure 2.14 shows the suggested cross section of a lateral drying colloidal film. Two

types of boundaries can be seen: one separates a wet dispersion from a flooded close-

packed array of particles which is called a particle-packing front, and the other

separates the flooded array from fully dry, packed particles, which is drying front

[39].

V i• • • i i

s s s x s s s s s s s s w / / / / / / / / / / /

x s V s s s s s s w s s X X s \ s s s s s s s w

•Wet dispersion

Particle-packing front

n—M Close-packed U— region X

Dry region

Drying front

Figure 2.14. Schematic diagram shows a lateral drying in a colloidal film with a central wet region and dry edges. Between these regions, water fills the void space between packed particles. The inner boundary is the particle packing front, and the outer boundary is the drying front [39].

41

Chapter 2

2.7. References:

1. Evens D.F., W.H., The Colloidal Domain. 1999: Wiley-VCH, New York.

2. Hunter, R.J., Introduction to Modern Colloid Science. 1993: Oxford

University Press.

3. Renn, J., . ., . , Einstein’s invention o f Brownian motion 2005: Ann. Phys. 23

-3 7 .

4. Cosgrove, T.e.b., Colloid Science: Principles, methods and applications.

2005: Blackwel Publishing, UK.

5. Ugelstad, J., Hansen, F.K., and Lange, S., Emulsion Polymerization o f

Styrene with Sodium Hexadecyl Sulfate/Hexadecanol Mixtures as Emulsifiers

- Initiation in Monomer Droplets. Makromolekulare Chemie-Macromolecular

Chemistry and Physics, 1974.175(2): p. 507-521.

6 . Keddie, J.L., Film formation o f latex. Materials Science & Engineering R-

Reports, 1997. 21(3): p. 101-170.

7. Jotischky, H., Coatings, regulations and the environment reviewed. Surface

Coatings International Part B-Coatings Transactions, 2001. 84(1): p. 11-20.

8 . Qiu, J., Charleux, B., and Matyjaszewski, K., Controlled/living radical

polymerization in aqueous media: homogeneous and heterogeneous systems.

Progress in Polymer Science, 2001. 26(10): p. 2083-2134.

9. Asua, J.M., Miniemulsion polymerization. Progress in Polymer Science,

2002. 27(7): p. 1283-1346.

10. Lovell, P.A., El-Aasser, M.S.,, Emulsion Polymerisation and Emulsion

Polymers. 1997: John Wiley & Sons Ltd. UK.

11. Bardosova, M. and Tredgold, R.H., Ordered layers o f monodispersive

colloids. Journal of Materials Chemistry, 2002.12(10): p. 2835-2842.

12. Mellon, V., Synthesis and characterisation o f Waterborne Polymer/Laponite

Nanocomposite Latexes through Miniemulsion Polymerisation, in PhD

Thesis. 2009, CNRS-LCPP (Laboratory of Chemistry and Processes of

Polymerization), France.

42

Chapter 2

13. Bouanani, F., Bendedouch, D., Maitre, C., Teixeira, J., and Hemery, P.,

Characterization o f miniemulsion polymerization by small-angle neutron

scattering. Polymer Bulletin, 2005. 55(6): p. 429-436.

14. Landfester, K., Synthesis o f colloidal particles in miniemulsions. Annual

Review of Materials Research, 2006. 36: p. 231-279.

15. Antonietti, M. and Landfester, K., Polyreactions in miniemulsions. Progress

in Polymer Science, 2002. 27(4): p. 689-757.

16. Wu, X.Q., Schorlc, F.J., and Gooch, J.W., Hybrid miniemulsion

polymerization o f acrylic/allcyd systems and characterization o f the resulting

polymers. Journal of Polymer Science Part a-Polymer Chemistry, 1999.

37(22): p. 4159-4168.

17. Erdem, B., Sudol, E.D., Dimonie, V.L., and El-Aasser, M.S., Encapsulation

o f inorganic particles via miniemulsion polymerization. I. Dispersion o f

titanium dioxide particles in organic media using OLOA 370 as stabilizer.

Journal of Polymer Science Part a-Polymer Chemistry, 2000. 38(24): p. 4419-

4430.

18. Tiarks, F., Landfester, K., and Anonietti, M., Encapsulation o f carbon black

by miniemulsion polymerization. Macromolecular Chemistry and Physics,

2001.202(1): p. 51-60.

19. Everett, D.H., Basic Principles o f Colloid Science. 1988: The Royal Society

of Chemistry.

20. Attard, P., Recent advances in the electric double layer in colloid science.

Current Opinion in Colloid & Interface Science, 2001. 6(4): p. 366-371.

21. Erkseliu, S., Film formation from Dispersions - Preparation and

M echan ism s in PhD Thesis. 2006, Lund University, Sweden.

22. Theo, G.M. and Ven, V.D., Colloid Science. 1989: Academic Press Limited,

London.

23. Israelechvili, J., Intermolecular & Surface Forces. 1992: Academic Press

Limited, London.

24. Evans, D.F., Wennerstrom, H.„ The Colloidal Domain. Second edition ed.

1999: Wiley-VCH, New York.

43

Chapter 2

25. Bennett, G., Gorce, J.P., Keddie, J.L., McDonald, P.J., and Berglind, H.,

Magnetic resonance profiling studies o f the drying o f film-forming aqueous

dispersions and glue layers. Magnetic Resonance Imaging, 2003. 21(3-4): p.

235-241.

26. Vanderho.Jw, Bradford, E.B., and Carringt.Wk, Transport o f Water through

Latex Films. Journal of Polymer Science Part C-Polymer Symposium,

1973(41): p. 155-174.

27. Croll, S.G., Drying o f Latex Paint. Journal of Coatings Technology, 1986.

58(734): p. 41-49.

28. Eckersley, S.T. and Rudin, A., Mechanism o f Film Formation from Polymer

Latexes. Journal of Coatings Technology, 1990. 62(780): p. 89-100.

29. Kessel, N., Physical and chemical Aspects o f the film formation o f Self-

Crosslinldng Acrylic acrylic latex. 2007, University of Surrey, UK.

30. Vanderhoff, J.W., Tarkowski, H.L., Jenkins, M.C., and Bradford, E.B.,

Theoretical consideration o f the interfacial forces involved in the coalescence

o f latex particles. J. Macromol. Chem, 1966.1(2): p. 361-397

31. Dobler, F., Pith, T., Lambla, M., and Holl, Y., Coalescence Mechanisms o f

Polymer Colloids .1. Coalescence under the Influence o f Particle Water

Interfacial-Tension. Journal of Colloid and Interface Science, 1992. 152(1):

p. 1 - 1 1 .

32. Sperry, P.R., Snyder, B.S., Odowd, M.L., and Lesko, P.M., Role o f Water in

Particle Deformation and Compaction in Latex Film Formation. Langmuir,

1994.10(8): p. 2619-2628.

33. Lin, F. and Meier, D.J., A Study o f Latex Film Formation by Atomic-Force

Microscopy . l . A Comparison o f Wet and Dry Conditions. Langmuir, 1995.

11(7): p. 2726-2733.

34. Brown, G.L., Formation o f Films from Polymer Dispersions. Journal of

Polymer Science, 1956. 22(102): p. 423-434.

35. Keddie, J.L., Meredith, P., Jones, R.A.L., and Donald, A.M., Kinetics o f Film

Formation in Acrylic Latices Studied with Multiple-Angle-of-Incidence

44

Chapter 2

EIlipsometry and Environmental Sem. Macromolecules, 1995. 28(8): p. 2673-

2682.

36. Sheetz, D.P., Formation o f Films by Drying o f Latex. Journal of Applied

Polymer Science, 1965. 9(11): p. 3759-&.

37. Routh, A.F. and Russel, W.B., Deformation mechanisms during latex film

formation: Experimental evidence. Industrial & Engineering Chemistry

Research, 2001. 40(20): p. 4302-4308.

38. Routh, A.F. and Russel, W.B., A process model for latex film formation:

Limiting regimes fo r individual driving forces. Langmuir, 1999. 15(22): p.

7762-7773.

39. Salamanca, J.M., Ciampi, E., Faux, D.A., Glover, P.M., McDonald, P.J.,

Routh, A.F., Peters, A.C.I.A., Satguru, R., and Keddie, J.L., Lateral drying in

thick films o f waterborne colloidal particles. Langmuir, 2001. 17(11): p.

3202-3207.

40. Routh, F. and Russel, W.B., Horizontal drying fronts during solvent

evaporation from latex films (vol 44, pg 2088, 1998). Aiche Journal, 2002.

48(4): p. 917-918.

45

Chapter 3

Magnetic Resonance Profiling

Out of all the techniques used in this research work, magnetic resonance profiling

(MRP) was employed to collect the majority of data. Therefore this chapter will give

a description of MRP with its background, principles and theory.

3.1. Historical Background of MRP

The nuclear magnetic resonance (NMR) phenomenon was first reported by Rabi and

co-workers [1] in 1938 when they discovered it during an ion beam experiment.

They experimented with a beam of lithium chloride molecules passing through a

magnetic field and observed the resonance peaks of lithium and chlorine, hi 1940s

two groups of scientists observed the phenomenon of NMR in bulk matter. It was in

1946 when Purcell, Torrey and Pound [2, 3] succeeded in their attempts to detect the

NMR of protons in paraffin. Independently, dming the same time, Bloch’s group [4]

detected the NMR of protons in water. In 1952 Bloch and Purcell were awarded the

Nobel Prize for Physics, for their contribution to NMR. Since then NMR has become

a common and routine technique for scientists. It has been used to study solids,

liquids and gases, as well as various combinations of the three; as a few examples,

emulsions, biological tissues and gases or liquids diffusing through solid materials.

The first commercial NMR spectrometer was available in 1953.

Chapter 3

Throughout the years scientists in various disciplines have contributed to this field.

Swiss Physical Chemist Richard Ernst [5, 6] won the 1991 Nobel Prize in Chemistry

for his contribution to the development of high resolution NMR spectroscopy. Later,

the 2003 Nobel Prize in Medicine was awarded to Sir Peter Mansfield [7, 8] and

Lauterbur [9], for their independent development of NMR.

Magnetic resonance imaging (MRI) was developed from the knowledge gained in the

study of NMR. An MR profile is a one dimensional map of intensity of the NMR

signal as a function of position in one direction. An MR image is a two dimensional

picture created by several MR profiles of an object. MRI has developed most rapidly

in clinical diagnosis where the importance of non-invasive, non-ionizing, high-

resolution (tens of micrometers) imaging technique is most obvious. Since the first

commercial MRI scanner was developed in 1978, MRI has become one of the

routinely performed procedures in hospitals. Furthermore, MRI has become

increasingly popular with numerous applications, as the MRI technique is completely

non-invasive and it allows spatial resolution down to a few micrometers. NMR is

successfully used to study drug release [10], drying of polymer films [11-13], drying

of glue layers [14], drying and skin development in PSAs [15, 16], water distribution

in semicrystalline polymer layers [17], photo-initiated cross-linking in latex

dispersions [18, 19], behaviour of water in water-swollen cellophane films [20],

cement hydration [2 1 , 2 2 ] and many more.

Medical MRI is based on imaging the nuclei in mobile water, which seems to be

in all places in the human body. The image contrast comes usually from the level of

water mobility. If the water becomes less mobile, the imaging becomes more

difficult. The magnetic resonance line widths of mobile are narrow, and very high

gradients are needed to separate the resonances of less mobile nuclei [23]. hi

addition, NMR Image contrast is governed by one of several NMR parameters and

one can get information about water mobility, chemical potential, self-diffusion

coefficient, coherent flow or temperature, depending upon the exact form of the MRI

measurement [24]. Even though MRI has evolved into a complex, interdisciplinary

science over the years, it’s still young and growing.

47

Chapter 3

3.2. The Principles of NMR

NMR relies upon the fact that the nuclei of many atomic isotopes, including the

hydrogen proton, are magnetic and are extremely sensitive to the local magnetic

environment [23], Any nucleus that contains air odd number of protons and/or of

neutrons has a spin greater than zero. The most commonly used nuclei in NMR are

]H and l3C. In NMR experiments, the sample is placed in an external magnetic field

in which the nuclei can precess like a spinning top in a gravitational field. The

resonance frequencies of different nuclei are proportional to the applied magnetic

field [24].

When placed within an external magnetic field, a sub-atomic particle, such as an

atomic nucleus (e.g. 13C), with a magnetic moment adopts one of a set of allowed

configurations with respect to that field. Each allowed configuration corresponds to a

different energy level of the particle. The number of allowed energy levels is dictated

by the intrinsic (i.e. non-orbital) angular momentum or spin quantum number, /. I

may have any of the values 0, %, 1, 3/2, 2, 5/2........ The actual magnitude of the

intrinsic angular momentum, I of the particle is given by:

| I |= & V / ( / + l ) (3.1)

where, and A is Plank’s constant (h) divided by I n which is equal to 1.055 x 10"34 J.s,

and is called the reduced Planck’s constant. The reduced Planck’s constant is taken

as the unit of angular momentum [24].

When there is no external magnetic field, the magnetic dipole moments are randomly

oriented, hi this situation, the net magnetisation is zero. If an external magnetic field,

B0, is applied along the z-axis, the magnetic moments of the nuclei align with this

field, as shown in the Figure 3.1.

48

Chapter 3

A BFigure 3.1. Schematic illustration of randomly oriented magnetic dipole moments where (A) no external magnetic field, and (B) when external magnetic field, Bo, is applied, the orientations align with the field.

When an external magnetic field, Bo is applied along the z-axis, the z component of

the angular momentum or spin, 7Z, can have mtl values, where m is the magnetic

quantum number. The values for m = -7, - 7 + 1 , . . . , I - 1 , I and there are 27+1

possible values of m. This generates (27+1) number of energy levels and they are

known as the Zeeman energy levels. For instance, the hydrogen nucleus which has

only one proton with spin, 7 = Yz has two Zeeman energy levels: m = + Y z and m = - Yz.

These two orientations can be separated by the application of an external magnetic

field, Bo. This is known as the Zeeman effect, which splits each level into its

component states, each having a different value of m. Figure 3.2 illustrates the

splitting of the Zeeman energy levels occurs due to the application of a static

magnetic field Bo. The intensity of the transition which can be induced between those

two levels is proportional to the population excess of the lowest level compared with

the highest level. In a real sample, there could be around 1026 nuclei. These nuclei

split into energy levels due to the application of static magnetic field Bo, according to

Boltzmann distribution [25]:

N high - A E= e x p --------

kT (3.2)

49

Chapter 3

where Nhigh.iow are the populations of the high and low energy states respectively, AE

is the energy difference between two energy levels, k is Boltzmann constant and T is

the absolute temperature of the system [25].

Bt

▼ ▼m,= -1/2

A E = Y ftB 0

m,= +1/2

Figure 3.2. Schematic illustration of the splitting of the Zeeman energy levels due to the application of a static magnetic field Bo. The lower energy level has m = +V2 where magnetic moment is parallel to the external magnetic field, B0 and higher energy level has m = -Vi where magnetic moment is anti-parallel to B0 [25].

Charged particles with spin / gain a magnetic moment proportional to that spin:

ju = y h l (3.3)

where y is magnetogyric ratio for the particle and it is a constant for a given isotope:

‘H, 13C, 3iP. For'H , — = 42.58 xlO 6 Hz T' 1 [26].2 K

The energy for the Zeeman interaction between an external magnetic field, Bo and

such a magnetic moment is:

E = - / ' A (3.4)

For the external magnetic field Bo in the z direction, with the magnetic moment

operator inserted, this can be written as:

50

Chapter 3

E = - y h I 2B 0 (3.5)

The energy of the low (equation 3.6) and the high (equation 3.7) levels are given by

F v =- y h B ,

2 2

E _ + Jftgp -X 2

The two energy levels of the hydrogen proton are separated by:

(3.6)

(3.7)

A £ = (3.8)

This is the energy required to induce a transition from the lower to the higher energy

state. The energy separation of the states depends linearly on the strength of applied

magnetic field B0.

The transition of the nucleus between the two energy levels is accompanied by

absorption of a photon. The energy of this photon must exactly match the energy

difference between the two states, The energy, E, of a photon is related to its

frequency, co, by Plank’s constant, % .

A E = yhB0 = hco0

a>0 = rB o (3.9)

where is the Larmor resonance frequency. The energy difference is influenced by

the chemical nature of the nucleus; therefore protons in different chemical

environments can be identified separately, because of small differences in the local

magnetic field applied to the nucleus. This is the principle of NMR spectroscopy.

51

Chapter 3

3.3. MR Profiling using GARField magnet

A simple way of explaining how MRI works is that when a sample composed of

water molecules, which each contain two hydrogen nuclei (or protons), goes inside a

powerful magnetic field, these protons align with the direction of the field. When a

radio frequency (RF) electromagnetic field (pulse) is briefly turned on with the use of

an RF coil, these protons now alter their alignment relative to this RF field. When

this field is turned off the protons return to the original magnetisation alignment. The

signal which is created by these alignment changes is detected by the same RF coil.

A magnetic field gradient across the sample allows the distribution of the resonant

frequencies of the nuclei according to their position. Applying additional magnetic

fields (pulses) during the experiment allows an image of the sample to be built up.

For this research work, the GARField magnet was used to study the mobile lH in a

drying sample. The same setup can be used to study the mobile 13C or 31P in a

sample. 13C or 31P will have its own magnetogyric ratio, y. The frequency of the RF

pulse, which used to excite the chosen nuclei, should be adjusted accordingly to the

chosen isotope.

GARField, standing for Gradient At Right-angles to the Field, is a small permanent

magnet with shaped pole pieces [11, 27]. These pole pieces which are shown in

Figure 3.3 create a strong magnetic field gradient in the vertical direction (Gy in

Figure 3.4.) with a constant magnetic field (Bo in Figure 3.4.) in the horizontal

direction. The field Bo has a curvature of less than ±5 pm over a 5 x 5 mm region in

the sample area [27]. This magnet can be used to investigate a few tens of pm thin

films to hundreds of pm larger samples. The main advantage of the GARField

magnet over conventional solid-state NMR techniques is its ability to study samples

containing both solid and liquid components. More importantly, GARField is

particularly suitable for the investigation of planar, waterborne colloidal systems

during their various stages of drying [1 1 ].

52

Chapter 3

y [cm]6 -

4 - 2 “

0 -

-2 “

-4“

-6 -

Figure 3.3. Cross section through the centre of the magnet which shows the shape of the pole pieces and the sample location in relation to the magnets and the pole pieces. The RF coil is located directly below the sample location. Image taken from [27].

Magnetic resonance profiling relies upon the fact that the magnetic nuclei of atoms

(e.g. the hydrogen proton) precesses in a magnetic field at a localised resonant

frequency directly proportional to the field strength (equation 3.9). A magnetic field

gradient, Gy, leads to the resonant frequencies within the sample depending on the

position. The Larmor resonant frequency B)(y) now encodes position along the y-

direction as: [24].

a>(y) = yBn + yGv.y (3.i0)

In an experiment, if the Larmor resonant frequency is known, one can

calculate the position along the y-direction (y) using this equation.

I 1--------------1-------------- 1

NdFeBr 0 2 6 12 m a g n e t

z[cm]

Pole Piece

Samplel o c a t i o n

53

Chapter 3

As shown in Figure 3.4, a latex sample (or other film) is horizontally placed on the

locator tape above the radio-frequency (RF) coil. This RF coil is used to excite and

detect the MR signal via a current arising from the transient response of the nuclei to

a resonant RF stimulus. The size of the RF coil is of the order of 3 mm in diameter.

The shaped pole pieces of the GARField magnet ensures the optimization of

profiling through thin, planar samples. The static magnetic field, B0, of 0.7 T, is

parallel to the sample plane. The excitation field, Bi is due to the RF coil. The

dBmagnetic field gradient, Gy = — - which is approximately 17.5 T/m, is

dy

perpendicular to the sample plane [1 1 ].

GravityBo

7Sample

Gy Bi Profile

A A A

Coverslip. \\ Locator tape

Height

Intensity

RF Coil

Figure 3.4. Schematic diagram of the GARField magnet. The magnetic field gradient, Gy, is perpendicular to the direction of the magnetic field, Bo [11]. B\ is the magnetic field generated by the planar radio frequency (RF) coil. The profiles of the sample are a measure of the intensity of the magnetisation signal as a function of the height of the sample, in the direction of the gradient which is shown on the right hand side.

For this research work, 20 mm x 20 mm and 180 pm thick glass cover-slips were

used as the substrate. The sample was cast on these cover-slips to get the desired

thickness. As a rough guide, around 70 pi of 50 wt. % solid content latex would

produce a 300-400 pm thick wet film. Access to the magnet pole piece to place the

sample is from above. Immediately after the casting, the wet sample on a glass

54

Chapter 3

cover-slip was placed inside the magnet. NMR profiles were obtained under room

temperature and humidity.

The NMR signal is obtained from an excitation using a quadrature echo sequence:

90x-x-(90y-x-echo-x-)„ [11, 23]. Consider a coordinate set rotating at a given

frequency on the x-axis. In this frame, following a 90x° pulse, all the magnetisations

lie along the y-axis. After a time (pulse gap) x, a 90 y° pulse is used. The 2x runs from

the centre of one pulse duration to the centre of the next pulse duration. After a

second x, all the magnetisations once again come along the y-axis, forming an echo.

(90y~x-echo-x-) is repeated n times to get n number of echos.

90x-x-(90y-x-echo-x-)„ is repeated NS (number of scans) times to average each

echo train. Each echo is recorded with SI number of points, and the time between

two of these points is the dwell time, dw. The intensity o f the successive echo

profiles follow an exponential decay described by the time constant 1/T2. T2 is the

spin-spin relaxation time which is sensitive to the mobility of the water. For instance,

T2 is longer for free water and shorter for polymer particles. As a result, the intensity

of each GARField profile recorded during an excitation sequence is differently

weighted to water and polymer particles [11]. Between each excitation pulse

sequence, a lapse of time, which is relaxation delay, RD is left to allow the system to

return to its equilibrium. Usually RD is of the order of three times Ti. Ti is the spin-

lattice relaxation time. For a typical latex experiments, n is 32, x is 75 ps, dw is 0.7

ps, SI is 256, NS is 32, RD is 3.5 s and the spectrometer frequency, SF, is 29.6 MHz.

These echoes are in the time domain (intensity as a function of time) and they are

Fourier transformed to get the signal in the frequency domain (intensity as a function

of frequency), hence position is determined using equation 3.10. The resulting data

are added together to obtain a profile of signal intensity as a function of position.

The resolution is calculated by [24]:

A r = ---------l- (3.11)G x y x SI xdw

55

Chapter 3

By increasing SI and dw one can increase the resolution, but it would significantly

increase the time taken to obtain an echo. This extra time is multiplied by n and NS

times for every obtained profile. Doing so, one could lose vital information on a

drying experiment, as the number of profiles that can be obtained would be reduced.

In addition, the resolution can be increased by increasing G.

An increase in gradient will decrease the field of view of the image ( Az), according

to [24]:

AQAz = — — (3.12)

G x y

where AQ is the bandwidth of the pulse. A smaller field of view has a very practical

importance. The thickness of the initial film should be smaller than the field of view

to obtain profiles which represent the whole of the film. The range of frequency -

and hence the maximum thickness - is also influenced by -7 - [27].dw

For each and every experiment, the time delay between profiles was changed as

desired. By choosing the same MR parameters, direct comparisons of profiles for

different samples are allowed. In order to correct for the decline in sensitivity over

the film thickness, the profile shapes are normalised by an elastometer standard.

56

Chapter 3

3.4. A typical GARField Profile

a>a£3</)a>*-•(0■4-*</>J33(/)

250 300

Thicknns of dry film HtiihtGim)

Initial thicknoss ofthofilm

Figure 3.5. Typical GARField profiles of a model acrylic latex (coating), where each profile was taken in 5 minute intervals. The arrow shows the increase of the drying time. On right is the film-air surface (top of the film) and on left is the film- substrate surface (bottom of the film).

A typical GARField profile is a map of the NMR signal-intensity as a function of the

height of the film, as shown in Figure 3.5. The time of the first profile was defined as

0 minutes and in this example the profiles were taken in 5 minute intervals. As the

drying time increases, the height (thickness) of the film decreases, mainly due to the

evaporation of water in the film. The film thickness at a given drying time can be

measured along the X-axis. On the other hand, as the drying time increases, the

intensity of the NMR signal decreases, and this is proportional to the density of

mobile water in the sample. When the intensity of the signal does not decrease as

drying time increases, it is assumed that all the mobile water has evaporated. The dry

film thickness can be calculated for the samples with known solids content. As an

example, if the initial film thickness of a sample is 300 pm and the solids content is

50 wt. %, then the dry film thickness is around 150 pm, assuming % voids are zero.

57

Chapter 3

More recently, a 3D waterfall type of plot has been introduced, where drying time is

plotted along the third-axis. The intensity of the NMR signal has been normalised to

the initial intensity, as shown in Figure 3.6. This ensures that the first profile is

relatively uniform. For latex with a low Tg, after the film reached its final thickness,

the NMR signal intensity is still very high as there is molecular mobility in the

polymer melt and one can obtain a NMR signal from this mobile polymer. Figure 3.5

shows GARField profiles of a coating formulation, where the intensity of the signal

drops to zero. Figure 3.6 shows GARField profiles of an adhesive formulation, where

intensity of the signal does not drop to zero, as GARField is sensitive to the mobility

of soft polymer particles. In an adhesive formulation, it is assumed that the film is

dry when the thickness and the intensity do not decrease with increasing drying time.

Figure 3.6. Typical GARField profiling of a latex with low Tg (adhesive) using 3D waterfall type of plotting.

The moments of statistics provide model-free information about a distribution and

they have been used to analyse the NMR profiles obtained from drying latex films.

The definition of the z'-th moment is given by [28]:

Height(gm)

N

(3 .1 3 )

58

Chapter 3

where N is the number of the points of a distribution, Xn is the value of the

independent variable, (which is the vertical position in an experiment), AX is the

pixel spacing, and In is the value of the distribution at Xn (NMR intensity in our case)

[29]. In the literature, the normalized moments are often used, and normalisation is

done by dividing the moment by the zero-th moment, m0. The definition of the

normalized z-th moment is given by [28]:

N

Y l „ ( X „ -ml)

where m* is the normalized first moment. To calculate the normalized first moment,

m* is set to zero in this equation [29].

The physical meanings of the first five moments are given in Table 3.1 [29].

Table 3.1. The meanings of the first five moments [29]

m0 integral

*mx average

m*2 variance

m \ / / skewness

r * / q-3 mV * kurtosis

59

Chapter 3

Software was used to calculate these moments using NMR data obtained by the

GARField magnet [29]. The zeroth moment, m0, which is the integral of a

distribution or the area under the NMR profile, is proportional to the water content of

the sample at the time when the profile was taken. The knowledge of the initial film

thickness and the solids fraction allows the zeroth moment to be converted to water

mass per unit area of the film. At the point where the zeroth moment no longer

changes over the time, it was assumed that the water content was zero. (Note that a

signal is often obtained from the polymer.) The plots of the zeroth moment and the

thickness as a function of time give an indication of any changes in drying rate [29].

When studying drying, it is important to know if the film dries homogeneously. For

homogeneous drying, the content of the mobile water within the drying film is

uniform with depth from the surface. Therefore, the NMR profile is symmetric. On

the other hand, for non-uniform drying, the NMR profile is asymmetric. Using the

same software, the skewness of each NMR profile from the second and third

normalised moments (m *2 a n d ) has been calculated. The skewness was used as a

measurement of non-homogeneity of the drying film [29].

60

Chapter 3

3.2. References:

1. Rabi, I.I., et al., A new method o f measuring nuclear magnetic moment.

Physical Review, 1938. 53(4): p. 318-318.

2. Pound, R.V., and Purcell, E.M., Measurement o f Magnetic Resonance

Absorption by Nuclear Moments in a Solid. Physical Review, 1946. 69(11-1):

p. 681-681.

3. Purcell, E.M., Torrey, H.C., and Pound, R.V., Resonance Absorption by

Nuclear Magnetic Moments in a Solid. Physical Review, 1946. 69(1-2): p.

37-38.

4. Bloch, F., Hansen, W.W., and Packard, M., The Nuclear Induction

Experiment. Physical Review, 1946. 70(7-8): p. 474-485.

5. Ernst, R.R., Nuclear Magnetic Double Resonance with an Incoherent Radio-

Frequency Field. Journal of Chemical Physics, 1966. 45(10): p. 3845.

6 . Ernst, R.R., and Anderson, W.A., Sensitivity Enhancement in Magnetic

Resonance .2. Investigation o f Intermediate Passage Conditions. Review of

Scientific Instruments, 1965. 36(12): p. 1696.

7. Mansfiel.P., and Grannell, P.K., NMR Diffraction in Solids. Journal of

Physics C-Solid State Physics, 1973. 6(22): p. L422-L426.

8 . Mansfield, P., and Grannell, P.K., Diffraction and Microscopy in Solids and

Liquids by NMR. Physical Review B, 1975.12(9): p. 3618-3634.

9. Lauterbur, P.C., Image Formation by Induced Local Interactions - Examples

Employing Nuclear Magnetic-Resonance. Nature, 1973. 242(5394): p. 190-

191.

10. Hyde, T.M., Gladden, L.F., and Payne, R., A Nuclear-Magnetic-Resonance

Imaging Study o f the Effect o f Incorporating a Macromolecular Drug in

Poly (Glycolic Acid-Co-Dl-Lactic Acid). Journal of Controlled Release, 1995.

36(3): p. 261-275.

11. Gorce, J.P., et al., Vertical water distribution during the drying o f polymer

films cast from aqueous emulsions. European Physical Journal E, 2002. 8(4):

p. 421-429.

61

Chapter 3

12. Salamanca, J.M., et al., Lateral drying in thick films o f waterborne colloidal

particles. Langmuir, 2001. 17(11): p. 3202-3207.

13. Ciampi, E., et al., Lateral transport o f water during drying o f alJcyd

emulsions. Langmuir, 2000.16(3): p. 1057-1065.

14. Bennett, G., et al., Magnetic resonance profiling studies o f the drying o f film-

forming aqueous dispersions and glue layers. Magnetic Resonance Imaging,

2003. 21(3-4): p. 235-241.

15. Mallegol, J., et al., Skin development during the film formation o f waterborne

acrylic pressure-sensitive adhesives containing tacldfying resin. Journal of

Adhesion, 2006. 82(3): p. 217-238.

16. Mallegol, J., et al., Origins and effects o f a surfactant excess near the surface

o f waterborne acrylic pressure-sensitive adhesives. Langmuir, 2002. 18(11):

p. 4478-4487.17. Ciampi, E., and McDonald, P.J., Skin formation and water distribution in

semicrystalline polymer layers cast from solution: A magnetic resonance

imaging study. Macromolecules, 2003. 36(22): p. 8398-8405.

18. Wallin, M., et al., Depth profiles o f polymer mobility during the film

formation o f a latex dispersion undergoing photoinitiated cross-lin/dng.

Macromolecules, 2000. 33(22): p. 8443-8452.

19. Hellgren, A.C., et al., New techniques fo r determining the extent o f

crosslinldng in coatings. Progress in Organic Coatings, 2001. 43(1-3): p. 85-

98.

20. Laity, P.R., et al., Structural studies and diffusion measurements o f water-

swollen cellophane by NMR imaging. Cellulose, 2000. 7(3): p. 227-246.

21. McDonald, P.J., et al., A unilateral NMR magnet fo r sub-structure analysis in

the built environment: The Surface GARField. Journal of Magnetic

Resonance, 2007.185(1): p. 1-11.

22. McDonald, P.J., et al., Two-dimensional correlation relaxometry studies o f

cement pastes performed using a new one-sided NMR magnet. Cement and

Concrete Research, 2007. 37(3): p. 303-309.

23. McDonald, P.J. and Newling, B., Stray field magnetic resonance imaging.

Reports on Progress in Physics, 1998. 61(11): p. 1441-1493.

62

Chapter 3

24. McDonald, P J ., Stray field magnetic resonance imaging. Progress in Nuclear

Magnetic Resonance Spectroscopy, 1997. 30: p. 69-99.

25. Canet, D., Nuclear Magnetic Resonance : Concepts and Methods. 1996: John

Wiley & Sons.

26. Hore P. J., Nuclear Magnetic Resonance 1995: Oxford University Press.

27. Glover, P.M., et al., A novel high-gradient permanent magnet for the

profiling o f planar films and coatings. Journal of Magnetic Resonance, 1999.

139(1): p. 90-97.

28. Konig, A.M., Weerakkody, T.G., ICeddie, J. L., Johannsmann, D.,

Heterogeneous Drying o f Colloidal Polymer Films: Dependence on Added

Salt. Langmuir, 2008. 24(14): p. 7580-7589.

29. Konig, A.M., Light Scattering and Magnetic Resonance Imaging -

Investigation o f the Film Formation Process o f Latexes. 2007, Clausthal

University of Technology: Clausthal-Zellerfeld.

63

Chapter 4

Physical Characterisation of Film Formation

and Film Properties of Organic/Inorganic

Nanocomposite Coatings

4.1. Introduction

In the earliest years of the polymer industry, inorganic fillers were used as extending

agents to reduce the cost of polymer-based products. Soon fillers were recognized to

be an integral component in many applications involving polymers, particularly for

reinforcement purposes. The reinforcement efficiency of inorganic fillers is strongly

related to their aspect ratio (diameter/thickness) therefore platelet-like fillers have

drawn significant interest. Among the different platelet-like fillers, a huge amount of

work has focused on clays [1 ].

Polymer/clay nanocomposites were first reported by Blumstein in 1965, when he

demonstrated the polymerisation of vinyl monomers with Montmorillonite (MMT)

[2, 3]. In 1992, the researchers at the Toyota Central Research laboratories reported

that the incorporation of small amounts o f MMT into nylon-6 resulted in a hugely

Chapter 4

surprising enhancement of thermal and mechanical properties of the resulting

nanocomposite material [4], Their results have drawn a remarkable renewal of

interest both academic and industrial, to this class of material; particularly due to

their enhanced thermal, chemical, mechanical and rheological properties [5-8], By

combining inorganic fillers with organic polymers, one can expect to combine the

inorganic properties with the properties of polymers, such as flexibility and

processability. Furthermore, the fact that a small amount of clay minerals could

cause a significant increase in properties makes these organic/inorganic hybrids

specially attractive to industry [9]. Advantages of clays are that they have active sites

such as hydroxyl groups and exchangeable interlayer cations [10, 11]. In addition,

the small dimensions of the individual layers and the high aspect ratio of clay

minerals render them particularly attractive in several areas of materials science [8].

In this current research work Laponite RD was chosen as the inorganic material to be

incorporated into the organic polymer nanocomposites.

Laponite RD is a synthetic disk-shaped clay which is available in very high purity

and constant quality. The discs are around 30 nm in diameter and around 1.2 nm in

thickness. It has been reported that the surface area is ca. 800 m2/g, with a density of

2.57 g/ml. The listed negative surface charge density is 0.014 e7A2, and the reported

hydroxyl group concentration is 0.36 rneq/g determined by titration with triethyl

aluminium [8 , 12, 13]. Like most of the clays, Laponite can be easily modified. Pre-

treatment is usually required in order to improve its compatibility with a polymer

matrix and to achieve a good dispersion. In addition, clay minerals also have a high

surface area to volume ratio, due to their small size. As a result, when Laponite is

incorporated into an organic matrix to form nanocomposites, the contact between the

two phases is increased and the properties of the resulting material are expected to be

improved [9].

The structure of Laponite consists of two-dimensional layers with a central sheet of

Mg306 octahedra (O) sandwiched between two external sheets of Si(0 ,OH)4

tetrahedra (T), in a stacking pattern of ‘T-O-T’. The tetrahedral shaped units (Figure

4.1 (a)) serve as the basic structural components of the tetrahedral sheets (Figure 4.1

65

Chapter 4

(b)). Each of these tetrahedral units consists of a central four-coordinated silicon

atom surrounded by four oxygen atoms. These oxygen atoms are linked with other

nearby silicon atoms, thereby serving as inter-unit linkages to hold the sheets

together. The octahedral-shaped units (Figure 4.1 (c)) serve as the basic structural

components of the octahedral sheets (Figure 4.1 (d)). Each of these octahedral units

consists of a central six-coordinated magnesium atom surrounded by six oxygen

groups, and once again, are linked with other nearby magnesium atoms, thereby

serving as inter-unit linkages to hold together the sheet [7, 9]

• and o = silicon; O and O = oxygen, hydroxyl

(c) (d)# magnesium; O and O = oxygen

Figure 4.1. (a) The structure of a single tetrahedral shaped unit of a central four-coordinated silicon atom surrounded by four oxygen atoms and (b) the resulting tetrahedral sheet, (c) is the structure of an octahedral unit with a central six- coordinated magnesium atom surrounded by six oxygen groups and (d) the resulting octahedral sheet. Image taken from [14].

Figure 4.2 shows a schematic illustration of the ‘T-O-T’ stacking pattern. Laponite

can swell in water by incorporation of water between these sheets. This swelling

capacity is reversible, meaning that Laponite can absorb or lose water depending on

the pressure and temperature of its media.

66

Chapter 4

Tetrahedral Sheet

Octahedral Sheet

Tetrahedral Sheet

Figure 4.2. A schematic illustration of the ‘T-O-T’ stacking pattern, where one octahedral sheet is sandwiched between two tetrahedral sheets (on the left). The same stacking pattern can be found in Laponite structure. On the right is the crystalline structure of a Laponite clay disk. Image taken from [7, 9].

An isomorphic substitution, which is the replacement of one atom by another of

similar size in a crystal lattice without disrupting or changing the structure, of Si4̂ by

Al3+, for instance, in the outer tetrahedral sheet will generate a negative charge on the

Laponite particle. Similarly, Al3+ in the central octahedral sheet can be substituted by

Mg2+ and also could generate a negative charge in the interlayer region. This excess

of negative charge adsorbs cations to the clay surface, in a similar way to how a

magnet attracts iron filings, and they are exchangeable. This phenomenon gives clay

minerals an important and interesting property, which is its cation exchange capacity

or CEC. The CEC is defined as the number in moles of monovalent cations that can

be substituted to counterbalance the negative charge of 100 grams of clay. Typical

CEC values are in the range of 60-120 meq/lOOg [15]. The Laponite used in this

study has a CEC of 75 meq/lOOg [7, 16]. CEC is a constant for a given clay mineral

and does not depend on pH. However, there is a second cause of charge on the

Laponite particles due to Si-OH (acid), Mg-OH and Al-OH (basic) groups at the

edge of the layers. The Laponite used in this study has a hydroxyl group

concentration of 36 meq/lOOg [16]. The hydroxyl group concentration depends on

pH. Depending on the pH of the media, the clay particles are positively or negatively

charged. In acidic media positive species are predominant, and in basic media it is

negatively charged species that are predominant. The acido-basic properties of these

sites can be explained by using the equilibrium equations shown in Figure 4.3.

10A

67

Chapter 4

X— OH + H+ T_ X— OH2+

X— OH ^ X— 0" + H+

Figure 4.3. Acido-basic equilibrium equations: The top equation is for acidic media, where positive species are predominant and the equilibrium shifts to the right, and as a result, clay particles are positively charged. In the basic media, negatively charged ions are predominant and according to the second equation, the resulting clay particles will be negatively charged.

In the literature, it was reported that several research groups proposed a variety of

clay modifications for nanocomposite synthesis. These treatments for clays are

performed in order to optimise the dispersion or to chemically link the polymer

matrix and the clay [1]. Three main clay modification systems have been reported:

cation exchange, silane grafting, and adsorption of polar polymers. In cation

exchange, the structural inorganic cations of the clay interlayer are exchanged by

organic cations that can bring, in some cases, a reactive or functional group. These

functional groups further improve chemical compatibility between the mineral and

the organic metrix. The amount of organic cations that can be exchanged depends on

the amount of the exchangeable sites, hence it depends on the CEC and the structure

of the clay [1]. In silane grafting, the hydroxyl groups on the edges of the clay plates

are used to covalently bond organosilanes onto the clay surfaces. Organosilanes are

molecules with the general formula of RnSiX4.11, where R represents the organic part

of it, and its functional group (for example acrylate, styrene etc.) will serve as an

anchor for the organic matrix to produce a covalent bond. X is a hydrolysable group

(for example halogen, allcoxy etc.). Organosilanes are known to improve the material

properties, such as chemical adhesion, wetting, rheology and mechanical resistance;

hence they are widely used in optics, coatings and catalysis [1,9, 17, 18]. It has been

reported that some polar* polymers adsorb onto the clay surfaces via hydrogen

bonding and van der Waals attraction forces [19, 20].

Clay modifications have further developed over the years. In 2006, Negrete-Herrera

and co-workers [7] reported the synthesis of clay-armoured latex particles via

emulsion polymerisation. In the process, they used Laponite which was organically

68

Chapter 4

modified through cation exchange by using 2 ,2 -azobis (2 -methylpropionamidine)

hydrochloride (AIBA) as the polymerisation initiator or 2-(methacryloyoxy) ethyl

trimethyl ammonium chloride used as a comonomer. These modified Laponite clays

were then dispersed in the water phase prior to emulsification with the monomer

phase. Cauvin, Colver and Bon [21] successfully synthesised Laponite armoured-

latex particles of polystyrene via a Pickering-stabilised miniemulsion polymerisation.

Sun and co-workers [22] developed a novel polystyrene-encapsulated Laponite

system via miniemulsion polymerisation. The Laponite particles were modified

through cation exchange using an ammonium salt, cetyltrimethylammonium bromide

(CTAB). Then the modified clay was mixed in the monomer phase prior to

emulsification with the water phase. In Figure 4.4, the Transmission electron

microscopy (TEM) image reveals the irregular coarse-edged latex particles

containing Laponite clay discs.

Figure 4.4. TEM image of polystyrene latex particles containing Laponite clay. Image taken from [22].

In 2006 Tong and Deng [23] synthesised polystyrene nano-saponite suspension via

miniemulsion polymerisation. They showed that the stability of the suspension and

the intercalation degree of the clay strongly depended on the pre-treatment of

saponite with the cationic surfactant, octadecyltrimethylammonium bromide

(OTAB). In addition, they argued that the pre-treatment process not only intercalated

69

Chapter 4

the clay layers but also converted the nanoclay particles from being hydrophilic to

hydrophobic. It is clear from the TEM image (Figure 4.5 left) that when unmodified

saponite was used in the polymerisation, the saponite particles were segregated

together in the polystyrene matrix. However, it was shown, that when saponite

particles were pre-modifid with OTAB, they were intercalated and uniformly

dispersed in the polystyrene matrix (Figure 4.5 right).

Figure 4.5. TEM images of polystyrene saponite composite films; on left with unmodified saponite and on right with saponite modified with OTAB. Image taken from [23].

4.1.1. Surface tension, surfactant concentration and CMC

It is well known that in the bulk of a condensed phase, the intermolecular forces act

between the molecules (or atoms) in a symmetric fashion. At the interface or surface,

there is an imbalance of the forces as the local chemical environment changes. This

imbalance in forces results in a surface tension, y . This surface tension acts to

minimise the surface area [24]. All surfactants possess the common property of

lowering the surface tension when added to water. Pure water has a surface tension

of 72 mN/m and gradually decreases with increasing concentration at low

concentrations and is a linear function of the logarithm of surfactant concentration

70

Chapter 4

until it reaches the critical micelle concentration, CMC of the surfactant. At

concentrations well below CMC, the hydrophobic ends of the surfactant molecules

orient with their hydrophilic tail in the water. As concentration of surfactant

increases, more and more molecules come to the surface. When the surface is

saturated with the hydrophobic ends of the surfactant, it is called a ‘monolayer’. At

the same concentration, micelles form. Micelles occur when a group of hydrophilic

ends surrounds their hydrophobic ‘tails’ and shield them from water. Now with the

increase of the surfactant concentration, the surface tension does not decrease. This

inflection point is called critical micelle concentration, CMC. Figure 4.6 illustrates a

typical plot of surface tension as a function of the logarithm of concentration of

surfactant [25-30].

Figure 4.6. Schematic illustration of surface tension as a function of the logarithm of the surfactant concentration. Surface tension of pure water or at 0 log concentration is 72 mN/m and the inflection point where surface tension no longer decreases with the increase of surfactant concentration is the CMC of a given surfactant.

4.1.2. Marangoni Flow

The Marangoni effect, first identified as ‘tears of wine’ by physicist James Thomson

in 1855, was later named after the Italian physicist Carlo Marangoni, a decade later.

Marangoni flows are those induced by a surface tension gradient generated either by

71

Chapter 4

a composition or a temperature variation along a free liquid surface [31-35], Xu and

Luo [36] reported that in an evaporation process, the nonuniform evaporation from

the liquid/vapour interface could produce a temperature gradient, which in turn, it

should generate a surface tension gradient along the liquid surface. They argued that,

this surface tension gradient usually drives a convective Marangoni flow inside the

liquid. Fanton and Cazabat [35] studied the spreading of liquid films driven by

surface tension gradients induced by evaporation from a mixture with two

components. Many researchers have studied the Marangoni effect over many years,

as it plays a key role in coating, thin film deposition, crystal growth and production

of photonic materials [37-39].

4.1.3. Overview of Nanocomposite Materials

For the current study, miniemulsion polymerisation was used to synthesise

polymer/Laponite latex dispersions. Two systems were adopted. First, using the fact

that the latex particles are the replica of the miniemulsion droplets, it was possible to

encapsulate the clay plates, if clay plates are initially dispersed inside the monomer

droplets. For this approach, the clay plates were functionalised with an organophilic

treatment in order to be compatible with the monomer phase. On the other hand,

clay-armoured latex particles were synthesised via miniemulsion polymerisation, by

anchoring a free initiator to the clay surface through cation exchange. This

modification gives the clay particles an amphiphilic status. This encouraged

subsequent clay interaction with the miniemulsion droplets. Then the clay with

attached radicals nucleated reactions in the nano-droplets, which then progressively

formed polymer particles with an inner polymer core surrounding an outer shell of

clay sheets [1]. Taking aboard these techniques, two systems of polymer/clay

(Laponite) nanocomposites were developed. They were both synthesised by

miniemulsion polymerisation at the CNRS-LCPP (Laboratory of Chemistry and

Processes of Polymerisation), Lyon, France, one of the academic partners of the

NAPOLEON Project. These two systems are hereafter referred to as Route I and

Route II [1, 16].

72

Chapter 4

In Route I, the clay was modified, using the cationic initiator AIBA, which was

incorporated in the Laponite layers through cation exchange. This modification gave

the Laponite plates a reactive ability to initiate the polymerisation process at the

surface of the Laponite plates [40]. Initially, the cation exchange was done at 100%

of CEC. These AIBA modified Laponite plates were then dispersed in water with

surfactant. The monomer, methyl methacrylate (MMA) and butyl acrylate (BuA)

were mixed with a hydrophobe, Behenyl acrylate (BylA) and poured into the clay

suspension. The polymerisation was carried out at 70 °C for three hours under

nitrogen flow. As Laponite carries the initiator, the polymerisation starts from the

clay surface and it is expected that the Laponite plates are located on the surface of

the droplets and therefore on the surface o f the polymer particles (Figure 4.7 left).

73

Chapter 4

Route I S LaPonite Powder/ ........... v

Functionalisation I exfoliation ^ 4

Route II

% ^ ^

* * •

A * *Water —

* * * * *

* * * * * *

Monomer(s) 'T T "

^ M iniemulsion Polym erisation ^

• 3 *

Polymerparticles

Laponitediscs

Water

Figure 4.7. Schematic illustration of Route I and II. In Route I (on the left), Laponite plates were functionalised with the cationic initiator, AIBA and dispersed into the water phase. In Route II (on the right) double functionalised Laponite by MPTMS and DDAB, was dispersed in the monomer phase. After the miniemulsion polymerisation in Route I it is expected that the clay will be at the surface of the latex particles, whereas in Route II, the clay will be encapsulated within the latex particles. Image taken from [41].

In Route II, an organosilane coupling agent, y-Methacryloxypropyl-trimethoxysilane

(y-MPTMS), was grafted to the edges of the Laponite plates. Laponite was dispersed

in the monomer phase. In addition, didodecyldimethylammonium bromide (DDAB)

[42], an alkyl ammonium surfactant, was integrated into the basal faces of the clay

through cation exchange. This double modification was carried out in order to render

a good compatibility to the Laponite plates with the monomer mixture. The double

74

Chapter 4

functionalised Laponite plates were dispersed in the monomers and hydrophobe. In

the resulting emulsion, the polymerisation was started by an injection of 4’-azobis (4-

cyanopentanoic acid) (ACPA) and was carried out at 70 C for three hours under

nitrogen flow. As the double modified Laponite plates show good compatibility with

the monomers, they are expected to remain inside the monomer droplets; thereafter

inside the polymer particles. Route II is shown in Figure 4.7 (right).

Figure 4.8 shows the chemical structure of (a) y-MPTMS and (b) DDAB molecules

and (c) a schematic of the double functionalised Laponite disc.

(a)

0 (C H 2)3-S i(0 C H 3) 3

O

MeO—

Figure 4.8. (a) the chemical structure of y-MPTMS molecule, (b) the chemicalstructure of the DDAB molecule and (c) a schematic diagram of a double functionalised Laponite disc with y-MPTMS molecules grafted on its edges and DDAB molecules on its surface.

Figure 4.9 shows cryo-TEM images of composite latex particles of (a) Route I, and

(b) Route II. In Figure 4.9 (a) dark filaments can be clearly seen at the surface of the

polymer particles and these are interpreted as being the clay plates. In (b) the

Laponite is encapsulated by the polymer phase as indicated by the dark lines within

75

Chapter 4

the particles [16]. Schematic representation of polymer particles by Route I and

Route II are shown by (c) and (d) respectively.

(d)

Figure 4.9. Cryo-TEM images of polystyrene/Laponite nanocomposite particles by Route I (a) and Route II (b). Images were taken from [43]. (c) and (d) are the schematic representations of the polymer particles by Route I and Route II, respectively.

76

Chapter 4

4.2. Materials

4.2.1. Latexes by Route I

The latexes were prepared at the CNRS-LCPP (Laboratory of Chemistry and Process

o f Polymerisation), Lyon, France, by Veronique Mellon. Methyl methacrylate

(MMA) and butyl acrylate (BuA) were used as monomers in a 50:50 weight ratio to

synthesise the nanocomposite latexes by miniemulsion polymerisation. A series of

latexes was prepared with 0 wt. % (pure latex - for comparison), 3 wt. %, 5 wt. %

and 7 wt. % Laponite based on monomers. For Route I, modified Laponite with

ABBA initiator, at 100% of the Cation Exchange Capacity (CEC) was used. In all

cases the solids content was around 20 wt. %. Table 4.1 summarises the different

characteristics of Route I latexes.

Table 4.1. Characteristics of Polymer/Laponite nanocomposite latexes synthesised through miniemulsion polymerisation with ABBA functionalised Laponite for Route I.

Monomer AIBA

(%

CEC)

Laponite

(wt.%)

Droplet

size

(nm)

Particle

size

(nm)

Conversion

(% )

MMA/BuA 0.3 0 119 83 82

MMA/BuA 10 0 3 119 1 0 1 73

MMA/BuA 1 0 0 5 116 104 75

MMA/BuA 10 0 7 149 128 96

77

Chapter 4

4.2.2. Latexes by Route II

For Route II, MPTMS-DDAB twice functionalised Laponite was used. The initiator

was ACPA (4,4’-azobis(4-cyanopentanoil acid); 0.5 wt. % based on monomers were

used. Again, the solids content for all cases was around 20 wt. %, and Table 4.2

summarises the different characteristics of Route II latexes.

Table 4.2. Characteristics of Polymer/Laponite nanocomposite latexessynthesised through miniemulsion polymerisation with MPTMS/DDABfunctionalised Laponite for Route II.

Monomer MPTMS/

DDAB

(% CEC)

Laponite

(wt.%)

Droplet

size

(nm)

Particle

size

(nm)

Conversion

(% )

MMA/BuA 0 0 152.5 97.4 96.4

MMA/BuA 2 0 0 3 133.5 97.0 96.4

MMA/BuA 2 0 0 5 116.3 105.4 80.9

MMA/BuA 2 0 0 7 167.0 129.2 51.9

In Table 4.1 and Table 4.2, the droplet size represents the size of the monomer

emulsion droplets before polymerisation and the particle size represents the size of

the polymer particle after the polymerisation. It is widely accepted that the droplet

size and the number should remain close to the size of particles during

polymerisation. A 1:1 ratio indicates there is neither monomer diffusion nor

coalescence before and during polymerisation, so that most nanodroplets keep their

identity while being converted into particles. If the droplet and particle sizes are

equal, the number of particles (Np) and the number of droplets (Nd) are also equal,

that is to say the ratio of (Np /Nd) should be close to one. It is widely accepted that the

Np /Nd ratio in the range of 0.8-1.2 is considered reasonable. If the Np /Nd ratio is

greater than one, then it indicates that the droplets are breaking up as a result o f a

secondary nucleation. On the other hand, if Np /Nd ratio is less than one, it indicates

78

Chapter 4

droplet coalescence, which results in bigger particles [9]. In the tables, the

conversion as a % represents the amount of monomer being converted into polymer.

The target normally is to reach close to 100% conversion. If the conversion is 96.4%,

it means that 3.6% of the monomer remains unpolymerised in the resulting product

after the polymerisation process.

79

Chapter 4

4.3. Techniques

4.3.1. Magnetic Resonance Profiling (MRP)

For MRP experiments, 20 mm x 20 mm and 180 pm thick glass cover-slips were

used as the substrate. The latex was applied on to the coverslips to get the desired

thickness. As a rough guide, around 70 pi of 50 wt.% solid content latex would

produce a 300-400 pm thick wet film. As soon as the latex was cast on to the glass

coverslip, it was placed inside the magnet and NMR profiles were taken at room

temperature and ambient humidity. The magnetic field, Bo, was 0.7 T when the

measurements were performed. The gradient strength, Gy, was approximately 17.5

T/m. The NMR signal is obtained from an excitation using a quadrature echo

sequence: 90x-x-(90y-x-echo-x-)n [44, 45]. For typical latex experiments, the

number of echoes, n, is 32; the pulse gap, x, 75 ps; the dwell time, DW, is 0.7 ps; the

number of points per echo, SI, is 128 and the spectrometer frequency, SF, is 29.6

MHz. For each and every experiment, the time delay between profiles and the total

drying time was changed as desired. The same MR parameters were used for all

experiments, so that direct comparisons of profiles for different samples were

allowed. The profile shapes were normalised by an elastometer standard in order to

correct for the decline in sensitivity over the film thickness.

4.3.2. Optical Transmission Measurements

Optical transmission measurements of the films were carried out with a

spectrophotometer (Campsec 350, Cambridge, UK). The resulting films from the

MRP experiments were used to measure the optical transmission at a wavelength of

600 nm.

80

Chapter 4

4.3.3. Profilometry

Thickness variations of the dried films were investigated using a stylus profilometer

(Dektalc 8 Surface Profiler by Veeco Instrument Limited, Cambridge, UK). The

Dektak Surface Profiler is an advanced surface texture measuring system that

accurately measures surface texture below the sub-micrometer scale. The instrument

has a vertical range of 5 nm to 0.262 mm. A scan length of up to 50 nun can be

scamied within 3-200 seconds and a vertical resolution of 4 nm for a 0.262 mm range

scan. Veeco Instruments Limited claims that the Dektak 8 Surface Profiler combines

high repeatability, low-force sensor technology which allows easier measurement of

soft materials and characterization of sub-micron lines and spaces. The advanced 3D

data analysis with a high aspect-ratio tip is also ideal for measuring shallow as well

as deep structures. The result of the profilometry is a map of the dry film’s thickness

as a function of lateral distance. As the maximum distance, which is from one end to

the other end of each film, varies between samples, the distance was normalised by

dividing each sample’s distance by it’s maximum distance and multiplied by 10 0

Photographs of the drying film were taken using a camera on a contact angle

analyser (Kruss Easy Drop Standard, Drop Shape Analysis System by Kruss GmBH,

Hamburg, Germany). The instrument uses a monochrome interline CCD 752 X 582

pixel camera with a 6.5x zoom. The system is capable of automatic recording of a

sequence of images at defined time intervals. The samples were prepared by using

around 70 pi of the dispersion applied on to a 20 mm x 20 mm glass coverslip, the

same way as for the GARField experiments. The automated recording system was set

up to record the images at desired time intervals. Figure 4.10 shows a photograph of

the setup.

[46].

4.3*4. Methods for obtaining photos of the films

81

Chapter 4

Sampleplatform Light

Figure 4.10. The Easy Drop Standard setup, which was used to take the photos from the side of a drying film. The image was taken and modified from [47].

82

Chapter 4

4.4. Results and Discussion

4.4.1. Original Route I and Route II

4.4.1.1. MR Profiling and Images from side

Figure 4.11 (a) compares a series of profiles obtained from 0 wt.% Laponite latex

synthesised by Route I to a series obtained with latex containing 3 wt.% Laponite,

prepared by Route I (Figure 4.11 (b)). It was found that in the pure latex (0 wt.%

Laponite), the film thickness decreased at a constant rate and there was no sign of

skin-like layer forming on the top layer of the film. Both films show non-uniformity

in the profiles at the later stages of drying. There is no significant difference in the

drying profiles for the two types of films. Photographs from the side of the films also

appear very similar for the two types of latex dispersions.

Figure 4.11. MR profiles of (a) 0 wt.% Laponite latex, synthesised by Route I and (b) 3 wt.% Laponite latex, synthesised by Route I over time. The profiles were obtained every five minutes. In both cases the thickness decreases at a constant rate, and non-uniformities in water concentration in the vertical direction do not develop until the later stages of drying.

o.o0 100 200 300 400

Height (11m)100 200 300 400

Height(tim)

Method 1 (AIBA) with 3% Laponite

1 .2 -[

0.8

Figure 4.12 shows a series of photographs of a drying film of 0 wt.% Laponite -

Route II latex. The film was cast on to a 2 cm x 2 cm x 0.18 mm glass coverslip. The

photos were taken from the side of the film and only the left half of the film is

83

Chapter 4

shown. The drying time of each photo is indicated on the left. Initially, the thickness

of the wet film decreases over the time. About 30 minutes into drying, the drying

front proceeds from the edge of the film, resulting in a thinner film near the edges.

These two processes simultaneously continue till the film is completely dried.

60 min.

70 min. — ■ ■ — i=a—

80 min. — — M

90 min. — ■■■

100 min. u

110 min. ------------

120 min. - — ,

Figure 4.12. Series of photographs taken from the side of a drying film of 0 wt.% Laponite - Route II latex. The drying times are indicated in the figure.

In comparison, the GARField profiles (Figure 4.13) and photographs taken from a

side view of a drying film (Figure 4.14) of 3 wt.% Laponite - Route II latex,

however, are noticeably different. The film thickness of the sample, determined by

MR profiling, initially decreases with no gradient in water concentration. That is, the

water concentration within the film from top to bottom is more or less the same.

After about 60 minutes into drying, the film thickness increases in the central region

of the film and simultaneously a gradient in the water concentration develops. At this

stage, images from the side (shown in Figure 4.14) confirm that the edge regions

continue to thin over time. As drying continues the film thickness decreases in the

central regions.

0 min.

10 min.

20 min.

30 min.

40 min.

50 min.

84

Chapter 4

Film thickness(iam)

Figure 4.13. The GARField profiles of the latex with 3 wt.% Laponite by Route II. The Film thickness initially decreases with no non-uniformities or gradient in water concentration in the vertical direction. At a later stage, the film thickness increases over time and simultaneously develops gradient in water concentration.

In order to describe the drying process, it was divided into four stages as follows:

Stage 1: The film thickness decreases at a constant rate. No gradient in water

concentration is found.

Stage 2: In the film centre, the change in film thickness slows and then the film

thickness is constant over time. A drying front simultaneously proceeds from the

edge of the film, and the film thickness becomes thinner near the edges.

Stage 3: The thickness in the centre of the film increases over time, as the edge

regions continue to thin over time. The contact angle between the central region and

the edge region increases over time. A gradient in water concentration develops.

Stage 4: The film appears dry near the edges, and the central region decreases in

thickness.

These four stages were shown in Figure 4.14, in a series o f photographs and in

Figure 4.15, with the GARField profiles.

85

Chapter 4

0 min.

10 min.

20 min

30 min.

40 min.

5 0 min. ___ „

60 min. __

iSta je 1

7 0 min.

80 min.

90 min.L

10 0 min.

1 10 min.

12 0 min.

Stajc 2

13 0 min.

14 0 min

St<age 3

Stqge 4

Figure 4.14. Series of photographs taken from a side view of the drying film of 3 wt.% Laponite - Route II latex on a 2 cm x 2 cm glass substrate. The drying times are indicated on the figure. Only the left-half of the film is shown.

Photographs in Figure 4.14 confirm at the end of Stage four that the centre looks

slightly thicker. One can argue that the vast amount of lateral flow is water, and with

it some polymer particles could have flowed as well. A ridge is observed near the

edge of the film.

86

Chapter 4

0 100 200 300 400 500» •*» « *•*"»> Height (^m)

Figure 4.15. A series of MR profiles obtained at five minute intervals, from 3 wt.% Laponite sample by Route II, showing the four stages of drying; Stage 1 - film thickness constantly decreases, no water concentration gradient, Stage 2 - film thickness decrease slows down to a constant thickness, Stage 3 - film thickness increases in the centre and water concentration gradient develops, Stage 4 - film thickness decreases in the central region. Arrows show the direction of increasing drying time.

A similar type of drying process was found in the other Route II nanocomposite

samples containing Laponite at higher concentrations (5 wt.% and 7 wt.%). As a

simple means to compare them, the thickness was normalised by dividing by the

initial thickness, and the drying time was normalised by dividing by the total drying

time. The resulting normalised film thickness as a function of normalised time is

shown in Figure 4.16 for dispersions of 0, 3, 5 and 7 wt.% of Laponite by Route II.

Film thickness increases are only seen in the dispersions that contain Laponite -

prepared by Route II - but not in the pure latex.

87

Chapter 4

Normalised Time

Figure 4.16. Comparison of normalised film thickness as a function of normalised drying time for latexes with various Laponite concentrations by Route II. The thickening effect is stronger with 3 wt.% Laponite, but it is still noticeable with 5 wt.% and 7 wt.% Laponite concentrations.

In order to understand the causes of this effect, experiments were conducted to

determine separately the effects of the latex solid content and the surface tension.

The solids content was increased in latex prepared by Route I and II by evaporating

the water. Known amounts of latex samples in open glass bottles were left on the

shaker, until they reached the desired solid content. The viscosity of the dispersion

was simultaneously increased by this procedure. Figure 4.17 shows the effect of

raising the solids content up to 30 wt.% in the latex with 3 wt.% Laponite by Route

II. After the solids was increased up to 30 wt.%, it was possible to see that the

thickness decreases at a relatively constant rate, although there is some non­

uniformity in the water distribution.

88

Chapter 4

Figure 4.17. Comparison of (a) GARField profiles for the as-received latex of 3 wt.% Laponite by Route II to profiles for (b) the same latex when its solids content was raised to 30 wt.%, by evaporating water.

Servoxyl [29], a commercial wetting agent, containing an anionic surfactant, was

added to adjust the surface tension and the spreading of the film on the substrate.

Servoxyl was received as a 35 wt.% solution in water from Elementis Specialities.

0.75 g of Servoxyl 35 wt.% was added to every 50 g of solids in the latex.

The time-dependence of the thickness of the latex with 3 wt.% Laponite by Route II

is compared in Figure 4.18 for four formulations. The effects of solids content and

wetting agent were determined individually and together. It was seen that raising the

solids content to 30 wt.% (and hence increasing the viscosity) has a pronounced

effect in leading to a constant rate of thickness decrease. The addition of wetting

agent (Servoxyl) leads to a slightly more uniform rate, but to a lesser extent.

Film thicknesslttm)

89

Chapter 4

Normalized drying time

Figure 4.18. The normalized thickness as a function of the normalized drying time of the latex with 3 wt.% Laponite by Route II. Results are shown for as-received latex (black), latex with a higher solids content of 30 wt.% (red), as received latex with added Servoxyl (blue), and for latex with a 30 wt.% solids content plus added Servoxyl (green).

4.4.1.2. Optical Transmission

A transparent sample is one that transmits light so as to render the objects beyond it

perfectly visible. At the other extreme, an opaque sample transmits no light, and all

information about the objects lying behind it is lost. A translucent sample lies

between these two extremes. They transmit a part of light, but diffuse, scatter or

absorb some of it, so that the objects beyond it are not readily visible [48, 49]. Three

factors could affect the optical transparency of the films studied in this work: the

scattering from the second phase, the scattering by the surface due to the surface

roughness and the absorption by the material. First of all, the optical transparency

could be reduced by the effects of the light scattering when films contain phases of

differing refractive indexes, with a size of the second phase far below than the

wavelength of light (600 nm). van Tent and Nijenhuis [48] have shown that smaller

the second phase, or in their case the pore sizes, the more transparent the film. By

increasing the pore size, the transmission would drop dramatically.

90

Chapter 4

Secondly, the surface roughness could also decrease optical transparency. If the

surface of the sample is perfectly smooth, the incident beam is split into a reflected

ray, (which is transmitted back) and a refracted ray which is transmitted into the

sample, upon incidence. If the surface is not smooth, it scatters light upon incidence.

In this case, in addition to the reflected and refracted rays, a significant amount of

light now propagates in multiple directions. If the surface is perfectly smooth or

glossy, then reflected rays predominate. If the surface is not smooth, the multiple

direction propagated rays predominate. Then the sample is commonly described as

having haze, haziness, milkiness or cloudiness [49].

Finally, absorption by the material could also reduce the optical transparency. For

instance, coloured films absorb certain wavelengths of light.

The transmitted light intensity can be calculated by the following equation [49]:

I = I 0exp"'" (4.1)

where I is the transmitted light intensity and Io is the incident light intensity and x is

the film thickness, p, the absorption coefficient, is a property of the material. It is

mathematically defined as [49]:

4 71M ~ — ' k (4,2)

where k is the extinction coefficient.

Optical transmission measurements were carried out to indicate the film quality. The

optical transmission of the films made from Route I and Route II with 0, 3, 5 and 7

wt.% of Laponite concentrations were measured. The latex films on glass coverslips

from the GARField experiments were used to measure the optical transmission. The

dry film thicknesses were between 300 - 500 pm. Figure 4.19 shows the

transmission (%) as a function of Laponite content (wt.%) for both Route I and Route

II. The graph shows that the increase in the concentration of Laponite did not

91

Chapter 4

decrease the transparency for the films from Route I latex, but there was a significant

drop in transparency in Route II films with increased Laponite concentration.

0 1 2 3 4 5 6 7 8 Laponite content ( wt.%)

Figure 4.19. Optical transmissions (at a wavelength of 600 nm) as a function of Laponite content for the films by Route I (red) and Route II (blue).

Furthermore, the transmission of the films by Route II - as received and with 30

wt.% solids and added Servoxyl - was measured as a function of Laponite

concentration. The intention was to identify if by raising the solids content and

adding Servoxyl there was an effect on the film quality of Route II latexes. The

results are shown in Figure 4.20. According to the graph, raising the solids content

and adding Servoxyl appears to further reduce the transparency for Route II latexes.

92

Chapter 4

Laponite content ( wt.%)

Figure 4.20. Optical transmission (at a wavelength of 600 nm) as a function of Laponite concentration for Route II latex as-received (red) and after raising the solids to 30 wt.% and adding Servoxyl wetting agent (blue).

Out of the causes given above, it is true that the films with Laponite plates have

materials with different reflective indexes, but the sizes of the second phase are very

much smaller than the wavelength of the light. Therefore, it is reasonable to rule out

the effects of Laponite plates or clusters made with few plates on transparency

variations. On the other hand, increased concentrations of Laponite in films could

have affected the absorption coefficient, which ultimately affects the intensity of the

transmitted light. In addition, thickness variations in the films also affect the intensity

of the transmitted light. The dry film thicknesses were between 350 - 400 pm. The

most significant factor affecting the transparency should be the film/air surface

roughness. From the photographs of the films (not shown), it was clear the surfaces

of the Route II samples were more rough or had more wrinkle-like unevenness

compared with Route I films. In addition, Route II samples were cloudier compared

with Route I films. This would give a clear explanation for the results shown in

Figure 4.19. The transmission variations between the samples with different Laponite

concentrations should have been the differences in the absorption coefficients, which

depend on the material. Different absorption coefficients should have been the key to

the results shown in Figure 4.20. By raising the solids and adding Servoxyl, the

93

Chapter 4

absorption coefficients of these films should have been affected. It is difficult to

explain the reasons for transmission trends which are shown in Figure 4.20.

4.4.2. Effect of the DDAB content on Route II latexes

As stated earlier, in Route II latex, Laponite plates were doubly functionalised by

MPTMS and DDAB before being incorporated into monomer droplets and

polymerised. The amount of DDAB content was 200% CEC of Laponite. There are

several advantages of higher DDAB content in a system, such as, better stability of

the monomer/clay dispersion, consequently a more stable latex and also a higher

monomer conversion and lower secondary nucleation [9]. As the amount of DDAB

in the system was two times the CEC, one can argue that DDAB was forming a

double layer on the clay surface (Figure 4.21 (a)). Furthermore, some of the DDAB

molecules which are adsorbed via their hydrophobic tail could migrate to the

monomer droplet/water interface (Figure 4.21 (b)). Then they could diffuse through

the water phase and form surfactant aggregates (Figure 4.21 (c)). It should be

mentioned that aggregates (micelles) would only be formed at high concentrations -

above CMC. They might form when water is evaporated and concentration increases.

Laponite disc

MPTMS DDAB

(a)

94

Chapter 4

(b)

(c)Figure 4.21. Schematic illustration of double functionalisation of a Laponite disc by 200% CEC of DDAB and MPTMS molecules, (a) A formation of a double layer of DDAB molecules on the surface of Laponite disc, (b) migration of excess DDAB molecules to the monomer droplet/water interface and (c) migrated DDAB molecules diffuse to the water phase and form surfactant aggregates when water is evaporated.

95

Chapter 4

Based on the above mentioned argument, the following process was put forward to

explain the drying behaviour of Route II acrylic Laponite nanocomposite latex.

1. The initial polymer particles with encapsulated Laponite discs arguably have

excess DDAB molecules at their surface (Figure 4.22 (a)).

2. In the first stages of drying, the evaporation of water brings the particles into

close packing at the edges of the film[50] (Figure 4.22 (b)).

3. At the edges of the film, due to the desorption of the excess DDAB, free

DDAB molecules can be found in the serum (Figure 4.22 (c)).

4. As desorption of DDAB mainly occurs at the edges of the film, a surfactant

concentration gradient could develop within the drying film. Higher DDAB

concentration at the edge of the film and a relatively lower concentration in

the central regions of the film develops (Figure 4.22 (d)). This concentration

gradient causes a lateral surface tension gradient within the drying film; a

lower surface tension at the edges and a higher tension in the central regions

develops.

5. There is a driving force to reduce this surface tension gradient. This force

naturally causes a flow of liquid away from the low surface tension regions.

In the literature, this mechanism is identified as the ‘Marangoni flow’ [31-34,

36]. Due to the Marangoni flow, one could expect a flow of water within the

drying film from the edges to the central regions (Figure 4.22 (e)).

6 . As evaporation of water from the drying film continues, this process

continues.

96

Chapter 4

InitialParticle packing at theedge of the film Desorption of D D A B :

N so u rce of DDAB at the edge only*

(a) _ (b)4mh • * i * •

DDAB

/Marangoni flow of surfactant ^

M ,

ly - Surface tension gradient develops

h i g h e r ylo w y lo w y

(e) (d)

Figure 4.22. Explanation of drying behaviour of the latexes by Route II. (a) Initial polymer/Laponite particle, where excess DDAB molecules stick out of the particle, (b) Due to water evaporation polymer particles are closely packed near the edges of the drying film, (c) Desorption of excess DDAB molecules to the surrounding serum, (d) Development of a DDAB concentration gradient, which causes a surface tension gradient between the edges and the centre of the film, (e) Marangoni flow of liquid from low to high surface tension regions. This process could continue as long as the drying continues.

To test this idea, new polymer/Laponite nanocomposite samples were synthesised by

miniemulsion polymerisation at the CNRS-LCPP. For the new formulation,

compared with the ‘original' Route II formulation, only 100% CEC of DDAB with

MPTMS was used to twice functionalise the Laponite plates. Then this twice

functionalised Laponite was incorporated into the monomer phase prior to

polymerisation. Two new samples were synthesised both with around 3 wt.%

Laponite, but one sample with a lower solid content (19.2 wt.%) and the other with

97

Chapter 4

relatively higher solid content (29.6 wt.%). Table 4.3 summarises the components of

the three formulations.

Table 4.3. Components and characteristics of the ''original Route II and new formulations (less DDAB/low solids and less DDAB/high solids).

Original New formulation New formulation

Route II / low solids / high solids

Amount of DDABCEC-cation exchange

capacity

2 x CEC* CEC* CEC*

Amount of DDAB

As a % of monomer11.42 x 10'3 5.71 x 10‘3 5.71 x 10"3

Solids content (wt.%) 2 0 19.2 29.6

ACPA as initiator Yes Not known Not known

Surfactant (wt.%) 2 1.7 1

Hydrophobe wt.%) 6 Not known Not known

Droplet size (nm) 133.5 93.6 1 1 2 . 1

Particle size (nm) 97 90.4 132.6

Conversion (%) 96.4 95.4 8 8 .1

Laponite content

based on monomers3 2 .8 3.4

4.4.3. Comparison of drying properties of ‘original’

and new polymer/Laponite latexes

4.4.3.1. MRP of ‘original’ and new polymer/Laponite

latexes

Two problems were identified with the ‘original’ Route II, 3 wt.% Laponite latex.

The thickness increased in the film centre and vertical non-uniformity in water

98

Chapter 4

distribution developed ( Figure 4.13). The latexes prepared with the new formulation,

where only 100% CEC of DDAB was used to functionalise the Laponite plates,

clearly show an improvement in drying properties. With less DDAB with low solid

content (19.2 wt.%) the sample shows a steady decrease in film thickness over time,

but develops non-uniformity in the water distribution towards the end of the drying

(Figure 4.23 (a)). The high solid content (29.9 wt.%) sample showed the most

improved drying properties; during drying the thickness decreases steadily over time

and the water distribution was uniform in the depth of the film (Figure 4.23 (b)).

Dry ng time (Mn)

(Profiles were taken every 3 rrin, total ctying time 132 rrins)

100 200 300 400 500

99

Chapter 4

50 100 150 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 Height (pm)

Figure 4.23. Drying profiles of: (a) new formulation with less DDAB and low solids content (19.2 wt.%), the film thickness decreases steadily over time and the water distribution was non-uniform towards the end of drying, (b) New formulation with less DDAB and high solids content (29.9 wt.%), the film thickness decreases steadily over time and the water distribution is uniform in the depth of the film.

As a simple means to compare the drying profiles of these samples, the thicknesses

were normalised by dividing by the initial thicknesses, and the drying times were

normalised by dividing by the initial thickness. Doing so, the effects of initial

thickness variations were eliminated. The resulting normalised film thickness as a

function of normalised drying time is shown in Figure 4.24 for these three samples.

Compared to the large thickness increase with the original Route II sample, the

samples made with the new formulation showed better drying properties. While the

low solids sample still showed a slight thickness increase, the high solids sample

illustrates the best drying properties of all three samples as it showed hardly any

thickness increase.

100

Chapter 4

Normalised Drying Time (min/ ̂m)

Figure 4.24. Normalised film thickness as a function of Normalised Drying Time for original Route II (blue), new formulation with less DDAB and low solids (red) and less DDAB with high solids (black). All three samples contained around 3 wt.% Laponite.

Furthermore, moments of statistics (zeroth moment and skewness) were used to

analyse the MRP data. The zeroth moment of a drying profile is proportional to the

water content of the sample (within the area of the RF coil) at a given time. The

skewness is a measurement of non-uniformity of a drying profile. New formulation

samples show a gradual decrease in the normalised zeroth moment or the water

content of the samples. The original Route II sample was unusual as it shows a

significant increase in the zeroth moment half away through the drying process

(Figure 4.25 (a)). This can be correlated with the thickness increase in the centre of

the sample.

101

Chapter 4

a>NTJo(fl15EL .oz

3> 0.6o>c| 0.5CO

0.4

0.3

0.2

0.1

0.0

• ■■■■■■

0.00 0.05 0.10 0.15 0.20 0.25

Normalised Drying time (minI p m)

0.30

Figure 4.25. (a) Normalised zeroth moment and (b) Skewness of original Route II nanocomposite (blue triangles), new formulation low solids (red circles) and high solids (black squares). All three samples contained around 3 wt.% Laponite.

When comparing the skewness of drying profiles, note that the higher the maximum

skewness, then the more non-uniform the drying is. Out of these three samples, the

high solids sample using the new formulation showed the most uniform drying and

has the lowest skewness of all three (Figure 4.25 (b)). However, the original Route

II sample and the new formulation with low solids sample do not follow this trend.

102

Chapter 4

4.4.3.2. Visual appearance of the ‘original’ and new

polymer/Laponite latexes

The visual appearance of the films cast for the GARField experiments were further

investigated by taking photographs from above. Figure 4.26 shows the photographs

of the films made from (a) the original Route II, (b) less DDAB and low solids

content dispersion and (c) less DDAB and high solids content dispersion. By

comparison of the photographs, it is revealed that the quality of the films with the

new formulation where less DDAB was used, is higher than the original Route II

films. To the naked eye, the films made from the new formulation with a high solids

sample showed the least irregularities out of all three samples. The lateral flow of

liquid in the original Route II film could have caused the wrinkle-like irregularities.

(a) (b) (c)

Figure 4.26. Comparison of the photographs of the films made by (a) original Route II, (b) new formulation with a low solids and (c) new formulation with a high solids dispersions. The original Route II sample illustrates the most irregularities on the film/air interface compared with the other two samples.

4.4.3.3. Optical transmission of ‘original’ and new

polymer/Laponite latexes

The quality of the films obtained by the GARField experiments was studied by

comparison of optical transmission at a wavelength of 600 nm. The optical

transmission of the three samples is illustrated in Figure 4.27.

103

Chapter 4

100

80

gc.2 60 w(A

E(A

« 40t-

20

0

Figure 4.27. Comparison of optical transmission of the films obtained from original Route II (red), new formulation with low solids (green) and new formulation with high solids (blue) nanocomposites at a wavelength of 600 nm.

The film made from the original Route II shows the least transmission, whereas the

other two samples transmit significantly more light. As mentioned earlier, in section

4.4.3.2, in comparison of their visual appearances, the film made from the original

Route II showed the most irregularities. Therefore it is expected the film made from

the original Route II would show the lowest transmission, as explained in section

4.4.1.2. Between these three films, the surface roughness or irregularities should

have been the predominant reason for the transmission variations. The original Route

II film should has the most scattering from the surface in multiple directions, hence

the least transmission.

As all three films contained 3 wt.% Laponite, scattering from the different second

phases and the Laponite’s contribution to the absorption coefficient should be more-

or-less the same. It is significant that the new formulation latexes contained less

surfactant DDAB. The original Route II samples contained excess DDAB, and it is

possible that this excess surfactant clustered together, to be large enough to scatter

light. In the new formulations only one times CEC of DDAB was used, as a result,

they lack this possible effect created by excess DDAB. This could be another

104

Chapter 4

explanation for the films prepared using new formulations to have higher

transmission, compared with the lower transmission of the original Route II film.

4.4.3.4. Thickness variations of the films of ‘original’ and

new formulation polymer/Laponite latexes

To further investigate the proposed argument of the original Route II samples having

excess DDAB in the system which leads to the development of a concentration

gradient within the drying film and flow of liquid, profilometry was used to study the

thickness variations across the dry films. With the Marangoni flow of surfactant,

water and polymer particles also could flow. This polymer particle movement could

result in a thickness variation across the dry films. A Dektak 8 stylus profiler was

used to investigate surface texture and the thickness of the dry films.

The thickness variation of the film from the original Route II exhibits more of a

“Mexican Hat” shape (Figure 4.28 (a)) in comparison to the other two films (Figure

(b) and (c)). This is evidence of inward flow of latex during drying, as a result of the

surface tension gradient development. The films obtained from the new formulations

also have a lower edge, but the increased thickness in the middle of the films was

noticeably missing. These thickness variations could have caused the visual

differences and the transmission differences observed in sections 4.4.3.2 and 4.4.3.3.

105

Chapter 4

Figure 4.28. Thickness variations (film thickness as a function of normalised distance, i.e. distance divided by total lateral distance) of the films by (a) original Route II, (b) new formulation with low solids and (c) new formulation with high solids using profilometry. It is significant that the original Route II sample (a) has a ‘Mexican Hat’ shape, in comparison to the other two films.

The proposed process in Figure 4.22 illustrates the situation when the wet film is

convex. Due to the evaporation of water, the particles at the edge of the film pack

into closed pack array. At the edges of the film, excess surfactant desorption into the

serum results in a low surface tension. Relatively high surface tension remains in the

middle of the film. This surface tension gradient induces Marangoni flow of

surfactant, polymer particle and water inwards; hence we observed a ‘Mexican Hat’

shape thickness variation in the dry film. If some how we manage to produce a

concave wet film (Figure 4.29 (a)), particles should start to pack into close-packed

array from the centre of the film. Now, there will be excess surfactant desorption in

the centre. As a result low surface tension could develop in the centre, compared to a

relatively high surface tension at the edges of the film (Figure 4.29 (b)). As a result,

now Marangoni flow should occur from the centre to the edge of the film (Figure

4.29 (c)). If this argument is valid, the dry film should have a relatively thin centre

and be slightly thicker somewhere between the centre-to-edge region (Figure 4.29

(d)).

106

Chapter 4

(a) (b)

(c) (d)

Figure 4.29. Schematic illustration of a (a) concave wet film, (b) low surface tension at the centre and high surface tension at the edges of the drying film, (c) Marangoni flow from centre to the edges, (d) dry film with relatively thin centre.

To test this idea, a mould was prepared by using poly(methyl mathacrylate)

(Perspex) as shown in Figure 4.30. The cylindrical reservoir for the wet latex is 20

mm in diameter and 0.2 mm deep. The mould was treated for 30 minutes in UV in

order to make the surface hydrophilic [51]. Into this reservoir, about two drops of

wet latex were applied, so that all the edges of the reservoir were covered with latex.

The sample was left to dry in ambient conditions and it was observed that the film

was starting to dry from the middle. Once the film was completely dry, its surface

was examined by the profilometry.

Figure 4.30. Illustration of the Perspex mould with the 20 mm diameter and 0.2 mm deep cylindrical reservoir to obtain a concave wet film.

107

Chapter 4

The surface variations of the dry concave films are presented in Figure 4.31. The

vertical axis is vertical distance and it’s somewhat different to the film thickness.

This was due to the way the profilometry was done on the samples. The scans were

done from one wall of the reservoir to the other wall across the centre of the reservoir

to obtain the hills and the valleys of the surface. It is clear from the similarity of

Figure 4.31 (b) and (c), that the new formulation films have thin centres and a

relatively smooth gradual decrease in thickness from the edge of the film. In

comparison, the original Route II film (Figure 4.31 (a)) also has a thin centre, but the

decrease in thickness is not a smooth gradual decrease. It is significant that there is a

circular ridge about 20 - 30% of the radial distance from the edge.

Figure 4.31. Surface variations (vertical distance as a function of normalised, i.e. distance divided by total lateral distance) of the dry films obtained by initially concave wet films of (a) original Route II, (b) new formulation with low solids and (c) new formulation with high solids using profilometry.

First of all, these results confirm that both of the new formulation films show a

similar pattern. The films are relatively thin in the centre, due to the concave wet

film; there is a smooth gradual thickness decrease from edge to the centre as there

was no lateral flow of any kind. In comparison, the original Method II film also has a

relatively thin centre, as one could expect from a concave wet film, but what’s

significant is the film thickness variation from the edge to the centre. One can easily

identify the thickness increase somewhere around 20 - 30% of the radial distance

from the edge the edge of the film on both sides. The other profilometry scans done

on this sample (not shown) confirm that this thickness increase is consistent all-

around the film. A possible explanation to this result is that original Method II wet

108

Chapter 4

latex had excess DDAB in the system, and as the wet film was concave, particles

first started to pack into a close-packed array at the centre. This would develop an

excess surfactant desorption in the centre which would lead a low surface tension in

the centre of the drying film compared to the edges of the film. As a result,

Marangoni flow should direct from the centre to the edge of the drying film.

Profilometry of the dry film confirms the thickness increase somewhere between

centre and the edge of the film. The consistency of the thickness increase in all

directions across the film confirms that the Marangoni flow occurs from centre to the

edges.

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Chapter 4

4.5. Conclusions

Physical characteristics o f film formation and film properties of organic/inorganic

nanocomposite coating formulations were studied. Samples from two main systems,

original Route I and original Route II, were examined. Each system comprised four

latexes with different Laponite contents (0, 3, 5 and 7 wt.%). In Route I latexes,

Laponite plates were located on the surface of the polymer particles and in Route II

latexes, the clay particles were encapsulated within the latex particles.

The drying measurements and the photographs from the side of drying films confirm

that the original Route I, pure polymer (0 wt.% Laponite) sample and the original

Route I with Laponite samples dry with no significant differences. In comparison,

the original Route II latexes with Laponite showed thickness increases in the centre

duiing drying, and 3 wt.% Laponite sample showed the strongest effect. It was

argued that the excess DDAB (two times CEC), which was used to fimctionalise the

Laponite plates before incorporation into the monomer droplets, caused this

thickening effect. Based on this argument, two new Route II latexes were developed

with less DDAB (only 100% CEC of Laponite) in the system, one latex with lower

solid content (19.2 wt.%) and the other having a relatively higher (29.6 wt.%) solid

content. During drying, the new formulation samples were uniform vertically and

showed no lateral flow. The optical transmission and structure of the films of original

Route II were affected by the lateral flow of liquid.

As it was argued that the excess DDAB in the original Route II latexes was the cause

for the thickening effect; the excess DDAB could have created a double layer on the

clay surfaces and some of it could have diffused through to the water phase.

Desorption of DDAB was more significant when the particles were closely packed.

As a result, a wet convex film develops a surfactant concentration gradient within the

drying film, with a higher concentration at the edges of the film and a lower

concentration towards the centre of the film. This causes a lateral surface tension

gradient, with a higher surface tension in the centre and lower surface tension

towards the edges of the drying film. The gradient induces a Marangoni flow of

110

Chapter 4

liquid from low to high surface tension regions (Figure 4.22). Thickness variations of

the dry films confirm a ‘Mexican Hat’ shape figure for the original Route II, 3 wt.%

Laponite sample. Further investigation of concave wet films confirmed this

argument. In a concave wet film, particles should closely pack at the centre before

the edges of the drying film. As a result, lower surface tension now should develop in

the centre and Marangoni flow is from the centre towards the edges of the drying

film. It was confirmed by profilometry, that the thickness increased in the middle of

the radial distance in the concave wet film made from the original Route II sample.

The film obtained from the original Route II sample showed the least transmission

and most irregularities on the film/air surface compared with the films from the new

formulation dispersions. It was suggested that the surface irregularities, the internal

structure of the film, and the affected absorption coefficient could have contributed

to the low transmission of the film obtained from the original Route II sample.

It was proven that excess surfactant in a system can affect the physical characteristics

of the film formation process, and by adjusting the amount o f surfactant in the

system the film characteristics can be improved.

I l l

Chapter 4

4.6. References

1. Faucheu, J., Gauthier, C., Chazeau, L., Cavaille, J-Y., Mellon, V., Bourgeat-

Lami, E.„ Short review and recent advances in clay/polymer nanocomposites

obtained by miniemulsion polymerisation. submitted.

2. Blumstei.A, Polymerization o f Adsorbed Monolayers .2. Thermal

Degradation o f Inserted Polymer. Journal of Polymer Science Part a-General

Papers, 1965. 3(7pa): p. 2665-&.

3. Blumstei.A, Polymerization o f Adsorbed Monolayers J. Preparation o f Clay-

Polymer Complex. Journal of Polymer Science Part a-General Papers, 1965.

3(7pa): p. 2653-&.

4. Kojima, Y., Usulti, A., Kawasumi, M., Olcada, A., Fukushima, Y., Kurauchi,

T., and Kamigaito, O., Mechanical-Properties o f Nylon 6-Clay Hybrid.

Journal of Materials Research, 1993. 8(5): p. 1185-1189.

5. Kickelbiclc, G., Schubert, U, Synthesis, Functionalization and Surface

Treatment o f Nanoparticles; Baraton, M.-I., Ed. 2002: American Scientific

Publishers, CA, USA.

6 . Bourgeat-Lami, E., Organic-inorganic nanostructured colloids. Journal of

Nanoscience and Nanotechnology, 2002. 2(1): p. 1-24.

7. Negrete-Herrera, N., Putaux, J.L., and Bourgeat-Lami, E., Synthesis o f

polymer/Laponite nanocomposite latex particles via emulsion polymerization

using silylated and cation-exchanged Laponite clay platelets. Progress in

Solid State Chemistry, 2006. 34(2-4): p. 121-137.

8 . Herrera, N.N., Letoffe, J.M., Putaux, J.L., David, L., and Bourgeat-Lami, E.,

Aqueous dispersions o f silane-functionalized laponite clay platelets. A first

step toward the elaboration o f water-based polymer/clay nanocomposites.

Langmuir, 2004. 20(5): p. 1564-1571.

9. Mellon, V., Synthesis and characterisation o f Waterborne Polymer/Laponite

Nanocomposite Latexes through Miniemulsion Polymerisation, in PhD

Thesis. 2009, CNRS-LCPP (Laboratory of Chemistry and Processes of

Polymerization), France.

112

Chapter 4

10. Lee, D.C. and Jang, L.W., Preparation and characterization o f PMMA-clay

hybrid composite by emulsion polymerization. Journal of Applied Polymer

Science, 1996. 61(7): p. 1117-1122.

11. Olphen, v., An Introduction to Clay Colloid Ahemistry. 1997: Wiley, New

York.

12. Al-Mukhtar, M., Touray, J.C., and Bergaya, F., Synthetic clay fo r the study o f

swelling behaviour o f clayey soils: Na-laponite. Comptes Rendus De L

Academie Des Sciences Serie Ii Fascicule a-Sciences De La Terre Et Des

Planetes, 1999. 329(4): p. 239-242.

13. Utracld, L.A., Sepehr, M., and Boccaleri, E., Synthetic, layered nanoparticles

fo r polymeric nanocomposites WNCO. Polymers for Advanced Technologies,

2007.18(1): p. 1-37.

14. Grim, R.E., Clay mineralogy. 1968, New York: McGraw-Hill.

15. Mackenzie, R.C., The Differential thermal investigation o f clays. 1966:

London : Mineralogical Society (Clay Minerals Group),.

16. Herrera, N.N., Letoffe, J.M., Reymond, J.P., and Bourgeat-Lami, E.,

Silylation o f laponite clay particles with monofunctional and trifunctional

vinyl alkoxysilanes. Journal of Materials Chemistry, 2005.15(8): p. 863-871.

17. Yang, J.T., Fan, H., Bu, Z.Y., and Li, B.G., The preparation ofMMT-

supported initiator and its application in the emulsion polymerization o f

styrene. Acta Polymerica Sinica, 2006(6): p. 779-784.

18. Ogawa, M., Okutomo, S., and Kuroda, K., Control o f interlayer

microstructures o f a layered silicate by surface modification with

organochlorosilanes. Journal of the American Chemical Society, 1998.

120(29): p. 7361-7362.

19. Ogata, N., Kawakage, S., and Ogihara, T., Poly(vinyl alcohol)-clay and

poly(ethylene oxide)-clay blends prepared using water as solvent. Journal of

Applied Polymer Science, 1997. 66(3): p. 573-581.

20. Shen, Y.H., Sorption o f non-ionic surfactants to soil: the role o f soil mineral

composition. Chemosphere, 2000. 41(5): p. 711-716.

113

Chapter 4

21. Cauvin, S., Colver, P.J., and Bon, S.A.F., Pickering stabilized miniemulsion

polymerization: Preparation o f clay armored latexes. Macromolecules, 2005.

38(19): p. 7887-7889.

22. Sun, Q.H., Deng, Y.L., and Wang, Z.L., Synthesis and characterization o f

polystyrene-encapsulated laponite composites via miniemulsion

polymerization. Macromolecular Materials and Engineering, 2004. 289(3): p.

288-295.

23. Tong, Z.H. and Deng, Y.L., Synthesis o f water-based polystyrene-nanoclay

composite suspension via miniemulsion polymerization. Industrial &

Engineering Chemistry Research, 2006. 45(8): p. 2641-2645.

24. Goodwin, J., Colloids and interfaces with surfactants and polymers : an

introduction. 2004,: New York J. Wiley.

25. Caria, A., Regev, O., and Khan, A., Surfactant-polymer interactions: Phase

diagram and fusion o f vesicle in the didodecyldimethylammonium bromide-

poly (ethyleneoxide)-water system. Journal of Colloid and Interface Science,

1998.200(1): p. 19-30.

26. Attwood, D., Florence, A.T., Surfactant Systems: Their chemictry, pharmacy

and biology. 1983: Chapman and Hall, London, New York.

27. Segota, S., Heimer, S., and Tezak, D., New catanionic mixtures o f

dodecyldimethylammonium bromide/sodium

dodecylbenzenesulphonate/water I. Surface properties o f dispersed particles

(vol 274, pg 91, 2006). Colloids and Surfaces a-Physicochemical and

Engineering Aspects, 2006. 280(1-3): p. 245-245.

28. Yang, J.P., Xie, J.Y., Chen, G.M., and Chen, X.C., Surface, Interfacial and

Aggregation Properties o f Sulfonic Acid-Containing Gemini Surfactants with

Different Spacer Lengths. Langmuir, 2009. 25(11): p. 6100-6105.

29. Ogitani, S., Bidstrup-Allen, S.A., and Kohl, P.A., Factors influencing the

permittivity o f polymer/ceramic composites for embedded capacitors. Ieee

Transactions on Advanced Packaging, 2000. 23(2): p. 313-322.

30. Svitova, T.F., Smirnova, Y.P., Pisarev, S.A., and Berezina, N.A., Self-

Assembly in Double-Tailed Surfactants in Dilute Aqueous-Solutions. Colloids

114

Chapter 4

and Surfaces a-Physicochemical and Engineering Aspects, 1995. 98(1-2): p.

107-115.

31. Hu, H. and Larson, R.G., Analysis o f the effects o f Marangoni stresses on the

microflow in an evaporating sessile droplet. Langmuir, 2005. 21(9): p. 3972-

3980.

32. Girard, F., Antoni, M., and Sefiane, K., On the effect o f Marangoni flow on

evaporation rates o f heated water drops. Langmuir, 2008. 24(17): p. 9207-

9210.

33. Hibiya, T., Nagafuchi, K., Shiratori, S., Yamane, N., and Ozawa, S., Attempt

to study Marangoni flow o f low-Pr-numher fluids using a liquid bridge o f

silver. Advances in Space Research, 2008. 41(12): p. 2107-2111.

34. Rongy, L. and De Wit, A., Marangoni flow around chemical fronts traveling

in thin solution layers: influence o f the liquid depth. Journal of Engineering

Mathematics, 2007. 59(2): p. 221-227.

35. Fanton, X. and Cazabat, A.M., Spreading and instabilities induced by a

solutalMarangoni effect. Langmuir, 1998.14(9): p. 2554-2561.

36. Xu, X.F. and Luo, J.B., Marangoni flow in an evaporating water droplet.

Applied Physics Letters, 2007. 91(12): p. -.

37. Wang, H.T., Wang, Z.B., Huang, L.M., Mitra, A., and Yan, Y.S., Surface

patterned porous films by convection-assisted dynamic self-assembly o f

zeolite nanoparticles. Langmuir, 2001.17(9): p. 2572-2574.

38. Nguyen, V.X. and Stebe, K. J., Patterning o f small particles by a surfactant-

enhanced Marangoni-Benard instability. Physical Review Letters, 2002.

88(16): p. -.

39. Truskett, V. and Stebe, K.J., Influence o f surfactants on an evaporating drop:

Fluorescence images and particle deposition patterns. Langmuir, 2003.

19(20): p. 8271-8279.

40. Herrera, N.N., Persoz, S., Putaux, J.L., David, L., and Bourgeat-Lami, E.,

Synthesis o f polymer latex particles decorated with organically-modified

laponite clay platelets via emulsion polymerization. Journal of Nanoscience

and Nanotechnology, 2006. 6(2): p. 421-431.

41. Bourgeat-Lami, E., Evaluation Meeting 2008, NAPOLEON Project. 2008.

115

Chapter 4

42. Chidambaram, M., Sonavane, S.U., de la Zerda, J., and Sasson, Y.,

Didecyldimethylammonium bromide (DDAB): a universal, robust, and highly

potent phas e-transfer catalyst fo r diverse organic transformations.

Tetrahedron, 2007. 63(32): p. 7696-7701.

43. Bourgeat-Lami, E., Mellon, V., Pardal, F., Putaux, J., Me Kenna, T.,

Bonnefond, A., Micusilc, M., Paulis, M., Leiza, J. R., Schreiber, E.,

Landfester, K., Lohmeijer, B.„ Acrylic/Clay Nanocomposite Latexes:

Synthesis, Structure and Properties. Submitted.

44. McDonald, P J . and Newling, B., Stray field magnetic resonance imaging.

Reports on Progress in Physics, 1998. 61(11): p. 1441-1493.

45. Mallegol, J., Bennett, G., McDonald, P.J., Keddie, J.L., and Dupont, O., SJdn

development during the film formation o f waterborne acrylic pressure-

sensitive adhesives containing tacldfying resin. Journal of Adhesion, 2006.

82(3): p. 217-238.

46. http://www.veeco.com/stvlus-t>rofiler-svstems/index.aspx. [cited.

47. http:/Avww.kruss.de/en/home.html, [cited.

48. van Tent, A. and Nijenhuis, K.T., The film formation ofpolymer particles in

drying thin films o f aqueous acrylic latices - II. Coalescence, studied with

transmission spectrophotometry. Journal of Colloid and Interface Science,

2000. 232(2): p. 350-363.

49. Meeten, G.H., Optical Properties o f Polymer. 1986: Elsevier Science

Publishers Ltd, UK.

50. Routh, A.F. and Russel, W.B., Horizontal drying fronts during solvent

evaporation from latex films. Aiche Journal, 1998. 44(9): p. 2088-2098.

51. Vig, J.R. and Lebus, J.W., Uv-Ozone Cleaning o f Surfaces. Ieee Transactions

on Parts Hybrids and Packaging, 1976.12(4): p. 365-370.

116

Chapter 5

The Effects of Acrylic Acid and pH on the

Physical Characteristics of Pressure Sensitive

Adhesives

5.1. Introduction

Due to increasing public awareness and tighter environmental legislation in recent

times, the importance of waterborne polymer colloids (i.e. latexes) for adhesives and

coatings has never been so high, as they discharge minimal amounts of volatile

compounds (VOCs) into the atmosphere [1, 2 ]. Since most of the solvent-borne

coatings and adhesives are based on water-insoluble polymers, aqueous dispersions

are needed as a way forward for development of these products [2] . In preparing

polymers in a form suitable for formation into waterborne coatings and adhesives,

emulsion polymerisation is a particularly convenient method [3, 4],

Polymers with low glass transition temperatures (in the range of -40 to -50 °C) which

demonstrate tack are known as adhesives [5]. Pressure sensitive adhesives (PSAs)

are a type of adhesive that form a bond between the adhesive and the adherent

Chapter 5

instantly and firmly under the application of light pressure, without covalent bonding

or activation [6-9]. PSAs are tacky at room temperature. The bond forms as the

adhesive is soft enough to flow and to wet the adherent. The adhesive must be stiff

enough to resist the flow when stress is applied [10]. The adhesive materials are

usually applied to one of the substrates (i.e. the backing) to be bonded. The adhesive

is protected by applying a release liner to the open adhesive surface. Paper labels and

tapes are the most common backings. When the label or tape is to be used, the

release liner is removed and the backing bonded to a desired surface by applying

light pressure [4].

In applications such as labels or tapes, it is vital that the PSA film remains optically

transparent, regardless of the humidity or exposure to water. The loss of optical

transparency when adhesive films are exposed to high humidity or soaked in water,

is referred to as ‘water whitening’ [11]. Water whitening is a common problem

encountered in waterborne PSAs. It is attributed to the scattering of light by pockets

of water between polymer particles. If the size of these pockets of water is greater

than the order of 1/10th of the wavelength of light or in the region of 50 nm, the film

develops opacity (Figure 5.1 (b)). A film with no water whitening also might absorb

water but if this water is evenly distributed at the particle boundaries and not creating

water pockets greater than on order of 1 / 1 0 th of the wavelength of light, then it is

expected that the film will remain transparent (Figure 5.1 (a)). An adhesive film with

no water whitening should also be a good barrier [12] to liquid water transport. To

minimise the water whitening, it is expected that a film should achieve complete

particle coalescence during drying.

Figure 5.1. Schematic illustration of (a) optically transparent film and (b) loss of transparency due to larger pockets of water between particles.

118

Chapter 5

However, early-stage good coalescence could lead to another problem, ‘skin

formation’ [13, 14]. The particle coalescence near the air interface during early-

stages of drying is referred to as skin formation, which is illustrated in Figure 5.2.

The transport of water through a polymer skin layer is significantly slower than

transport around the particles [15]. This retarded water loss can cause water

entrapment within the drying film. Trapped water is also associated with inhibited

interdiffusion and hence weaker films.

Skin formation

water

Figure 5.2. Schematic illustration of ‘skin formation’ or particle coalescence near the air interface during drying of a latex film.

Prevention of early-stage good coalescence should avoid the skin formation and

water entrapment within the film during drying. On the other hand, good coalescence

at the later stages of drying should prevent water whitening and increase the barrier

properties. Therefore, there has been increased industrial as well as academic interest

to meet these conflicting requirements.

In recent years there has been enhanced interest and much work on water absorption

in coatings and PSAs being reported. Selections of reports are presented here under

three headings:

1 . Water uptake [16-19]

2. pH and PAA dependence [20, 21]

3. Properties [22-26]

Each of these will be considered separately here after.

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Chapter 5

5.1.1 Water uptake

Agarwal and Farris [16] reported the water absorption by films prepared from blends

of soft (low glass transition temperatures - low Tg) and hard (high Tg) acrylic-based

latex particles. They have shown that the blends with large proportions of the soft

particles absorb much larger amounts of water compared with those with hard

particles. These films turn white/opaque upon water absorption but regain their

transparency upon redrying. SEM images of freeze-fractured surfaces of wet films by

larger proportions of soft particles show micrometer size pockets of water. Figure 5.3

shows the freeze-fractured SEM images of blend films of (a) 70% and (b) 50% soft

particles. Blend films with a higher amount of soft particles (a) show much larger

holes compared to a film made from a relatively lower amount of softer particles.

The implications are that the PSAs used in current study also should have relatively

large, presumably micrometre size, pockets of water upon water absorption.

Figure 5.3. SEM images of freeze-fractured surfaces of blend films of (a) 70% and (b) 50% soft particles. Images taken from Reference [16].

In relation to Agarwal and Farris’s observations, one can conclude that acrylic-based

latex blend films absorb a significant amount of water from liquid water. These films

were easily removed from their substrates (glass plates) indicating that they lost

significant amount of their mechanical properties and turned white/opaque or lost

transparency upon water absorption. Furthermore, the absorbed water created water

120

Chapter 5

reservoirs or pockets like clusters between particles which can be attributed to

scattering of light. This could be because the polymers they used, (poly(methyl

methacrylate-co-ethyl acrylate - P(MMA-co-EA) and poly(methyl methacrylate-co-

buthyl acrylate - P(MMA-co-BA)), were significantly hydrophobic. If the particle

boundaries were hydrophilic, the absorbed water should have been evenly distributed

along the boundaries and reservoir type water clusters should have been less obvious.

In addition, clusters of surfactant between particles also could have been contributed

to these water reservoirs, upon hydration of surfactant salts. Furthermore, the blends

with a higher percentage of softer particles create larger holes upon water absorption

which confirms that the flexibility of the softer particles allow the water reservoirs to

grow bigger compared to the less flexible harder particles. The fact that these films

easily regain their original weight and transparency upon re-drying confirms that the

water absorption is reversible and no bonds were formed between the absorbed water

and the particles.

Van der Wei and Adan [17] studied the transport and equilibrium adsoiption of

moisture in polymer films and organic coatings. Three different types of absorption

curves were discussed. Firstly, the typical sorption curve, where mass varies linearly

as a function of the square root of time. With increasing time, the sorption curve

smoothly levels off to a saturation level of mass. Secondly, there can be a ‘two-stage’

sorption curve, where the first part is a typical sorption followed by a slow

absorption before ultimately reaching the saturation level. They argued, in this case

that there is more than one contribution to the absoiption process; a diffusion part

and one or more structural parts, which result from polymer relaxations. Therefore,

the total weight gain is the sum of these contributions. Thirdly, there can be the

‘Sigmoidal’ sorption curve, where the curve is ‘S’ shaped with a point of inflection.

Due to the slow establishment of equilibrium at the surface of the film, the kinetics

appears somewhat different than the standard. Figure 5.4 illustrate these three types

of soiption curves.

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Chapter 5

Figure 5.4. Three different types of absorption curves of relative mass uptake as a function of square root if time; (a) typical sorption curve, (b) the ‘Two-stage’ sorption curve and (c) the ‘Sigmoidal’ sorption curve, where the curve is ‘S’ shaped. Drawn after [17].

hi 2006 Chen and co-workers [18] investigated the wetting-resistance, the water-

resistance and the thermal stability of coating formulations. They found that after

perfluoroalkyl groups were introduced into polymer chains, these properties were

evidently enhanced. According to their calculations, the surface free energy of the

film with fluorine was much lower than that of the film without fluorine. When

perfluoroalkyl groups with low surface tension were introduced into polymer chains,

the surface energy of the latex film was greatly lessened and the latex film could not

be wetted by water easily. Furthermore, their water absorption results suggested that

the water-resistance of the film was successfully modified by introducing fluorine

into the polymer. Hydrophobic perfluoroalkyl groups prevented water molecules

entering into the film’s inner, and thus enhanced the water resistance of the films. In

addition, they found the water absorption of the latex films increased with increasing

PAA amount, due to the hydrophilicity of PAA. However, they foimd that the water

absorption was decreased first and then increased for the latex films which contained

very hydrophilic sulfosalt groups. Overall, more sulfosalt, led to higher water

absorption of the latex films. An increasing sulfosalt amount would decrease the

latex particle sizes, which results in a dense latex film. Therefore, the water

molecules find it difficult to penetrate through the film surface. Thus, the water

absorption of the latex film decreased. Under these two contrary effects, the authors

found that the lowest water absoiption occurred at 0.95% of sulfosalt, from their

range of 0 .8% to 1 .2 %.

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Chapter 5

Implications are that the PSAs with PAA shells in current study also show higher

water absorption, in comparison to the PSAs without PAA, due to the hydrophilicity

of PAA.

Feng and Winnik [19] reported the role of water in polymer diffusion in latex films.

For a hydrophobic polymer, water has little influence on the polymer diffusion rate.

Water absorption in those films does increase the film turbidity, both for recently

formed and for well-annealed films. For hydrophilic polymer films, the presence of

water increases the diffusion coefficient by a factor of five at 60 °C. They found that,

upon neutralisation of the carboxylic acid groups with NaOH, the polymer diffusion

is much retarded in dry films but greatly enhanced in wet films. Furthermore, the

polymer diffusion coefficients for the wet films are about two orders of magnitude

larger than those for the corresponding dry films. In addition, neutralisation with

NH3 results in intermediate diffusion rates, between those of im-neutralized and

NaOH-neutralised films in both dry and wet conditions. It was reported when dense,

crack-free films were formed from latex dispersions, they are often transparent, or

sometimes semitransparent. This transparency often depends on the film micro­

heterogeneity, film thickness or water content. It was reported that due to the micro-

heterogeneous nature of the newly formed films, the transparency of latex films in

many cases is lower than that of films cast from homogeneous solutions. In other

cases relatively highly transparent films were obtained, especially when the

temperature of film formation is well above the MFFT. A possible explanation could

be that, in the films formed at high temperatures, some interparticle polymer

diffusion may have occurred during the film formation process.

Furthermore, Feng and Winnik [19] found that their films which were dried close to

room conditions were fairly transparent, and transparency was higher if the films

were relatively thinner. In addition, annealing at elevated temperatures in dry

environment led to an increase in transparency. When their films were exposed to

water vapour or immersed in liquid water for some time (e.g. two days), they became

turbid. They found that the water content for their film exposed to 100% RH, was 3.5

wt.% and that for a film immersed in liquid water was as high as 15 wt.%. They

123

Chapter 5

confirmed that water becomes more concentrated in the interparticle boundary

regions richer in polar groups, in their case, -OSO3H groups. This in-homogeneity of

water distribution in the films would lead to turbidity for the films exposed either to

high humidity or to liquid water.

Feng and Winnik [19] have seen when these water-containing, turbid films were

heated in low humidity environment, that water was evaporated and the film

transparency increased and the films became clearer. When these heated, clear films

were once again placed into high RH or water, they once again became highly turbid.

These visible transmittance results for newly formed film, a film exposed to 100 %

RH, then heated for two hours at 60 °C at low humidity, and then placed into high

RH again are presented in Figure 5.5.

_ 120 PgUig 80<H

<cr

-..... 1 1 1

^ ( 3 )

—T 1

- N ( i )

^ (4 )

—

!---------- 1---------- 1 1V 2 ) -

— j_______ 1________400 500 600

WAVELENGTH (nm)700

Figure 5.5. Visible transmission spectra for films: (1) a newly formed film, (2) a recently formed film exposed to 100 % RH for two days, (3) a film heated in low humidity at 60 °C for two hours, (4) the well-heated film exposed once again to 1 00 % RH for two days. Figure taken from [19].

One would expect that as polymer molecules were well mixed by interdiffusion, the

heterogeneity in the initial latex films would gradually disappear and the

hydrophobic polymer matrix would become more resistance to water. However,

Feng and Winnik have seen that while the molecular mixing increases the overall

homogeneity of the system, some degree of nonuniform distribution of polar groups

124

Chapter 5

exists in their polymer. When these films were exposed to water, domains rich in

polar groups were found to take up more water and these regions were large enough

to scatter light. Therefore, Feng and Winnik confirmed that the water-containing

films remained turbid, no matter how much polymer diffusion has taken place.

There are couple of observations in Feng and Winnik work which could apply to the

current study. First of all, Feng and Winnik showed that the films made from the

latexes with heterogeneous particles had lower transparency than that of films made

from homogeneous solutions. As the PSAs used in this study are homogeneous

(average particle diameter between 162 mn - 164 nm, section 5.3.), the transparency

between the samples should not be affected by the particle size distribution. Then,

they found that the films which were exposed to high humidity absorbed less water

compared with the films which were immersed in liquid water. This study could also

show higher water absorption from liquid water than vapour.

5.1.2. pH and PAA dependence

In most PSA applications, several monomers can be incorporated into the polymer,

in order to impart desired properties [7]. PAA or Methacrylic Acid (MAA) is often

used in both coating and adhesive formulations to impart colloidal stability, freeze-

thaw stability, and improved film forming properties [24], The carboxylic acid

comonomer forms a major component of water-soluble chains on the surface of the

latex particle. It will provide both steric and electrostatic stabilisation of the colloid.

The surface hydrophilic chains are referred to as a ‘hairy layer5 [26]. This hairy layer

of poly PAA chains is sensitive to the pH. With increasing pH the charge repulsion

by the PAA chains increases. As a result the PAA chains are widely spread (Figure

5.6 a) and the hydrophobicity of the surface decreases [26]. As pH decreases, the

hairy layer will collapse (Figure 5.6 b) and the hydrophobicity of the surface

increases.

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Chapter 5

A B

Figure 5.6. Schematic representation of polymer particle with ‘hairy layer’, (a) At high pH the charge repulsion by PAA chain increases and the chains are widely spread; (b) as pH decreases, the hairy layer collapses. Drawn after [26].

De Bruyn and co-workers [20] reported work on electrosterically stabilized latex

having a polystyrene core and a poly PAA hydrophilic layer as the shell. They

observed that the hairy layer thickness of PAA on particles increases when the pH is

increased. They performed small angle neutron scattering measurements over a range

of contrasts for three latexes with different high and low pH values. They reported

that the parameters obtained by fitting to standard core/shell models were consistent

with the shell being highly hydrated. Furthermore, they found that the shell was

about 89% hydrated at low pH and about 94% hydrated at high pH. The core was

found to contain about 3% acrylic acid. By doubling the proportion of acrylic acid in

the reaction, it was shown that shell thickness was increased by about 2 0 %.

In the current study, the samples with PAA shells at a higher pH should be expected

to dry slower than the samples at a lower pH, as PAA shells are more hydrated, and

release of water from the drying film should be slower.

Rharbi and co-workers [21] studied adhesive films made by core-shell particles with

hydrophobic cores made of a poly(styrene) and poly(butyl acrylate) copolymer and

thin hydrophilic shells made of poly(acrylic acid) (PAA) and poly(butyl acrylate)

126

Chapter 5

pBA) copolymer. In addition to these dispersed polymer particles, the aqueous phase

also contained soluble amphiphilic polymers, surfactant and salts. These soluble

species were eliminated through dialysis and equilibrium with ion exchange resins.

The ionisation of the PAA groups at the shell was controlled through the addition of

a base, NaOH. They found that the films made with un-dialysed latexes showed

greater water uptake, as the soluble species that were initially in the aqueous phase

has become trapped in the membranes. The films made with dialysed latexes also

showed greater water uptake, if they were neutralised by NaOH in the aqueous

phase. In these films, PAA shells are polyelectrolytes, and therefore the membranes

remained hydrophilic. They observed the water uptake by the films made with

dialysed latexes, but when kept in the acid form, the uptake was very low. They

suggested their results were due to the fact that the surface polymers remained in the

acidic form, and the membranes became hydrophobic in the dry state. Figure 5.7

summarises their results. The films adjusted to pH 10 by NaOH at aqueous stage

gained the most water compared with the acidic form at pH 2 which gained the least

amount of water. Latex neutralised by NaOH to be pH 7 was intermediate [21].

127

Chapter 5

% Weight gain

1) 500 1000 1500 2000Time (b)

Figure 5.7. Comparison of the water uptake of films by latex dispersions of core/shell particles (poly styrene and poly butyl acrylate hydrophobic core surrounded by a thin layer of hydrophilic poly acrylic acid and poly butyl acrylate shell): % weight gain as a fimction of time. Films from latex kept in the acidic form at pH 2 (filled circles), films from latex partly neutralised at pH7 by NaOH (unfilled diamonds), films from latex fully neutralised at pH 10 by NaOH (filled triangles), films from latex neutralised by Ba(OH)2 at pH 7 (+). Figure taken from [21].

Implications from Rharbi and co-workers [21] are that in current study, samples with

PAA shells at a higher pH should absorb more water compared with the samples at a

lower pH, as PAA shells in acidic form should become hydrophobic in the dry state.

5.1.3. Properties

Wang and co-workers [22] investigated the effects of pH on film drying, mechanical

and adhesive properties of waterborne poly(butyl acrylate-co-acrylic acid) films. In

films cast from acidic colloidal dispersions, hydrogen bonding between carboxylic

acid groups dominates the particle-particle interactions, whereas ionic dipolar

interactions are dominant in films cast from basic dispersions. They have shown by

force spectroscopy using atomic force spectroscopy and macroscale mechanical

measurements that those latex films with hydrogen-bonding interactions have lower

elastic moduli and are more deformable. In addition, these films yielded higher

adhesion energies. In comparison, in basic latexes, ionic dipolar interactions

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Chapter 5

increased the moduli of the dried films. These films were stiffer and less deformable

and, consequently, exhibited lower adhesion energies. Furthermore, they have shown

that the rate of water loss from acidic latex was slower, because of hydrogen bonding

with the water. They concluded that although acidic latexes offer greater adhesion,

there is a limitation in the film formation.

The implications are that the PSAs in the current study are also expected to show a

relation between adhesion and drying properties.

Yang, Li and Wang [23] reported difference in adhesion properties and the water

resistance of four different acrylic PSAs with the same composition of their

constituent co-polymers but stabilised by four different anionic surfactants. They

used two conventional low-molecular-weight surfactants (a sodium salt and an

ammonium salt) and two anionic monomers (a sodium salt and an ammonium salt).

Water absorption of PSA films was determined by a gravimetric method. The peel-

strength retention of PSA tapes after immersion in water was compared. They

reported that both the adhesion properties and the water resistance of the acrylic

PSAs stabilised by anionic monomers were better than the acrylic PSAs stabilised by

low-molecular-weight surfactants. Furthermore, these properties of PSAs with

ammonium surfactants were better than the sodium surfactants. These differences

were mainly caused by the different migration ability and their different hydrophilic

nature of the four surfactants in the PSA layers. They found that low-molecular

weight surfactant can easily migrate and concentrate but the migration ability and the

degree of enrichment of the surface by an ammonium surfactant is less than that of a

sodium surfactant.

In 1999 Tzitzinou and co-workers theoretically modelled the dependency of the

optical transmission on the radius of the voids of a film for different volume fractions

of voids [25]. They used Rayleigh scattering theory [27] to relate optical

transmission, T, to volume fraction of voids/, using the equation :

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Chapter 5

In T =-32 ;r 4rv3f vd m -1

2̂

A4V ^ nt2 +1 y J(5.1)

Where rv is the radius of the voids, assuming that the voids are spherical in shape, d

is the optical path length which is the film thickness, X is the wavelength of light, and

m is the relative refractive index; which is equal to n fnp> with nv being the reflective

index of air voids (nv = 1 ) and np the reflective index of the fully dense polymer (the

value of np would depend on the polymer). The number of voids per unit volume, N,

is related to f v by

N -(5-2)

Using the equations 5.1 and 5.2, one can, in principle, determine N from a

measurement of T. They noted that equation 5.1 applies only for certain ranges of

void size and refractive index. Hence, they tested its applicability by measuring T

over a wide range of A, and reported that the dependence of ln(T) is weaker than

(-AT4), predicted by equation 5.1. Nevertheless, they proved that optical transmission

can be qualitatively related to void size and concentration in their films.

130

Chapter 5

Radius of Void (nm)

Figure 5.8. Predicted optical transmission of 100 pm thick films of a continuous medium with n - 1.5 and containing spherical air voids («v =1.0). The volume fraction of voids (fv) is taken to be 0.25 (—); 0.025 (—) and 0.0025 (•••). Figure taken from [25].

Figure 5.8 shows the dependency of the optical transmission on radius of voids

predicted by the equation 5.1, assuming that the volume fraction of voids remains

constant with increasing radius of the voids. It was predicted that transmission

decreases with increasing void radius. The loss of transmission was highest with the

largest volume fraction of voids (0.25) and was lowest with the smallest. For all three

volume fraction of voids, as the radius of void decreased the transmission was

increased. Furthermore, it is shown that, for a void radius of 30 mn, the film can be

nearly transparent, nearly opaque or partially transparent, depending on the void

volume fraction (and hence the number of voids per unit volume) [25].

131

Chapter 5

5.2. Aims of present research work

In the literature review, it was shown that much research has been done on water

uptake [16-19], pH and PAA dependence [20, 21] and properties [22-26] of latex

films. It was shown that the pH value of the wet latex influences these properties in

the dry films. Little attention was given to the drying properties [22]. However, no

extensive studies of how the pH and PAA shell of wet core/shell PSAs in a single

system affect the drying (i.e. water coming out from a wet latex film), water

whitening and water sorption (i.e. liquid water or vapour water going into a dry latex

film) and adhesion properties have been reported. A systematic study of these

relations will lead to a better understanding of the correlation between these

properties. One could expect that latex systems which achieve good coalescence

during drying would show good water resistance properties as well. The reason

being, that good coalesced particles would be expected to show better water

resistance properties, hence less liquid and vapour water absoiption from the

environment. In addition, adhesion properties also have been studied. It is expected

that the films with well coalesced particles will have greater adhesion energy. The

water uptake could interfere with coalescence of particles and decrease cohesive

strength of the films. Furthermore, attention was given to control the conditions of

the wet latex in order to achieve the desired qualities of the dry PSAs.

This study investigates the effects of PAA and pH on drying, water whitening, water

sorption and adhesion properties of PSA films. Core-shell latex particles, with a

poly(isodecyl acrylate) (PIDA) core and a PAA shell, were used as a model PSA.

Particles with only PIDA core were expected to resist water whitening and water

sorption as PIDA is hydrophobic due to its long C10H21 chain. In comparison, the

particles with hydrophilic PAA shells were expected to absorb more water. The pH

of the latex dispersions was adjusted from acidic to basic by NaOH and NH4OH. In

the basic latex dispersions adjusted by NaOH Na+ cations would remain in the

dispersion, hi comparison, the latex dispersions adjusted by NH4OH would not have

any (or less) cation effect as volatile NH3 would evaporate to the atmosphere.

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Chapter 5

5.3. Materials

The poly (isodecyl acrylate) (PIDA) latexes, with and without acrylic acid (PAA),

were prepared and dialysed at the University of Manchester, UK by Andrew Foster

and Michael Rabjohns. As a means of adjusting the hydrophilicity, 3 wt.% acrylic

acid (PAA) was co-polymerised in the shell of the PIDA particles. Two commercial

surfactants (Rhodapex AB/20 and Dowfax 2A1) in an active ratio of 1:1, 2 wt.% on

monomer has been used. The solid content of the latexes, before dialysis is in the

range from 54 -56 wt.% and the particle diameters were between 156 nm - 159 nm.

The latexes were dialysed by pouring lOOg of latex into a length of Visking tubing,

which was then sealed and immersed in a large excess of de-ionized water and left

for seven to ten days. During this time, the de-ionized water was changed to fresh

water at least once a day. The dialysed latexes were then re-concentrated by rotary

evaporation at around 35 °C until the solids contents were reached around 45 %.

After dialysis, the solids content was 42.6 - 43.1 wt.% and the average particle

diameter was between 162 nm -164 nm. A schematic diagram of a latex particle with

PIDA core and an PAA shell is shown in Figure 5.9.

PIDA core

AA shelloFigure 5.9. A schematic diagram of a latex particle with PIDA core and a PAA shell.

PIDA and PIDA with PAA core/shell (97 wt.% / 3 wt.%) latexes were at a pH of 3

and solids content was around 50 wt.%. The pH was adjusted to 8 by 1M NaOH or

1M NH4OH solutions in water. For this study, six samples were used: PIDA at a pH

of 3, PIDA at a pH of 8 adjusted with NaOH, PIDA at a pH of 8 adjusted with

NH4OH, PIDA/PAA core/shell (97 wt.% / 3 wt.%) at a pH of 3, PIDA/PAA at a pH

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Chapter 5

of 8 adjusted with NaOH, PIDA/PAA at a pH of 8 adjusted with NH4OH. Table 5.1

surnmarises the compositions and characteristics of the latexes.

Table 5.1. Summary of the compositions and characteristics of the latex samples.

SamplepHof

wet latex

Solidscontent(wt.%)

Average Particle diameter

(nm)

Overall amount of

PAA (wt.%)

OPAA 3 42.6 162 0

0 PAA, pH 8 by NaOH

8 42.6 162 0

0 PAA, pH 8 by NH4OH

8 42.6 162 0

3 PAA 3 43.1 164 3

3 PAA, pH 8 by NaOH

8 43.1 164 3

3 PAA, pH 8 by NH4OH

8 43.1 164 3

134

Chapter 5

5.4. Methods

5.4.1. Magnetic Resonance Profiling (MRP)

MRP experiments were carried out using the GARField magnet. For these

experiments 2 0 mm x 2 0 mm and 180 pm thick glass cover-slips were used as the

substrate. The latex was applied on to the coverslips to get the desired thickness. As a

rough guidance around 70 pi of 50 wt.% solid content latex would produce a 300-

400 pm thick wet film. As soon as the latex was cast on to the glass coverslip, it was

placed inside the magnet and NMR profiles were taken at the room temperature and

humidity. The magnetic field, Bo, is 0.7 T when the measurements were performed.

The gradient strength, Gy, is approximately 17.5 T/m. The NMR signal is obtained

from an excitation using a quadrature echo sequence: 90x-T-(90y-T-echo-T-)„ [28,

29]. For typical latex experiments, the number of echoes, n, is 32; the pulse gap, t,

75 ps; the dwell time, DW, is 0.7 ps; the number of points per echo, SI, is 128 and

the spectrometer frequency, SF, is 29.6 MHz. For each and every experiment the

time delay between profiles and the total drying time can be changed as desired. The

same MR parameters were used, so that direct comparisons of profiles for different

samples were allowed. The profile shapes were normalised by an elastometer

standard in order to correct for the decline in sensitivity over the film thickness.

The GARField data were further analysed as follows [30]. The solids content, at

a given drying time, t was calculated by the following equation.

where mo(t) is the zeroth moment at drying time t, mo(0) is the zeroth moment at

t = 0 , <J>0 is the solids content at t = 0 , H(t) is the thickness at time t, and Ho is the

initial thickness. The initial thickness, Ho, was determined from the MRP profiles. In

(5.3)

135

Chapter 5

doing so, it was assumed that the upper edge of the water profile coincides with the

upper edge of the film.

5.4.2. Measurements of Water Whitening by Optical

Transmission

The latex dispersions were cast onto PET substrates (30 cm x 20 cm) using a hand­

held bar applicator with a defined wet thickness of 40 pm. The films were dried in

ambient conditions for 24 hours and under laminar air flow on heated plates at 110

°C for three minutes. A strip of the film was cut from the centre of the film and was

submerged in de-ionized water in a square cuvette. The film face was positioned

parallel with the cuvette walls. The optical transmission of the films, while in the

water, was obtained at various time intervals. Optical transmissions of the films were

performed with a UV/visible spectrophotometer (Campsec M350, Cambridge, UK).

During a measurement, the wavelength of light was varied from 300 to 900 nm. The

percent transmission was recorded to indicate the transparency of the film.

5.4.3. Probe-Tack Adhesion Measurements

The probe tack test usually characterises instant adhesion properties of PSAs under

light pressure [9, 31-34]. The time of contact between the adhesive and the probe can

be specifically controlled and is usually very short, around one second. The

parameters that influence adhesion are the rheological properties of the adhesive

layer and the nature of its interactions with the substrate or the probe. The shape of

the probe is also important. As spherical probe is more favourable than a flat-ended

probe, as it gives more reproducible results [6 , 35]. The reason is that the spherical

probe avoids the difficulty of a proper parallel alignment between the probe and the

film. On the other hand, a flat probe gives a much more uniform stress field and

strain rate under the probe surface.

136

Chapter 5

In 1985 Zosel reported [36] the development of probe-tack experiments. The

pulling force needed to detach a flat, solid punch from an adhesive film was recorded

during the entire separation process which was performed at a constant velocity. The

traction curves obtained showed that the force increases sharply and reaches a peak

value, then it drops suddenly and stabilizes at a plateau value and eventually

vanishes. The adhesion energy is the work done during the entire separation process

[32].

Figure 5.10 (a) Photograph of the probe-tack analyser, (b) The appearance offibrils between the PSA and the probe during the debonding of the probe from the PSA. Image (b) taken from [34].

Probe-tack adhesion measurements were followed on a commercial instrument

(Figure 5.10 (a)) (Microsystems Texture Analyser, Godalming, UK) using a high

energy spherical, stainless steel surface probe with 2.5 cm diameter. The samples

were prepared on glass as substrate using a 2 0 0 pm cube applicator; hence the wet

film thickness was 200 pm. Films were dried in the ambient conditions. The probe

was pressed in contact with the PSA film with a 4.9 N force for 1 second, before

being removed at a constant velocity of 100 pm/sec. Force and displacement

measurements were used to generate force-distance curves [37, 38].

137

Chapter 5

The debonding process can be separated into several stages which illustrated in

Figure 5.11. During the probe-tack test, when the debonding process of PSA film

begins an initial stress will appear in the bulk of the film (Figure 5.11 (a)) [32],

When the initial stress reaches a critical stress, cavities will form in the bulk of the

film or at the interface between the PSA film and the probe (Figure 5.11 (b)) [31,

39]. As internal stress continues to increase, more cavities will appear and existing

cavities will expand simultaneously (Figure 5.11(c)). Theses cavities further grow,

and the inter-cavity distance reaches the same order as the initial thickness of the

film (Figure 5.11 (d)). The thin walls between the cavities extend mainly in the

direction of traction [32] and extend into filaments known as ‘fibrils’ (Figure 5.10

(b)) [36], before detaching (Figure 5.11 (e)). This process, known as fibrillation,

enormously contributes to the energy of adhesion [33, 39]. For high performance

PSAs, it is vital to have high cavitation and fibrillation [7].

Figure 5.11 Schematic illustration of cavitation and fibrillation development in PSA debonding process, (a) Initial stress is imposed on the bulk of the film, (b) at critical stress cavities will form in the bulk or at the interface of the probe and the film, (c) formation of new cavities and expansion of existing cavities, (d) the inter­cavity distance reaches the initial film thickness, (e) fibrils starting to appear before detaching. Image taken from [7].

138

Chapter 5

A typical plot of force and displacement as a function of time of a probe-tack curve

is shown in Figure 5.12 [31]. At time zero, ‘start’ is when the probe is starting to

move towards the PSA film. At point 1, the probe is touching the PSA film. From

point 1 until 2 is the force applied by the probe; in this case it is 80 N. From point 2

to 3, the probe is held in contact with the PSA film; in this case it is for about five

seconds.

Figure 5.12. Typical force ( ------------) and displacement ( ------------ ) curves as afunction of time. Image was taken from [31] and modified. Details of the process is given in the main text.

At point 3, the probe is starting to pull back and the displacement starting to

progress. At point 4 the load becomes positive, which means that the compressed

layer has returned to its original value. The displacement corresponds to zero. Point

4 till 5, the film is starting to stretch and cavities will start to form (Figure 5.11 (b)

indicate this stage). Point 5 is where the most cavities are initiated (Figure 5.11 (d)

indicates this stage). Point 5 until 6 is when the cavities are continuing to expand to

create thin walls. As the internal stress continues to increase the expanded cavities

139

Chapter 5

start to open into fibrils (Figure 5.11 (e) corresponds to this stage). Around point 6 ,

fibrils are well formed. From point 6 until 7, stretching of fibrils or fibrillation

occurs. Point 7 to 8 indicates the detachment of fibrils from the probe.

The experimental data of force-distance probe - tack curve was converted to a

nominal stress (a) - nominal strain (s) curve using equations 5.4 and 5.5

F(5.4)

h - F

e = ~ T ~ (5'5)o

where F is force (N), Ao is probe contact area, around 2 mm2, h is distance travelled

by the probe above the PSA surface and ho is the initial film thickness. A stress -

strain curve of PIDA without PAA at a pH of 3 after five days of drying time is

shown in Figure 5.13 [9].

140

Chapter 5

0 1 2 3 4 5 6

Strain

Figure 5.13. The stress as a function of strain for PIDA without PAA at a pH of 3. In this probe-tack curve (Figure 5.13), the point when most cavities are initiated is

indicated by the maximum stress, a max. The plateau stress, a p, is related to the stress required to draw the fibrils. The maximum strain, smax or failure strain Sf is the end of the deformation.

The energy dissipated during the debonding process, Ea, is proportional to the area

under the probe-tack stress-strain curve and can be calculated by using equation 5 .6 .

efE a = K J a{s)ds (5 6 )

0

If the initially formed cavities coalesce and form a crack, the interfacial debonding of

the PSA will be rapid and the practical work of adhesion and the energy dissipation

will be low. On the other hand, if the coalescence of neighbouring cavities does not

occur, the walls between cavities will be extended as fibrils during the debonding

process. As a result, a relatively large work of adhesion can be achieved [7]. Figure

5.14 summarises different stress-strain curves corresponding to different works of

adhesion.

141

Chapter 5

Figure 5.14. Stress-strain curves corresponding to different adhesion energies of PSAs. Curve I is for low, Curve II is for intermediate and Curve III is for high work of adhesion. Image taken from [7].

In Figure 5.14, Curve I is for a low work of adhesion. In this case, during the

debonding, an interface crack propagates and these PSAs display very low energy

dissipation. Curve II has an intermediate work of adhesion. During the debonding of

the PSA, cavities propagate but these cavities do not transform into fibrils or else

they detach prematurely. The overall work of adhesion is relatively higher than curve

I, but still the adhesive does not perform as a high energy PSA. Curve III represents

high work of adhesion. During debonding cavities form and develop into fibrils.

These fibrils extend to very large distances before PSA detachment from the probe

and high adhesion energy can be obtained. Curve III behaviour is associated with a

more dissipative material. The type of the stress - strain curve depends on the

viscoelastic properties of a given PSA [7].

5.4.4. Moisture Sorption Measurements

Gravimetric analysis of moisture sorption measurements were performed using the

IGAsorp Moisture Sorption analyser (by Hiden Isochema Limited, Warrington, UK)

142

Chapter 5

[40]. This instrument continuously monitors the mass of a sample and is able to

control both the relative humidity and the temperature of the chamber. It is fully

automated and computer controlled. The sample container for the IGAsorp Moisture

Sorption analyser is a gas permeable micromesh stainless steel pan, in the shape of a

cone. The pan hangs from the balance with a gold chain. The balance capacity is 5 g,

but the maximum sample capacity is 4.5 g when using the sample container and

hang-down chain. The typical sample size is 10-200 mg and the resolution is 0.1 pg

for 100 mg. The instrument has a maximum flow rate of 500 ml/min and minimum

flow rate of 100 ml/min humidity. The method used for humidity is laminar flow

wet/dry gas mixing at a constant total mass flow rate with feed back control. Inlet

pressure is 45-90 psi gauge or 3-6 bar gauge. The instrument has a humidity

measurement accuracy of +/- 1% (0-90% RH) and +/- 2% (90-95% RH).

Temperature is measured by a platinum resistance thermometer and the measurement

accuracy is +/- 0 .1 °C.

For moisture soiption experiments, the latex dispersions were cast onto the silicon-

coated paper using a pipette to get the desired thickness. As a rough guide, 70 pi

latex on a 20 mm x 20 mm area would produce a film with 300-400 pm thickness.

The films were dried in ambient conditions for 24 hrs and then dried in an oven at

110 C for three minutes. The samples were stored in a sealed desiccator with silica

gel in it. For these experiments, the pans were pre-cleaned with acetone and dried.

The mass of the empty pan was measured. Then the latex film was peeled off from

the substrate and placed in the pan and was loaded into the chamber of the

instillment. Initially the sample was dried in zero humidity air at 25 °C for four hours.

The mass of the sample at this stage was taken as the reference mass for the uptake

of water. Then the humidity of the chamber was increased to 70 % and the

temperature was set to be 25 °C and the mass change was recorded until it was

deemed that equilibrium had been reached, which is when there is no mass change. If

the equilibrium camiot be predicted successfully, the instrument will continue for a

given maximum time at the given conditions. For these experiments, this given

maximum time was four hours. The instrument provides the results in a form of a

143

Chapter 5

variation of mass of the sample with time (kinetic information). Also the equilibrium

data can be presented in the form of an isotherm [40].

The % mass change at a given time, t, M was calculated by the following equation.

(M, -M „)M = j — ----- ^-4x100% (5.7)K - mJ

where Mt is the total mass at a given time t (mg), Mp is the mass of the pan (mg), M0

is the initial mass (mg).

Figure 5.15 shows the water sorption kinetics for PIDA with 0 wt% PAA at a pH of

3, plotted as % mass change of the sample as a function of time (in minutes).

Time (minutes)

Figure 5.15. Water sorption kinetics for PIDA with 0 wt% PAA at a pH of 8

adjusted by NaOH (blue), undergoing a humidity step change from 0% to 70% RH at 25 C, (% mass change of the sample as a function of time in minutes). The red line show the data fitted using the Equation 5.8.

144

Chapter 5

Using the following equation to fit to the data:

l - e x pf - t \ \

(5.8)

one can calculate x, the characteristic time, which is the time it takes to reach around

63 % ofM m, the equilibrium or plateau value. M w gives an indication how

hydrophilic the dry film is. If M m is lower, a lesser amount of water was absorbed by

the dry film, hence the film is less hydrophilic. If is higher, the dry film was

absorbed larger amount of water, hence it should be more hydrophilic, in

comparison. In addition, the characteristic time gives an indication of how slow or

fast the water uptake by the dry film is. A higher x means it takes a longer time to

reach around 63% ofM m, therefore, a slower uptake of water by the dry film, hi

comparison, a lower x indicates that it takes a shorter time to reach 63% ofM w;

therefore, the water uptake by the dry film is faster.

145

Chapter 5

5.5. Results and Discussion

First of all, the drying results (water going out from a wet film) are presented. Then,

water whitening, which is water going into a dry film from liquid water followed by

water absorption results, which is water going into a dry film from vapour, are

presented.

5.5.1. Drying measurements

Figure 5.16 (a), shows the series of MR profiles acquired at five minute intervals for

pure PIDA, latex sample. Attention is drawn to the NMR signal at the beginning of

the drying, where the intensity of the signal is contributed by the signal from the

mobile polymer and the water. When the drying is completed and the film reached its

final thickness, the intensity of the NMR signal is from the molecular mobility in the

polymer melt.

D r y in g t i m e (m in )

Figure 5.16. (a) MR profiles of PIDA , acquired every five minute intervals. When the film reached its final thickness, the intensity of the NMR signal is from the mobile polymer, (b) the zeroth moment (the area under each profile), which is proportional to the water content of the sample at that time, as a function of drying time, (c) the film thickness as a function of time. Both the zeroth moment and the film thickness have a final value due to the signal from the mobile polymer.

NMR signal from mobile polymer + water

200 300 400Film Thickness (nm)

NMR signal from mobile polymer

1.01„ „ C 0.8 «

061 0.40.2 |0.0 z

146

Chapter 5

Figure 5.16 (b), shows the zeroth moment, which is proportional to the water content

of the film at the time when the profile was taken, as a function of the drying time.

Figure 5.16 (c), shows the film thickness as a function of the drying time. Even

though the drying has completed, the zeroth moment and the film thickness have a

positive value due to the signal from the mobile polymer. When the intensity remains

unchanged with the increasing drying time, it is taken that the film is completely dry.

5.5.1.1. Effect o f PAA shell at a low er pH (pH = 3)

By comparing latexes of pure PIDA at a pH of 3 and PIDA with an PAA shell also at

a pH of 3, one can find the effect of the PAA shell in an acidic form on drying.

Figure 5.17 (a), shows the comparisons of the solids fraction as a function of drying

time and (b), as a function of drying time normalised by initial film thickness for

PIDA with 3 wt.% PAA (at a pH of 3) and for PIDA with no PAA (at a pH of 3).

100 150 200

D rying tim e (m in)

250 02 0.4 0.6 08

Drying tim e/ Initial th ick n ess (rriiVum)

Figure 5.17. (a) Solid fraction as a function of drying time and (b) as a function of drying time normalised by the initial film thickness, for as receive, therefore pH = 3, for PIDA with 3 wt.% PAA (filled squares) and for PIDA with no PAA (empty squares). Pure PIDA reach the maximum solid fraction earlier then PIDA with PAA sample, therefore, pure PIDA completed the drying before PIDA with PAA.

By normalising drying time by the initial thickness, it eliminates the effect of the

small differences in the initial thicknesses of the films on drying. As PIDA with no

147

Chapter 5

PAA sample reaches the maximum solids fraction sooner than the PIDA with 3 wt.

% PAA sample, it confirms that pure PIDA dries faster than PIDA with PAA.

However, it was expected that the hydrophilic boundaries of PAA shells would help

the water transport.

5.5.I.2. Effect of PAA shell at a higher pH adjusted with NaOH (pH = 8)

By comparing latexes of pure PIDA at a pH of 8 adjusted by NaOH and PIDA with

PAA shell also at a pH of 8 adjusted by NaOH, one could compare the effect of an

PAA shell in a basic form on drying.

Figure 5.18 compares the solids fraction as a function of drying time normalised by

the initial thickness of the films for pure PIDA and PIDA with 3 wt.% PAA, both at a

pH of 8 adjusted by NaOH. The results show that there is no significant difference

between them. This emphasises that at a higher pH adjusted by NaOH, the PAA shell

does not contribute to drying properties. This result is different from the results of the

same samples at a lower pH of 3. One explanation could be the effect of Na+ from

NaOH in the system. The repulsion between Na+ ions in the system could be keeping

the water pathways open so that the water can reach to the top of the film. If the Na+

effect is stronger, the effect of hydrophilic boundaries of PAA is less important. As a

result, both the films dry in a similar way.

148

Chapter 5

Drying time/ Initial thickness (min/nm)

Figure 5.18. Solids fraction as a function of drying time normalised by the initial film thickness for pure PIDA and PIDA with PAA at a pH of 8 adjusted by NaOH. The effect of PAA shell at a higher pH adjusted by NaOH is insignificant.

5.5.1.3. Effect o f pH on latex w ith PAA shell

To investigate how the hydrophilic channels created by PAA shells at different pH

values affect drying, the drying of PAA shell samples at a pH of 3, and at a pH of 8

adjusted by NaOH and NH4OH were compared. On one hand, the hydrophilic

channels created by PAA could led to a faster drying, as they facilitate pathways for

the water to reach to the top of the film. On the other hand, as they are hydrophilic,

they could hold on to the water and lead to a relatively slower drying process.

Figure 5.19 presents the effect of pH on drying properties of PIDA with PAA PSAs.

PIDA with PAA at a pH of 3 was compared with a pH of 8 adjusted by N aO H and

N H 4O H . In N H 4O H aqueous solution, there will be an equilibrium between N H 4+ +

O H ' and aqueous NH3 + H20 . During drying, N H 3 will evaporate. In N a O H aqueous

solution, the counterion concentration is unaffected. The results reveal that at the

149

Chapter 5

lower pH (pH = 3) the PIDA sample dries faster then the higher pH (pH=8 ) samples.

An explanation could be that, at a low pH, PAA brushes collapse on to the particle

surfaces and create hydrophilic channels or pathways for the water to reach the top of

the film.

1.00

0.96 J

2 0.92o

CO

0.88 -

3AA : pH = 33AA : pH = 8 by NaOH3A A : pH = 8 by NH4QH

0.1 0.2 0.3 0.4 0.5 0.6

Drying time/ Initial thickness (min/nm)

0.7

Figure 5.19. Solids fraction as a function of drying time normalised by the initial film thickness for PIDA with PAA (pH = 3), a pH of 8 adjusted by NaOH and NH4OH.

5.5.2. W ater w hitening m easurem ents

All the PSA films that were submerged in de-ionized water in a square cuvette

gradually turned white, as shown in Figure 5.20. Figure 5.20 is a photograph of (a)

PET substrate with a latex film as soon as it was submerged in water, followed by (b)

after 1 hour, (c) after 7 hours and (d) after 24 hours in the water.

150

Chapter 5

Figure 5.20. Photograph to compare the change in the optical transparency over time: (a) PET substrate with the latex film as soon as it was submerged in water, (b) after 1 hour, (c) after 7 hours and (d) after 24 hours. Image from C.-H. Lei, University of Surrey.

The change in the transparency was measured with a UV/visible spectrophotometer

at a wavelength of 600 nm and plotted as a function of the time duration of the film

immersion in water. The results of all six PSA samples are shown in Figure 5.21.

The PIDA with PAA sample at a pH of 3 (named as 3PAA : pH = 3 in Figure 5.21)

lost the least of its transparency. In comparison, pure PIDA at a pH of 8 adjusted

with NaOH (named as OPAA+NAOH in Figure 5.21) lost its transparency within

around 100 hours of immersion in water. One would expect, PSA samples with

higher pH (i.e. a pH of 8) adjusted by NaOH or NH4OH, to be more hydrophilic,

hence absorb more water in comparison with lower pH (i.e. a pH of 3) samples [19].

The results from current research are in line with this argument. The contradictory

result was the higher transmission shown by the acidic form PIDA with PAA sample

in comparison with the lower transmission by more hydrophobic pure PIDA film at a

pH of 3.

151

Chapter 5

Time in water (hr)

Figure 5.21. Optical transmission at a wavelength of 600 nm as a function of the time the film immersion in water. Open symbols are for the PSA films by pure PIDA latexes and the filled symbols are for the PIDA with PAA samples. The squares for low pH latexes (pH of 3), diamonds are for a pH of 8 adjusted with NH4OH and the circles are for a pH of 8 adjusted with NaOH. PIDA with PAA (pH of 3; filled squares) least lost its transparency and pure PIDA with a pH of 8 adjusted with NaOH the fastest to lose its transparency.

It is clear from the Figure 5.21 that within all three groups of different pH values,

that is pH of 3, pH of 8 adjusted by NaOH and pH of 8 adjusted by NH4OH, the

films with PAA have higher transparency compared to their counterparts. This

should be due to the fact that the PAA in the shell is hydrophilic and the water which

was absorbed stays along the boundaries. Water along the particle boundaries should

not scatter light. In comparison, in the samples with no PAA, the particle boundaries

are more hydrophobic due to the hydrophobic PIDA. When these films were

submerged in water, the absorbed water is more likely to be pushed into pockets

between hydrophobic boundaries. These pockets of water are more likely to scatter

light, hence relatively low transmission compared to the samples with PAA on their

shells would result.

152

Chapter 5

The higher pH samples adjusted by NaOH (with and without PAA) have a lower

transmission than the samples adjusted by NH4OH. It is concluded that the counter

ions are playing a part. In the higher pH samples adjusted by NH4OH, volatile NH3

leaves to the atmosphere, hi comparison, in the samples with higher pH adjusted by

NaOH, the Na+ remains within the system, preferably between polymer particles.

There are two possibilities: there could be water hydration associated with Na+ ions,

or repulsion between these Na+ ions also encourages the creation of water pockets, as

the films were immersed into the water. It is justifiable that either or both of these

possibilities or any other reason related to Na+ ions are significant, as both films with

pH adjusted by NaOH lost their transparency relatively within very short periods of

time compared to the other samples.

Even though, the transparency was used as an indicator of the amount of water

uptake, it should be noted that there is a question of accuracy of this practice, as the

relation between water uptake and transparency is not fully known. As an example, a

film could uptake a large amount of water, but if this water was evenly distributed

along the boundaries, without creating water pockets, then the film should still

transmit light [11]. Tzitzinou and co-workers [25] predicted the relation between

optical transmission and radius of air voids and volume fraction of those voids

(Figure 5.8). Nevertheless, it’s not proven that these air voids can be used as a direct

measurement of water uptake. As a step forward, the moisture sorption

measurements were performed. Now, the dry films were exposed to high humidity

and the moisture soiption was measured as a weight gain. The weight increase within

the dry film was only caused by the water vapour absorbed by the film. Therefore,

moisture soiption measurements are a direct method of measuring the water uptake

by the dry films. Transmission measurements were also carried out on the films

which were exposed to high humidity, but the changes in the transparency were too

small to detect by the UV/visible spectrophotometer.

153

Chapter 5

5.5.3. M oisture Sorption M easurem ents

Moisture from high humidity going into a dry film was measured, and the results are

presented in a similar order as the drying measurements, to look separately at the

effects of PAA and pH on moisture uptake. Figures 5.22, 5.23 and 5.24 represent the

sample mass response to a step change in humidity from 0 - 70% RH, [17]. Note that

at short times the initial mass uptake will be dominated by adsorption, and at later

times the mass increase arises due to diffusion of water into the film.

5.5.3.1. Effect o f PAA at a low er pH (pH o f 3)

Figure 5.22 shows a comparison of the % mass change as a function of square root of

time for films with and without PAA. The sample with 3 wt.% PAA in the shell

showed a higher % mass change compared with the sample with 0 wt.% PAA. It is

likely that the sample with PAA in its shell had higher hydrophilicity on its particle

surfaces compared with the sample with no PAA, hence it showed greater % mass

change at high humidity.

0 10 20 30 40TimeA1/2 (minA1/2)

Figure 5.22. % mass change as a function of square root of time for 0 wt.% PAA sample (empty squares) and 3 wt.% PAA sample (filled squares). Sample with PAA in its shell shows higher % mass change, compared to no PAA sample.

154

Chapter 5

5.5.3.2. Effect of PAA at a higher pH adjusted with NaOH (pH of 8)

Figure 5.23 compares the effect of PAA in the shell at a higher pH of 8 adjusted with

NaOH. At a higher pH of 8 , the sample with PAA in the shell showed the greater %

mass change, compared with no PAA. Figure 5.22 and 5.23 show that having PAA in

the shell makes the samples more hydrophilic in both acidic and basic form.

0 10 20 30 40Tim e1'2 ( (m in)1'2)

Figure 5.23. % mass change as a function of square root of time for 0 wt.% PAA sample (empty squares) and 3 wt.% PAA sample (filled squares) at a pH of 8

adjusted by NaOH. Sample with PAA in its shell shows higher % mass change, compared to no PAA sample, in a basic form.

5.5.3.3. Effect of pH on latex with PAA shell

Figure 5.24 compares the effect of pH on % mass change for the samples with PAA

in their shells. The lower pH film shows the highest % mass change. Furthermore,

of the pH of 8 adjusted by NaOH is very close to that of lower pH. The

155

Chapter 5

difference between these two samples is the characteristic time, x, which is 160

minutes for lower pH and 280 minutes for higher pH adjusted by NaOH. In other

words, the lower pH film shows fast uptake of moisture compared with higher pH. In

contrast, a pH of 8 adjusted by NH4OH shows the lowest % mass change; but its x is

slightly higher than that of low pH but significantly lower than the sample with a pH

of 8 adjusted by NaOH.

0.9 |-----

0.8 -

0.7 -a>» 0.6 -no 0.5 -tn| 0.4 -

SS 0.3 -

0.2 -

0.1 -

0.0 - L.

0

Figure 5.24. % mass change as a function of square root of time for the samples with PAA in their shells. Red squares for a pH of 3, blue circles for a pH of 8

adjusted by NaOH and black diamond for a pH of 8 adjusted by NH4OH.

Table 5.2 summarises the calculated M ^ a s a %), equilibrium or plateau value

obtained from the water sorption kinetics and characteristic time for the different

samples which were examined.

3 AA pH33 AA pH8 by NaOH 3 AA pH8 by NH40H

10 20 30 40

Time172 ((min)172)

156

Chapter 5

Table 5.2 Calculated M ^ and characteristic time, r for different samples.

Sample (% ) t (min)

0 PAA (pH=3) 0.35 103

0 PAA (pH=8 , by NaOH) 0.58 2 0 0

0 PAA (pH=8 , by

NH4OH)0.38 184

3 PAA (pH=3) 0.79 160

3 PAA (pH=8 , by NaOH) 0.75 280

3 PAA (pH=8 , by

NH4OH)0.53 180

■ 3AA PH s= 3□ 0AA PH 5= 3♦ 3AA PH := 8 by NH4 OH

<0 0AA PH := 8 by NH4 OH• 3AA PH s= 8 by NaOHO 0AA PH = 8 by NaOH

0.2 -

0.0 —1---------- ■ 1 * 1 * 1 *--------100 150 200 250 300

Characteristic time (min)

Figure 5.25. as a function of calculated characteristic time, x, for all sixsamples. The samples without PAA showed lower M ^ compared with the samples which contained PAA in their shells.

157

Chapter 5

M m is an indication of how hydrophilic the sample is. M m values in Table 5.2 and

Figure 5.25 show that the samples with 0 wt.% PAA have lower values

compared with higher values of 3 wt.% PAA samples. A possible explanation could

be that the hydrophilic PAA on the particle shells creates hydrophilic pathways in the

dry films, and water is easily adsorbed into the films through these pathways. The

suggestions of De Brayn and co-workers [20], which was that the hairy layer

thickness of PAA shell increase with the increase of pH, was less obvious in this

current work. It was found that was higher at a pH of 3. Further more, Rharbi

and co-workers [2 1 ] reported that their un-dialysed adhesive films by core-shell

particles with PAA shells showed greater water uptake. The films made with

dialysed latexes also showed greater water uptake, if they were neutralised by NaOH,

but water uptake was very low for the samples kept in the acidic form. The current

results are partially in an agreement with Rharbi and co-workers’ observations. All

the samples in the current work are dialysed. For 3 wt.% PAA, a pH of 8 adjusted by

NaOH shows a higher compared to 0 wt.% PAA. In contradiction to their results,

the current results show the films made with dialysed acidic form of PAA also show

a higherMro compared to basic form PAA, adjusted by NH4OH.

5.5.4. Correlation of drying, water whitening and moisture

sorption measurements

Possible correlations between water loss, liquid water uptake and vapour uptake are

investigated. The results are presented against each other. Desirable properties of a

given parameter are indicated by the direction of an arrow head, and an expected

trend of a chosen parameter is shown by a coloured, dotted line.

158

Chapter 5

5.5 .4 .I. C orrelation o f drying and w ater w hitening

From the GARField measurements, the time to lose all the water from each PSA

sample was obtained. It was assumed to be when the intensity of the signal continued

to remain constant with increasing drying time that all the mobile water was

evaporated. This time was normalised by the initial film thickness of each sample.

Using the optical transmission measurements, the transmission at 600 nm was

obtained for each sample after each was immersed in water for 200 minutes. Figure

5.26 compares the transmission after 200 minutes in water as a function of

normalised time to lose water for all six PSA samples.

If a wet film develops a skin during drying, the normalised time to lose water should

be relatively higher. If this skin acts as a barrier, then the film should show a higher

transmission after soaking in water. This correlation is represented by the green

dotted line on Figure 5.26. If we argue that hydrophilic films hold water, then the

samples with higher normalised time to lose water should have lower transmission

after 200 minutes in water. In comparison, more hydrophobic films should hold less

water, hence there will be a lower normalised time to lose water and higher

transmission. This correlation is represented by the yellow dotted line.

159

Chapter 5

u 1-05 13| 0.8<A C

E 0.6o oCM

| 0.4 re

■i 0.2wE(Ag 0.0H

0.3 0.4 0.5 0.6 0.7

Normalized time to lose water, tdry, (minI nm)

Figure 5.26. Optical transmissions of the films after 200 minutes in water as a function of normalised time to lose all the mobile water within the films. The drying properties get desirable towards the arrow 1 and the water resistance properties get desirable towards the arrow 2 .

The lower the normalised time to lose water, then the faster is the drying, which is

indicated by the direction of the arrow head one. The greater the transmission of the

film after 2 0 0 minutes in water, then the higher is the water resistance properties,

which is indicated by the direction of the arrow head two. Therefore, out of all six

samples, the two PSA latex samples with a pH of 3 show the most desirable

combined properties. These two samples have a relatively higher transmission after

2 0 0 minutes in water and a relatively lower normalized time to lose water compared

with the other remaining four samples.

From the results presented in Figure 5.26, it can be concluded that pH of the wet

latex affects the transmission of the dry film after it was immersed in water for 2 0 0

minutes. Both the lower pH samples, at a pH of 3, showed higher transmission

properties compared with the higher pH samples. In the same manner, the lower pH

samples also showed better drying properties compared with higher pH samples. No

obvious correlation of drying and water whitening is to be seen. Very roughly, the

samples with and without PAA, follow the argument that hydrophobic films hold less

■ ■ 3AA : pH = 3□ 0A A : pH = 3

.•** + 3A A : pH = 8 by NH4OH

, ■ x. „• O 0A A : pH = 8 by NH4OH’D ' '' S • 3A A : pH = 8 by NaOH

O 0A A : pH = 8 by NaOH

♦0

I •' .- % ■ y V • 0

1 . 1 . 1 ,

160

Chapter 5

water, hence lower normalised time to lose water, tdry, and higher transmission. The

basic samples form films also roughly following the argument that hydrophilic films

hold on to the water, hence higher tdry, and lower transmission.

5.5.4.2. C orrelation o f drying and m oisture sorption

Again, if we consider the skin development during drying as a barrier to lose water

(hence normalised time to lose water is higher) and a barrier for water to go in to the

film (low M x ), then this inverse correlation would be represented by the yellow

dotted line on Figure 5.27 (a). If we compare characteristic time, i, and drying

together, one could expect the skin development to hold water within the film during

drying, hence a higher normalised time to lose water would correlate with a higher x.

This positive correlation is represented by the green dotted line on Figure 5.27 (b).

0.3 0.4 0.5 0.6 0.7Norm alized tim e to lose water, t ^ , (m in/ ^m)

■ 3AA PH ; 3□ 0AA PH = 3♦ 3AA PH = 8 by n h 4o h

o 0AA PH = 8 by n h 4o h

• 3AA PH = 8 by NaOHo 0AA PH - 8 by NaOH

161

Chapter 5

(b)■ 3AA PH == 3□ OAA PH := 3♦ 3AA PH == 8 by NH4OH

o OAA PH == 8 by NH4 OH• 3AA PH 5= 8 by NaOHo OAA PH = 8 by NaOH

Normalized time to lose water, tdry) (minI ^m)

Figure 5.27. (a) M ^ as a function of normalised time to lose all mobile water with in the films. The drying properties get better towards the arrow 1 and the dry films adsorb less moisture from high humidity towards the arrow head two. (b) t as a function of normalised time to lose all the mobile water with in the films. Towards the direction of arrow head two, the dry films adsorb moisture faster from high humidity.

In addition to the drying properties getting more desirable towards the direction of

arrow 1 in Figure 5.27 (a), the dry films show less hydrophilic behaviourin the

direction towards arrow 2 with smallerM x . Out of all six samples, the PSA film

without PAA with a pH of 3 shows the best combined drying and water adsorption

properties. In addition, except for this PSA film, the rest of the films somewhat

follow the expected trend (yellow dotted line) between and normalised time to

lose water, tdry. With low values of x (Figure 5.27 (b)), the dry films show fast uptake

of moisture. One could argue that a fast drying film should show fast water uptake as

well. It is observed in Figure 5.27 (b), the PSAs with a pH of 3 are in a greater

agreement with this concept and the samples with a pH of 8 adjusted by NH4OH are

in least agreement. However, it should be noted that during drying of a wet film, the

film formation is not yet completed. As the water leaving the drying film, close

packing of particles, particle deformation, and coalescence/interdiffusion/aging of

162

Chapter 5

particles (Figure 2.9) still exist. When a dried film is exposed to high humidity or

submerged in water, the film formation is complete. Therefore, a fast drying film

does not necessarily show fast uptake of moisture/water as the nature of the films in

the two different situations is different.

S.5.4.3. C orrelation o f m oisture absorption and w ater

w hitening

Correlation between water going into a dry film from liquid water and water going

into a dry film from vapour was studied. It was expected that films with high

transparency would have lower , and the samples with higher M m would be less

transparent. This correlation is represented by the yellow dotted line in Figure 5.28

(a), furthermore, it was expected that the films with higher transmission after 2 0 0

minutes in water would have better barrier properties. The same films are to be

expected to have higher t, as the films have better barrier properties. This correlation

is represented by the green dotted line in Figure 5.28 (b).

As discussed earlier, the greater the transmission of the film after 200 minutes in

water, the higher are the water resistance properties, indicated by the direction of the

arrow head one, in Figure 5.28 (a) and (b). It is more desirable to have a l o w e r ;

then, these films should be less hydrophilic. This correlation is indicated by the

direction of the arrow head two, in Figure 5.28 (a). Even though, it is indicated

towards the direction of arrow head two, in Figure 5.28 (b), that lower t is more

desirable, it is open for discussion.

It was observed in Figure 5.28 (a), that was affected by PAA; for all three

different pH values, the samples with PAA had a higher (unbroken brown curve

in Figure 5.28 (a)) compared with the samples without PAA (dashed brown curve in

Figure 5.28 (a)). The films with PAA were more hydrophilic, hence they absorbed

163

Chapter 5

more water, and as a result, higher M ̂ . With the exception of the acidic form film

with PAA, there is a general downward trend between A/^and transmission. The

correlation between t and transmission after 2 0 0 minutes in water shows a rough

negative connection.

Transmission after 200 mins in water

■ 3AA PH == 3□ 0AA PH 1= 3♦ 3AA PH ss 8 by NH4 OH

0 0AA ■■za. = 8 by NH4 OH• 3AA PH == 8 by NaOH0 0AA PH s= 8 by NaOH

Transmission after 200 mins in water

■ 3AA : pH = 3 □ 0AA : pH = 3 + 3AA : pH = 8 by NH4OHo 0AA : pH = 8 by NH4OH• 3AA : pH = 8 by NaOH O 0AA : pH = 8 by NaOH

Figure 5.28. (a) M ^ and (b) x as a function of optical transmissions (at 600 nm) of the films after 2 0 0 minutes in water.

164

Chapter 5

There is a possibility of phase inversion occurring during drying. Phase inversion is

the phenomenon whereby the dispersed phase inverts to become the continuous

phase and vice-versa under conditions determined by the system properties, volume

ratio and energy input [41]. If, phase inversion occurs during drying in PAA samples,

there is a possibility o f having PIDA, hydrophobic pathways instead of PAA,

hydrophilic pathways. As a result PAA samples also should behave like pure PIDA

samples.

5.5.5. Probe-Tack A dhesion M easurem ents

Experiments were carried out to determine whether slow drying has any

consequences for adhesion properties. Differences in the drying times were observed.

One could argue that the presence of water in a PSA film could interfere with

coalescence and decrease the cohesive strength.

Figure 5.29 (a), reveals adhesion properties of the PSA film with 3 wt.% PAA in the

shell at a pH of 3. Tack measurements have been performed on the same film at

different points in time: at 90 minutes, 24 hours, and 6 days after it was cast. £-max and

Ea both were elevated with increasing drying time. The adhesion properties of PSA

film with 3 wt.% PAA, at a pH of 8 by NH4OH are shown in Figure 5.29 (b). As the

drying time increases, £^ax and E& both were increased. The elevation was significant

between one and five days. Figure 5.29 (c), summarises the adhesion energy of PSA

film with 3 wt.% PAA in the shell at pH 3 and at pH 8 by NH4OH for different

drying times. Earlier, using drying measurements it was shown that films prepared

by low pH were fast-drying compared with the films prepared at high pH. As a

result, the low pH film lost a significant amount of water during the first day

compared to the period between day one to day six. The adhesion energy follows the

same trend; there is a significant increase during the first day, and a lesser increase

from day one to day six. In comparison, slow drying higher pH films lost a

significant amount of water from day one to day six. It is speculated the adhesion

energy was significantly increased during the periods when most water was lost.

165

Chapter 5

90 min 24 hrs 6 days

Strain

□ 3AA: pH = 3 o 3AA: pH = 8 by NH4OH

Drying time (days)

Figure 5.29. (a) The adhesion properties of PSA film with 3 wt.% PAA in the shell, at a pH of 3, performed at 90 minutes, 24 hours and 6 days after it was cast, (b) The adhesion properties of PSA film with 3 wt.% PAA, at a pH of 8 by NH4OH for different drying times and (c) summarises the adhesion energy of PSA film with 3 wt.% PAA in shell at a pH of 3 and at a pH of 8 NH4OH for different drying times.

166

Chapter 5

5.6. Conclusions

The effects of PAA and pH on drying, water whitening, water sorption from high

humidity and adhesive properties have been investigated. The drying measurements

confirm that pure PIDA samples dry faster than the samples with PIDA with 3wt.%

PAA shells, at a lower pH of 3. An explanation could be that the films with more

hydrophobic PIDA should have hydrophobic particle boundaries and during drying

these hydrophobic boundaries could push the remaining water to the top of the film,

resulting in faster drying. Physically, there could be a de-wetting of the water from

the polymer. The sample with 3 wt.% PAA in their shells should have hydrophilic

boundaries, and during drying water molecules could hang on to these hydrophilic

boundaries, hence a slower drying is observed. On the other hand, one could argue

that these hydrophilic boundaries facilitate pathways for the water to be transported

to the top of the film. But, there is little evidence for the second argument. An

important conclusion o f the current work is, at least fo r this system, that hydrophilic

pathways do not contribute to faster drying.

Furthermore, the investigations of the effects of pH on PAA revealed that in an

acidic form, the sample dries faster than in a basic form. De Bruyn and co-workers

[20] reported that the hairy layer thickness of PAA on particles increases when the

pH is increased and the shell was more hydrated. The current research findings are in

an agreement with De Bruyn’s findings, as slower drying was observed with high

pH. When the shell was more hydrated, release of water from the drying film was

slow. On the other hand, one would argue that hairy layers could speed up drying by

keeping the boundaries “open”. Current investigations disprove, at least fo r this

system, the idea that a hairy layer o f PAA keeps the film more open and leads to

faster drying.

Water whitening experiments revealed that the samples with a higher pH of 8

adjusted by NaOH or NH4OH, lost their transparency faster and more than the

samples with a lower pH of 3. This difference in transmission could be due to the

fact that higher pH samples are more hydrophilic, hence absorbed more water and

lost their transparency. The lower pH samples are more hydrophobic and absorbed

167

Chapter 5

less water, therefore they remained transparent. These results are in accordance with

the findings of Chen and co-workers [18], who performed wetting-resistance

experiments using contact angle measurements of the films which were modified by

introducing hydrophobic perfluaroalkyl groups into the polymer. These groups

prevented water molecules into the film’s interior, and improved the water resistance

properties. The water whitening results of the films prepared using 3 wt.% PAA and

0 wt.% PAA at a pH of 3 were somewhat surprising, as 3 wt.% PAA is more

hydrophilic, but it is also the most transparent. The relation between water uptake

and transparency is not fully known. A film which uptakes a large amount of water

could still transmit light, if this water was evenly distributed along the particle

boundaries (Figure 5.1). In fact, in the current study, the PSAs with 3wt.% PAA

shells should even support the distribution of absorbed water along their hydrophilic

boundaries. In addition, the moisture absorption by high humidity experiments

shows that M m is the highest for 3 wt.% PAA in an acidic form. An important

conclusion o f the current work is that the films with hydrophilic boundaries absorb a

large amount o f moisture, but still retain their transparency, as absorbed moisture is

evenly distributed along the boundaries. Transparency or water whitening is not

necessarily a reliable measurement o f water uptake, at least fo r this system.

Water sorption curves of % mass change as a function of square root of time in the

current study shows the typical soiption curve type which was discussed by Van der

Wei and Adan [17]. The soiption curves show an initial linear dependency of mass as

a function of square root of time, and then smoothly level off to a saturation level of

mass, with increasing time. The soiption results were comparable with the rest of

these current findings and earlier literature findings alike. More hydrophilic films

(PIDA with 3 wt.% PAA shells) adsorbed more water compared to less hydrophilic

films (0 wt. % PAA).

The results from current research of drying measurements (td,y), water whitening

measurements (T), water soiption measurements (M ro and x) and the adhesion energy

after one day of drying from probe-tack adhesion measurements for all six samples

are summarised in Table 5.3.

168

Chapter 5

Table 5.3 A summary of the results for all six samples of PSAs.

Sampletdry

(min/pm)T 00

Tau

(min)

adhesion

energy

(J/m2)

OPAA

(pH=3)0.31 0.67 0.35 103 50.3

OPAA

(pH=8, by 0.55 0 .0 1 0.58 2 0 0 109.6

NaOH)

OPAA

(pH=8, by 0.65 0.26 0.38 184 74.5NH4OH)

3 PAA

(pH=3)0.38 0.99 0.79 160 64.6

3 PAA

(pH=8, by 0.5 0 .0 1 0.75 280 48.5NaOH)

3 PAA

(pH=8, by 0.65 0.33 0.53 180 50.1

NH4OH)

In the Table 5.3, the results marked with blue were the most desirable for

applications out of all the six samples, for that particular type of measurement, and

the results marked with green were second best. The PIDA sample without PAA at a

pH of 3 shows the better results in more cases than the rest of the samples. The

effects of calculated characteristic time, 1 (min), from water sorption measurements,

on performance of PSAs need to be discussed and investigated further. We are

unable to conclude whether high or low value of x would be desirable in a PSA.

From these results it can be concluded that the PIDA sample without PAA at a pH of

3 shows the best compromise with the most desirable properties out of this series.

This film showed the most desirable transparency measurement result and the second

169

best results for drying andM ^. The worst compromise are the PSAs with 3 wt.%

PAA, with higher pH adjusted by NaOH and NH40H, as these films failed to

perform well in any type of measurement.

________________________________________________________________ Chapter 5

170

Chapter 5

5.7. R eferences

1. Jotischky, H., Coatings, regulations and the environment reviewed. Surface

Coatings International Part B-Coatings Transactions, 2001. 84(1): p. 11-20.

2. Jovanovic, R. and Dube, M.A., Emulsion-based pressure-sensitive adhesives:

A review. Journal of Macromolecular Science-Polymer Reviews, 2004.

C44(l): p. 1-51.

3. Lovell, P.A., El-Aasser, M.S.,, Emulsion Polymerisation and Emulsion

Polymers. 1997: John Wiley & Sons Ltd. UK.

4. Garrett, J., Lovell, P.A., Shea, A.J., and Viney, R.D., Water-borne pressure-

sensitive adhesives: Effects o f acrylic acid and particle structure.

Macromolecular Symposia, 2000.151: p. 487-496.

5. Fitch, R.M., Oplymer Colloids: A Comprehensive Introduction. 1997:

Academic Press, New York.

6 . Satas, D.E., Handbook o f Presure-Sensitive Adhesive Technology. Vol. 2nd

edition. 1989: Van Nostrand Reinhold, New York.

7. Wang, T., Interfacial Control in Colloidal Nanocomposites fo r Presure-

Sensitive Adhesives. 2008, University of Surrey, Guildford, UK.

8 . Lindner, A., Lestriez, B., Mariot, S., Creton, C., Maevis, T., Luhmann, B.,

and Brummer, R., Adhesive and Theological properties o f lightly crosslinked

model acrylic networks. Journal of Adhesion, 2006. 82(3): p. 267-310.

9. Creton, C., Pressure-sensitive adhesives: An introductory course. Mrs

Bulletin, 2003. 28(6): p. 434-439.

10. Wang, T., Lei, C.H., Dalton, A.B., Creton, C., Lin, Y., Fernando, K.A.S.,

Sun, Y.P., Manea, M., Asua, J.M., and Keddie, J.L., Waterborne,

nanocomposite pressure-sensitive adhesives with high tack energy, optical

transparency, and electrical conductivity. Advanced Materials, 2006. 18(20):

p. 2730-+.

11. Donkus, L.J., Solvent-like performance from emulsion PSAs: Advances in

water-whitening resistance. Adhesives Age, 1997. 40(10): p. 32-+.

171

Chapter 5

12. Aramendia, E., Barandiaran, M.J., Grade, J., Blease, T., and Asua, J.M.,

Improving water sensitivity in acrylic films using surfmers. Langmuir, 2005.

21(4): p. 1428-1435.

13. Ciampi, E. and McDonald, P.J., Sian formation and water distribution in

semicrystalline polymer layers cast from solution: A magnetic resonance

imaging study. Macromolecules, 2003. 36(22): p. 8398-8405.

14. Keddie, J.L. Film Formation o f Waterborne Coatings 2006. Emulsion

Polymerisation Processes Course.

15. Narita, T., Hebraud, P., and Lequeux, F., Effects o f the rate o f evaporation

and film thickness on nonuniform drying o f film-forming concentrated

colloidal suspensions. European Physical Journal E, 2005.17(1): p. 69-76.

16. Agarwal, N. and Farris, R.J., Water absorption by acrylic-based latex blend

films and its effect on their properties. Journal of Applied Polymer Science,

1999. 72(11): p. 1407-1419.

17. van der Wei, G.K. and Adan, O.C.G., Moisture in organic coatings - a

review. Progress in Organic Coatings, 1999. 37(1-2): p. 1-14.

18. Chen, Y.J., Zhang, C.C., and Chen, X.X., Emulsifier-free latex o f fluorinated

acrylate copolymer. European Polymer Journal, 2006. 42(3): p. 694-701.

19. Feng, J.R. and Winnik, M.A., Effect o f water on polymer diffusion in latex

films. Macromolecules, 1997. 30(15): p. 4324-4331.

20. De Bruyn, H., Gilbert, R.G., White, J.W., and Schulz, J.C., Characterization

o f electrosterically stabilized polystyrene latex; implications fo r radical entiy

Idnetics. Polymer, 2003. 44(16): p. 4411-4420.

21. Rharbi, Y., Boue, F., Joanicot, M., and Cabane, B., Deformation o f cellular

polymeric films. Macromolecules, 1996. 29(12): p. 4346-4359.

22. Wang, T., Canetta, E., Weerakkody, T. G., Keddie, J.L., Rivas, U.„ pH

Dependence o f the properties o f waterborne pressure-sensitive adhesives

containing acrylic acid American Chemical Society, 2009.1(3): p. 631-639.

23. Yang, Y.K., Li, H., and Wang, F., Studies on the water resistance o f acrylic

emulsion pressure-sensitive adhesives (PSAs). Journal of Adhesion Science

and Technology, 2003.17(13): p. 1741-1750.

24. Blackley, D.C., Polymer latices/ 2, Types oflatices. Vol. 2. 1997: Chapman

& Hall, London

172

Chapter 5

25. Tzitzinou, A., Keddie, J.L., Geurts, J.M., Peters, A.C.I.A., and Satguru, R.,

Film formation o f latex blends with bimodal particle size distributions:

Consideration o f particle deformability and continuity o f the dispersed phase.

Macromolecules, 2000. 33(7): p. 2695-2708.

26. Vorwerg, L. and Gilbert, R.G., Electrosteric stabilization with poly(aciylic)

acid in emulsion polymerization: Effect on Idnetics and secondary particle

formation. Macromolecules, 2000. 33(18): p. 6693-6703.

27. Gabriel, G.J., Microscopic Theory o f Rayleigh-Scattering. Physical Review

A, 1973. 8(2): p. 963-990.

28. McDonald, P.J. and Newling, B., Stray field magnetic resonance imaging.

Reports on Progress in Physics, 1998. 61(11): p. 1441-1493.

29. Mallegol, J., Bennett, G., McDonald, P.J., Keddie, J.L., and Dupont, O., Skin

development during the film formation o f waterborne acrylic pressure-

sensitive adhesives containing tacldfying resin. Journal o f Adhesion, 2006.

82(3): p. 217-238.

30. Koenig, A.M., Weerakkody, T.G., Keddie, J.L., and Johannsmami, D.,

Heterogeneous drying o f colloidal polymer films: Dependence on added salt.

Langmuir, 2008. 24(14): p. 7580-7589.

31. Lakrout, H., Sergot, P., and Creton, C., Direct observation o f cavitation and

fibrillation in a probe tack experiment on model acrylic Pressure-Sensitive-

Adhesives. Journal of Adhesion, 1999. 69(3-4): p. 307-359.

32. Chikina, I. and Gay, C., Cavitation in adhesives. Physical Review Letters,

2000. 85(21): p. 4546-4549.

33. Gay, C. and Leibler, L., Theory o f tacldness. Physical Review Letters, 1999.

82(5): p. 936-939.

34. Wang, T., Lei, C.H., Liu, D., Manea, M., Asua, J.M., Creton, C., Dalton,

A.B., and Keddie, J.L., A molecular mechanism fo r toughening and

strengthening waterborne nanocomposites. Advanced Materials, 2008. 20(1):

p. 90-+.

35. Chuang, H.K., Chiu, C., and Paniagua, R., Avery Adhesive Test yields more

performance data than traditional probe. Adhesives Age, 1997. 40(10): p.

18-23.

173

Chapter 5

36. Zosel, A., Adhesion and Tack o f Polymers - Influence o f Mechanical-

Properties and Surface Tensions. Colloid and Polymer Science, 1985. 263(7):

p. 541-553.

37. Chiche, A., Dollhofer, J., and Creton, C., Cavity growth in soft adhesives.

European Physical Journal E, 2005.17(4): p. 389-401.

38. Creton, C., Hooker, J., and Shull, K.R., Bulk and interfacial contributions to

the debonding mechanisms o f soft adhesives: Extension to large strains.

Langmuir, 2001. 17(16): p. 4948-4954.

39. Shull, K.R. and Creton, C., Deformation behavior o f thin, compliant layers

under tensile loading conditions. Journal of Polymer Science Part B-Polymer

Physics, 2004. 42(22): p. 4023-4043.

40. Cleaver, J.A.S. and Wong, P., Humidity-induced surface modification o f

boric acid. Surface and Interface Analysis, 2004. 36(13): p. 1592-1599.

41. Yeo, L.Y., Matar, O.K., Hewitt, G.F., Ortiz, E.S., , Phase Inversion and

associated phenomena : A review. To appeal* in Multiphase Science and

Technology.

174

Chapter 6

Conclusions and Future Work

6.1. C onclusions

The overall task of this research, which was to investigate the physical characteristics

of the film formation process and film properties of latexes, was achieved. A series

of organic/inorganic waterborne nanocomposites for coating formulations and a

series of waterborne pressure sensitive adhesive formulation samples were studied.

The samples from two main systems, Original Route I and Original Route II, were

studied. In Route I latexes, Laponite plates were located on the surfaces of the

polymer particles; and in Route II latexes, the clay particles were encapsulated within

the latex particles. For each system four different latexes with different Laponite

contents (0, 3, 5 and 7 wt.%) were studied. Each of these samples was investigated to

6.1.1. C onclusions about organic/inorganic

nanocom posite coating form ulations

Chapter 6

determine their drying properties, as well as their visual appearance and the optical

transmission of dry films. It was found that the samples by Route I dried with no

significant differences in comparison to a standard acrylic latex. In contrast, latexes

by Original Route II with Laponite showed thickness increases in the centre of the

films during drying. All latexes with Laponite, showed this effect, but the 3 wt.%

sample showed the strongest effect. This was confirmed by the GARField profiles

and the images taken from the side of the drying films.

It was understood that for Route II, Laponite plates were doubly functionalised by

DDAB and y - MPTMS in order to render a good compatibility to the Laponite plates

with the monomer mixture. The amount of DDAB used was significant, which is

twice the CEC of Laponite. It was argued that the excess DDAB in the system could

cause a surface tension gradient within the drying film which would induce a

Marangoni flow of liquid [1-3]. These dried films were proven to have a ‘Mexican

Hat’ shape thickness variation. This concept was further investigated with a concave

wet film; where particles were expected to close pack from the centre of the drying

film and to induce Marangoni flow from the centre towards the edges of the film. It

was confirmed that the dry film thickness increase was in the middle of the radial

distance from the centre to the edge of the dried film.

Two new formulations for Route II, with 3 wt.% Laponite were developed with only

100 % of the CEC of Laponite in their systems; one with low solids (20 wt.%) and

the other with a relatively high solid content (30 wt.%). The drying properties of

these two new formulation Route II samples were dramatically improved. Films cast

from concave wet films showed no evidence for any lateral liquid flow, indicationg

that Marangoni effects were not active.

It was concluded that excess surfactant in a system could affect the physical

characteristics of the film formation process, and by adjusting the amount of

surfactant in the system, these properties could be improved.

176

Chapter 6

6,1.2. C onclusions about pressure sensitive

adhesive form ulations

An extensive study on water going out of a wet film (i.e. drying), liquid water going

into a dry film (i.e. water whitening), and vapour going into a dry film (i.e. vapour

absorption from high humidity) along with the adhesion properties of PSA films has

been completed.

Two sets of PSA formulations, one set with hydrophobic PIDA and the other set with

a PIDA core surrounded with a PAA shell, were investigated for the effects of PAA

and pH on drying, water whitening, vapour sorption from high humidity and

adhesive properties. Three pure PIDA samples with a pH of 3, a pH of 8 adjusted by

NaOH, and a pH of 8 adjusted by NH4OH were investigated. In parallel, three PIDA-

core PAA shell samples with the same pH values were also studied.

It was clear that pure PIDA samples at a pH of 3 showed the most desirable drying

properties. One would expect that the hydrophobic PIDA boundaries would push or

de-wet the remaining water from the drying film to the top of the film, resulting in

faster drying. There was less experimental evidence to support the idea that the

hydrophilic boundaries of PIDA/AA core/shell particles enable water transport

during drying. Among the samples with a PAA shell, the lower pH samples dried

faster than the higher pH. These results can be explained using De Bmyn’s findings

[4]. It was found that the hydrophilic pathways created by PAA shells do not

contribute to faster drying. In addition, this investigation disproves the idea that the

hairy layer of PAA keeps the film more open and leads to faster drying.

Moisture soiption experiments proved that more hydrophilic films (PIDA with PAA

shells) absorbed more moisture from high humidity compared to less hydrophilic

films with no AA (Figure 5.28 (a)). It was shown that the extent of drying affects the

adhesion properties of PSAs. Adhesion energy increases as drying proceeds.

177

Chapter 6

Water whitening experiments confirmed that the more hydrophilic, higher pH

samples lost their transparency faster and more than the low pH samples, as one

would expect if hydrophilic boundaries would allow water transport more than the

less hydrophilic boundaries. These findings can be explained using the earlier reports

of Chen and co-workers [5]. Drawing on the water vapour sorption experiments, it is

clear that films with hydrophilic boundaries (PSA film with 3 wt.% PAA at a pH of

3) absorbed more water than the film without PAA. However, the hydrophilic films

still retain their transparency. A likely explanation is that the 3 wt.% PAA film at a

pH of 3 absorbs water uniformly along the particle boundaries, so that light is not

scattered. It was shown that transparency or water whitening is not necessarily a

reliable measurement of water uptake, at least for this system.

178

Chapter 6

6.2. Future Work

6.2.1. Future work for organic/inorganic

nanocomposite coating formulations

To progress further in this study, additional insight into the excess surfactant in the

system is needed, hi the proposed process (Figure 4.22.), desorption of excess

DDAB molecules into the surrounding serum is significant. Attempts to confirm this

movement of surfactant is needed. This study would be most complete if one could

model the experimental results of Marangoni flow of fluid where surface tension

gradient develops in a system. Microscopy of film cross-section could be used to

explain the loss of optical clarity.

6.2.2. Future work for Pressure Sensitive Adhesive

formulations

In Chapter Five, the liquid water going into a dry film was investigated by optical

transmission, and the moisture sorption was investigated by a gravimetric method. In

the future, attempts could be made to verify the water whitening by a gravimetric

method and, as a comparison, moisture absorption from vapour could be supported

by optical transmission. As mentioned in Chapter Five, efforts were made to carry

out transmission measurements on the films which were exposed to high humidity.

The procedure was quite challenging as the changes in the transparency were too

small to detect by the UV/visible spectrophotometer. More sensitive equipment could

resolve the difficulties of transparency measurements of the PSA films which are

exposed to high humidity.

179

Chapter 6

6.3. References

1. Girard, F., Antoni, M., and Sefiane, K., On the effect o f Marangoni flow on

evaporation rates o f heated water drops. Langmuir, 2008. 24(17): p. 9207-

9210.

2. Hu, H., and Larson, R.G., Analysis o f the effects o f Marangoni stresses on the

microflow in an evaporating sessile droplet. Langmuir, 2005. 21(9): p. 3972-

3980.

3. Hibiya, T., et al., Attempt to study Marangoni flow o f low-Pr-number fluids

using a liquid bridge o f silver. Advances in Space Research, 2008. 41(12): p.

2107-2111.

4. De Brayn, H., et al., Characterization o f electrosterically stabilized

polystyrene latex; implications fo r radical entry Idnetics. Polymer, 2003.

44(16): p. 4411-4420.

5. Chen, Y.J., Zhang, C.C., and Chen, X.X., Emulsifier-free latex o f fluorinated

acrylate copolymer. European Polymer Journal, 2006. 42(3): p. 694-701.

180