Hydrophobically Modified Polymers 2002Tesis

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Hydrophobically Modified PolymersRheology and Molecular Associations

Leif Karlson

Avhandling för Filosofie Doktorsexamen

Matematisk-Naturvetenskapliga Fakulteten

Avhandlingen kommer att försvaras vid en offentlig disputation fredagen den 4 oktober 2002 kl. 13.15 ihörsal C, Kemicentrum, Lund

© Leif Karlson 2002ThesisPhysical Chemistry 1Center for Chemistry & Chemical EngineeringLund UniversityP.O. Box 124

SE-221 00 LundSweden





Contents

List of Papers 2

Chapter 1 Introduction 3

1.1 Hydrophobically modified polymers in paint 5

1.2 References Chapter 1 11

Chapter 2 Hydrophobically modified polymers 13

2.1 Structure and synthesis of hydrophobically modified polymers 14

2.1.1 HEUR thickeners 14

2.1.2 HM-EHEC 15

2.1.3 Comb HEUR thickeners 18

2.2 Hydrophobically modified polymers in aqueous solution 19

2.2.1 HM-PEG in aqueous solution 23

2.2.2 HM-EHEC in aqueous solution 26

2.2.3 Interaction between HM-Polymers and surfactants 28

2.2.4 Clouding 30

2.2.4.1 Cloud point of HM-PEG 32

2.2.4.2 Cloud point of HM-EHEC 33


Chapter 3 Inhibition of hydrophobic associations as a tool to study

cross-linking mechanisms 37

3.1 Inhibition of hydrophobic interactions by changing solvent quality 38

3.2 Inhibition of hydrophobic interactions by addition of surfactant 40

3.3 Inhibition of hydrophobic interactions by addition of cyclodextrin 41

3.3.1 Structure and properties of cyclodextrin 42

3.3.2 Formation of inclusion complex between lipophilic guestmolecules and cyclodextrin 43

3.3.3 Cyclodextrin and HM-Polymers 45

3.3.3.1 Cyclodextrin and HM-EHEC 46

3.3.3.2 Cyclodextrin and HM-PEG 49


Main conclusions 57

Popular summary (in Swedish) 58

Acknowledgements 61

List of commercially available hydrophobically modified polymers 62

1



2

List of Papers

I Rheology of an aqueous solution of an end-capped poly(ethylene glycol) polymer

at high concentration

Karlson, L.; Nilsson, S.; Thuresson, K. Colloid Polym. Sci. 1999, 277 , 798-804.

II Clouding of a cationic hydrophobically associating comb polymer

Thuresson, K.; Karlson, L.; Lindman, B. Colloid and Surfaces A: Physiochem. Engin.

Aspects 2001, 201, 9-15

III Phase behavior and rheology in water and in model paint formulations thickened

with HM-EHEC: influence of the chemical structure and the distribution of

hydrophobic tails

Karlson, L.; Joabsson, F.; Thuresson, K. Carbohydrate Polymers 2000, 41, 25-35.

IV A rheological investigation of the complex formation between hydrophobically

modified ethyl (hydroxy ethyl) cellulose and cyclodextrin

Karlson, L.; Thuresson, K.; Lindman, B. Carbohydrate Polymers 2002, 50 , 219-226.

V Cyclodextrins in HM-PEG Solutions. Inhibition of Rheologically Active Polymer-

Polymer Associations

Karlson, L.; Thuresson, K.; Lindman, B. Submitted

VI Complex formed in the system hydrophobically modified polyethylene glycol /

methylated α-cyclodextrin / water. An NMR diffusometry study

Karlson, L; Malmborg, C.; Thuresson, K.; Söderman O. Submitted



Chapter 1

Introduction

Aqueous solutions thickened with polymers are common in our daily

life. Shampoo, for instance, is a water-based solution of surfactants

that should have high viscosity, since a low viscosity would mean that

it would flow between the fingers when you poured it out of the bottle.

In cooking there are many examples of how water-soluble polymers

are used for thickening. Starch from potatoes or corn can be used for

thickening of a sauce and gelatin gives the jelly consistency to many

desserts. Polymers are also used as thickener in many low fat

products. Some pharmaceutical formulations are water-based

systems that gain their flowing properties from polymers.

Water-borne paint is another example of an aqueous system that has

to be thickened to behave in the way we want. In fact the use of

Hydrophobically Modified Polymers (HM-P) in paint is the basis for

this thesis and has therefore got a separate section (section 1.1

below).

The aim of this thesis is to provide useful knowledge for the

development of new hydrophobically modified polymers with

improved properties primarily for the paint application. In order to fulfill

this goal the first part of the work is dealing with how hydrophobic

modification influences the properties of the polymers in solution

(PaperI,II

, andIII

). In the second part of the thesis the thickeningmechanisms of HM-polymers in aqueous systems have been

investigated (Paper IV, V and VI).

The discussion in this thesis is based upon two types of HM-

polymers, Hydrophobically Modified Ethyl Hydroxyethyl Cellulose

(HM-EHEC) and Hydrophobically Modified Ethoxylated Urethane

(HEUR). HM-EHEC is an example of a HM-polymer with a water-

soluble backbone, and hydrophobic groups attached along the

backbone (Figure 1.1.a). HM-EHEC has a relatively high molecular

3



weight (mw) and the thickening mechanism of HM-EHEC may include

contributions both from chain entanglement and associations

between different hydrophobic parts of the molecule.

a

b

Figure 1.1. Schematic

illustration of the structure

of a HM-EHEC and b

HEUR. White necklace

represents hydrophilic

monomers and the bold lines

represent hydrophobic

groups.

Hydrophobically modified Ethoxylated Urethane (HEUR) polymers

have a water-soluble backbone with relatively low mw and

hydrophobic groups attached at both ends of the backbone (Figure

1.1.b). In a solution of a HEUR polymer the thickening effect relies

mainly on hydrophobic associations and entanglements are expected

to be of very small importance.

One way to obtain information about the thickening mechanisms of

HM-polymers in aqueous systems is to synthesize both the HM-

polymer as well as the unmodified version of the same polymer and

study the difference in solution behavior. This has been the subject of

numerous studies.1-7

Another way to study the thickening mechanism is by addition of a

third component capable of selectively inhibiting one or more of the

mechanisms that contribute to the thickening effect. For instance it is

well known that, depending on the concentration, addition of

surfactant can either increase or decrease viscosity of a solution of a

HM-polymer.1,8-23 At high surfactant concentrations associations

between hydrophobic parts of the polymer chains are disrupted.

However, this method is unselective and is expected to inhibit all

types of hydrophobic interactions (including both interactions from

polymer hydrophobic tails as well as from hydrophobic patches of the

main chain). A much more selective method to disrupt only some

types of hydrophobic interactions is offered by addition of

4



cyclodextrins, a group of cyclic substances with a hydrophobic cavity

in an otherwise hydrophilic molecule.24,25 In an aqueous

environment the hydrophobic cavity of the cyclodextrin can host a

hydrophobic molecule or a hydrophobic part of a molecule provided

that it fits into the geometry of the cavity. A hydrophobic group of a

HM-polymer that has formed a complex with a cyclodextrin molecule

does not take part in the thickening mechanism.26-28 In this way it is

possible to distinguish between the contributions to the hydrophobic

associations by different parts of the HM-polymer.

1.1 Hydrophobically modified polymers in paint

This section will summarize some properties that are important for the

paint industry and that can be controlled by the choice of thickener. A

water borne paint consists of several ingredients and an example of a

simple recipe for a water borne paint can be found in Table 1.1. Even

though the thickener constitutes less than 1 % of the paint it is a veryimportant ingredient since it influences many of the paint properties.

10-3

10-2

10-1

100

101

102

103

104

HM-EHEC

High mw EHEC

Low mw EHEC

V i s c o s i t y

Shear rate (s-1)


viscosity profile for threemodel paints formulated with

0.45 %w/w high mw EHEC,

0.9 %w/w low mw EHEC, or

0.45 %w/w HM-EHEC

respectively

5



Normally water borne paint is formulated aiming at a certain Stormer

viscosity. The Stormer viscosity corresponds to the viscosity at a

shear rate (10 – 100 s-1) similar to the shear rate when stirring the

paint in the can, or when pouring the paint. A correct Stormer

viscosity is also important when loading the brush since a too low

viscosity means that the paint will drip off the brush. The Stormer

viscosity is adjusted by the amount of the polymer. For this reason

the polymer concentration may vary widely and depends on the

thickening efficiency of the polymer. Conventional thickeners, with a

high molecular weight, (mw) normally have a high thickening

efficiency and give the required Stormer viscosity with a small

addition of the thickener. However at the same time they give a

strongly shear thinning behavior (Figure 1.2). This means that the low

shear viscosity (<2 s-1) is high whereas the high shear viscosity (>104

s-1) is low. Many important paint properties are influenced by the

shear profile. The low shear viscosity (<2 s-1) is important since it

influences the sedimentation of particles in the can. It also influences

the flow properties in the paint film after application of the paint. The

leveling is improved by a decreased low shear viscosity (Figure 1.3)

but on the other hand the newly applied paint film will start to sag on a

vertical surface if the low shear viscosity is too low (Figure 1.4). The

high shear viscosity influences the thickness of the paint film during

roller application, since the shear rate in the thin layer between the

surface and the roller is high (>104 s-1). Increased high shear viscosity

means that the applied paint film is thicker resulting in better hiding

properties (Figure 1.5) and thereby reducing the number of coats

required. The main advantage of a conventional high molecular

weight thickener is the low concentration that is needed and therebythey become cost effective. However, the strong shear thinning

behavior that results in bad leveling and bad hiding power is a

problem.

Table 1.1. Example of a simple

recipe for a water borne paint

Ingredient (wt‰)

Water 242

Thickener 1

Defoamer 5

Dispersing Agent 6

Preservative 1

Filler 110

Pigment 180

Binder (Latex) 455

Figure 1.3. Example of a

panel from a leveling test. In

this test the surface should

be as smooth as possible.

Figure 1.4. Example of a

panel from a sagging test. In

this test the thickness of the

paint film gradually

increases from the top line to

the bottom line. The sagging

is measured as the film

thickness where the paint starts to sag.

A less pronounced shear thinning viscosity profile can be obtained by

using a thickener with a lower mw and compared to the high mw

thickeners the leveling and the hiding power are improved. A

disadvantage with this approach is that in order to achieve a required

6



Stormer viscosity a much higher polymer concentration is required,

which generates a higher cost.

In general hydrophobically modified polymers combine high

thickening efficiency with a less marked shear thinning viscosityprofile. By varying the length of the hydrophobic groups and

molecular weight of the polymer the viscosity / shear profile can be

controlled. The associative thickeners have a strong thickening effect

and give the required Stormer viscosity already at low addition levels.

Actually in most cases their thickening efficiency is comparable to

what is achieved with non-associative thickeners with a high mw. Both

high shear and low shear viscosities are influenced. Compared to the

type of conventional thickeners with high mw the HM-P:s have a much

less shear thinning profile (Figure 1.2).

Figure 1.5. Result from a

hiding power test

In the paint industry, HM-P:s are often referred to as associative

thickeners. Here hydrophobically modified cellulose derivatives (HM-

HEC and HM-EHEC), HEURs and HM-acrylates are the most

commonly used associative thickeners. There are also important

differences within the group of associative thickeners (Figure 1.6).The

HEUR thickeners together with low mw HM-acrylates give the lowesttendency to shear thinning. They have the lowest low shear viscosity

and they retain a virtually constant viscosity up to high shear rates

where the viscosity suddenly drops off. HM-HEC, HM-EHEC and high

mw HM-acrylates show rheology profiles that are in-between the

HEUR thickeners and the non-associative thickeners. The less shear-

thinning behavior of the associative thickeners results in improved

hiding power and leveling properties.

7



10-3

10-2

10-1

100

101

102

103

10-1

100

101

102

.

2%w/w HM-acrylate

3.5%w/w HM-PEG

3%w/w HM-EHEC

,

* ( m P a s )

(s-1)

Figure 1.6. Viscosity, η ,

(filled symbols) and complex

viscosity, η *, (open symbols)

as a function of shear rate

for three different HM-

polymers

One important advantage of HM-polymers is that the spatter from the

roller when the paint is rolled on a wall or a ceiling is drastically

reduced when the paint is thickened with a HM-polymer compared to

when a conventional thickener is used (Figure 1.7). Improved gloss is

another important parameter that is influenced by the use of an

associative thickener compared to when non-associative ones are

used. In light of this the associative thickeners seem to be a goodchoice.

It has, however, to be recognized that with the associative thickeners

the properties of the paint may change quite dramatically. The major

problem for associative thickeners is their sensitivity to variations in

coating composition. Changes in type of latex, surfactant or co-

solvent concentration, or addition of colorants, can have a

pronounced effect on paint viscosity. This is due to the thickening

mechanism of the associative thickeners that to a large extent is

dependent on associations between the hydrophobic groups on the

thickener, since these also associate with other ingredients in the

paint. The associations are very sensitive to variations in paint

composition. For example the monomer compositions of latex

particles, type of surfactant, and the surfactant concentration all have

a large impact on the paint viscosity. The non-associative thickeners

rely mainly on chain entanglements which are much less influencedby changes in paint composition.

Figure 1.7. Results of

spatter tests with two paints

thickened with HM-polymer

(upper) and conventional

thickener (lower).

8



As will be discussed in section 2.2.3 the addition of surfactants can

either increase or decrease the viscosity of the associative thickener

solution depending on the surfactant concentration in the solution and

what type of surfactant is used. One problem is that the surfactant

content and the type of surfactants included in the paint are often

unknown, even to the paint producer. A large fraction of the surfactant

content in the paint originates from the synthesis of the latex, and

details behind the commercial production of latex are well-hidden

secrets. During the production of paint more surfactant is often added

as a wetting agent for the pigment or to improve the stability of the

paint. Normally the surfactant concentration in the paint is on a level

above where the viscosity maximum occurs, as exemplified in Figure

1.8. Additional surfactant therefore causes a reduction of the

viscosity. Paints formulated with HEUR thickeners are in general the

most sensitive to addition of surfactant since associations of

hydrophobic groups are the only effective thickening mechanism for

the HEUR thickeners in the concentration range used in paint

formulations. Hydrophobically modified acrylates and cellulose

derivatives are less sensitive since they obtain a considerable part of

their thickening power from chain entanglements.

csurf

csurf


illustration of the viscosity ,

η , of a HM-polymer solution

as a function of surfactant

concentration, c surf,.

Colorants used for tinting the paint contain high amounts of

surfactant. The additions of colorants can have a strong impact on

viscosity. In the worst case a paint can lose as much Stormer

viscosity as 30 to 40 KU (30 to 40%) when tinted to a deep-tone

color.

Color acceptance is another parameter of great importance to the

paint industry. A tinted paint can show variations in shade depending

on the magnitude of the shear during the application of the paint. Bad

color acceptance appears as brush-marks, which make the surface

look striped, when the paint is applied with varying shear force from

the paintbrush. In the paint industry the color acceptance is evaluated

in a 'rub-out test' in which one part of the surface of the painted chart

is rubbed while another part is untouched (Figure 1.9). The color

acceptance is judged by means of differences in shade between the

two parts (Figure 1.10). The color acceptance problem becomes

9



more pronounced when hydrophobic pigments are used. The color

acceptance has been attributed to phase separation caused by the

polymer but the problem is not fully understood.

When formulated in paints the associative thickeners are often usedin combinations, both with other associative thickeners and / or non-

associative thickeners. One example is when a HEUR thickener is

added to a paint thickened with a high mw non-associative thickener

to increase the high-shear viscosity.29 But formulating a paint with

several different thickeners can be full of uncertainties since mixtures

of polymers often phase separate. The phenomenon with phase

separation is even more pronounced if one of the polymers is

hydrophobically modified and the other is not.30 This is probably the

cause of some of the flocculation problems that occur when

associative thickeners are tested in paint formulations that contain

more than one thickener.

Figure 1.9. The ”rub-out-

test” for color acceptance

Figure 1.10. Results of color

acceptance test. For a good

result the paint should be as

little affected as possible by

the “rub-out test”. The far

right panel shows a good

result whereas the far left

panel shows a relatively

poor result.

10



1.2 References Chapter 1

(1) Williams, P. A.; Meadows, J.; Phillips, G. O.; Senan, C.Cellulose: Sources and Exploration 1990, 37 , 295-302.

(2) Thuresson, K.; Lindman, B. J. Phys. Chem. 1997, 101, 6460-6468

(3) Wang, K. T.; Iliopoulos, I.; Audebert, R. Polymer Bulletin 1988,20 , 577-582.

(4) Valint, J., P.L.; Bock, J. Macromolecules 1988, 21, 175-179.

(5) Bock, J.; Siano, D. B.; Valint Jr., P. L.; Pace, S. J. In Polymersin aqueous media; Glass, J. E., Ed.; American ChemicalSociety: Washington DC, 1989; Vol. 223, p 411-424.

(6) Glass, E. J. Coatings Technology 2001, 73, 79-98.

(7) Winnik, M. A.; Yekta, A. Current Opinion in Colloid & InterfaceScience 1997, 2 , 424-436.

(8) Gelman, R. A. In 1987 International dissolving PulpsConference; TAPPI, Ed. Geneva, 1987, p 159-165.

(9) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B.Progr. Colloid Polym. Sci. 1992, 89, 118-121.

(10) Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991, 7 , 617-619.

(11) Annable, T.; Buscall, R.; Ettelaie, R.; Shepherd, P.;Whittlestone, D. Langmuir 1994, 10 , 1060-1070.

(12) Loyen, K.; Iliopoulos, I.; Olsson, U.; Audebert, R. Progr. ColloidPolym. Sci. 1995, 98 , 42-46.

(13) Piculell, L.; Thuresson, K.; Ericsson, O. Faraday Discuss. 1995,101, 307-318.

(14) Chen, M.; Glass, J. E. Polym. Mater. Sci. Engin. 1995, 73, 449-450.

(15) Aubry, T.; Moan, M. J. Rheol 1996, 40 , 441-448.

(16) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson,O. Adv. Colloid Interface Sci. 1996, 63, 1-21.

(17) Persson, K.; Wang, G.; Olofsson, G. J. Chem. Soc. FaradayTrans. 1997, 90 , 3555-3562.

(18) Panmai, S.; Prud'homme, R., K.; Peiffer, D., G.; Jockusch, S.;Turro, N., J. Polym. Mater. Sci. Engin. 1998, 79, 419-420.

(19) Nilsson, S.; Thuresson, K.; Hansson, P.; Lindman, B. J. Phys.Chem. 1998, 102 , 7099-7105.

(20) Olesen, K. R.; Bassett, D. R.; Wilkerson, C. L. Progress OrganicCoatings 1998, 35 , 161-170.

(21) Jiménez-Rigaldo, E.; Selb, J.; Candau, F. Langmuir 2000, 16 ,8611-8621.

(22) Chronakis, I. S.; Alexandridis, P. Marcomolecules 2001, 34,5005-5018.

11



12

(23) Steffenhagen, M. J.; Xing, L.-L.; Elliott, P. T.; Wetzel, W. H.;Glass, J. E. Polym. Mater. Sci. Engin. 2001, 85 , 217-218.

(24) Immel, S.; Lichtenthaler, F. W. Starch/Stärke 1996, 48 , 225-232.

(25) Connors, K. A. Chem. Rev. 1997, 97 , 1325-1357.

(26) Akiyoshi, K.; Sasaki, Y.; Kuroda, K.; Sunamoto, J. ChemistryLetters 1998, 1998 , 93-94.

(27) Zhang, H.; Hogen-Esch, T. E.; Boschet, F.; Margaillan, A.Langmuir 1998, 14, 4972-4977.

(28) Gupta, R. K.; Tam, K. C.; Ong, S. H.; Jenkins, R. D. In XIIIthInternational Congress on Rheology Cambrige, UK, 2000, p335-337.

(29) Howard, P.; Leasure, E.; Rosier, S.; Schaller, E. J. CoatingTechnology 1992, 64, 87-94.

(30) Tsianou, M.; Thuresson, K.; Piculell, L. Colloid Polym. Sci. 2001, 279, 340-347.



Chapter 2

Hydrophobically modified polymers

Hydrophobically modified water-soluble polymers (HM-P) are

polymers with hydrophobic groups chemically attached to a

hydrophilic polymer backbone. They are often also referred to as

associative polymers or associative thickeners. The first studies on

HM-P were made by Strauss and coworkers more than 50 years ago.

They are described in a review article.1 The work was done with

hydrophobically modified polyelectrolytes. The idea behind the

studies was that since soap molecules associate to form micelles in

aqueous solution also surfactants chemically grafted to a water

soluble polymer would form micelles. That indeed was what they

found. In addition they found that the “polysoaps” gave unique

solubilizing effects and a surprisingly large increase of the viscosity to

an aqueous solution. These two effects of HM-P are widely utilized.

The largest application for HM-P is as rheology modifier in water

borne paint. Landoll and his coworkers described the first associative

thickeners for water borne paint in the eighties.2-4 They worked with

hydrophobically modified (hydroxyethyl) cellulose (HM-HEC) which is

a nonionic cellulose ether. Hydrophobically modified ethyl

hydroxyethyl cellulose (HM-EHEC), hydrophobically modified

ethoxylated urethanes (HEUR) and hydrophobically modified

polyacrylates (HM-PA) are other examples of associative thickeners

that have been developed for the paint application. Thehydrophobically modified cellulose derivatives are, still after 20 years,

the largest class of associative thickeners for water borne paint.

13



2.1 Structure and synthesis of hydrophobicallymodified polymers

Depending on how the hydrophobic groups are situated in the

molecule HM-polymers can be divided into two categories. The first

has the hydrophobic groups attached at the ends of the polymer

backbone and they are referred to as hydrophobically end-capped

polymers (Figure 1.1.b). The second category has the hydrophobic

groups grafted along the polymer backbone. These are called comb

like HM-polymers (Figure 1.1.a).

a

b

2.1.1 HEUR thickeners

Hydrophobically modified ethoxylated urethanes (HEURs) are

examples of end-capped water-soluble polymers. They consist of a

hydrophilic polyethylene glycol (PEG) segment in the middle with

hydrophobic groups attached at both ends. Compared to other

polymers used as thickeners the molecular weight (mw) of a HEUR

thickener is normally relatively low, 15,000 to 50,000.5 Often the

molecular weight distribution of a commercially available HEUR is

broad due to the synthesis procedure used for the manufacture of the

polymer. Polyethylene glycol of low molecular weight, e.g. 6000, is

reacted with a slight excess of diisocyanate. The resulting polymer

chains with isocyanate groups at both ends are then reacted to a long

chain alcohol (Figure 2.1.a).5 A way to synthesize a HEUR with a

more narrow distribution is offered by the reaction of an alcohol

ethoxylate to diisocyanate (Figure 2.1.b). The HEUR-polymer from

this process has a polydispersity index (weight average molecular

weight ( M w) / number average molecular weight ( M n)) of about 1.1.

This type of HEUR has been used in the present studies and is

referred to as “Triblock” or HM-PEG. It should be mentioned that even

though the present polymers have a low polydispersity index, model

HEUR thickeners with even lower polydispersity index ( M w /M n =1.01)

have been synthesized.6 Here the starting material was PEG, with

narrow molecular weight distribution, which was reacted to alkyl

p-toluene sulphonate at both ends.


picture of the synthesis of

HEUR-polymers. The

necklaces represent

polyethylene glycol chains. Bold lines represent

hydrophobic end groups and

filled balls represent

diisocyanate groups or

diurethane linkages.

14



2.1.2 Hydrophobically modified EHEC

The base for ethyl hydroxyethyl cellulose (EHEC) and for

hydrophobically modified EHEC (HM-EHEC) is cellulose, one of the

most common natural polymers. Cellulose is a polysaccharide built upfrom 1,4-anhydroglucose units (AHG). The cellulose molecules in

native cellulose form large crystalline regions, and therefore cellulose

is insoluble in water. To make cellulose soluble it has to be modified

to split up the crystalline packing. The process for making cellulose

derivatives starts with an alkalization step. The alkalization has two

purposes. Firstly by introducing charges into the molecules, the

cellulose swells. This makes individual cellulose chains available for

the chemical reaction. Secondly it also acts as catalyzation for the

modification reactions. During the synthesis of EHEC the alkalized

cellulose is modified by a reaction with ethylene oxide and then with

ethyl chloride. Both reaction steps are performed at elevated

temperature. Since both ethylene oxide and ethyl chloride are volatile

compounds a pressurized reaction vessel is required.O

OO

O

OH

OH

OO

O

O

OH

OH

O

O

Na+

Na+

Each AHG has three hydroxyl groups available for reaction. The

reaction of one ethylene oxide molecule to one of the hydroxyl groupson an AHG results in a new hydroxyl group that is also reactive

(Figure 2.2). The newly formed hydroxyl group has a reactivity

comparable to that of the hydroxyl groups on the AHG which means

that besides the reaction of the hydroxyl groups on the AHG there is

also a chain growth reaction going on. The outcome is that short oligo

(ethylene oxide) chains are formed.7 The molar substitution of

ethylene oxide ( MS EO) is the average total number of ethylene oxide

groups per AHG (Figure 2.3). For practical reasons the upper limit for

MS EO is about 2.5 to 3 since the efficiency of the reaction decreases

dramatically above that level due to side reactions. Up to this point

about 70 % of the ethylene oxide reacts with cellulose to form ether

groups. The remainder forms glycols by reaction with water, or ethers

of glycols by reaction with ethyl chloride.7

Figure 2.2. The reaction of

ethylene oxide to alkalized

cellulose.

15



OH

O

O

O

OH

O

OH

OH

OOO

OH

O

OH

O

OH

OH

OOO

O

O

O

O

O

OH

O

O

O

O

OH

O

OH

O

Figure 2.3. Possible

structure element of an

EHEC molecule.O

represents a hydroxyethyl group. Ethyl groups are

represented by bold lines.

In this example

MS EO= (4+3+0+2+1)/5 =2

DS ethyl =(2+2+0+1+1)/5=0.8

In contrast to the reaction with ethylene oxide where new hydroxyl

groups form, the ethyl chloride reaction consumes sodium hydroxide

and the hydroxyl group that has reacted to an ethyl chloride is

terminated for further reaction (Figure 2.4). The number of hydroxyl

groups per AHG that has reacted is expressed as degree of

substitution ( DS ) and the figure ranges from 0 to 3. Practically the

upper limit for DS ethyl is about 1 since the water solubility of the final

EHEC polymer decreases dramatically with increasing DS ethyl .8 Of

course the reaction does not give a perfectly homogeneous

substituent-distribution over all AHGs. It is likely that the synthesis

process for EHEC gives an uneven distribution of the hydroxyethyl

and ethyl substituents. Therefore the numbers of DS ethyl and MS EO are

average values. Segments of anhydroglucose units that have a high

degree of ethyl substituents are slightly hydrophobic. In water solution

the ethyl groups can give rise to hydrophobic interactions provided

that they are situated in long sequences. This is an origin of thebackbone associations and the reason why the unmodified EHEC is

surface active and shows an associative behavior.9,10 The situation

is similar for other short hydrophobic groups (C6 or shorter) where an

anhydroglucose unit bearing hydrophobic groups can be seen as a

hydrophobic monomer unit of a copolymer. The cellulose backbone is

relatively stiff and the associations from the short hydrophobic groups

are too weak to force the polymer backbone to bend into a loop

where the hydrophobic groups could intra-aggregate. Instead the

result is inter-associations between hydrophobic segments on

OO

O

O

O

OH

OH

Cl

Na+

OO

O

O

O

OH

OH

Na+

Cl

Figure 2.4. The reaction of

ethyl chloride to alkalized

hydroxyethyl cellulose.

16



different polymer chains, which can be detected as increased solution

viscosity.3,11,12 If the polymer concentration or the flexibility of the

polymer backbone changes the situation may be different.

By reacting aliphatic groups to the EHEC polymer a hydrophobically

modified EHEC is obtained (Figure 2.5). The HM-EHEC obtained in

this way is an example of a comb like HM-P. It has hydrophobic

groups grafted along the water-soluble EHEC backbone. Only a small

amount of hydrophobic groups are required to totally change the

properties of the polymer.3,11 In our study less than 1% of the

glucose units of the EHEC backbone have hydrophobic groups

attached and this was enough to substantially change the solution

properties as compared to those of the corresponding unmodified

EHEC.

OO

ny

OO

y

OO

y

OO

y

n = 1 or 2

O

OO

O

OH

O

O

O

O

O

O

O

O

O

OH

OH

O

OH

O

OOH

O

O

R

OH

OH O

O

O

OH

O

OH

OO

O

O

OH

O

OHOO

OH

OH

O

O

OO

x

(NP)

(C12)

(C14)

(C16)

(C1618)

R=

Figure 2.5. Possible structure segment of the HM-EHEC:s studied in paper III.

R=(NP) for HM-(NP)-EHEC, R=(C 12 ) for HM-(C 12 )-EHEC, R=(C 14 ) for HM-(C 14 )-EHEC,

R=(C 16 ) for HM-(C 16 )-EHEC, and R= a blend of (C 16 ) and (C 18 ) for HM-(C 1618 )-EHEC

17



In paper III we have investigated the effect of various chain lengths of

the hydrophobic groups. Alkyl groups varying from C12 to C16 or a

blend of C16 and C18 or nonylphenol have been used. The HM-EHEC

polymers that were obtained with these hydrophobic groups arereferred to as HM-(C12)-EHEC, HM-(C14)-EHEC, HM-(C16)-EHEC,

HM-(C16-18)-EHEC and HM-(NP)-EHEC, respectively.

The values of MS EO, DS ethyl , and MS hydrophobe for the HM-EHEC:s

included in this study are presented in table 2.1.2

Table 2.1.2. The substitution degrees of ethylene oxide (MS EO ), ethyl (DS ethyl ), and hydrophobic tails

(MS hydrophobe ) of each of the polymer samples given as average numbers of substituents per repeating glucose

unit. Independent repeated determinations render an uncertainty in the numerical values of about 5%. Theabbreviations given in the 'Hydrophobic group' column refers to the unmodified parent EHEC (0), HM-

EHEC modified with, nonylphenol groups (NP), C 12 groups (C 12 ), C 14 groups (C 14 ), C 16 groups (C 16 ), and

with C 16 – C 18 groups (C 1618 ). The values for concentration of hydrophobic groups in the solution, chydrophobe ,

are calculated for 1% w/w solutions.

Hydrophobicgroup

MS EO DS ethyl MS hydrophobe mw/AHG

(g/mol)chydrophobe

(mmolal)

0 2.1 0.8 0 277.0 0

NP 2.1 0.8 0.008 279.7 0.28

C12 2.1 0.8 0.0086 279.9C14 2.1 0.8 0.0082 280.0 0.29

C16 2.1 0.8 0.0081 280.1

C1618 2.1 0.8 0.009 280.9

2.1.3 Comb HEUR

The comb-like HEUR polymers have some interesting properties but

they have not yet received much attention. This may be because they

are complicated to synthesize in a well-characterized way.5 One

possible route to synthesize them is offered by reacting ethoxylated

monoalkylamines (EMAA) to a diisocyanate (Figure 2.6). It is a step

growth reaction and by changing the reaction conditions the

molecular weight of the polymer is varied. The molecule consists of a

number of EMMA-units, each bearing one hydrophobic group and

one amine function. This means that the polymer at low pH has a

18



positive net charge, located close to each hydrophobic group. Two

comb HEUR:s with this structure were studied in Paper II.

+ +

+

+

+++ +

+

+

+

+

+

+++

+++

+++

++++ + + + +Figure 2.6. Schematicillustration of the synthesis

process for comb HEUR

polymers. White necklace

represents a sequence of

hydrophilic monomers and

the bold lines represent the

hydrophobic groups. Filled

balls represent diisocyanate

monomers and balls with a

plus sign represent

protonated amino groups.

Their alkyl group is in both cases a C12 chain, while the length of the

polyethylene oxide spacer between the alkyl groups has been varied.

The polyethylene oxide chains on the alkylamine contain on average

51 or 74 units, respectively. The M w was estimated at about 25 000

for both of the polymers, indicating that they on average consist of

roughly four units. The way they have been produced suggests that

they should have a wide distribution in molecular weight and it was

found that M w /M n was about 2.2 for both these polymers.

2.2 Hydrophobically modified polymers inaqueous solution

The behavior of the polymer molecules in solution depends to a large

extent on the polymer concentration, c. To describe how the behavior

of a HM-P varies with the polymer concentration it is easier to start

the discussion on the behavior of the unmodified parent polymer. The

polymer concentration interval can be divided into three different

regimes, the dilute, the semidilute and the concentrated regime

(Figure 2.7).13 In the dilute regime c is low and the mean centrum to

centrum distance between the polymer coils is larger than the mean

radius of a single polymer coil denoted as the radius of gyration, R g .

19



The individual polymer chains are expected to move independently of

each other in the solution.

In the semidilute regime R g is larger than the mean distance between

the coils. Since the total volume of all polymer coils exceeds the

volume of the solution the polymer coils are forced to overlap and the

concentration where this occurs is often referred to as the overlap

concentration and is denoted c*. The chain of one polymer molecule

will entangle with other polymer molecules (Figure 2.8). The result is

entanglements of the polymer chains and the formation of a transient

polymer network which can be detected as a dramatic increase in the

viscosity of the polymer solution. The overlap concentration can

roughly be estimated as the reciprocal of the intrinsic viscosity,

c*≈1/[η ], and is for most polymers in the region 0.1 to 10 %w/w. The

importance of the entanglements to the dynamics increases with

increasing polymer concentration. The chemical structure of the

polymer is very important for the coil size and thereby for the behavior

of the polymer in solution. An increased mw results in larger coils and

more chain entanglements, which can be seen as increased

viscosity.13 The coil size is also influenced by the chemical

composition of the backbone. A polyethylene glycol based polymer is

more flexible than a polymer with a cellulose origin and has therefore

a smaller coil size.14 The repulsion between the ionic groups makes

the polymer backbone of a polyelectrolyte stiff. The electrostatic

repulsion is strongly influenced by the ionic strength in the solution.

The fact that the viscosity of a polyacrylate solution decreases when

Figure 2.7. Polymer

concentration intervals

dilute solution (c<c*),

semidilute solution (c>c*)

and concentrated solution

(c>>c*)

c<c* c>c* c>>c*

Figure 2.8. Entanglements

of polymer molecules.

20



salt is added can be explained by reduced coil sizes due to increased

flexibility of the polymer chains.11,15

In the concentrated region the system consists of highly entangled

polymer chains. The behavior of the polymer molecules is more

similar to that in a polymer melt than to the behavior in the polymer

network in the semidilute solution.

Describing the behavior of hydrophobically modified polymers it is

important to notice that according to the properties of the unmodified

analogue the HM-P molecule also has the possibility to associate with

other HM-P molecules. The association of the hydrophobic groups is

very similar to self-association of surfactants. To minimize the contactbetween water and hydrophobic groups the hydrophobic groups

associate to each other and form a water-poor domain, which is the

interior of a micelle. The surface of the micelle is covered by the

hydrophilic polymer backbone. In aqueous solution the hydrophobic

groups of a hydrophobically modified polymer associate with each

other resulting in physical bonds holding different parts of the polymer

chains together (Figure 2.9). In a snapshot picture it can be described

as a cross-linked gel but in contrast to covalent bonds the physical

bonds are reversible. They break and reform continuously. A

hydrophobic group on one polymer molecule can either take part of

an intra-molecular association, i.e. it interacts with another

hydrophobic group on the same polymer chain, or interacts with a

hydrophobic group on another polymer molecule (inter-molecular

association) (Figure 2.10). At low concentrations the probability for

interaction between different HM-polymer molecules is small. Intra-

aggregation results in a reduced coil size.1,16-20 The intrinsic

viscosity for a HM-P is therefore often lower than for the unmodified

analogue of the same polymer. Upon increasing polymer

concentration inter-molecular associations become more important

and the three-dimensional network is formed. This gives rise to a

dramatic increase of the solution viscosity. The onset concentration of

inter-molecular association is often well below the overlap

concentration, c*, of the corresponding unmodified polymer with the

same molecular weight.21,22

Figure 2.9. Associations of

hydrophobic groups of HM-

polymer molecules.

a)

b)

a)

b)

Figure 2.10. Illustrations of

inter-molecular association

(a) and intra-molecular

associations (b).

21



The strength of the hydrophobic interactions between polymer chains

is influenced by:

the length of the hydrophobic groups

the molar substitution of hydrophobic groups ( MS hydrophobe)

the distribution of the hydrophobic groups along the polymer

backbone.

Longer hydrophobic groups give an increased residence time of a

hydrophobic group within the micelle and also increased lifetime of

the aggregates of hydrophobic groups. This was illustrated by Sau et

al, who found that if two identical polymers are substituted with

different hydrophobic groups the polymer with the longer hydrophobic

groups gives the highest viscosity to a water solution.4 The results in

section 2.2.2 also illustrate this.

The influence of MS hydrophobe on the solution viscosity can be divided

into three different regions: At low MS hydrophobe there is a positive

correlation between MS hydrophobe and viscosity. This can be explained

by an increased number of inter-connection points holding the

polymer network together. Depending on the structure of the polymer

backbone and the length of the hydrophobic groups there is a

viscosity maximum somewhere typically in the range of 1 to 5

hydrophobic groups per 100 repeating units of the polymer backbone

if a comb like polymer is investigated (Figure 2.11).3 The reason for

the decrease is a conversion of intermolecular associations to intra-

molecular association and a gradual degradation of the polymer

network.20,23 At even higher MS hydrophobe the HM-P becomes insoluble

in water.

0

100

200

300

400

500

0 1 2 3 4

% w/w C 12 hydrophobe

c P

2

Figure 2.11. Brookfield

viscosity of 2% w/w solution

of HM-HEC substituted with

1,2-epoxydodecane as a

function of degree of

hydrophobic modification.

Reproduced from3

The synthesis of HM-P is often performed in a two-phase system

where one phase is an aqueous solution of the polymer backbone

and the other phase consists of the hydrophobic reagent. This gives

rise to a HM-P with a more or less blocky distribution of the

hydrophobic groups along the polymer backbone. Depending on the

type of hydrophobic groups and the length of the hydrophobic

segments the more blocky structure can favor the formation either of

22



intra-associations or inter-associations (Figure 2.10). Provided that

the hydrophobic associations are strong they can force the polymer

backbone to adopt conformations that give rise to intra-molecular

associations. Selb et al have shown that for HM-P with C16-alkyl

groups the viscosity of the polymer with the blocky structure can be

several times less than that of the corresponding polymer with a more

random distribution of the hydrophobic groups (compare to the right

part of the diagram in Figure 2.11).23,24 This is in contrast to what is

described in section 2.1.2 for short hydrophobic groups.0 1 2 3

102

103

104

( c P )

csalt

(%w/w)Hydrophobically modified ionic polymers like HM-PA are strongly

influenced by the salt content in the solution. As mentioned above

increasing salt concentration reduces the repulsion between ionic

groups on the polymer backbone. At the same time the addition of

salt makes the solvent more polar which promotes the hydrophobic

associations (Figure 2.12). At low salt concentrations the increased

interchain cross-linking predominates leading to a viscosity increase.

At higher ionic strength the electrostatic effects prevail and a

reduction in the viscosity occurs (Figure 2.12).15,25

Figure 2.12. Viscosity as a

function of NaClconcentration for 2% w/w

solution of HM-polyacrylate

substituted with C 18

hydrophobic groups (3% of

the repeating units covered).

Reproduced from11

2.2.1 HM-PEG in aqueous solution

The commonly accepted mechanism for the association of the HEUR

thickeners is somewhat different from the one for the comb like

polymers described in the previous section (Figure 2.13).6,26-28 At

very dilute conditions the HM-polymer molecules exist as free

molecules (unimers) or as oligomers with low aggregation

numbers.27 With increasing polymer concentration the polymer

molecules start to form small micelle-like structures with the

hydrophobic parts of the thickener looping back into the micelle,

forming flower-like structures. The onset of micelle formation

generally occurs already at polymer concentrations far below the

overlap concentration of the unmodified analogue of the polymer (c*).

The formation of micelles becomes more cooperative with increasing

length of the hydrophobic groups.28 The unfavorable entropy caused

23



by bending the hydrophilic backbone into a loop conformation

opposes the micelle formation. Consequently the formation of flower

micelles is favored by longer hydrophobic groups and by increased

length of the PEO-spacer as can be seen as a decrease of the

concentration where aggregation starts to occur. Fluorecence

quenching techniques have been used to determine the average

number of hydrophobic groups per micelle ( N R) on a variety of HEUR

polymers.28,29,30,31 It was found that flower micelles are very

uniform in size and contain in the range of 20 to 30 hydrophobic

groups per micelle. Over a wide concentration range N R is

independent of the polymer concentration. N R for HEUR polymers is

considerably lower compared to the aggregation number for related

surfactants forming spherical micelles, which is typically 60 to 80.13

This can probably be explained by the fact that the polymer

backbones of HM-PEG are large head groups which limits the

number of hydrophobic groups that can participate in the same

micelle.

Unimers Flower

Micelles

Clusters Network

increased cHM-PEG

Unimers Flower

Micelles

Clusters Network

increased cHM-PEG

Figure 2.13. Schematic representation of the self-aggregation of HM-PEG as function of increasing

c HM-PEG

24



With increasing polymer concentration the average distance between

the flower micelles becomes smaller and larger aggregates are

formed. The flower micelles can be seen as building blocks for the

formation of larger aggregates. Transient bridges consisting of

HM-polymer molecules with one hydrophobic group in one micelle

and the other end in a neighboring micelle are formed resulting in

clusters of micelles. The driving force for this cross-linking is the

lowering in free energy achieved by allowing some of the thickener

molecules to attain more flexible conformations of the hydrophilic

backbones with no strict need for looping back. In the case of long

hydrophobic groups the aggregation into clusters starts at a

concentration far below close packing of micelles. For the situation

where the attraction forces are weaker (shorter hydrophobic groups)

the aggregation starts at higher concentrations, but still below the

concentration for close packing of micelles. Semenov et al predicted

that these systems at concentrations below close-packed micelles

would phase separate into one phase containing closely packed

micelles and one phase impoverished in polymer.22 However, in our

studies on aqueous solutions of HM-PEG with C16-18 hydrophobic

groups (the structure is described in paperI) no macroscopic phase

separation occurred at room temperature. Instead a microscopic

phase separation has been suggested with polymer rich

microdomains (clusters) in a diluted bulk phase.6,27,28 Contrary to

the micelles, which have rather well defined aggregation numbers, it

is reasonable that the clusters appear in a wide range of sizes and

that the average cluster-size increases with increasing HM-PEG

concentration.6,27,32 The polymer concentration inside the clusters

differs from the average concentration in the solution. One indication

for the solution being inhomogeneous is given by the phase behavior

of triblock solutions which is further described in section 2.2.4.1.

Upon increasing polymer concentrations the distances between

different clusters become smaller which gives the possibility for the

polymer chains to more frequently connect micelles located in

different clusters and a three-dimensional network that extends over

macroscopic distances is formed. This can be detected as a dramatic

increase of the solution viscosity. It occurs at a polymer concentration

25



where the solution is still likely to be very inhomogeneous with large

concentration fluctuations. Due to the large concentration fluctuations

the polymers that connect micelles located in different clusters and

have to span polymer depleted regions are likely to be rare. In

contrast the inter-micellar links within the clusters are much more

numerous. All the physical bonds are temporary and the clusters

continuously break and reform. Therefore the HM-P:s that are

involved in forming bridges between the clusters at one moment can

change to be an intra-micellar link at the next moment .

2.2.2 HM-EHEC in aqueous solution

In paper III we found that the hydrophobic group chain length had a

dramatic effect on the low shear viscosity of aqueous solutions of

HM-EHEC. The C12-group only has a minor effect on the viscosity,

and experiments with shorter hydrophobic groups (not presented)

have shown that the hydrophobic groups should have at least 12

carbon atoms to have any noticeable effect on the viscosity. By

increasing the length of the hydrophobic chains from C12 to C16 the

viscosity increased two orders of magnitude (Figure 2.14). This is in

good agreement with results from earlier studies.4,25,26 This effect is

ascribed to the residence time of the hydrophobic chains in the

“polymer micelles”, which increases for longer hydrophobic groups

and results in slower motions of the polymer molecules and thereby a

higher viscosity.26,33

102

103

104

(C16)(C14)(C12)(NP)(0)

( m P a s )

Figure 2.14. Viscosity of 1%

w/w solutions of HM-EHEC

with varying length of the

hydrophobic groups. (0) represents unmodified EHEC

For grafted HM-P with low MS hydrophobe, like HM-HEC and HM-EHEC,

the average number of hydrophobic groups per micelle ( N R) is low.

The low aggregation number is likely to result from the polymer chain

being a very large head group. The relatively stiff backbone from

cellulose ether prevents formation of loops and the consequence is

that only a small number of hydrophobic groups can take part in the

formation of each micelle. N R for HM(NP)-EHEC and HM-HEC

micelles have been determined to be about five to ten9,33 compared

to 60 to 80 for surfactants forming spherical micelles13 and 20 to 30

for the more flexible HEUR thickeners. The consequence is that

26



rather poor micellar structures are formed with a high degree of

contact between water and hydrophobic groups.

(a)

(b)

(c)

(a)

(b)

(c)

From what has been discussed above, it follows that there are at

least three types of interpolymer crosslinks, that contribute to theformation of the three dimensional network of a HM-EHEC solution

(Figure 2.15). Apart from chain entanglements and associations

between hydrophobic side groups also associations of hydrophobic

segments of the polymer backbone play an important role. The

hydrophobic segments on the EHEC and HM-EHEC backbones have

been ascribed to patches with high substitution density of ethyl

groups, described in section 2.1.2.34 Earlier when different

HM-EHEC batches have been compared it has been assumed that

since all studied HM-EHEC:s were synthesized according to the

same process the substitution pattern should be similar and that the

interactions of hydrophobic backbone segments contribute almost

equally for all HM-EHEC:s. To give a clearer picture of the

contribution from the different types of crosslinks it would be helpful to

have methods to study the contributions separated from each other.

This will be discussed in chapter 3. Figure 2.15. Interpolymeric

cross-links in HM-EHEC solutions. (a) chain

entanglements, (b) associations between

hydrophobic side-chains and

(c) associations between

hydrophobic segments of the

polymer backbone.

27



2.2.3 Interaction between hydrophobically modified polymers

and surfactants

log csurf

log csurf

log csurf


illustration of the influenceof surfactant concentration

on the viscosity of solutions

of HM-polymers.

Hydrophobically modified polymers in aqueous solution interact

strongly with surfactants leading to the formation of mixed micelles. At

concentrations of HM-P corresponding to the semidilute regime of the

unmodified parent polymer it is found that the viscosity passes via a

pronounced maximum when the surfactant concentration is gradually

increased (Figure 2.16).17,18,35-40 The degree of interaction is

determined both by the structure of the surfactant and the nature of

the polymer. As described in section 2.2.1 and 2.2.2 the micellar

structures of HM-P normally have low aggregation numbers

compared to surfactant micelles and the consequence is a quite large

degree of contact between water and the hydrophobic groups. At lowsurfactant concentrations, already far below the cmc of the surfactant,

the surfactant molecules are incorporated in the existing micelles

from the HM-P. Incorporation of surfactant molecules into the micelles

reduces the water hydrocarbon contact. This increases the activation

energy for detachment of a hydrophobic group from the micelle

thereby increasing the residence time of the hydrophobic groups

within the micelles thus leading to stronger associations.9,40 The

viscosity in an aqueous solution of a HM-P depends on the number of

interconnecting links in the network and on the relaxation time. A

28



changed viscosity can be the result of a variation of either of these

parameters, or both. For the end-modified polymers an increased

number of active links has been observed29,41 while for

hydrophobically modified cellulose ethers the effect of increased

viscosity upon addition of surfactant is suggested to be caused mainly

by increased relaxation times.9,33,42 Besides the increased viscosity

the stronger association can also be detected as a dramatic shift to

lower T Cp (compare section 2.2.4.2).

At surfactant concentrations above the viscosity maximum the

number of micelles in the solution increases. This results in an

increased ratio between micelles and hydrophobic groups of the

polymer. In this process the decreased viscosity is a consequence of

the physical network losing some of its connectivity. At high surfactant

concentrations where the number of micelles exceeds the number of

polymer hydrophobic groups in the system there is only one polymer

hydrophobic group in each micelle. At this stage the viscosity is

independent of the surfactant concentration and has a value that is

even lower than for the HM-P solution before addition of surfactant.

How strong the effect is depends on the structure of the surfactant.

Normally nonionic polymers interact more strongly with anionic

surfactants than with nonionic or cationic surfactants. In line with this

it has been found that anionic surfactants give the most pronounced

viscosity increase and also the largest reduction of the viscosity at

excess surfactant.34

Hydrophobically modified polyelectrolytes, for instance HM-PA,

interact strongly with oppositely charged surfactants. The interaction

is caused by a combination of electrostatic attraction and hydrophobic

forces. The strength of the hydrophobic associations increases with

increasing length of the hydrophobic groups on the polyelectrolyte.

With long hydrophobic groups the hydrophobic interactions can be

strong enough to overcome the electrostatic repulsion between the

polymer backbone and surfactants of the same charge resulting in a

net attraction.36,43

29



2.2.4 Clouding

For most substances the solubility increases with increasing

temperature. This is not the case for EHEC and HEUR thickeners.

They both belong to a family of polyethylene oxide containingsubstances that have a reversed relationship between solubility and

temperature.44 The solubility of these substances decreases with

increasing temperature. At temperatures above a critical value a

water solution containing any of these polymers phase separates into

one polymer rich phase and one phase depleted in polymer. The

phase separation can be detected by the scattering of light resulting

in a cloudy appearance of the solution. The temperature where the

solution first becomes hazy is referred to as the cloud point

temperature, T Cp. The process is reversible and decreasing the

temperature below T Cp results in a one-phase situation and a

transparent solution. Many attempts to explain the reversed solubility

phenomenon have been done. One reasonable explanation builds on

conformational changes of the polymer molecules with changing

temperature. The polyethylene oxide chain has a large number of

possible conformations. The conformation with the lowest free energy

in a polar environment (conformation A in Figure 2.17) has a low

statistical weight. At low temperature the low energy conformation will

dominate. Conformation A has a large dipole moment. With

increasing temperature other conformations with higher energy but

also with higher statistical weight will be more and more important.

The higher energy conformations have a lower dipole moment and

conformation B in Figure 2.17, for instance, has virtually no dipole

moment. The consequence is that the polyethylene oxide chain

becomes less and less polar with increasing temperature. This gives

an increasing tendency to phase separation since water-polymer

interactions become less favorable with increasing temperature.44,45

CO

CC

OC

HH

H H

A

B

O

C C

O CC

HH

HH

Figure 2.17. Different

conformations of an ethylene

oxide group. Conformation

A has low energy and is

more polar compared to

conformation B .

Phase behavior studies give the possibility to study the influence of

other substances on the interaction between the polymer chains.

Addition of a third water soluble component can have a large impact

on the T Cp

.45 For instance most salts decrease the T Cp

(salting out)

but some salts with large anions, e.g. I- and SCN-, have the opposite

30



effect (Figure 2.18). The addition of a salt that does not interact with

the polymer molecules results in a more polar environment and

thereby stronger hydrophobic interactions and increased tendency for

phase separation. On the contrary the large polarizeable anions I- and

SNC- interact with the unpolar parts of the polymer molecules

resulting in an increased entropic penalty of phase separation.

0 2 4 6 8 1060

70

80

90

( T

C p

)

( ° C )

csurf

(mmolal)

0 1 2 330

40

50

60

70

80

NaCl

NaSCN

T C p

( ° C )

c Salt

(M)

Figure 2.18. T Cp as a function of salt

concentration for 0.9% w/w solution of

EHEC. Reproduced from45

Figure 2.19. T Cp as a function of sodium

dodecyl sulphate concentration for 0.9%

w/w solution of EHEC. Reproduced

from45

Surfactants are another type of substance that strongly influences the

phase separation temperature. Depending on the surfactant

concentration, c surf , and type of surfactant, addition of surfactants can

either increase or decrease the T Cp. Upon progressively increasing

the surfactant concentration, c surf , of the ionic surfactant C12SO4Na

(SDS), T Cp is found to decrease initially (Figure 2.19). At slightly

higher c surf the T Cp passes through a minimum and at even higher c surf

T Cp increases. The trend is similar for addition of other micelle forming

surfactants provided that the surfactant molecules associate with the

polymer. If there is no association between polymer and surfactant

the result can be a segregative phase separation with the polymer

enriched in one phase and the surfactant in the other phase.

20 30 40 50 6010

20

30

40

50

( P a s )

T (°C)

Figure 2.20. Complex

viscosity as a function of

temperature for a model

paint thickened with EHEC.

The phase separation can be

detected as a step decrease

in viscosity.

Since T Cp is strongly influenced by added surfactants it is also likely

that other surface-active ingredients have a large impact on the

phase separation temperature. It is therefore not sufficient to measure

the T Cp in water to predict the phase separation temperature for a

31



paint. Since the paint is a dispersion of particles rather than a clear

solution it is not possible to use the normal cloud point measurements

to detect the phase separation. As illustrated in Figure 2.20 the phase

separation can instead be determined as a dramatic viscosity

decrease when the temperature is increased.

2.2.4.1 Cloud point of HM-PEG

0 1 2 3 4 5 6 7 80

20

40

60

80

100

120

1

2

T C p

( ° C )

c polymer

(% w/w)

Figure 2.21. T Cp as a

function of polymer

concentration for C 1618-

(EO)140 polymer (open

circles) and for C 1618-

(EO)140-IPDU-(EO)140-C 1618

polymer (filled circles).

HM-PEG with hydrophobic groups at both end show a dramatic drop

in the T Cp compared to unmodified PEG or PEG that is only modified

at one end (Figure 2.21). 6,30,46 T Cp also strongly depends on the

polymer concentration and the cloud point curve as a function of

polymer concentration passes via a minimum. The effect of T Cp

depression by introducing hydrophobic groups to the polymer

structure is very strong and cannot be explained only by the small

shift in hydrophobic/hydrophilic balance between the polymers. It is

more likely that it depends on the strength of the hydrophobic

associations holding the polymer network together and restricting the

swelling of the polymer matrix. The formation of one concentrated

phase in equilibrium with one phase depleted in polymer requires the

hydrophobic associations to be strong enough to compensate for the

entropic loss following the formation of the concentrated phase.

32



2.2.4.2 Cloud point of EHEC and HM-EHEC

0

20

40

60

(C16

)(C14

)(C12

)(NP)(0)

T C p

( ° C

)

For EHEC T Cp is correlated to MS EO and DS ethyl , and T Cp increases with

increasing MS EO and decreases with increasing DS ethyl .8 The cloud

point is dramatically influenced by the introduction of hydrophobic

groups on the EHEC polymer. As an example T Cp decreased by 15°C,

from 65 to 50°C, when on average about one out of 120 glucose units

of the unmodified EHEC (0) was grafted with nonyphenol groups

(NP). On a typical HM-EHEC molecule this corresponds to five to ten

hydrophobic groups. As can be seen in Figure 2.22 the shift in T Cp is

even stronger when the EHEC is modified with alkyl groups. The

longer the alkyl chain, the more pronounced is the shift in T Cp. The

large difference in T Cp between the polymers indicates that the

strength of the hydrophobic association is much larger for longer

hydrophobic groups. Also the effect on the solution viscosity of the

polymers reveals large variations in the strength of the associations

(compare section 2.2.2).47

Figure 2.22. T Cp of 1% w/w

solutions of HM-EHEC with

varying length of the

hydrophobic groups. (0) represents unmodified EHEC

33





(21) Schaller, E. Surface Coatings Australia 1985, 6-13.

(22) Semenov, A. N.; Joanny, J.-F.; Khokhlov, A. R. Macromolecules 1995, 28 , 1066-1075.

(23) Volpert, E.; Selb, J.; Candeau, F. Macromolecules 1996, 29,1452-1463.

(24) Selb, J.; Candau, F. In Associative polymers in aqueous media;Glass, J. E., Ed.; American Chemical Society: Washington DC,2000; Vol. 765, p 95-108.

(25) Wang, T. K.; Iliopoulos, I.; Audebert, R. In Water-soluble polymers: synthesis, solution properties and applications;Shalaby, Ed.; American Chemical Society: Washington D.C.,1991, p 218-231.

(26) Annable, T.; Buscall, R.; Ettelaie, R.; Whittlestone, D. J. Rheol. 1993, 37 , 695-726.

(27) Alami, E.; Almgren, M.; W., B. Macromolecules 1996, 29, 2229-2243.

(28) Winnik, M. A.; Yekta, A. Current Opinion in Colloid & InterfaceScience 1997, 2 , 424-436.

(29) Persson, K.; Wang, G.; Olofsson, G. J. Chem. Soc. FaradayTrans. 1997, 90 , 3555-3562.

(30) Alami, E.; Almgren, M.; Brown, W.; Francois, J.Macromolecules 1996, 29, 5026-5035.

(31) Vorobyova, O.; Winnik, M. A. In Associative polymers inaqueous solution; Glass, J. E., Ed.; American Chemical

Society: Washington DC, 2000; Vol. 765, p 143-162.(32) Xu, B.; Li, L.; Yekta, A.; Masoumi, Z.; Kanagalingam, S.;

Winnik, M., A.; Zhang, K.; Macdonald, P., M.; Menchen, S.Langmuir 1997, 13, 2447-2456.

(33) Piculell, L.; Nilsson, S.; Sjöström, J.; Thuresson, K. In Assosciatve polymers in aqueous media; Glass, J. E., Ed.; American Chemical Society: Washington DC, 2000; Vol. 765, p317-335.

(34) Thuresson, K.; Lindman, B. J. Phys.Chem. 1997, 101, 6460-6468.

(35) Shaw, K. G.; Leipold, D. P. J. Coatings Technology 1985, 57 ,63-72.


(37) Dualeh, A. J.; Steiner, S. A. Macromolecules 1990, 23, 251-255.

(38) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B.Progr. Colloid Polym. Sci 1992, 89, 118-121.

(39) Nystrom, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11,1994-2002.


35



36



(43) Iliopoulos, I. Curr. Opin. Colloid Interface Sci. 1998, 3, 493-498.

(44) Karlström, G. J. Phys. Chem. 1985, 89, 4962-4964.

(45) Karlström, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990,94, 5005-5015.

(46) Thuresson, K.; Nilsson, S.; Kjoniksen, A.-L.; Walderhaug, H.;Lindman, B.; Nystrom, B. J. Phys. Chem. 1999, 103, 1425-1436.

(47) Thuresson, K.; Joabsson, F. Colloids and Surfaces A:Physicochem. Eng. Aspects 1999, 151, 513-523.



Chapter 3

Inhibition of hydrophobic associations as a

tool to study cross-linking mechanisms

Many studies have tried to investigate how the hydrophobic

modification influences the solution properties of a polymer. Of course

it is possible to get an idea of the strength of the associations of

hydrophobic groups by synthesizing both the hydrophobically

modified polymer and its unmodified analogue. This was the

approach of many of the early studies.1-5 However, if both polymers

are synthesized in separate reactions it is possible that their

structures differ by more than just the hydrophobic modification.

Sometimes this problem can be circumvented by using the

unmodified polymer as starting material in the synthesis. It is likely

that with this approach the HM-P and the parent polymer differ in

molecular weight since an additional reaction step often leads to a

degradation of the polymer backbone. By decoupling the polymer

network it is possible to gradually move in the direction of the

unmodified system. The decoupling can be achieved by changing the

solvent quality or by the addition of a third component (co-solute).

Surfactants and cyclodextrins are examples of co-solutes that

dramatically change the strength and number of the associations.

Different information can be achieved by using the different methods.

By changing the solvent quality or by the addition of an excess of

surfactant all types of hydrophobic associations can be decoupled. It

would therefore be desirable to have a method to specifically

disconnect associations caused by hydrophobic side chains. The

addition of cyclodextrin, on the other hand, offers the possibility to

specifically decouple the associations originating from hydrophobic

side chains. This is for example particularly useful for the evaluation

of HM-EHEC since the associative interactions originate both from

associations of hydrophobic segments of the polymer backbone and

from associations of hydrophobic side groups. The different methodsare discussed in more detail in the following sections.

37



3.1 Inhibition of hydrophobic interactions bychanging solvent quality

Hydrophobically modified polymers have a much stronger tendency to

associate in water than in other (less) polar solvents, e.g. alcohols

and glycols.4,6 This is to be expected since the driving force for

association is to minimize contact between the hydrophobic moieties

of the HM-P and the solvent molecules and this becomes less

important when the polarity of the solvent is reduced. It is also in

agreement with what is found for self-assembly of surfactants.7 Upon

gradual addition of a less polar solvent to an aqueous polymer

solution the intermolecular hydrophobic associations are broken since

it becomes less important to avoid the contact between the

hydrophobic tails and the solvent. In Figure 3.1 the viscosity of 1 %

w/w HM-(C14)-EHEC and 1 % w/w HM-(NP)-EHEC solutions are

given as a function of the concentration of diethylene glycol

monobutylether (BDG), c BDG, in the solvent. Since the viscosity of the

solvent changes with changing ratio between BDG and water the

viscosity is presented as the relative viscosity, η rel = η / η solvent , where

η solvent in each point is the viscosity of the solvent at that specific

BDG/water ratio.

0 5 10 15 20 25 3010

1

102

103

HM-(C14

)-EHEC

HM-(NP)-EHEC

r e l

c BDG

(%w/w)

Figure 3.1. The influence of

c BDG on the viscosity of 1%w/w

solutions of HM-EHEC.

A “saturation level” where η rel is independent of c BDG is reached at

about 15 % w/w BDG. Above that BDG concentration the relative

38



viscosity of the polymer solution is constant. In paper III a BDG/water

ratio of 20 / 80 of was used, and in the following text viscosity

measured in such a solution is referred to as η BDG. The fact that both

HM-(C14)-EHEC and HM-(NP)-EHEC have a η BDG that is almost the

same as η BDG for the corresponding unmodified EHEC indicates that

the hydrophobic interactions from the hydrophobic modification are

totally decoupled. A way to represent the influence by the

hydrophobic interaction is Q BDG which is the ratio between the value of

the (Newtonian) viscosity in water η to that observed in water/BDG,

η BDG (equation (3.1)).

BDG

BDGQ η

η

= (3.1)

Q BDG can be regarded as a phenomenological measurement of the

influence of hydrophobic associations on the viscosity in the aqueous

solution. In this way different polymer samples (regarding chemical

structure of the hydrophobic tails, modification degree, modification

pattern etc.) can be ranked. With this method it is evident that the

unmodified EHEC also has a contribution to the viscosity originating

from hydrophobic interactions. This was observed as a small but

significant Q BDG of about 1.2 (Figure 3.2). Since this polymer has no

hydrophobic grafts the origin of the interactions has to be sought

elsewhere. As described in section 2.2.2 the uneven distribution of

ethyl substituents results in hydrophobic segments along the

backbone and it is likely that the blocky structure causes hydrophobic

associations. The low Q BDG of 1.2 indicates that the strength of these

interactions is much weaker than those given by the hydrophobic

grafts, provided that the length of the hydrophobic groups is C12 or

longer. Since all hydrophobic associations are disconnected and

chain entanglements are the only remaining interpolymer cross-links

in the solution, η BDG can be used as a measure of the chain

entanglement contribution to the viscosity.

39



Figure 3.2 shows the influence of the length of the hydrophobic tails

on the solution viscosity and Q BDG of some HM-EHEC:s. The strength

of the association of the hydrophobic grafts is strongly dependent of

the length of the hydrophobic groups as can be seen from the Q BDG.The values of Q BDG for HM-EHEC grafted with short hydrophobic

groups (C12 or NP) are 2.5 and 3.4 respectively whereas it increases

dramatically for the longer hydrophobic groups (C14).

0

2

4

6

8

10

12

14

16

(C12

)(0) (NP) (C14

)

Q B D G

Figure 3.2. Q BDG for

HM-EHEC with different

hydrophobic groups. From left

to right; unmodified EHEC (0) , HM-EHEC modified with

nonylphenol groups (NP) , withC 12 and C 14 groups.

With this method it is not possible to separate contributions to the

viscosity from associative interactions of different origin, since it was

found that the contribution from grafted hydrophobic groups as well

as the contribution from a hydrophobic polymer backbone wasaffected by the addition of BDG.

3.2 Inhibition of hydrophobic interactions byaddition of surfactant

In section 2.2.3 is described the influence of surfactant on the

associative behavior of HM-polymers. At high concentration of

surfactant the number of micelles exceeds the number of hydrophobic

groups of the polymer which means that on average each micelle

contains only one hydrophobic group from a HM-polymer, as

illustrated in Figure 3.3. The result is that the hydrophobic

associations between HM-polymers are decoupled and the polymer

network is disconnected. This can be detected as a decreased

solution viscosity and increased self-diffusion of the polymer

molecules.4,8-23

The viscosity and self-diffusion in a solution of aHM-polymer at excess surfactant are expected to attain the same

values as for a solution of the corresponding unmodified polymer

(provided that the molecular weight is the same). This has for

example been illustrated for HM-HEC and unmodified HEC.19 Figure 3.3. Schematic picture

of HM-P at high c surf where the

associations between

hydrophobic side chains are

decoupled by surfactant.

As described in section 2.2.2 the hydrophobic associations of

HM-EHEC consist of associations between hydrophobic side chains

as well as interaction of hydrophobic segments of the polymerbackbone. Thuresson et al have shown that not only HM-EHEC but

40



also unmodified EHEC is affected by the addition of surfactant.5 The

suggested explanation is that the surfactant associates both to the

hydrophobic segments of the main chain and to hydrophobic side

chains. At excess surfactant the viscosity of solutions of the

hydrophobically modified polymer and the unmodified parent polymer

attains the same value. In analogy with the effect of addition of BDG,

discussed above, the observation that the viscosity for a solution of

the parent polymer is lower compared to when no surfactant is added

indicates that the associations from hydrophobic segments of the

backbone are decoupled by addition of surfactants. Surfactants

cannot be used to selectively decouple any of the types of

hydrophobic associations. Similarly to the solvent approach (section

3.1) it is therefore not possible to distinguish between the contribution

from hydrophobic associations of the polymer backbone and the

contribution from associations of hydrophobic side chains by this

method.

3.3 Inhibition of hydrophobic interactions by

cyclodextrinCyclodextrin (CD) is a cyclic molecule with a hydrophobic cavity

(Figure 3.4 and 3.5). CD binds selectively to hydrophobic molecules

or parts of molecules that fit into the cavity. In aqueous solutions of

HM-polymers the CD molecules bind primarily to hydrophobic side- or

end-groups and not to hydrophobic segments of the backbone.

Therefore addition of CD provides a unique possibility to specifically

decouple the association caused by hydrophobic groups grafted to

the polymer backbone. The deactivation of hydrophobic associations

by CD gives unique information about the association mechanisms of

HM-polymers that cannot be achieved by deactivation at excess

surfactant or by changed solvent quality.

O

OH

OHOH

O

O

OH

OH

OH

O O

OHOH

OH

O

O

OH

OH OH

O

O

OH

OH

OH

OO

OHOH

OH

O

Figure 3.4. Chemical structure

of α-cyclodextrin.

41



3.3.1 Structure and properties of cyclodextrin

O

H

CH

C

O

O

H

OH

CH

C

C

H

C

O

H

H

H

H

Hydrophobic cavity

O

H

CH

C

O

O

H

OH

CH

C

C

H

C

O

H

H

H

H

O

H

CH

C

O

O

H

OH

CH

C

C

H

C

O

H

H

H

H

Hydrophobic cavity

Cyclodextrins (CD:s) are cyclic oligomers of α-D-glucose. Three

different CD:s, denoted α-, β-, or γ - cyclodextrin, are naturally

occuring and they consist of 6, 7 or 8 glucose units respecively.24,25

They are synthesized by enzymatic degradation of starch. Their

chemical structure is very rigid and the three-dimensional shape can

be described as a shallow truncated cone with a cavity in the center

extending from one end to the other (Figure 3.5).24,25 The exterior of

the cone is hydrophilic since all the hydroxyl groups of the AHGs are

located there while the cavity has non-polar properties. The size of

the cavity varies depending on whether it is α-, β-, or γ - cyclodextrin.

Some useful physical properties of the different cyclodextrins are

listed in table 3.1.


representation of the geometry

of a cyclodextrin molecule.

By substitution, the physical properties of the cyclodextrins can be

changed. Substitution with methyl- (M-) or hydroxypropoxyl (HP-)

groups has been used to increase the solubility of CD in organic

solvents. As a natural consequence of the location of hydroxyl groups

the substituents will be located on the rims of the molecule, resulting

in an increase in the height of the torus. More surprising is that the

diameter of the cavity is reduced by the derivatization.26 The overall

result of the derivatization is that the cavity volume increases. Acetyl

(Ac-) is used to increase the solubility of β-cyclodextrin in water,

which in its natural form has a quite poor aqueous solubility.

Somewhat unexpected it is found that also “slighly hydrophobic”

substituents like methyl and hydroxypropyl increase the water

solubility (Table 3.2.). A reduced possibility to form crystalline

structures is the most probable reason (compare with native cellulose

that is insoluble while methyl cellulose and hydroxypropyl cellulose

are soluble).27

42



Table 3.1. Properties of α -, β -, and γ - Cyclodextrin.

Cyclodextrin Number ofglucose units

Molecularweight

Cavitydiameter (Å)

Torus hight(Å)

α- 6 972 4.7 – 5.3 8

β- 7 1135 6.0 – 6.5 8γ - 8 1297 7.5 – 8.3

Per-O-methyl-α 6 1224 4.2 11

Per-O-methyl-β 7 1429 5.8 11

Data obtained from26 and 24

Table 3.2. Solubility in water at 25°C for α -, β -, and γ - cyclodextrin substituted with methyl (M-),

hydroxypropyl (HP-), and acetyl- (Ac-) groups

Cyclodextrin Degree of

substitution

Solubility in

water at 25°C(g/100ml)

α- - 14a)

β- - 2a)

γ - - 23a)

M-α- 1.8 388b)

M-β- 1.8 300b)

HP-β- 0.75 200b)

Ac-β- 1 220b)

M-γ - 1.8 330b)

HP-γ - 0.6 180b)

a) Data obtained from24

b) Data supplied by Dr Stephan Neuman, Wacker-Chemie GmbH, Germany

3.3.2 Formation of inclusion complex between lipophilic guest

molecules and cyclodextrin

In an aqueous solution a less polar guest molecule readily substitutes

the polar water molecules inside the cavity provided that the unpolar

molecule has the correct dimensions to fit within the cavity (Figure

3.5). This hydrophobic attraction drives the formation of an inclusion

complex. The complex formation has frequently been studied and

surfactant/cyclodextrin systems especially have received a lot of

attention. Various methods, e.g. calorimetry or surfactant selective

electrodes, have been used to determine complex constants.28-34

The complex constants for the formation of an inclusion complex

43



between α-CD or β-CD and some commonly used surfactants are

listed in Table 3.3.


representation of the inclusion

of a lipophilic group into the

cavity of a cyclodextrin

molecule. The filled balls

represent water molecules.

The changed shape of the cavity, as a result of the derivatization of

the CD, influences the ability for the modified CD:s to form a complex

with another substance. Therefore the complex constants for the

modified CD:s differ from the constants from the corresponding

unmodified CD:s.26 An increased length of the cavity often results in

a stronger tendency for complex formation. On the other hand a

reduced cavity diameter from the derivatization results in a reduced

ability to form complex with bulky hydrophobes e.g. aromatic

groups.26

The fact that lipophilic molecules can hide inside the cavity of an

otherwise hydrophilic and water-soluble molecule has given

cyclodextrins many technical applications. One obvious application is

to enhance water solubility of poorly soluble substances but it has

also been used to mask unpleasant odors and tastes and to reduce

the vapour pressure of volatile organic compounds dissolved in

water.24

44



Table 3.3. Complex formation constant, K 1 (mM -1 ), for α -CD and β -CD in combination with sodium

dodecyl sulphate (SDS), sodium tetradecyl sulphate (STS), sodium hexadecyl sulphate (SHS), dodecyl

trimethyl ammonium bromide (DTAB), tetradecyl trimethyl ammonium bromide (TTAB) and cetyl

trimethyl ammonium bromide (CTAB).

Surfactant K 1 Reference

-CD β-CD

DTAB 23.7 30

TTAB 61.0 39.8 30

CTAB 99.2 67.7 30

SDS 25.6 33

STS 48.2 33

SHS 53.3 33

3.3.3 Cyclodextrin and HM-P

The hydrophobic tails of an HM-polymer in an aqueous solution can

form inclusion complexes with added cyclodextrin molecules. This

leads to a disruption of the physical bonds holding the three

dimensional polymer network together (Figure 3.6). This can be

detected as a reduction in the viscosity of the polymer solution. This

is similar to the effect of the addition of excess surfactant or by

changes of the solvent quality to a less polar system as discussed in

section 3.1 and 3.2. Eisenhart and Lau and their coworkers were the

first to report the viscosity reducing effect by the addition of

cyclodextrin in two patents.35,36 They used the inhibition of

hydrophobic interactions to reduce the viscosity in highly

concentrated solutions of associative thickeners. The reduced

viscosity is desired during production and handling (pumping etc.) of

the thickener or at other occasions when the polymer is present at

high concentration and therefore gives very high viscosity. The

complexation is reversible and by addition of e.g. a surfactant with

higher affinity to the cyclodextrin the thickening effect can be

regained.

Figure 3.6. A schematic

representation of the

disruption of the polymer

network following the complex

formation between

cyclodextrin and polymer

hydrophobic tails.In some later papers it has been reported that the degree of

association in solutions of hydrophobically modified polymer can be

45



controlled by addition of cyclodextrin.37-39 The viscosity is reduced

with increasing CD concentration (cCD) and levels off at a CD/HM-P

ratio where all hydrophobic interactions are inhibited.37,38 At excess

CD the HM-P molecules are expected to be unable to associate to

each other. This can be used if the molecular weight of the

HM-polymers should be determined by techniques such as light

scattering methods or by gel permeation chromatography (GPC).

Islam and coworkers have demonstrated how the use of cyclodextrins

simplifies the determination of the molecular weight of a

hydrophobically modified polyacrylate by preventing self-

association.40

3.3.3.1 Cyclodextrin and HM-EHEC

In paper IV we have studied the formation of an inclusion complex in

aqueous solution between cyclodextrin and the hydrophobic groups

grafted on EHEC. We found, in agreement with earlier studies37,38

that in the region where cCD is low compared to the total concentration

of polymer hydrophobic groups in the solution (chydrophobe), the viscosity

decreases with increasing CD concentration in the solution (Figure

3.7). At cCD > chydrophobe the viscosity levels off and attains a constant

value. Three different cyclodextrins, methyl-α-CD, β-CD and methyl-

β-CD, were used in combination with two HM-EHEC samples with

either nonyl phenyl (HM-(NP)-EHEC) or tetradecyl (HM-(C14)-EHEC)

hydrophobic groups. By representing the complex formation within a

Langmuir adsorption model and assuming that 1:1 “nut and bolt”

complexes are formed the concentration of “adsorption sites”, B, and

the complex constant, K , can be determined. A detailed description of

how equation (3.2) is derived is found in the appendix of paper IV.

B

Bc K c B K c B

CDCDCD −

++−

++

−=−

−

∞

∞ 4

)/1(

2

/1

1

2

0 η η

η η (3.2)

46



The viscosity without CD is represented by η 0 and the viscosity at

excess CD by η ∞ . From fitting equation (3.2) to our experimental data

points (∞( ) η 0 ∞( ) vs. cCD) with K and B as fitting parameters, B

and K could be determined.

η −η −η

0 1 2 3

0.0

0.2

0.4

0.6

0.8

1.0

8

8

(

)

0

)

cCD

(mmolal)

Figure 3.7. Relative viscosity

as a function of the

concentration of methylated- β -

cyclodextrin, cCD , of 1% w/w

solutions of HM-EHEC. Open

symbols represent HM-(NP)-

EHEC and filled symbolsrepresent HM-(C 14 )-EHEC.

The full lines represent a fit of

Equation (3.2) to the data.

In table 3.4 it can be seen that the complex constant, K , is very much

influenced both by the shape of the polymer hydrophobic group and

the structure of the CD. For the HM-EHEC with C14-hydrophobic

groups the highest values of K are found for the methylated

cyclodextrins. As described in chapter 3.3.1 methylation of a CD

makes the cavity deeper and narrower. This indicates that the long

and relatively thin C14 hydrophobe fits better into the longer and more

narrow cavity of a methylated cyclodextrin. The values of K are

slightly lower but in the same range as those found for the complex

formation between CD and surfactants containing C14 alkyl groups

(see table 3.3.). This is reasonable since the backbone of HM-EHEC

is an extremely large and bulky head-group that is likely to oppose

the complex formation.

Compared to the HM-(C14)-EHEC the values of K are in general lower

for the HM-(NP)-EHEC. On the other hand it seems that the more

bulky nonyl phenol group fits best into the wider cavity of the β-CD as

indicated by the highest K for β-CD in combination with HM-(NP)-

EHEC.

47



Table 3.4. Complex formation constant, K, and concentration of “adsorption sites”, B, obtained

by fitting Equation (3.2) to the experimental data. η ∞ is the viscosity at excess CD and η 0 is the

viscosity when no CD is added .

HM-EHEC CD K B 0 ∞

(mmolal-1

) (mmolal) (mPa s) (mPa s)NP M-α 2.7 0.27 440 115

β 22.6 0.25 440 105

M-β 17.0 0.26 440 105

C14 M-α 44.0 0.32 1439 50

β 11.2 0.31 1439 80

M-β 66.0 0.30 1439 80

0 M-α - - 45 45

For HM-(C14)-EHEC the concentration of binding sites, B, obtained

from the model (table 3.4) almost perfectly matches the concentration

of hydrophobic tails (chydrophobe = 0.29 mmolal) obtained from chemical

analysis. This gives an indication that all hydrophobic groups are

potentially important for the formation of the polymer network and that

all hydrophobic tails can form a complex with CD. With NP the

situation is different, and it can be concluded that the values of B are

lower than the total concentration of hydrophobic groups for HM-(NP)-

EHEC. Judging from the values of B and (chydrophobe = 0.28) 5 to 10% of

the hydrophobic groups are not available for complex formation with

CD. The nonylphenol used for the synthesis of HM-(NP)-EHEC is of

technical quality that contains both mono- and di-nonyl phenol. In

mono-nonyl phenol the nonyl group can be situated either in ortho

position or para position on the phenol ring. It is possible that the 5 to

10% that is not available for complex formation consists of di-nonyl

phenol and ortho-nonyl phenol. They have the most bulky structure

and are therefore more difficult to fit into the cavity of CD.

Considering the size of the hydrophobic segments of the backbone of

HM-EHEC it is reasonable to assume that CD is not capable of

decoupling the associations of such segments. The fact that the

viscosity of a solution of the unmodified EHEC was not affected at all

by addition of CD suggests this.

48



The viscosity at excess CD, η ∞ , also tells something about the ability

of the CD to decouple the polymeric network. For M-α-CD the value

of η ∞ is almost equal to the viscosity for a solution with the same

polymer concentration of the unmodified EHEC (HM-(0)-EHEC) with

the same molecular weight. This is an indication that all associations

that stem from the grafted hydrophobic groups are disconnected. We

note that η ∞ is higher than η BDG (Figure 3.1) where also associations

between hydrophobic segments are disconnected. For the other

combinations η ∞ is somewhat higher than the viscosity of the

unmodified polymer and especially for the HM-(NP)-EHEC this is

obvious. The reason is that all polymer hydrophobic tails are not

available for complex formation with CD in these cases. From thevalues of η ∞ and by using equation (3.2) the fraction of hydrophobes

in the solution of HM-(NP)-EHEC that is not available for complex

formation can be estimated to be about 16%, which is quite close to

what was found above when B was compared to chydrophobe.

CD offers a selective way of decoupling the associations of

hydrophobic side chains, provided that the hydrophobic side chains

have a structure that fits into the cavity, but leaving the associations

from hydrophobic patches of the backbone intact. The quotient, QCD,

between η ∞ and η 0 (equation 3.3) can be used as a phenomeno-

logical measurement of the contribution to the viscosity caused by

association between hydrophobic side chains.

∞

=η

η 0CDQ (3.3)

3.3.3.2 Cyclodextrin and HM-PEG

In analogy with the results for HM-EHEC the addition of cyclodextrin

to an aqueous solution of HM-PEG results in a degradation of the

polymer network as indicated by a reduction of the solution viscosity

and an increased mean self-diffusion coefficient of HM-PEG ( D HM-PEG)

(Figure 3.8). In papers V and VI we have studied the degradation ofthe polymer network in the HM-PEG system in the polymer

49



concentration range 3 to 10% w/w. We adopted the same model as

we used for the HM-EHEC – CD system (equation 3.2).

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

cCD

(mmolal)

0

10-13

10-12

10-11

D ( m 2 / s )

Figure 3.8. Relative viscosity

(filled symbols) and mean self-

diffusion coefficient (open

symbols) of a 3% w/w solution

of HM-PEG as a function of

the concentration of

methylated-α-cyclodextrin, cCD.

Low concentration of CD

Figure 3.9 shows that the viscosity decreases dramatically with the

addition of methylated α-cyclodextrin (M-α-CD) to the HM-PEG

solution. The change is most pronounced at small additions of CD,

below 1 mmolal. In an attempt to determine the number of binding

sites, B, in the same way as described for the CD /HM-EHEC system

in section 3.3.4 equation (3.2) was fitted to the viscosity data points

(0 vs. cCD). The best representation of the experimental results

was obtained for B = 0.4 mmolal which constitutes only 10% of the

total number of hydrophobic groups. The results show that

deactivation of the first few hydrophobic associations has a much

stronger influence on the viscosity than would be expected if allassociations were equally important for the viscosity. This is

supported by the measurements of D HM-PEG also included in Figure

3.9. The increase in D HM-PEG is steep at cCD < 0.5 mmolal and at higher

concentrations it levels off. This shows that it is enough to terminate

about 10% of the hydrophobic tails to change the viscosity and D HM-

PEG almost to the levels achieved at excess CD where the network is

totally decoupled.

/η η

50



0 1 2 3

0.0

0.2

0.4

0.6

0.8

1.0

B1

cCD

(mmolal)

0

10-13

10-12

10-11

D ( m 2 / s )

Figure 3.9. Relative viscosity, η / η0, (filled symbols) and mean

self-diffusion coefficient,

D HM-PEG

, (open symbols) as a

function of the concentration

of methylated-α-cyclodextrin,

cCD , for a 3% w/w solution of

HM-PEG. The full line

represents a fit of Equation

(3.2) to the relative viscosity

data. B1 was obtained by

extrapolation to η / η0=0 from

the behavior at low cCD

To explain this we must go back to the model of the network

formation of HEUR thickeners in aqueous solution (section 2.2.1). 3%

w/w HM-PEG is in the region where the HM-PEG is expected to be

present in a percolated network built of clusters of flower micelles

(Figure 2.13). At this HM-PEG concentration the solution is expected

to be inhomogeneous with rather large concentration fluctuations

where inter-micellar links inside the clusters are numerous while thepolymers that connect micelles located in different clusters are rare. It

is likely that the polymers that connect different clusters give a

relatively more important contribution to connectivity of the network

and therefore are more important to the viscosity and D HM-PEG than the

polymers involved in associations inside the clusters. The dramatic

change in0 and D HM-PEG can be understood if primarily

hydrophobic associations responsible for connecting different clusters

are deactivated at low cCD.

/η η

Viscosity measurements show that B is virtually independent of the

polymer concentration in the concentration range between 3 and 10

%w/w ( B = 0.45 mmolal at 3% w/w, B = 0.42 mmolal at 5% w/w and B

= 0.53 mmolal at 10% w/w). This indicates that the number of

linkages between the clusters stays almost unchanged with

increasing polymer concentration whereas the clusters grow in size.

This has been suggested before by Alami et al.41

51



Intermediate concentrations of CD

At intermediate concentrations, where B < cCD < chydrophobe, a new region

appears, as can be seen in Figure 3.10. The changes in0 and in

D HM-PEG are much less dramatic in this region. The break-point

between the two regions in the viscosity curve almost coincides with

what is found from the self-diffusion measurements. It is reasonable

that it is the size of the “decoupled” clusters that influences the

viscosity of the solution and D HM-PEG in this region indicating that the

size of the clusters decreases with increasing concentration of CD.

This indicates that it is the “inter-micellar” linkages inside the clusters

that are disconnected leading to a degradation of the clusters into

separate micelles and further into separate polymer molecules

bearing a CD molecule at each end. The distribution in self-diffusion

coefficients, σ , reflects the size distribution of the polymer

aggregates.42,43 As can be seen in Figure 3.12, σ decreases with

increasing cCD. This also indicates that the clusters are degraded

since the clusters are expected to have a broad distribution in sizes

while the size of the micelles is rather uniform.

/η η

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0

cCD

(mmolal)

0

10-13

10-12

10-11

B(3%)

D ( m 2 / s )

Figure 3.10. Relative viscosity,

η / η0, (filled symbols) and mean

self-diffusion coefficient,

D HM-PEG, (open symbols) as a

function of the concentration

of methylated-α-cyclodextrin,

cCD , for a 3%w/w solution of

HM-PEG in the intermediate

region of cCD.

52



High concentration of CD

In the region cCD > chydrophobe both the viscosity and D HM-PEG are

expected to be independent of cCD and to be on same level as that

found in a solution containing the corresponding unmodified PEG with

similar molecular weight. In fact this is what we found.

0 5 10 15 20 25 3010

-13

10-12

10-11

10-10

10-9

0

1

2

D PEG

for unmodified PEG

DCD

for free CD

D H M - P E G , D C D

( m 2 / s )

cCD

(mmolal)

σ

Figure 3.11. Mean self-

diffusion coefficients for

HM-PEG, D HM-PEG; (open

circles) and for M-α-CD, DCD ,

(triangles) and the distribution

in D HM-PEG , σ , (filled circles) as

a function of CD concentration

in 3%w/w solution of HM-PEG.

The lower dashed line

represents the mean self-

diffusion coefficient forunmodified PEG (D PEG ) (mw=

20000g/mol) in 1% w/w

solution of PEG. The upper

dashed line represents DCD

when no HM-PEG or PEG is

present.

Since the self-diffusion at high cCD does not attain a plateau value

until cCD is above twice B this indicates that more than one

cyclodextrin molecule can bind to each hydrophobic group. Another

explanation could be that the complex formation is not quantitative

and free cyclodextrin is present in the solution in this region.

The self diffusion of M-α-CD ( DCD) has also been measured (Figure

3.11). DCD changed moderately with cCD and reached a plateau at

about 10 to 15 mmolal. The value of DCD at the plateau was low

compared to DCD in a solution of CD where no HM-PEG was present.

Experiments where HM-PEG was substituted by unmodified PEG

showed that interactions between the PEG chain and CD are of minor

importance. This indicates that the reduction of DCD at high cCD is

mainly caused by obstruction effects.

The fraction of CD that is bound to HM-PEG ( P b) can be determined

by the use of equation 3.4 where DCD,obs is the observed self-diffusion

53



of CD at the actual cCD and DCD,free is the self-diffusion of CD at excess

CD.

freeCDb PEG HM bobsCD D P D P D ,, )1( −+= − (3.4)

The fraction of bound CD decreases with increasing cCD. A calculation

at cCD = 10.7 mmolal gave the result that the average number of

bound CD per hydrophobic group (CD/hydrophobe) was 1.4 which

supports that more than one CD molecule can bind to each

hydrophobic group. In line with this Olson et al have shown by NMR-

measurements that two or even more α-CD molecules can bind to a

C12-hydrophobic group attached to a PEG chain.44

A model for the degradation of the HM-PEG network

From the results presented above a model for the degradation of the

polymer network in the HM-PEG solution is suggested (Figure 3.12).

In the region cCD < B the CD primarily breaks the linkages between

different clusters. In the region B < cCD < chydrophobe where the change in

viscosity is less pronounced the viscosity is mainly influenced by the

size of the clusters. At high CD concentration, cCD > chydrophobe, the

HM-PEG appears mainly as small aggregates or as individual

molecules with the hydrophobic groups hidden inside the interior of

the CD molecules.

cCD

cCD


representation of the suggested

model for the degradation of

HM-PEG network with

cyclodextrin

54



3.4 References Chapter 3

(1) Wang, K. T.; Iliopoulos, I.; Audebert, R. Polymer Bulletin 1988,20 , 577-582.

(2) Valint, J., P.L.; Bock, J. Macromolecules 1988, 21, 175-179.(3) Bock, J.; Siano, D. B.; Valint Jr., P. L.; Pace, S. J. In Polymers

in aqueous media; Glass, J. E., Ed.; American ChemicalSociety: Washington DC, 1989; Vol. 223, p 411-424.

(4) Williams, P. A.; Meadows, J.; Phillips, G. O.; Senan, C.Cellulose: Sources and Exploration 1990, 37 , 295-302.

(5) Thuresson, K.; Lindman, B. J. Phys. Chem. 1997, 101, 6460-6468

(6) Gelman, R. A.; Barth, H. G. Adv. Chem. Ser. 1986, 213, 101-110.

(7) Jönsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B.Surfactants and polymers in aqueous solution; John Wiley &Sons Ltd: Chichester, England, 1998.

(8) Gelman, R. A. In 1987 International dissolving PulpsConference; TAPPI, Ed. Geneva, 1987, p 159-165.

(9) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B.Progr. Colloid Polym. Sci 1992, 89, 118-121.



(12) Loyen, K.; Iliopoulos, I.; Olsson, U.; Audebert, R. Progr. ColloidPolym. Sci. 1995, 98 , 42-46.


(14) Aubry, T.; Moan, M. J. Rheol. 1996, 40 , 441-448.

(15) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson,O. Adv. Colloid Interface Sci. 1996, 63, 1-21.

(16) Persson, K.; Wang, G.; Olofsson, G. J. Chem. Soc. Faraday

Trans. 1997, 90 , 3555-3562.

(17) Macdonald, P., M. In Polymeric materials: Sci. Eng. Springmeeting1997 ; ACS, Ed. San Francisco, 1997; Vol. 76, p 27-28.

(18) Panmai, S.; Prud'homme, R., K.; Peiffer, D., G.; Jockusch, S.;Turro, N., J. Polym. Mater. Sci. Engin. 1998, 79, 419-420.


(20) Olesen, K. R.; Bassett, D. R.; Wilkerson, C. L. Progress OrganicCoatings 1998, 35 , 161-170.

(21) Jiménez-Rigaldo, E.; Selb, J.; Candau, F. Langmuir 2000, 16 ,8611-8621.

55



56

(22) Chronakis, I. S.; Alexandridis, P. Marcomolecules 2001, 34,5005-5018.

(23) Steffenhagen, M. J.; Xing, L.-L.; Elliott, P. T.; Wetzel, W. H.;Glass, J. E. Polym. Mater. Sci. Engin. 2001, 85 , 217-218.

(24) Loftsson, T.; Brewster, M. E. J. Pharm. Sci. 1996, 85 , 1017-1025.

(25) Connors, K. A. Chem. Rev. 1997, 97 , 1325-1357.

(26) Immel, S.; Lichtenthaler, F. W. Starch/Stärke 1996, 48 , 225-232.

(27) Wentz, G. Angew. Chem. Int. Ed. Engl. 1994, 33, 803-822.

(28) Amiel, C.; Sebille, B. J. Inclusion Phenomena MolecularRecognition in Chem. 1996, 25 , 61-67.

(29) Mwakibete, H.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1994, 10 ,3328-3331.

(30) Mwakibete, H.; Bloor, D. M.; Wyn-Jones, E.; Holzwarth, J. F.Langmuir 1995, 11, 57-60.

(31) Junquera, E.; Tardajos, G.; Aicart, E. Langmuir 1993, 9, 1213-1219.

(32) Funasaki, N.; Yodo, H.; Hada, S.; Neya, S. Bull. Chem. Soc.Jpn. 1992, 65 , 1323-1330.

(33) Park, J. W.; Song, H. J. J. Phys. Chem. 1989, 93, 6454-6458.

(34) Sasaki, K. J.; Christian, S. D.; Tucker, E. E. Fluid PhaseEquilibria 1989, 49, 281-289.

(35) Eisenhart, E. K.; Johnson, E. A. In U.S. Patent 5137571; Rohmand Haas: United States, 1992.

(36) Lau, W.; Shah, V. M. In U.S. Patent 5376709; Rohm and Haas:United States, 1994.

(37) Akiyoshi, K.; Sasaki, Y.; Kuroda, K.; Sunamoto, J. ChemistryLetters 1998, 93-94.

(38) Zhang, H.; Hogen-Esch, T. E.; Boschet, F.; Margaillan, A.Langmuir 1998, 14, 4972-4977.

(39) Gupta, R. K.; Tam, K. C.; Ong, S. H.; Jenkins, R. D. In XIIIthInternational Congress on Rheology Cambrige, UK, 2000, p

335-337.

(40) Islam, M. F.; Jenkins, R. D.; Bassett, D. L.; Lau, W.; Ou-Yang,H. D. Macromolecules 2000, 33, 2480-2485.


(42) Nydén, M.; Söderman, O. Macromolecules 1998, 31 (15), 4990-5002.

(43) Nydén, M.; Söderman, O.; Karlström, G. Macromolecules 1999,32 , 127-135.

(44) Olson, K.; Chen, Y.; Baker, G. L. J. Polym. Sci. Part A: Polym.Chem. 2001, 39, 2731-2739.



Main conclusions

One intention of this thesis has been to support the development of improved associative

thickeners for water borne paint and it is my opinion that novel information has been obtained.

It has been shown that the viscosity of HM-PEG solutions as a function of polymer

concentration passes via a maximum. At concentrations above 50% w/w the viscosity decreases

considerably. This was referred to a gradual transition from a state containing micelle-like

structures to a more meltlike state (Paper I). This is important when the goal is to have high

concentration of polymer while keeping the viscosity moderate, and may be utilized to minimize

handling and transportation costs of the product.

The dynamics, and the strength, of hydrophobic associations of hydrophobically modified

polymers in aqueous solution are very much influenced by the length of the hydrophobic

groups. Longer hydrophobic groups give, due to slower dynamics and increased relaxation

times, an increased viscosity. When formulated in a paint a HM-polymer with long hydrophobic

groups gives a more elastic consistency compared to when a HM-polymer with shorter

hydrophobic groups is used. (Paper III)

In an aqueous solution a cyclodextrin (CD) molecule can form an inclusion complex with a

hydrophobic group on a HM-polymer. This prevents the hydrophobic group from associating

with other hydrophobic groups, and it leads to a degradation of the physically cross-linked

polymer network. This can be detected as a reduction of the viscosity. At excess CD the

viscosity attains the same value as for a solution of the unmodified polymer with the same

molecular weight. This can be used to deduce the part of the total thickening effect that has its

origin in associations of hydrophobic side chains (Paper IV). This observation has already been

implemented in analysis methods for quality control in the production of HM-EHEC.

In a HM-PEG solution it is enough to terminate only a small fraction of the total amount of

associative linkages to reduce the viscosity almost to the same level as that for a solution of anunmodified PEG. The results were confirmed by self-diffusion measurements. The changes in

viscosity and self diffusion are for instance much more dramatic compared to what can be

observed when surfactant is added. The suggested interpretation is that it is primarily

hydrophobic associations involved in connecting different clusters of micelles that are

disconnected (Paper V and VI). These results have supplied new information that can be useful

for the understanding of the thickening mechanisms of HM-PEG, both in water solution and in

more complicated systems like a paint.

57



Populärvetenskaplig sammanfattning

Tre begrepp som är viktiga för denna avhandling är viskositet,

polymer och hydrofob grupp. Ett materials viskositet är ett mått på hur

trögt eller hur lätt materialet flyter. Låg viskositet betyder att materialet

flyter lätt medan hög viskositet betyder att det flyter trögt. En polymer

är en stor molekyl, som bildas genom kemisk reaktion där små

molekyler, monomerer, kopplas samman till en mycket större

kedjemolekyl. Vissa polymerer är lösliga i vatten och kan användas

som förtjockare för vattenbaserade system, d.v.s. att de höjer

viskositeten hos vattenlösningen. Begreppet ”hydrofob grupp” antyder

att den inte tycker om vatten. (hydro- är ett förled som anger att något

innehåller eller har samband med vatten och -fob kommer av pho´bos

som på grekiska betyder 'fruktan', 'skräck'.) I själva verket är det så

att det är vattenmolekylerna som hellre omger sig med andra vatten-

molekyler än att komma i kontakt med den hydrofoba gruppen. För att

minimera kontakten med vatten söker sig den hydrofoba gruppen tillandra hydrofoba grupper i lösningen. Man säger att de hydrofoba

grupperna associerar till varandra.

Figur 1. Schematisk bild av

polymermolekyler som

trasslar in sig i varandra

En bra bild för att förstå hur förtjockningen med polymerer går till är

en tallrik spagetti. Trådarna av spagetti trasslar in sig i varandra och

det är svårt att röra runt med gaffeln. Polymermolekylerna i en lösning

uppträder på samma sätt. De är långa trådar som trasslar in sig i

varandra och hindrar varandra från att röra sig vilket resulterar i en

förhöjd viskositet (Figur 1). Om man skär spagettin i mindre bitar går

det lättare att röra omkring med gaffeln. På samma sätt är det med

polymerlösningar. Korta polymermolekyler (låg molekylvikt) förtjockar

mindre än långa polymermolekyler.

I en hydrofobmodifierad polymer (HM-polymer) har en liten mängd

hydrofoba grupper reagerats fast längs polymerkedjan. De hydrofoba

grupperna associerar till varandra och ger tvärbindningar mellan

polymerkedjorna (Figur 2). Det betyder att alla polymerkedjorna

hänger ihop i ett enda stort nätverk. Resultatet blir en avsevärd


polymermolekyler med

hydrofoba grupper somassocierar till varandra.

58



förhöjning av viskositeten. Stora hydrofoba grupper ger starkare

tvärbindningar än små grupper och därför högre viskositet. I liknelsen

med spagetti kan man säga att de hydrofoba grupperna är som riven

ost som klistrar ihop spagettin och gör det ännu svårare att röra runt.

Vattenlösningar förtjockade med polymerer är vanliga i vårt dagliga

liv. Ett exempel är schampo, som är en vattenlösning som bör ha hög

viskositet. Om den inte hade det skulle den rinna ut mellan fingrarna

när den hälldes ur flaskan och ner i handen. Andra exempel kan man

hämta från matlagningen. Stärkelse från potatis eller majs, används

för att reda (förtjocka) såser och gelatin används i många efterrätter

för att ge dem dess konsistens.

Figur 3. Färgen rollas på en

svartvitrutig panel när

täckförmågan skall bedömas.

Vattenbaserad målarfärg är ytterligare ett exempel på en vatten-

lösning som måste förtjockas för att den skall uppföra sig som vi vill. I

en färg med för låg viskositet sjunker alla partiklar snabbt till botten på

burken och när man målar kan man bara ta lite färg i penseln om inte

färgen skall droppa. För att färgen skall få rätt viskositet tillsätts

vattenlösliga polymerer.

Polymerer med hög molekylvikt är effektiva förtjockare vilket betyder

att bara lite polymer behöver tillsättas för att ge den önskade

viskositeten. Nackdelen är att färg förtjockad med polymer med hög

molekylvikt har dålig täckförmåga vilket betyder att man måste göra

flera strykningar för att få bra täckning (Figur 3). Andra nackdelar är

att färgen har dålig utflytning d.v.s. att den målade ytan får märken av

penseldrag (Figur 4) och att den skvätter mycket när man rollar den

på väggen eller i taket (Figur 5). Polymerer med lägre molekylvikt ger

bättre färgegenskaper men i gengäld måste mycket mer polymer

tillsättas för att man skall få önskad viskositet.

Figur 4. Panel från

utflytningsförsök. Brautflytning ger en jämn yta

medan dålig utflytning ger en

yta med tydliga linjer efter

penseldrag.

Figur5. När man skall avgöra hur

mycket en färg skvätter rollas

färgen på väggen. På ett svart

papper som har placerats

horisontellt en bit nedanför kan

man avgöra hur mycket färgen har

skvätt. Panelen till vänster är ett

exempel på när en färg skvätter

lite medan färgen som använts till

panelen till höger skvätter mycket.

59



När HM-polymerer används som förtjockare i färg ger de en

kombination av de goda egenskaperna från polymerer med hög och

låg molekylvikt. Samtidigt som de ger bra färgegenskaper såsom bra

täckförmåga, bra utflytning och lite skvätt ger de hög förtjocknings-

effekt d.v.s. lite polymer behöver tillsättas.

Hydrofilutsida

Hydrofobthålrum

Hydrofilutsida

Hydrofobthålrum

Hydrofobmodifierade polymerer förtjockar både genom intrassling av

polymerkedjorna (spagetti) och genom associationer mellan

hydrofoba grupper (smält ost). Arbetet i denna avhandling har gått ut

på att försöka förklara hur förtjockningen går till och hur polymerens

struktur påverkar dess egenskaper. Ett sätt att studera detta som jag

har använt i det här arbetet är att tillsätta cyklodextrin till

vattenlösningar av polymerer och se hur det påverkar lösningarnasviskositet. Cyklodextrinmolekylen liknar en mutter i formen (Figur 6).

På utsidan är den hydrofil (tycker om vatten) medan hålet i mitten är

hydrofobt (tycker inte om vatten). En hydrofob grupp på polymeren

kan gömma sig inuti hålrummet på en cyklodextrinmolekyl förutsatt att

den inte är för stor för att få plats i hålet. Det finns olika cyklodextriner

med olika storlek på hålrummet. Med rätt cyklodextrin får det bara

plats en hydrofob grupp i varje cyklodextrinmolekyl och bara en

cyklodextrin får plats på varje hydrofob grupp. En hydrofob grupp somhar gömt sig inuti hålrummet i en cyclodextrinmolekyl kan inte längre

delta i att bilda tvärbindningar. Resultatet blir att polymernätverket

faller sönder och viskositeten sjunker. Eftersom varje cyclodextrin-

molekyl tar hand om en hydrofob grupp kan man bryta tvär-

bindningarna i polymernätverket på ett mycket kontrollerat sätt och

därmed få en detaljerad bild av hur förtjockningen går till.


en cyklodextrinmolekyl

Figur 7. Schematisk bild avhur polymernätverket bryts

ner av cyklodextrin.

60



Acknowledgements

I wish to thank:

- Sture and Magnus, my bosses during these years, for giving me the opportunity to use

a lot of working hours for this work and for the great freedom in choosing the subjects ofmy study, almost total freedom as long as it concerned associative thickeners. I shareyour conviction that this work someday will pay back in the form of new and improvedproducts.

- my supervisors, Krister and Björn for excellent guidance, encouragement and patience.Even though my first attempt to do a PhD came to nothing you encouraged me to try asecond time. Special thanks for the hospitality Krister, Maria and Thea have shownduring my many visits to Lund.*

- all the people at the paint lab and the analysis lab at Akzo Nobel Surface Chemistry. Icould not have done this without the help from you. Especially, Barbro and “Myran”,

your results are all over this thesis.

- all the people at Physical Chemistry 1 for the stimulating and friendly atmosphere at thedepartment. The co-authors Fredrik, Susanne, Carin and Olle for measurementresults, discussions and advice. Majlis, Monica, Martin and Maria for all small thingsyou have helped me with.

- the people who contributed in putting the thesis together. Peter interpreted the paintresults and helped with the confusing terminology of the paint industry. David, our manin England, helped with an extra spell and grammatical check of this thesis. I love thecomments, like “”water poor domain” doesn’t sound like English – but I don’t have analternative!”

- my family for all the love, support and patience.

*Krister and I, in Krister’s

and Maria’s living room,

making the last adjustmentsbefore submitting one of the

papers

61



List of commercially available hydrophobically modified polymers

used as associative thickeners in the paint industry.

More than 100 associative thickener products exist of which the majority is sold in very small volumes.

This is not a complete list of all products but a selection of some of the most commercially important. The

column Producer’s comment contains information the producers use to characterize their products. All

data are obtained from internet.

Product Type Producer’s comment Solvent Producer

Acrysol TT 615 HASE1 high low shear visc/low high shear visc water R & H

10

Acrysol TT 935 HASE1 high low shear visc water R & H

10

Acrysol DR 1 HASE1 high low shear visc/low high shear visc water R & H

10

Acrysol DR 73 HASE1 high low shear visc/high high shear visc water R & H10

Acrysol RM 5 HASE1 high low shear visc/low high shear visc water R & H

10

Acrysol RM 55 HASE1 high low shear visc/low high shear visc water R & H

10

Acrysol DR 72 HASE1 high low shear visc/ pseudoplastic water R & H

10

Acrysol RM825 HEUR2 KU efficient BDG/w

6 R & H10

Acrysol SCT275 HEUR2 KU efficient BDG/w

6 R & H10

Acrysol RM2020 HEUR2 high low shear viscosity water R & H

10

Acrysol RM 8 W HEUR2 KU efficient water R & H

10

Acrysol RM 12 W HEUR2 high low shear visc/ pseudoplastic water R & H

10

Aquaflow NLS-200 HM-PE4 low shear efficient BDG/w

6 Aqualon

Aquaflow NLS-210 HM-PE4 low shear efficient BDG/w

6 Aqualon

Aquaflow NHS-300 HM-PE4 high shear efficient water Aqualon

Bermocoll EHM200 HM-EHEC3 250 – 600 mPa s (1% solution) none ANSC

11

Bermodol HAC 2000 HASE1 high medium shear visc. water ANSC

11

Bermodol HAC 2001 HASE1 newtonian water ANSC

11

Bermodol PUR 2102 HEUR2 high low shear visc. BDG/w

6 ANSC

11

Bermodol PUR 2110 HEUR2 newtonian.. none ANSC

11

Bermodol PUR 2130 HEUR2 newtonian. water ANSC11

Bermodol PUR 2150 HEUR2 high low shear visc. surfactant w/surf.

7 ANSC

11

DSX 1514 HEUR2 low structural viscosity BTG/w

8 Cognis

DSX 1550 HEUR2 structural viscosity BDG/w

6 Cognis

DSX 2000 HM-PE4 newtonian BDG/w

2 Cognis

DSX 3000 HM-PE4 newtonian w Cognis

DSX 3256 HEUR2 pseudoplastic w/diluent

9 Cognis

DSX 3290 HEUR2 high low shear viscosity w/diluent

9 Cognis

Natrosol Plus 100 HM-HEC5 5 – 25 cP (1% solution) none Aqualon


62



63

Product Type Comment Solvent Producer



Rheolat 255 HEUR2 antisettling BDG/w

6 Elementis

Rheolat 278 HEUR2 antisettling BDG/w

6 Elementis

Rheolat 420 HASE1 antisettling water Elementis

Tafigel PUR 40 HEUR2 pseudoplastic BTG/w

8 Münzing

12

Tafigel PUR 45 HEUR2 newtonian BTG/w

8 Münzing

12

Tafigel PUR 50 HEUR2 pseudoplastic water Münzing

12

Tafigel PUR 60 HEUR2 strongly psedoplastic BTG/w

8 Münzing

12

Tafigel PUR 61 HEUR2 strongly psedoplastic water Münzing

12

Ucar Polyphobe 202 HASE1 highly associative water Dow

Ucar Polyphobe 203 HASE1

low associative nature water Dow

Ucar Polyphobe 205 HASE1 low high shear viscosity water Dow

Ucar Polyphobe 206 HASE1 high low shear viscosity water Dow

1 Hydrophobically modified polyacrylate (Hydrophobically modified Alkali Swellable Emulsion)2 Hydrophobically modified urethanes3 Hydrophobically modified ethyl hydroxyethyl cellulose4 Hydrophobically modified polyether5 Hydrophobically modified hydroxyethyl cellulose6 mixtures of diethyleneglycol monobutylether and water7 water with surfactant

8

mixture of triethyleneglycol monobutylether and water9 water with viscosity reducing agent10 Rohm & Haas11 Akzo Nobel Surface Chemistry12

Münzing Chemie

















Colloids and Surfaces

A: Physicochemical and Engineering Aspects 201 (2002) 9–15

Clouding of a cationic hydrophobically associating combpolymer

K. Thuresson a,*, L. Karlson b, B. Lindman a

a Physical Chemistry 1, Center for Chemistry and Chemical Engineering , Lund Uni ersity, P.O. Box 124 ,

SE -221 00 Lund , Swedenb Akzo Nobel Surface Chemistry AB , SE -444 85 Stenungsund , Sweden

Received 16 June 1999; accepted 29 September 2000

Abstract

A novel cationic hydrophobically associating comb polymer, containing poly(oxyethylene) chains in the back-bone,

is described and investigated with respect to the aqueous solubility by cloud point measurements. Cloud points were

investigated as a function of polymer charge, poly(oxyethylene) chain length and concentrations of added NaCl and

could be interpreted in terms of the aqueous behavior of poly(oxyethylene) chains, general electrostatic effects,

salting-out effects for nonionic polymer chains and polymer association. © 2002 Elsevier Science B.V. All rights

reserved.

Keywords: Associating polymer; Hydrophobically modified polymer; Comb polymer; Phase behavior; Cloud point

www.elsevier.nl/locate/colsurfa

1. Introduction

Hydrophobically associating polymers have in

recent years become a very active field of research.

Both block and graft copolymers as well as more

complex architectures have been investigated,

with both fundamental and applied aspects as a

starting point [1–4]. With respect to applications,

rheology control and adsorption on solid surfaces

are in the foreground, but control of phase behav-ior is always of key importance.

There is an enormous variation in polymers

investigated with respect to architecture, back-

bone, substituents and charge but due to limita-

tions in methods of synthesis and modification as

well as in characterization it is difficult to system-

atically vary the different parameters of polymer

chemical structure, which are relevant. Instead

progress is dependent on the synthesis and

physico-chemical studies of polymers with fea-

tures which are novel with respect to previouswork.

The present study introduces a novel polymer,

characterized by alkyl chains grafted onto a back-

bone composed of poly(oxyethylene) (POE)

chains and cationic ammonium groups; the back-

bone is obtained by coupling fatty amines with

* Corresponding author. Tel.: +46-46-2220112; fax: +46-

46-2224413.

E -mail address: [email protected] (K. Thures-

son).

0927-7757/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 7 - 7 7 5 7 ( 0 0 ) 0 0 7 7 5 - 5



K . Thuresson et al . / Colloids and Surfaces A: Physicochem. Eng . Aspects 201 (2002) 9 – 15 10

POE chains by diisocyanate. Furthermore, an ini-

tial physico-chemical characterization with respect

to phase behavior is presented. The polymer dis-

plays a clouding behavior in water at elevated

temperature and the conditions of clouding are

investigated and give an insight into interpolymer

association.

2. Experimental

2 .1. Materials and sample preparation

The amphiphilic polymers used in this investi-

gation were synthesized at Akzo Nobel Surface

Chemistry AB, Stenungsund, Sweden. The prepa-

ration was started by ethoxylating fatty amines.

The amines were produced from coconut oil that

mainly contains saturated hydrocarbons (C12).

Two different polymer fractions were made, andthe ethoxylation reaction was continued until the

fatty amine contained, on average, 51 or 74 re-

peating ethylene oxide units, respectively. This

compound, which has a diblock structure (DB),

may be viewed as a surfactant molecule with a

polymeric-sized head group (Fig. 1). This surfac-

tant is positively charged at low pH, while it is

uncharged at high pH, due to the titrating amine-

group located between the C12 hydrocarbon tail

and the poly(oxyethylene), POE, headgroup. In a

condensation reaction between the surfactant andisophoronediisocyanate, IPDI, a hydrophobically

modified poly(oxyethylene) polymer with a comb

structure was produced (Fig. 1). During the reac-

tion IPDI was present in small excess and

dibutyltindilaurat was used as a catalyst. Unre-

acted IPDI was eliminated by termination with

small amounts of ethanol. A rough estimate of the

weight-average molecular weight (M w) was deter-

mined by size-exclusion chromatography (SEC)

on a packing material made from styrene-divinyl-

benzene. Three columns were coupled in series(Phenogel 10 000 A (600×7.8 mm), Phenogel

1000 A (600×7.8 mm) and Phenogel 500 A

(600×7.8 mm) (Phenomenex)) and the mobile

phase was tetrahydrofuran. The flow-rate was 1.0

ml min−1, the injection volume 100 l and the

temperature 25°C. For detection a refractive index

detector was used. A narrow standard calibration

was made with PEG/PEO with M p=4250,

12 600, 27 250 and 50 400. The M w was about the

same for both polymer fractions and was esti-

mated to be roughly 25 000. Thus, each polymer

chain is composed of, on average, four DB

molecules. Apart from a distribution in the length

of the POE chains of the DB compound, the

number of DB molecules in a polymer chain may

vary significantly. It is not surprising that the

polydispersity index of M w/M n2.2 suggests a

wide distribution in molecular weight. Below, the

two different polymer fractions will be referred to

as -(OE)51- and -(OE)74-, respectively.

Before use the polymers were purified in several

steps. In order to remove hydrophobic com-

pounds, such as unreacted fatty amines or alco-

hol-terminated isophoronediisocyanate, the

polymer sample was allowed to swell or partly

dissolve in acetone for a few hours under vigorous

Fig. 1. The chemical structure of the investigated comb co-polymer together with the structure of the ethoxylated amine

(DB) and the coupling agent, isophoronediisocyanate (IPDI),

from which the final polymer was synthesized. The polymer

was made with two different lengths of the POE block ( x=51

or 74 OE-units, respectively). Calculated from the molecular

weight, M w=25 000, size-exclusion chromatography (SEC)

suggests n=4.




stirring. Later the polymer was precipitated by an

addition of hexane. After repeating this procedure

(three to five times) the precipitate was collected

and carefully dried, and later dissolved in water to

a concentration of a few percent. This solution

was dialyzed against deionized water, which had

been passed through a Millipore™ water purifica-

tion unit. This treatment reduces the amount of

low molecular weight impurities (such as salt and

oligo(oxyethylene)). Finally the polymers were re-

covered by freeze-drying. The samples for the

phase studies, each containing a volume of about

3 ml, were prepared by weight from a polymer

stock solution (with a concentration of ca. 10

wt.%), Millipore water, and NaCl. Glass tubes

sealed with Teflon tightened screw caps were used.

Each sample was equilibrated at least overnight

before measurement. The pH was adjusted by

adding small volumes of either HCl or NaOH

(with a concentration of approximately 1 M) witha microsyringe.

2 .2 . Phase behaior

The temperature of the samples was controlled

to within 0.1°C by immersion in a jacketed

glass vessel connected to a temperature-controlled

water-bath. At temperatures significantly differing

from room temperature there may be a large

difference between the temperature in the water

bath and the actual temperature in the samplecell. Therefore, a thermocouple was used for a

direct measurement in the sample. The cloud

point temperature, T cp, was determined by visual

inspection of the sample. The temperature was

varied in steps of 1°C, and T cp was taken as the

mean value from determinations where the tem-

perature was increased or decreased, respectively.

This procedure rendered an estimated uncertainty

of 1°C. During temperature equilibration and

measurement, the sample was stirred with a mag-

netic bar. For samples where temperatures below0°C had to be used a mixture of water and

ethanol was used as the cooling medium. In order

to facilitate the determination of T cp at low tem-

peratures, the sample cell was sprayed with etha-

nol on the outside to prevent condensed water

from freezing.

3. Results and discussion

Many nonionic polymers, as well as nonionic

surfactants of the oligo(oxyethylene) type, show a

reverse temperature dependence in their solubility

in aqueous systems, i.e. solubility decreases with

increasing temperature [5,6]. In the region of the

onset of phase separation, the scattering of light

from the solutions increases dramatically; the so-

lutions get ‘cloudy’, hence the notion of a ‘cloud

point’. The cloud point gives useful information

on the molecular interactions and, since it can be

accurately and easily measured, it offers a very

useful characterization of these systems.

Probably the simplest example of clouding of

an aqueous polymer system is offered by poly(-

oxyethylene); the phase behavior of POE – water

has been investigated in detail over the entire

mixing range for different molecular weights [7,8].

The phase diagram displays the expected behaviorof a closed-loop two-phase region (thus with mu-

tual miscibility between polymer and water both

at low and at high temperatures, but not at

intermediate), which increases in magnitude, both

along the temperature axis and along the compo-

sition axis, as polymer molecular weight increases.

Nonionic surfactant phase behavior has similar-

ities with that of POE, but the behavior is more

complex due to the self-assembly which leads to

several additional phases, than the water-enriched

and the polymer-enriched solution phases [9].Clouding is also displayed quite generally by

nonionic graft and block copolymers containing

oxyethylene groups. The phase behavior is inter-

mediate between what is displayed by POE and

nonionic surfactants, the complexity being deter-

mined mainly by the self-assembly of the polymer.

In general, the more strongly associating the poly-

mer the lower is the cloud point.

The solubility and the clouding of nonionic

polymers and surfactants is strongly dependent on

the addition of cosolutes [10 – 14]. Depending onwhether there is an enrichment or depletion of a

cosolute at the polymer molecules, either an in-

creased, i.e. increased cloud point, or decreased

solubility can result [15]. Ionic surfactants are the

most ef ficient in raising the cloud point and asso-

ciate to the nonionic micelles or the nonionic




Fig. 2. The cloud point temperature, T cp, as a function of the

polymer concentration for the two different comb copolymers.

The NaCl concentration was kept constant at 100 mM. The

insert shows the same data in a semi-logarithmic representa-

tion.

tions T cp increases. The polymer containing the

longest poly(oxyethylene) blocks, -(OE)74, has the

highest T cp of the two investigated polymer frac-

tions. It is obvious that the solubility decreases as

a result of a decreased length in the hydrophilic

OE block and the concomitant increased influence

of the hydrophobic IPDI units and fatty chains.

The data in Fig. 2 were obtained at a rather

high concentration of screening electrolyte in or-

der to reduce the expected strong influence of

charged amine groups. At polymer concentrations

slightly above 1 wt.%, T cp is rather invariant with

concentration. Therefore, a polymer concentra-

tion of C p=2 wt.% was chosen for further inves-

tigations of the influence of a screening electrolyte

and a variation in pH.

As can be expected, a strong decrease in T cpwas observed at NaCl concentrations up to about

C NaCl=30 mM, while at higher concentrations

the decrease in T cp is less pronounced. The firststeep part can be ascribed as a screening of the

electrostatic forces, which in turn is expected to

decrease the solubility of the polymer chains; as

salt is added the entropic penalty of phase separa-

tion arising from the counterions is reduced. The

rather slow decrease in T cp at NaCl concentra-

tions above C NaCl=50 mM is not caused by a

simple electrostatic effect. This weak decrease in

solubility is ascribed to a salting-out effect. Such a

behavior is indeed observed when NaCl is added

to aqueous solutions of a range of non-chargedcompounds, like POE and nonionic cellulose

ethers. The solubility is reduced on addition of

cosolutes depleted in the vicinity of the polymer.

As indicated in Section 2, the charge density of

the present polymer compounds can be varied by

a change in the pH. Because the charge is associ-

ated with a titrating amine-group, the polymers

can be expected to have either a zero-charge or to

be polyelectrolytes with a positive charge. At low

pH the polymer is expected to be highly charged,

while an increasing pH is expected to reduce thecharge density. In line with the expectation of a

titrating amine-group the pH-range where the

charge changes strongly seems to be located at

easily accessible pH-values, around pH 7.5 – 8.5

(Figs. 4 and 5). The effect of screening electrolyte

is pronounced at high charge densities (low pH),

polymer and effectively bring about a charging;

for a charged system the important contributionfrom the mixing entropy of the counterions

strongly promotes solubility [16]. However, this

effect is strongly reduced by added electrolyte.

Solubility can also be strongly enhanced by

charging up the polymer itself by the same mecha-

nism. Therefore, polyelectrolytes are generally

highly soluble and do not show clouding at ele-

vated temperature. However, for many polyelec-

trolytes, the solubility is strongly reduced in the

presence of electrolyte.

In the present investigation, we have consideredin some detail the clouding of a novel polymer,

which contains long POE chains, at the same time

as it is charged; the charge of the polymer can be

varied by varying the pH of the system. For two

polymers with different lengths of POE chains, we

have investigated the clouding as a function of

polymer concentration, pH and concentration of

added salt.

Generally it is thus found that poly(-

oxyethylene)-containing compounds have a re-

versed temperature dependency with a decreasedsolubility at elevated temperatures. This is also

the case with the present amphiphilic polymers

(Fig. 2). At low concentrations there is a pro-

nounced decrease in T cp with increasing polymer

concentration. At approximately 1 wt.% polymer

a minimum is reached, and at higher concentra-




while at high pH when the polymer chains ap-

proach zero-charge the effect of salt is small and

T cp is virtually the same independent of the NaCl

concentration. (A closer examination reveals

small variations. This will be discussed below.)

For electrostatic reasons, the high charge end of

the titration curve (low pH) extends to lower

pH-values when the concentration of screening

electrolyte is decreased (Fig. 4). This effect can be

traced to the fact that titrating groups on each

polymer chain are close enough not to be inde-

pendent. It is interesting to see that also at very

high concentrations of screening electrolyte, at

which electrostatics should have a vanishing con-

tribution to the solubility, an increasing pH (and

decreasing charge density) decreases the solubility

of the comb polymers. We will return to this issue

below.

In Figs. 6 and 7, we have assembled data for

the polymer concentration dependency of T cp fortwo different fixed pH values (pH 6 and 10). At

these two pHs, the polymer chains are expected to

have a high and a low charge density, respectively

(cf. Figs. 4 and 5). However, at the employed salt

concentration, C NaCl=100 mM, long range elec-

trostatic repulsions are screened and the polymers

should have a similar behavior as related non-

ionic compounds. In these figures we have also

included the T cp for an uncharged end-capped

poly(ethylene glycol). From earlier investigations,

it is known that this polymer forms micellar-likestructures. Thus, the hydrophobic part of the

molecules is protected from water exposure by the

hydrophilic poly(ethylene glycol) head group,

which forms the corona of the micelle. In a first

approximation such micelles are expected to have

a T cp similar to that of an unmodified

poly(ethylene glycol) of a corresponding molecu-

lar weight. Following this the cloud point temper-

ature of the micelles is expected to be

T cp100°C. The pronounced difference to the

measured data points was ascribed to the bifunc-tionality of the end-capped polymer [17]. Due to

connectivity this renders an attraction between

micelles that promotes a formation of a concen-

trated phase. A reduced T cp is expected. When the

polymer concentration increases this mechanism

becomes less important (because the average dis-

tance between micelles decreases), and T cp againincreases (strongly).

A similar behavior is anticipated for the presentcomb polymers. However, this is a more compli-

cated system, with a pH-sensitive charge densitywhich influences the solubility strongly when

C NaCl50 mM (cf. Fig. 3). To rationalize thedata in Figs. 6 and 7, we may speculate that

despite the fact that long-range electrostatic repul-sions are screened at C NaCl=100 mM, there is anon-negligible contribution from the electrostatic

repulsions in the headgroup region of the micelles.This opposes aggregation, and should, therefore,

favor solubility at concentrations where the ‘re-stricted swelling’ mechanism otherwise operates.

This explains the difference in T cp between pH 6and 10 (Figs. 6 and 7), and, therefore, also the

variation with pH at high salt concentrationswhich was pointed out in connection with Figs. 4

and 5. In this context, a close examination of Figs. 4 and 5 also reveals that changing the salt

concentration from 100 to 300 mM has a some-what more pronounced influence at low pH (high

charge density of the polymer) than at high. Thisobservation can possibly be related to an in-

creased aggregation situation and accompanyingconnectivity. However, T cp is also influenced athigh pH (where the polymer is expected to be

uncharged). We relate the latter observation to asalting-out effect.

Fig. 3. The cloud point temperature, T cp, as a function of

added NaCl for the two different comb copolymers at a

concentration of 2 wt.%. The insert shows the same data in a

semi-logarithmic representation.




Fig. 4. The cloud point temperature, T cp, for the comb copoly-

mer with the short PEO-block (containing 51 U) as a function

of pH at three different NaCl concentrations (0, 100, and 300

mM). The arrows indicate samples that had T cp100°C. The

polymer concentration was kept constant at 2 wt.%.

Fig. 6. The cloud point temperature, T cp, for the comb copoly-

mer with the short PEO-block (containing 51 U) as a function

of polymer concentration for two fixed pH values (pH 6 or 10,

respectively). Also included is the data from Fig. 2, where the

pH was not controlled. The NaCl concentration was kept

constant at C NaCl=100 mM. Included in the figure is T cp for

an ordinary end-capped PEG polymer (from Ref. [17]).Figs. 6 and 7 also comprise data from Fig. 2. It

follows that when the pH was not adjusted the-(OE)74- polymer produces solutions with pH

slightly above pH 6 (cf. Fig. 7). A comparison

with data presented in Fig. 5 suggests a pH 6.5 – 7

in a 2 wt.% solution (data point marked in Fig.

7). The corresponding data for the -(OE)51- poly-

mer suggests that this polymer produces solutions

with a higher pH. By comparison to Fig. 4 the

marked data point in Fig. 6 (2 wt.% polymer)

corresponds to pH 8.1.

If the qualitative behavior of the present comb

polymers is similar to that of the end-capped PEG

polymer, with a decrease in T cp at low concentra-

tions and thereafter an increase at higher concen-

trations, there are also differences. The

end-capped polymer has a much more pro-

nounced minimum in T cp, and we assign this to

the strong attraction between micelles that is ex-

pected from the C18 moieties. This association is

also expected to produce rather well-defined mi-

celles. With the present comb polymers, the hy-

drophobic attractions are weaker (C12 moieties).

We may also speculate that due to geometrical

constraints and the expected chemical differences

between different polymer chains, the micellarstructures that form are less well defined. One

obvious complication to the aggregation process

Fig. 5. As in Fig. 4 but for the comb copolymer with the long

PEO-block (containing 74 U). With this polymer, T cp was not

determined at C NaCl=0 mM.

Fig. 7. As Fig. 6 but for the comb copolymer with the long

PEO-block (containing 74 U).




is that two POE chains in the corona accompany

each hydrophobic tail aggregated in a micelle.

4. Conclusions

A novel hydrophobically-associating cationic

comb copolymer is described. Main features of

the copolymer are alkyl chains grafted onto a

backbone containing poly(oxyethylene) chains

and a variable (with pH) cationic charge. The

solubility in aqueous systems shows a complex

behavior with features of both a polyelectrolyte

and of a POE copolymer. The temperature-depen-

dent solubility, as studied by cloud point measure-

ments, has the following main features:

Solubility decreases with increasing

temperature.

Solubility increases with increasing lengths of

the POE blocks.

Solubility increases with increasing charge den-

sity of the polymer.

Solubility decreases with increasing NaCl con-

centration both for charged and uncharged

polymer and both at low and high salt

concentration.

The observations can be understood in terms of

a combination of the effects of interpolymer asso-

ciation, of a decreased polarity of the polymer athigher temperatures and two types of electrostatic

effects. Salt addition has dual effects: For a

charged polymer there is a strongly decreased

solubility due to small additions of salt, which can

be understood as a general electrostatic effect

related to the entropy of the counterion distribu-

tion. In addition, there is a general salting-out

effect, displayed at high salt concentration and

also for the nonionic polymer, attributed to the

depletion of ions in the vicinity of the polymer

chains. The latter effect is well-established for awide range of nonionic polymers, including POE

and cellulose ethers.

Acknowledgements

K. Thuresson thanks the Centre for Am-

phiphilic Polymers (CAP) for financial support.

References

[1] J.E. Glass, Polymers in aqueous media, in: Advances in

Chemistry Series, vol. 223, American Chemical Society,

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[2] D.N. Schulz, J.E. Glass, Polymers as rheology modifiers,

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(1997) 424 – 436.

[4] I. Iliopoulos, Curr. Opin. Colloid Interf. Sci. 3 (1998)

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[5] F.E.J. Bailey, J.V. Koleske, Poly(ethylene oxide), Aca-

demic Press, New York, 1976.

[6] G. Karlstrom, B. Lindman, Phase behavior of nonionic

polymers and surfactants of the oxyethylene type in water

and in other polar solvents, in: S.E. Friberg, B. Lindman

(Eds.), Organized Solutions, Marcel Dekker, New York,

1992.

[7] S. Saeki, N. Kuwahara, M. Nakata, M. Kaneko, Polymer

17 (1976) 685.

[8] G. Karlstrom, J. Phys. Chem. 89 (1985) 4962 – 4964.

[9] D.J. Mitchell, G.J.T. Tiddy, L. Waring, T. Bostock, M.P.

MacDonald, J. Chem. Soc. Faraday Trans. 1 (79) (1983)

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[10] G. Karlstrom, A. Carlsson, B. Lindman, J. Phys. Chem.

94 (1990) 5005 – 5015.

[11] B. Lindman, A. Carlsson, G. Karlstrom, M. Malmsten,Adv. Colloids Interf. Sci. 32 (1990) 183 – 203.

[12] P. Bahadur, K. Pandya, M. Almgren, P. Li, P. Stilbs,

Colloid Polym. Sci. 271 (1993) 657 – 667.

[13] K. Pandya, K. Lad, P. Bahadur, J.M.S. Pure Appl.

Chem. A30 (1993) 1 – 18.

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(1996) 2412 – 2417.

[15] L. Piculell, S. Nilsson, Prog. Colloid Polym. Sci. 82 (1990)

198 – 210.

[16] L. Piculell, B. Lindman, G. Karlstrom, Phase behavior of

polymer – surfactant systems, in: J.C.T. Kwak (Ed.), Poly-

mer – Surfactant Systems, Marcel Dekker, New York,

1998, pp. 65 – 141.

[17] K. Thuresson, S. Nilsson, A.-L. Kjøniksen, H. Walder-

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..



Phase behavior and rheology in water and in model paint formulationsthickened with HM-EHEC: influence of the chemical structure and the

distribution of hydrophobic tails

L. Karlsona, F. Joabssonb, K. Thuressonb,*

a Akzo Nobel Surface Chemistry AB, SE-444 85 Stenungsund, SwedenbPhysical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, PO Box 124, SE-221 00 Lund, Sweden.

Received 13 November 1998; received in revised form 24 February 1999; accepted 28 April 1999

Abstract

The phase behavior and rheology of aqueous solutions of hydrophobically modified ethyl(hydroxyethyl) cellulose have been investigated.

Effects of variations in the chemical structure of the hydrophobic tails grafted to the polymer backbone were followed. When the length of the

polymer hydrophobic tails was increased the effects caused by association between different polymer chains became more pronounced. This

was manifested by an increased tendency of the solution to phase separate, a higher viscosity, and a more elastic rheological response. The

higher elasticity and viscosity was ascribed to slower polymer dynamics following from stronger hydrophobic associations. A separation of

chemically different polymer chains into two coexisting phases was strongly promoted by modification with long hydrophobic tails. It was

found that one of the coexisting phases contained highly substituted polymer chains, while in the other phase, less substituted polymer chains

were found. It is proposed that this type of phase separation occurs because the highly substituted polymer chains have a pronounced

tendency to form a network.

Model paint formulations prepared with the different polymers showed that an increasing length of the polymer hydrophobic tails slowed

down the dynamics of the formulation. This was manifested as a higher thickening efficiency (a smaller amount of polymer material was

needed to obtain the desired viscosity), and a more pronounced shear-thinning behavior of formulations comprising polymers with long

hydrophobic tails. Compared to the simpler systems, which only contained polymer and water, the model paint formulations were less prone

to phase separation. It is suggested that, in the paint formulation, surfactants, latex particles, pigment, and fillers increase the number of

possible association sites for the polymer hydrophobic tails. 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Hydrophobically modified polymer; Associative thickener; Ethyl(hydroxyethyl) cellulose; EHEC; Phase behavior; Viscosity; Rheology; Paint

formulation

1. Introduction

Associative polymers are a class of polymers that have

found a widespread use in technical formulations. Their

main function is as thickening compounds, and thus, they

are added in order to obtain a desired consistency. Polymersbelonging to this group are amphiphilic, containing lyophi-

lic as well as lyophobic segments (Glass, 1989; Landoll,

1982). In an aqueous solution hydrophilic segments are

responsible for the hydration and swelling, while the hydro-

phobic parts of the polymer chain minimize the contact with

water by assembling in aggregates (cf. the micellization

process of surfactants) (Cabane, Lindell, Engstrom &

Lindman, 1996). As a result, hydrophobic modification of

the polymer chains may decrease the viscosity of its

aqueous solution. This is commonly seen at low concentra-

tions where different polymer chains have a low probability

to interact. This observation has been attributed to intra-

aggregation of individual polymer chains, and the accom-panying reduced size of the polymer coils (Bock, Siano,

Valint & Pace, 1989; Gelman, 1987; Strauss, 1989; Tanaka,

Meadows, Philips & Williams, 1990). However, at higher

concentrations inter -aggregation becomes increasingly

more important, and already at a concentration in the vici-

nity of the overlap concentration, c, of the unmodified

parent polymer the viscosity is increased substantially by

hydrophobic modification (Schaller, 1985; Semenov,

Joanny & Khokhlov, 1995; Shaw & Leipold, 1985).

The polymer used in the present investigation,

hydrophobically modified ethyl(hydroxyethyl) cellulose

Carbohydrate Polymers 41 (2000) 25–35

CARP 1351

0144-8617/99/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.

PII: S0144-8617(99) 00067-3

www.elsevier.com/locate/carbpol

* Corresponding author. Tel.: 46-46-222-0112; fax: 46-46-222-

4413.

E-mail address: [email protected] (K. Thuresson)



(HM-EHEC),is a technically important associative thick-

ener that can be obtained in different grades. A common

feature of different HM-EHEC fractions is that they all

are cellulose ethers to which a low amount of hydro-

phobic side chains have been attached by a chemical

reaction (Bostrom, Invarsson & Sundberg 1992). A typi-

cal grafting density of hydrophobic moieties is less than

5 per 100 repeating glucose units of the polymer chain.

A growing commercial interest, but also more basic

scientific interests regarding associative polymers, has

recently prompted investigations regarding HM-EHEC.

These studies dealt with the rheology and phase beha-

vior of HM-EHEC in an aqueous solution. Focus was

on the influence by a third additive (such as salt, alco-

hol (Thuresson, Nilsson & Lindman, 1996), or surfac-

tant (Joabsson, Rosen, Thuresson, Piculell & Lindman,

1998; Thuresson & Lindman, 1997; Thuresson, Lind-

man & Nystrom, 1997). Comparisons were always

done with corresponding solutions based on the unmo-

dified analogue in order to trace the effects of the modi-fication step. Less work has been devoted to effects of a

variation in the hydrophobic modification (density or

type). Therefore, we have in the present investigation

varied the length of the hydrophobic moiety in a

controlled way, and followed changes in phase behavior

and rheology of the quasi-binary solution (polymer

water). From a technical point of view it is important to

know how an associative thickener influences more

complex solutions. For this reason, some simple experi-

ments (stability and rheology) in model paint formula-

tions were also performed. Here interactions of the

polymer chains with surfactants, latex particles,pigments, fillers, etc. can be anticipated.

2. Experimental

2.1. Materials

The cellulose used for the synthesis of the (HM-)EHEC

polymers was supplied by Borregaard (Borregaard SVS

19T). Chloroethane was supplied by Hydro Polymers AB

(Stenungsund, Sweden). Epichlorohydrin of Puriss grade

was obtained from Fluka. Tintetrachloride (SnCl4) of Puriss

grade was purchased from Merck. Epoxyethane (EO) andnonylphenol (NP) were supplied by Akzo Nobel Surface

Chemistry AB (Stenungsund, Sweden). Dodecanol (C12),

tetradecanol (C14), hexadecanol (C16), and a mixture of

hexadecanol and octadecanol (C16–18) were all obtained

from Condea. Their trade names are Nacol 1296, Nacol

1495, Nacol 1695 and Nafol 1618, respectively. All have

a purity which is better than 95% (w/w), except Nafol 1618

which is sold with thespecification that beside 63^ 4% (w/w)

hexadecanol and 33 ^ 4% (w/w) octadecanol, the mixture

may also contain 0.2% (w/w) C12, 2% (w/w) C14,

3% (w/w) C20, and 0.2% (w/w) C22. Diethyleneglycol

monobutyl ether (BDG) was of 97%-purity and purchased

from BDH. Acetone of pro analysis grade was obtained

from Merck.

The following components were used to formulate the

model paints; a defoamer from BYK Chemie (trade name

Byk 022), a dispersing agent from Rohm & Haas (trade

name Tamol 731), a preservative from Angus Chemie

GmbH (trade name Canguard), a filler from Omya (trade

name Hydrocarb), a pigment from Kronos AS (trade name

Kronos 2190), and a binder from Vinamul (trade name

Vinamul 3650).

2.2. Synthesis

The hydrophobically modified EHECs were synthesized

according to a standard procedure (Bostrom et al., 1992).

2.3. Purification

The purification of the synthesized polymers from by-products originating from the reaction process was made

in several steps. First the polymer products were washed

with hot water containing 10% (w/w) Na2SO4. The tempera-

ture (95C) of the water was far above the cloud point of the

polymer. At these conditions only a small fraction of the

polymer material is soluble in water. In a recent investiga-

tion it was found that at equilibrium (at such high tempera-

tures) the polymer-rich phase, which has a concentration of

20–30% (w/w), is in equilibrium with a phase of a much

lower polymer concentration containing only about 20% dry

weight of the total polymer material (Joabsson et al., 1998).

Salt, and in particular salt with highly charged ions, likeSO42, is expected to reduce the solubility of EHEC signifi-

cantly. By taking into account that the equilibrium situation

is far from reached we expect to lose insignificant quantities

of the polymer via the rinsing water, while reducing the

content of NaCl and low molecular weight poly(ethylene-

glycol) (PEG). After washing with hot water each polymer

sample was dried and ground into granules with a size of ca.

0.3 mm. To get rid of remaining PEG and hydrophobic

material which not has been successfully attached to the

polymer chains, the dry granules were washed with acetone

under vigorous stirring for 30 min. After washing, the

product was collected by vacuum filtration with a Buchner

funnel. This procedure was repeated two or three times.After the final acetone aliquot had been removed by filtra-

tion the polymer powder was dried, and later dissolved in

water to a concentration of ca. 1% (w/w). This solution was

centrifuged during 1 h at ca. 10 000g to eliminate water

insoluble impurities, such as non-modified cellulose. In

the final step of the purification procedure, the supernatant

was dialyzed against an excess of Millipore water for

several days with a repeated exchange of the dialysis

water, to get rid of small water soluble impurities (remain-

ing salt, traces of PEG etc.). The dialysis was performed by

using a Spectra/Por membrane tubing with a molecular

L. Karlson et al. / Carbohydrate Polymers 41 (2000) 25– 3526



weight cut off of 6–8000. After the dialysis the polymer

material was recovered by freeze-drying.

2.4. Characterization

The degrees of substitution of hydroxyethyl and ethyl

groups, MSEO 2.1 and DSethyl 0.8, were determined

by gas chromatography following degradation of the

products with HBr in glacial acetic acid (Stead & Hindley,

1969. The given values (MSEO and DSethyl) correspond to the

average numbers of EO and ethyl groups per sugar unit,

respectively. The substitution degrees of hydrophobic tails

(C12, C14, C16, C16–18), MShydrophobe, were determined by aprocedure given by Landoll (1982). In one HM-EHEC

sample (hydrophobically modified with NP), MShydrophobe

was determined by measuring the absorption of light at

275 nm with a UV-vis spectrophotometer using phenol in

aqueous solution as a reference. All numbers (MSEO, DSethyl,

MShydrophobe) for the six different polymers are summarized

in Table 1. Below the different polymers will be referred to

as EHEC, HM(NP)-EHEC, HM(C12)-EHEC, HM(C14)-

EHEC, HM(C16)-EHEC, and HM(C16–18)-EHEC, respec-

tively.

2.5. Methods

All samples (except the model paint formulations) were

prepared by weighing the different components into test

tubes which were sealed with Teflon tightened screw caps.

If possible, the samples were carefully mixed for an

extended time (several days) at a temperature where they

did not phase separate. After complete mixing each sample

was equilibrated at the temperature of interest.

The model paint formulations were prepared by slowly

adding the thickener (the (HM-)EHEC polymer) to the

water under vigorous stirring with a magnetic bar. After

about 3 h the thickener was completely dissolved. A

‘pigment paste’ was prepared by adding part of the defoa-

mer, the dispersing agent, the preservative, the filler, and the

pigment to the thickener solution. The mixture was

dispersed with a mixer from Diat. The rotating part of the

mixer had a diameter of 4 cm and the mixing was performed

at 3000 rpm for 20 min. The binder and the remaining part

of the defoamer were then added to the pigment paste undervigorous stirring. Finally, the paint was stirred for another

20 min (see also Table 3).

(HM-)EHEC polymers in aqueous solution have a

reversed temperature dependency and a lower critical

consolute temperature. The behavior is unexpected from

simple thermodynamic considerations, and in passing we

note that different theoretical models have dealt with this

subject (Goldstein, 1984; Karlstrom, 1985; Karlstrom,

Carlsson & Lindman, 1990; Kjellander & Florin, 1981).

Because of the concomitant scattering of light when the

solution phase separates this temperature is often referred

to as the cloud point, T cp. In the present investigation the T cpwas taken as the temperature where the solution first became

hazy as determined by visual inspection. The determinations

were conducted on 1% (w/w) solutions in spectrophot-

ometer cuvettes immersed in a temperature controlled

water bath. The T cp was approached by increasing the

temperature in steps of 2C with a waiting time of 20 min

after each temperature step to allow for thermal equilibrium

before observation of the samples.

In the phase studies the samples were carefully equili-

brated and macroscopically separated at the temperature

L. Karlson et al. / Carbohydrate Polymers 41 (2000) 25– 35 27

Table 1

The substitution degrees of ethyleneoxide (MSEO), ethyl (DSethyl), and

hydrophobic tails (MShydrophobe) of each of the polymer samples given as

average numbers of substituents per repeating glucose unit. Independent

repeated determinations render an uncertainty in the numerical values of

about 5%. The abbreviations given in the ‘Hydrophobe’ column refers to

the unmodified parent EHEC (0), HM-EHEC modified with, nonylphenol

groups (NP), C12 groups (C12), C14 groups (C14), C16 groups (C16), and with

C16–C18 groups (C16–18). Values within brackets are determined for polymersamples after the dialysis, while the other numbers refer to the polymer

samples before the dialysis step

Hydrophobe MSEO DSethyl MShydrophobe

0 2.1 0.8 0

NP 2.1 0.8 (0.0079)

C12 2.1 0.8 0.0086 (0.0090)

C14 2.1 0.8 0.0082 (0.0074)

C16 2.1 0.8 0.0081 (0.0068)

C16–18 2.1 0.8 0.0091

Table 2

The relative weight of the two coexisting phases (including water) and the concentration of HM-EHEC, cp, in the top and the bottom phases in samples thatphase separated macroscopically at 30C. The values of the average hydrophobic modification degree, MShydrophobe, of the HM-EHEC chains in each phase is

also given. ‘TP’ refers to top phase, ‘BP’ to bottom phase and ‘mean’ refers to the average value of both phases. Two numbers given in the same box

corresponds to independent phase separations and composition analyses. Upper and lower halves is for HM-EHEC substituted with C14 and C16 hydrophobic

moieties, respectively

Weight fraction, BP cp, TP cp, BP cp, mean MShyd, TP MShyd, BP MShyd, mean

C14

42% (w/w) 0.83% (w/w) 1.37% (w/w) 1.05% (w/w) 0.006 0.010 0.009

30% (w/w) 0.83% (w/w) 1.33% (w/w) 0.99% (w/w) 0.010 0.013 0.011

C16

47% (w/w) 0.62% (w/w) 1.48% (w/w) 1.03% (w/w) 0.004 0.011 0.009

52% (w/w) 0.59% (w/w) 1.35% (w/w) 0.99% (w/w) 0.004 0.010 0.009



of interest (30C). If needed, the separation process was

speeded up with a centrifuge. During centrifugation the

temperature was kept at 30 ^ 0.5C. The polymer concen-

tration in each phase was determined by freeze-drying and

weighing. The average hydrophobic modification degree of

the polymer material in each phase was determined by either

of the two procedures described above.

The rheological measurements were performed with a


Fig. 1. (a) The viscoelastic response (G and G vs. angular frequency) of a 2% (w/w) EHEC solution (0) and a 2% (w/w) HM-EHEC solution modified with

C14 chains (C14). The cross over angular frequencies used to calculate the characteristic times are indicated. For the EHEC solution the cross over was

estimated by a linear extrapolation of G and G

. Note that G

and G

for HM(C14)-EHEC (upper set of curves) have been shifted upwards. The insert shows the

suggested evolution of G and G by the Maxwell model (see text). G∞ 1 Pa and

1 s were used in the calculation. The measurements were performed at

3C. (b). The complex viscosity, , from oscillatory shear experiments vs. angular frequency for 2% (w/w) aqueous solutions of the different (HM-)EHECs.

The angular frequency, 2 f #, used to calculate the characteristic time # is indicated for the solution prepared with HM(C14)-EHEC. The full lines represents,

from bottom to top, unmodified EHEC (0), HM-EHEC modified with nonylphenol groups (NP), with C12, C14 and C16 groups. The measurements were

performed at 3C.



StressTech rheometer from Rheologica equipped with a

4 cm, 1 cone and plate system. The temperature of the

sample during a rheological measurement was controlled

to within ^ 0.1C by an external water bath connected to

the measuring geometry. Measurements were performed at

(HM-)EHEC concentrations of 1% (w/w) or 2% (w/w). For

samples with a polymer concentration of 1% (w/w), and for

measurements on the solvent mixtures containing BDG and

water at different mixing ratios, the rheometer was put in the

constant shear mode. For these solutions the Newtonian

plateau, characterized by a shear rate independent viscosity,

was always reached.

In order to slow down polymer dynamics and to facilitate

the determination of the cross-over times, and #,

(defined below) a (HM-)EHEC concentration of 2% (w/w)

was employed. The rheometer was put in the oscillatory

shear mode, and all measurements were performed in the

‘linear’ regime, where the response was independent of the

applied stress. An oscillatory shear measurement reports the

storage, G , and loss, G , moduli as a function of thefrequency, f , of the oscillation. The complex viscosity, ,

was calculated as G2 G 2

2 f . The low frequency

limit characterized by a Newtonian behavior (viscosity

independent of f ) was reached for all samples except for

the aqueous solution containing 2% (w/w) HM(C16)-

EHEC (see Fig. 1(b)).

In this investigation two methods, which both use a cross-

over from a viscous to an elastic behavior of the solution,

have been used to indirectly probe polymer dynamics. is

the characteristic time corresponding to the inverse angular

frequency, 1/2 f , where G G

. At frequencies higher

than f

the response is mainly elastic. An elastic responseindicates that on the experimental time scale, the polymer

chains do not have the time to relax to the new equilibrium

position due to the applied deformation. In the low-viscous

samples (i.e. the samples prepared with EHEC, HM(NP)-

EHEC, and HM(C12)-EHEC) this frequency is not directly

accessible with the present rheometer. Here, the presented

values have been estimated by linearly extrapolating the

evolution of G and G vs. frequency in a log–log representa-

tion, Fig. 1(a). An alternative way to probe the crossover from

a viscous to an elastic behavior of the solution is to determine

the angular frequency at which the viscosity profile ( vs.

2 f ) changes from being Newtonian to shear thinning, Fig.

1(b). The characteristic time, # 1 2 f #, corresponding tothe inverse of thisangular frequency, is for the present samples

longer than , and the two methods become complemen-

tary. Indeed and # follow the same trend (cf. Fig. 6).

If the rheological data from a polymer system are deter-

mined by only one relaxation time, the simplest model of a

viscoelastic fluid, namely the Maxwell model, can be used to

describe the frequency dependencies of the dynamic moduli.

Then the evolutions of G and G with f are given by:

G G

∞

2 f 2

1 2 f 2 and G

G∞

2 f

1 2 f 2

Here G∞

is the plateau value of G at high frequencies. In this

model equals # In particular it follows from the Maxwell

model thatlog(G) has the slope 2asafunctionoflog( f )and

log(G) has the slope 1 in the terminal zone (low frequen-

cies), while at frequencies above , log(G ) has the slope 1

while G approaches G

∞. However, this model cannot satis-

factorily describe our data as is evident from the different

shapes of the curves (compare data in Fig. 1(a) with insert).

Rather the frequency dependencies of the dynamic moduli

indicate that a range of relaxation times has to be used to

give a proper representation. In light of the expected polydis-

persity regarding the molecular weight as well as the chemical

structure of the (HM-)EHEC polymers this is not surprising.

Thus, the reason for the differences in and

# can probably

be traced to the fact that the relaxation in solutions of

(HM-)EHEC is due to a range of different processes with

different characteristic times.

In the industry one method to characterize paints is to

measure the viscosity at one low and one high shear rate.

The two values are referred to as the ‘Stormer’ and ‘ICI’viscosities, respectively. Here the Stormer viscosity was

measured with a Stormer viscometer at a shear rate in the

range 10–100 s1. The values are given in Krebs units

(KU). The conversion of the numerical values from KU to

the SI-quantity Pa s can to our knowledge only be made

with an approximate formula (Reisser, M. & Oleinitec,

A.B., private communication).1 These measurements were

made in accordance with the ASTM D 562-81 standard. The

ICI viscosity was obtained at a shear rate corresponding to

approximately 12 000 s1 and is usually given in Poise.

However, here we have chosen to present these data in

Pa s to facilitate comparison with other measurements(10 P 1 Pa s). The measurements were made in accor-

dance with the ASTM D 4287-94 standard.

The model paints were characterized according to color

acceptance (ASTM D 5326), leveling (ASTM D 4062-81),

spatter resistance, gloss (ASTM D 523-89), and storage

stability. For the color acceptance test a universal colorant

was added to the paint formulation. After shaking the paint

for 5 min it was applied to a white chart. A ‘rub-out test’ in

which one part of the surface of the painted chart was

rubbed while another part was untouched was performed.

The paint was judged by means of color differences between

the two different parts, and by a comparison with standard

charts the paint formulation was given a rating of 1–10; 1corresponds to a poor resistance, while 10 to an excellent

resistance to mechanical rubbing.

Leveling of the model paint formulations was measured

by the ‘Lenata draw-down method’. The results were


1 Over the viscosity range of 0.2–2.1 Pa s the following equation may be

used to convert Stormer viscosity values from Krebs units to Pa s: ln(KU) 1.1187 0.8542 ln(193.8 Pa s 36) 0.0443[ln(193.8 Pa s 36)]2.

Over the viscosity range of 2.1–5.0 Pa s it is more appropriate to use:

ln(KU) 1.8118 0.596 ln(193.8 Pa s 36) 0.0206[ln(193.8 Pa s

36)]2. Here KU corresponds to the viscosity at 25C in Krebs units, while

Pa s is the viscosity at 25C in Pa s.



compared with standards and given ratings from 1 to 10

where 1 is a low degree of leveling and 10 is an excellent

degree of leveling.

The spatter resistance was measured by applying the paint

formulation with a roller onto a ‘wall’ with an area of

0.25 m2 at a frequency of 60 strokes/min. The spatter was

collected on a test chart located below the rolled surface,

and the result was compared with a standard. Again the

paint formulation was given a rating from 1 to 10 where 1

is a poor performance and 10 corresponds to a low spatter

and excellent spatter resistance.To evaluate the gloss properties of the paint formulation it

was applied to a glass panel. The reflectance was measured

after 10 days at an angle of 60 and is reported in percentage

of the reflectance of a standard surface of polished glass.

The storage stability of the paint was estimated by a

repeated measurement of the Stormer viscosity after 28

days of storage at 50C.

3. Results and discussion

This section is divided into three parts, each of which

contains a discussion of the effect of hydrophobic modifica-

tion of the polymer chains.

3.1. Phase behavior

It has been found that for EHEC (without hydrophobic

modification) the T cp can be correlated to MSEO and DSethyl

in a way such that T cp increases with increasing MSEO, while

it decreases with increasing DSethyl, Fig. 2 (Thuresson, Karl-

strom & Lindman, 1995). It was also found that the conver-

sion of EHEC to HM(NP)-EHEC had a drastic influence on

T cp. This is also found in the present investigation, Fig. 3.

The longer the aliphatic chain, the more pronounced is the

shift in T cp (relative to that of the unmodified EHEC). It is

important to note that the overall hydrophilic/hydrophobic

balance is only slightly changed by a variation in hydropho-

bic tail length, and consequently this cannot be the explana-tion to the pronounced shifts in T cp. It is therefore reasonable

to invoke a mechanism in which the phase separation is

influenced by the association of polymer hydrophobic tails

into micellar aggregates (Thuresson & Joabsson, 1999).

These connect different polymer chains into a network.

The swelling of the polymer matrix is then restricted by

finite extension of the part of the cellulose chains connecting

different micellar aggregates. This favors the formation of

one phase that is concentrated in polymer. This phase is in

equilibrium with a phase depleted in polymer (cf. a bridging

flocculation in a polymer/particle system or the restricted


Fig. 2. The cloud point of a collection of different EHEC samples (1% (w/w)) plotted against the fraction of hydroxyethyl groups, MSEO /(MSEO DSethyl). The

data is reproduced from Thuresson et al. (1995). An open circle and a filled circle represent the unmodified EHEC and the HM(NP)-EHEC corresponding to the

samples in the present investigation, respectively. The dashed line is only drawn as a guide to the eye.

Fig. 3. The cloud point, T cp, for the different (HM-)EHEC samples in 1%

(w/w) aqueous solutions. From left to right; unmodified EHEC (0), HM-

EHEC modified with nonylphenol groups (NP), with C12, C14 and C16

groups, and with C16–C18 groups(C16–18). The one phase region of the last

sample, HM(C16–18)-EHEC, is not experimentally accessible (T cp 0C).



swelling of a covalently crosslinked gel). The outlinedscenario requires the hydrophobic associations to be strong

enough to overcome the decrease in entropy that accompa-

nies the formation of the phase concentrated in polymer.

However, the unfavorable reduction of the entropy is signif-

icantly reduced if both phases contain approximately the

same polymer concentration. This has recently been

discussed by Annable and Ettelaie (1994) in an experimen-

tal and theoretical study, based on a simple Flory–Huggins

approach. They discovered that a mixture of a hydrophobi-

cally modified polymer with its unmodified analogue might

phase separate. After the phase separation the two different

polymers are enriched in separate phases. Below we will

refer to this behavior as a segregative phase separation, or

alternatively that the polymers phase separate segregatively

(Piculell & Lindman, 1992).Based on this knowledge, mixtures prepared with the

HM(C16)-EHEC and the HM(C14)-EHEC polymers, respec-

tively, were macroscopically phase separated at 30C into

two liquid phases in equilibrium. In each sample, the two

phases had similar volumes (Table 2). Each phase was

analyzed with respect to polymer concentration and the

average hydrophobic modification degree. Despite the

uncertainty in the data, the trend is clear and it was found

that the samples separated segregatively with one phase

containing HM-EHEC chains with a higher substitution

degree than the chains residing in the other phase. Thus,

the HM-EHEC sample contains a range of polymer mole-cules differing in molecular weight, a normal situation for

all polymer samples, and also in chemical composition, and

the ‘binary’ aqueous solution has a behavior reminiscent of

a multi-component system. This conclusion was also drawn

in a previous publication in which the phase behavior of

EHEC and HM(NP)-EHEC at high temperatures was inves-

tigated (Joabsson et al., 1998). In connection with this

discussion it is valuable to comment on the fact that

although T cp for a 1% (w/w) HM(C14)-EHEC solution is

reported to be ca. 32C (Fig. 3), a macroscopic segregative

phase separation is observed already at 30C (Table 2). This

reflects the difficulties in observing this type of phase

separation by means of the clouding phenomenon. Wethink the disagreement is due to the fact that in the vicinity

of the phase separation temperature the two phases have

similar compositions (segregative phase separation with

similar polymer concentrations), differing mainly in a

small variation of hydrophobic modification as shown

above. Thus, at such conditions the two phases can be

expected to have a similar refractive index and light is not

scattered (no clouding is observed). The conjecture is that

there is a risk of overestimating the true phase separation

temperature by visual detection of T cp. In the present HM-

EHEC systems we estimate the uncertainty in the phase


Fig. 4. (a) The bar to the left in each collection (gray) reports the viscosity

of the polymer in water, and the bar to the right (striped) reports the

viscosity in a 80:20 mixture of water and BDG (see text). To compensate

for the higher viscosity of the solvent (water/BDG compared to water), the

numerical values for the viscosity in water/BDG mixtures were divided

with 2.7 before presentation (cf. insert Fig. 5). The line reports on the‘Q-value’, which is the ratio between the viscosity of the aqueous polymer

solution and the viscosity of the polymer dissolved in the water/BDG

mixture. The different groups of bars correspond from left to right to,

unmodified EHEC (0); EHEC modified with nonylphenol groups (NP),

with C12, C14, and C16 groups. All solutions contain 1% (w/w) polymer

and the measurements were performed at 3C. The reported values corre-

spond to the viscosity at the Newtonian plateau and they were obtained with

the rheometer put in the constant shear mode. (b) Same as Fig. 4(a), but at

20C. At this temperature it was impossible to measure the viscosity of the

aqueous solution of HM(C16)-EHEC due to a phase separation. Note that

the solution of this polymer in the water/BDG mixture has a one phase

behavior.

Table 3

Recipe for the preparation of the model paints formulation. x is the weight

per mille (‰w/w) of thickener in the model formulation. The different

components are added to the mixture in the same order as listed in the

table (top to bottom). Note that the defoamer is added in two portions.

For details, see Section 2.5

Water (‰ w/w) 243.2 x

Thickener, HM-EHEC (‰ w/w) xDefoamer, Byk 022 (‰ w/w) 2

Dispersing agent, Tamol 731 (‰ w/w) 6.5

Preservative, Canguard (‰ w/w) 1

Filler, Hydrocarb (‰ w/w) 110

Pigment, Kronos 2190 (‰ w/w) 180

Binder, Vinamul 3650 (‰ w/w) 454.3

Defoamer, Byk 022 (‰ w/w) 3 (‰ w/w) 1000



separation temperature, by using the clouding phenomena,

to ^ 5C.

3.2. Rheology in quasi-binary solutions

Usually the expectation is that the viscosity of an aqueous

solution of a hydrophobically modified polymer increases

with the length of the hydrophobic tails. The higher viscos-

ity has been ascribed to slower motions of individual poly-mer chains as the residence time of the hydrophobic tails in

‘polymer micelles’ increases (Annable, Buscall, Ettelaie &

Whittlestone, 1993; Leibler, Rubinstein & Colby, 1991).

The present results follow that trend. Measurements

performed on aqueous solutions at two different tempera-

tures (3 and 20C) are reported in Fig. 4. The viscosity for

the HM(C16)-EHEC solution at 20C could not be measured

as a natural consequence of the phase separation which

occurred already at a much lower temperature (cf. Fig. 3).

Fig. 4 also gives the viscosity of the different polymers in a

solvent mixture composed of 80% (w/w) water and 20% (w/

w) BDG. The knowledge that a ‘saturation level’ of BDG

usually is observed was decisive when the ratio betweenwater and BDG was chosen. This is exemplified in Fig. 5,

where data for 1% (w/w) HM(C14)-EHEC and 1% (w/w)

HM(NP)-EHEC solutions are presented. Initially the visc-

osity decreases strongly, but at about 15% w/w BDG the

decrease levels off and above that BDG concentration the

relative viscosity of both the investigated polymer solutions

is constant within the experimental uncertainty. The effect

of BDG can be rationalized by comparison with the well-

known influence of surfactants on aqueous solutions of

hydrophobically modified polymers. These are regarded

to break intermolecular hydrophobic associations by

‘individually dissolving’ hydrophobic patches (or hydro-

phobic tails) within micellar-like aggregates (Piculell, Thur-

esson & Ericsson, 1995). Indeed, BDG has an amphiphilic

structure and is expected to adsorb at polar/non-polar inter-

faces. As a consequence of decreased hydrophobic associa-

tions, addition of BDG can also be anticipated to decrease

the tendency to segregative phase separation. In particular

the phase separation of an aqueous solution of HM(C16)-

EHEC was inhibited in the water/BDG mixture and theviscosity of that solution could be determined (Fig. 4(b)).

It is also interesting that all (HM-)EHEC polymers, inde-

pendent of the nature of the polymer hydrophobic tail, give

virtually the same viscosity when they are dissolved in the

water/BDG mixture. This indicates that the different

(HM-)EHEC polymers have similar molecular weights,

and that the differences observed between the aqueous solu-

tions can be referred to variations in the aggregation process

of the polymer hydrophobic tails. Thus, the Q-value, which

is the ratio between the value of the (Newtonian) viscosity

in water to that observed in water/BDG, can be regarded as a

phenomenological measurement of the influence of hydro-

phobic associations on the viscosity of the aqueous solution.In this way different polymer samples (variation in chemical

structure of the hydrophobic tails, variation in modification

degree, variation in modification pattern etc.) can be ranked.

In line with this, we note that the unmodified EHEC has a Q-

value close to 1. However, comparisons of Q-values should

be done with care because the evolution of Q with polymer

molecular weight has not been investigated.

Generally a shift towards longer relaxation times is

expected when the polymer chains in a solution become

more entangled (increased polymer concentration or higher

polymer molecular weight) or when the life time of


Fig. 5. The influence on viscosity of the composition of the solvent in solutions of associating HM-EHECs as exemplified with 1% (w/w) solutions of HM-

EHEC modified with nonylphenol groups (NP) or with C14 groups (C14). It can be seen that the viscosity levels off at an approximate weight mixing ratio of BDG:water corresponding to 15:85. The data are presented as a relative viscosity (the viscosity of the polymer solution divided with the viscosity of the solvent

mixture). The insert reports the viscosity of the BDG:water mixture at different compositions. All measurements were performed at 20 C with the rheometer

put in the constant shear mode.



intermolecular hydrophobic associations becomes longer

(in case of hydrophobically modified polymers). Extracted

from an oscillatory shear experiment, which reports indir-

ectly on polymer dynamics, the change in the characteristictimes, and #, may be taken to reflect such variations. At

experimental times exceeding and # the viscous beha-

vior of the solution dominates, while at shorter times the

elastic behavior prevails. Indeed, and # increase with an

increasing length of the hydrophobic tail (Fig. 6). We stress

that and # not are expected to correspond to any char-

acteristic time of any process on a molecular level, but we

do believe that they capture trends correctly and therefore

can be used to compare different HM-EHEC polymers. We

note that a shift towards longer relaxation times, following

an increased hydrophobicity of the hydrophobic tails, will

on a fixed experimental time scale (as in applications) showup as a more elastic behavior of the solution.

3.3. Model paint formulations

A recipe of the formulations is given in Table 3. The

concentration of (HM-)EHEC in the final mixtures varies

because a paint is formulated aiming at a certain (Stormer)

viscosity, rather than a certain polymer concentration (Table

4). The amount of polymer that is needed decreases when

the polymer hydrophobic tails becomes longer. In other

words; HM-EHECs with strongly associating hydrophobic

tails have a high thickening efficiency. It is interesting to

note that despite the rather extreme conditions which themodel paint formulations were exposed to (50C for 28

days was used in the storage stability test) no phase separa-

tion was observed. Rather the model paint formulations had

a good stability and only minor changes in the viscosity with

time could be detected (compare the storage stability

column with the Stormer viscosity column in Table 4).

This is in direct contrast to the results obtained in the

simpler ‘binary’ solutions. We recall that certain HM-

EHEC solutions phase separated already at room tempera-

ture (see Fig. 3). This can be taken as an indication of the

fact that surfactants, pigments, latex particles, fillers etc.

influence the aggregation process. The phase separation

may be inhibited by an increased number of sites where

the polymer hydrophobic tails can adsorb. However, at

this stage it is too early to draw any further conclusions.Another interesting result, which follows from Table 4, is

that HM-EHEC with long hydrophobic tails give paints with

a more elastic consistency as compared with paints prepared

with thickeners with shorter hydrophobic tails. This corre-

lates to the slower relaxation process with increasing

strength of the inter-molecular associations which was

observed as a higher elasticity by oscillatory shear measure-

ments already in the ‘binary’ solutions. The slower relaxa-

tion process of solutions prepared with HM-EHEC modified

with long aliphatic chains is also reflected in the viscosity

values reported in Table 4. The more pronounced shear-

thinning behavior of solutions of HM-EHEC comprisinglong hydrophobic tails follows from lower ICI viscosity

values.


Table 4

Summary of the observations in the model paint formulations. The different columns refer to unmodified EHEC (0), HM-EHEC modified with nonylphenol

groups (NP), with C12, C14, and C16 groups, and with C16–C18 groups (C16–18). The numbers given for color acceptance, spatter resistance, and leveling are

ratings obtained by visual comparisons to standards. 1 is poor and 10 is excellent. x is the ‰ (w/w) of the thickener that is needed to obtain the desired Stormer

viscosity

0 NP C12 C14 C16 C16–18

x (‰ w/w) 7.5 4.5 5.75 4.5 4 3.9Stormer viscosity (KU) 110 113 112 112 113 109

ICI viscosity (Pa s) 0.18 0.15 0.16 0.11 0.10 0.10

Colour acceptance 10 8 6 6 6 6

Spatter resistance 4 7 7 7 7 6

Levelling 1 1 1 1 1 1

Gloss (%) 24 27 26 24 26 24

Storage stability (KU) 123 120 119 120 121 119

Consistency flowing flowing flowing flowing jelly jelly

Fig. 6. The variation of (to the left in each collection) and # (to the right

in each collection) with the length of the hydrophobic tail. The cross over

times were determined as illustrated in Fig. 1. From left to right; unmodified

EHEC (0), HM-EHEC modified with nonylphenol groups (NP), with C12,

C14, and C16 groups. The line over each bar represents an estimated error.

The polymer concentration was kept constant at 2% (w/w) and the tempera-

ture was 3C.



4. Conclusions

One of the reasons why hydrophobically modified poly-

mers have appeared on the market, and sold as thickeners, is

that they often have been found to provide a better thicken-

ing efficiency in aqueous solutions than their unmodified

analogues. In this paper we have in a systematic way varied

the chemical structure of the hydrophobic moieties that

were grafted to the polymer backbone when ethyl(hydrox-

yethyl) cellulose, EHEC, was converted to obtain the hydro-

phobically modified HM-EHEC. By changing the length of

hydrophobic tails the strength of the associations between

different polymer molecules was modulated. At the condi-

tions in the present investigation—HM-EHEC with a

substitution degree of MShydrophobe 0.008 dissolved in an

aqueous solution to a concentration of 1% (w/w)—we

observed that the hydrophobic tails should contain more

than 12 carbon atoms to give a thickening efficiency signif-

icantly better than that of the unmodified analogue (see Fig.

4(a)). It was also found that the strong hydrophobicassociations that accompanied the modification with long

hydrophobic tails resulted in solutions that were more

elastic on a given time scale. This may be a problem in

applications (such as water borne paints). Thus, in the

design of an effective associative thickener, other aspects

apart from high viscosity following strong hydrophobic

associations have to be taken into account, and it is not

sufficient to rank different HM-polymers only by thickening

efficiency.

The literature reports that, provided inter molecular asso-

ciations are strong enough; increasing water content of an

aqueous solution of HM-EHEC induces a phase separation.Under such circumstances one of the phases contains

virtually pure water, while most of the polymer material

can be found in the other phase. The observation was ratio-

nalized in terms of a behavior reminiscent of restricted swel-

ling of a covalently bonded gel (Thuresson & Joabsson,

1998). In the present investigation we found that also a

modification of the polymer chains with tails of a quite

modest hydrophobicity can induce a phase separation. The

reason is an inhomogeneous distribution of hydrophobic

tails among the polymer chains. At such conditions, the

solution has a tendency to phase separate segregatively

into one phase containing polymer chains with a high substi-

tution degree, and the other phase containing polymerchains with a lower substitution degree. The mechanism is

related to that of the ‘restricted swelling’, but in addition the

tendency towards phase separation is increased because

both phases have similar polymer concentration.

Finally, it is interesting to note that when the HM-EHEC

polymers were in model paint formulations the tendency to

segregative phase separation was decreased. One explana-

tion to this observation can be that surfactants, latex parti-

cles, pigment, and fillers supply association sites for the

polymer hydrophobic tails. This is expected to promote a

single-phase behavior if the reason for a phase separation

can be traced to a restricted swelling of the HM-EHEC

matrix.

Acknowledgements

F.J. and K.T. thank the Centre for Amphiphilic Polymers

(CAP) for financial support. Dr. Lennart Piculell is grate-fully acknowledged for comments on this manuscript.

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Cyclodextrins in HM-PEG Solutions.

Inhibition of Polymer-Polymer Associations

L. Karlson,* ,1 K. Thuresson,2 and B. Lindman.2 1 Akzo Nobel Surface Chemistry AB, SE-444 85 Stenungsund, Sweden

2 Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University,

P.O. Box 124, SE-221 00 Lund, Sweden.

* To whom correspondence should be addressed.

fax: +46 303 839 21

e-mail: [email protected]

Abstract

In an aqueous solution of a hydrophobically end-modified PEG-polymer, HM-PEG, the

thickening effect is dependent on intermolecular hydrophobic associations and the formation

of a network structure. In the present investigation cyclodextrin, CD, has been added to an

aqueous HM-PEG solution and a decrease in Newtonian viscosity has been followed. The

decreased viscosity is referred to polymer-polymer associations becoming less numerous

when complexes between polymer hydrophobic tails and CD become more frequent;CD-decorated polymer hydrophobic tails have no possibility to contribute to the network. It

was found that deactivation of the first few hydrophobic tails has very large consequences for

the viscosity. A termination of a fraction as small as 10% (or below) of the total amount of

polymer hydrophobic tails may reduce viscosity to a level almost corresponding to that of the

unmodified parent polymer. This can be understood by taking into account that a solution of

a HM-PEG polymer is expected to be inhomogeneous with large concentration fluctuations,

and that the viscosity is likely to be strongly decreased by reducing the probability for

hydrophobic associations responsible for connecting different clusters. The effect of CD on

rheology is very different for different architectures of hydrophobically modified polymer, in

particular between graft copolymers and end-capped ones. This can be understood from the

differences in net-work structure.

-1-



Introduction

Hydrophobically modified polymers (HM-P:s) or water-soluble associative polymers

(WSAP:s) are used in a variety of technical formulations that we meet in our daily life.

Examples are water-borne paints and shampoos.1,2

A common reason for adding a HM-P toa formulation is that it gives a different rheological behavior as compared to a normal

hydrophilic thickener. Another reason to choose a HM-P is that the amphiphilic properties

can help to increase stability of dispersions.

To be able to predict performance of a hydrophobically modified polymer in an application it

is important to have knowledge about the effect of the hydrophobic modification on

macroscopic properties. Such knowledge is also interesting for more fundamental reasons

and is often correlated with molecular interactions when HM-P:s are subject to more basic

research. One way to obtain information about the effect of a hydrophobic modification is to

synthesize both the unmodified and the hydrophobically modified version of the thickener.

This is a route that has been employed in several investigations.3-9 Besides that this

requires extra synthesis work it is difficult to control the reaction so that the only difference

between the two polymers is the hydrophobic modification. Therefore it would be desirable if

the effect caused by a hydrophobic modification instead could be studied by inhibition and

decoupling of the hydrophobic associations.

In a recent paper, we reported on the addition of diethyleneglycol monobutylether (BDG) to

aqueous polymer solutions to obtain such decoupling.9 In other investigations, surfactants

have been added at high concentration, which decouples hydrophobic polymer-polymer

associations by encapsulating each polymer hydrophobic tail in a micelle.10,11 Another

efficient way, which will be employed here, to decouple the hydrophobic associations is

offered by addition of cyclodextrin (CD). CD is a cyclic oligomer of glucose with the shape of

a truncated cone that has a hydrophilic exterior and a hydrophobic cavity in the center,

Figure 1. Three different sizes are available; α-, β-, and γ -cyclodextrin consist of six, seven,

or eight glucose units, respectively. In aqueous solutions CD molecules form “nut and bolt”

(or inclusion) complexes with substances containing lipophilic groups, e.g. surfactants or

HM-P:s, provided that the hydrophobic group has a shape that fits in the cavity. Principles of

such complex formation have been studied thoroughly by several groups, and in particular

has complexation between CD and surfactants been in focus.12-18

-2-



O

H

CH

C

O

OH

OH

CH

C

C

H

C

O

H

H

H

H

Hydrophobic cavityOO

OO

O

O

OH

O

O

O O

OOH

O

O

O

O

OH OH

O

O

O

OH

O

OO

OHOH

O

OO

H

CH

C

O

OH

OH

CH

C

C

H

C

O

H

H

H

H

Hydrophobic cavity

O

H

CH

C

O

OH

OH

CH

C

C

H

C

O

H

H

H

H

O

H

CH

C

O

OH

OH

CH

C

C

H

C

O

H

H

H

H

Hydrophobic cavityOO

OO

O

O

OH

O

O

O O

OOH

O

O

O

O

OH OH

O

O

O

OH

O

OO

OHOH

O

O

Figure 1. To the left is shown the chemical structure and to the right a schematic

representation of the geometry of an α -cyclodextrin molecule.

CD has also been used together with HM-P with the purpose to reduce the viscosity of an

aqueous solution. This was described for the first time in the beginning of the nineties.19,20

Here the purpose was to reduce viscosity of a highly concentrated aqueous solution of a

HM-P thickener to facilitate incorporation of the thickener into technical formulations, e.g.

paint. More recently the concept of controlling the self-association of HM-P by addition of CD

has been more thoroughly studied in two papers by Zhang et al and Akiyoshi et al. .21,22 A

conclusion from these investigations was that CD molecules form complexes with the

hydrophobic end groups of the HM-P polymers, which means that polymer hydrophobic tails

are hidden within the CD-cavities, and only the hydrophilic outer shell of the CD molecules is

exposed to the aqueous environment. In HM-P solutions the viscosity then decreases since

the three-dimensional polymer network is disrupted when the possibility to form inter-

polymeric hydrophobic associations is reduced. The concept to encapsulate the hydrophobic

tails of HM-P with CD has also been used to avoid disturbing hydrophobic interactions when

properties of individual polymer molecules were in focus and investigated with techniques

such as gel permeation chromatography and static light scattering.23

In a recent investigation we focused on how the viscosity in an aqueous solution of a graft

copolymer, hydrophobically modified ethyl (hydroxy ethyl) cellulose (HM-EHEC), decreased

in the transition region where the concentration of CD (c CD) was lower than the total

concentration of hydrophobic groups (c hydrophobe), and the effect of various cyclodextrins was

-3-



investigated.24 The complex formation and the concomitant disruption of the polymer

network was followed by measuring the viscosity as a function of the CD concentration. The

data were rationalized in a simple association model from which the effective binding

constant could be extracted together with the concentration of binding sites. We found a

reasonable agreement between the model and the data, and in particular the concentration

of binding sites equaled the concentration of hydrophobic tails. The interpretation of this was

that all hydrophobic tails were important for the network formation in the HM-EHEC system.

In the present study we have used methylated α-cyclodextrin (M-α-CD) to decouple

associations in aqueous solutions of an associating polymer with a different architecture, a

water-soluble polymer hydrophobically end-capped, the choice of polymer being

hydrophobically modified poly (ethylene glycole), HM-PEG. The viscosity has been

rationalized within the same model as we used for the HM-EHEC/CD system, and it was

found that initially the decrease in viscosity is much stronger than what is expected from

results in the previous investigation. The observations can be understood by taking into

account that the HM-PEG solution is likely to be inhomogeneous with large concentration

fluctuations.

Experimental

Materials

Hydrophobically end-modified polyethylene glycol (HM-PEG) with the structure C1618-EO140-

IPDU-EO140-C1618 was used in this study. C1618-EO140 denotes an ethoxylate of a mixture of

unsaturated alcohols (C16 to C18), and IPDU represents an isophorone diurethane group

connecting two ethoxylated alcohol molecules. The synthesis and characterization methods

are described elsewhere.25 The weight average molecular weight (Mw = 13.500) and the

polydispersity index (Mw

/ Mn = 1.1) were determined by size exclusion chromatography

(SEC). The HM-PEG was purified from low molecular weight impurities (salt originating from

the catalyst, low molecular weight PEG etc.) by dialysis; a 3% w/w solution of HM-PEG was

dialyzed against an excess of Millipore water for several days with repeated exchange of the

dialysis water. The dialysis was performed by using Spectra/Por® molecularporous

membrane tubing with a molecular weight cut off of 6-8 000. After the dialysis the polymer

material was recovered by freeze-drying.

Methylated α-cyclodextrin (M-α-CD) was supplied by Wacker-Chemie (under the trade name

Cyclodextrin Alpha W6 M1.8). The degree of methylation per glucose unit was 1.6 – 1.9, as

-4-



given by the supplier. The M-α-CD was of pharmaceutical quality and was used without

further purification. To all samples water of Millipore quality was used.

Methods.

Aqueous HM-PEG solutions of three concentrations, 3, 5 or 10%w/w, were prepared, without

and with 0.5%w/w M-α-CD. These six stock solutions were prepared by weighing the

components in test tubes that were sealed with Teflon tightened caps. Before proceeding,

the stock solutions were left to equilibrate for at least 24 hours. Furthermore, in order to

facilitate preparation of test solutions with a CD concentration below 0.1%w/w, a dilution with

respect to CD of the more concentrated stock solutions was made. From these, in all nine

stock solutions, samples with desired compositions were prepared by weight. The M-α-CD

concentration was varied in the range 0.002 to 0.5%w/w (corresponding to 0.018 to 4.5

mmolal). Before any rheological measurements were started the final samples were left to

equilibrate at room temperature for at least 12h.

The rheological measurements were performed with a StressTech rheometer from

Rheologica, Sweden. A 4 cm, 1° cone and plate geometry was used, and the temperature of

the sample was controlled to within ±0.1°C by an external water bath. Measurements were

performed at 20°C, both as continuous, and oscillatory, shear measurements. The viscosity,

η or η *, was determined as a function of the shear rate, or 2 π f, where f denotes the

frequency. Within a range of shear rates the viscosity data from the two methods coincide,

and are independent of or 2 π f , Figure 2. Values from this Newtonian plateau are reported

in the following figures.

•

γ

•

γ

-5-



10-3

10-2

10-1

100

101

102

103

100

101

102

.

η ,

η *

( P a

s )

γ , 2πf (s-1)

Figure 2. Complex viscosity, η * (filled diamonds), from oscillatory shear measurements and

viscosity, η (open circles), obtained from continuous shear measurements for a solution

containing 10% HM-PEG (14.8 mmolal hydrophobes) and 0.005 % w/w CD (0.047 mmolal).

Model Considerations

Our results were interpreted in a simple model were CD-molecules are regarded to bind to

the hydrophobic tails of the polymer chains with a complex formation constant K . It is

assumed that 1:1 “nut and bolt” complexes are formed, and we represent this complex

formation within a Langmuir adsorption model. The concentration of “adsorption sites”, B, in

the model is restricted by the concentration of polymer hydrophobic tails, and cannot exceed

this value. To obtain the K and B values from our rheological data we have assumed that

τ τ η T nk Gb

∝∝ ∞.26,27 k b is the Boltzmann constant, and T is the absolute temperature.

Furthermore, we have assumed that the characteristic time,τ , of the relaxation process, that

is important for the viscosity at the Newtonian plateau, is independent of the CD

concentration, c CD. By this latter assumption the main contribution to the viscosity in the

HM-PEG solutions is regarded to stem from associations of the polymer hydrophobic tails,

and the effect from entanglements of the PEG backbone is regarded insignificant. I.e. a

change in viscosity at the Newtonian plateau is only dependent on the concentration of

rheologically active chains, n ( n∝η ). In fact, an earlier study of HM-PEG has shown that

disruption of hydrophobic associations is likely to be the main contributor to the relaxation

time,27 and since the coil size of an unperturbed PEG chain with similar molecular weight

suggests an overlap concentration far above the concentrations used in this study the effect

from chain entanglements is likely to be negligible.

28

-6-



From this follows that:24

η −η ∞η

0 −η ∞

=1− Θ =1−

B

+cCD +

1 K

2− B + c

CD

+ 1 K

( )

2

4− BcCD

B (1)

Here η 0 and η ∞ are the viscosities that are obtained without CD and at excess CD,

respectively, and Θ is the fraction of occupied binding sites in the Langmuir model.

In a previous paper we have studied the complex formation between a graft hydrophobically

modified polymer, HM-EHEC, and CD.24 In this work we fitted equation (1) to ourexperimental data points ( η −η ∞( ) η

0−η ∞( ) vs. c CD) with K and B as fitting parameters. In that

way we could determine K for several combinations of different hydrophobic groups and

CDs. We also found a very good correlation between the number of adsorption sites, B, and

the total amount of hydrophobic groups in the solution, c hydrophobe. This observation was not

too surprising and this was taken as an indication of all hydrophobic groups being equally

important and contributing in a similar way to the network formation and to the viscosity.

Figure 3 is obtained by using equation (1) and illustrates how the viscosity ( η ) as a function

of c CD is influenced by a variation in B and K . From this figure it appears that the initial

behavior at low c CD is largely determined by B, and provided K has a sufficiently high value

(>10 mmolal-1) B strongly influences the curve in a large viscosity range. In such a situation

the initial behavior (at low c CD) is well represented by a straight line with the slope of -1/B

(see Equation 2). The viscosity at excess CD (η∞ ) is expected to be virtually independent of

c CD and to be the same as that found in a solution containing an unmodified polymer with a

corresponding molecular weight. K influences the curve in the intermediate region where a

transformation from the slope –1/B to the plateau value at high c CD

(slope 0) appears. High

values of K cause a very abrupt transition, while a low value leads to a more extended

transition. In our previous work we determined K for the complexation between M-α-CD and

C14-alkyl hydrophobes of HM-C14-EHEC to K =44 mmolal-1,24 and since similar hydrophobes

(C16-18) are used also in the present polymer we have no reasons to believe that K is lower

here. The value of B can therefore be obtained from the initial behavior via a simplified

equation:

B

cCD

−∝≈−

−

∞

∞1

00 η

η

η η

η η (2)

-7-



0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20

cCD (mmolal)

/

0

−

K=3 mmolal-1

B=4.4 mmolal

K=44 mmolal-1

B=4.4 mmolal

K=44 mmolal-1

B=14.8 mmolal

Figure 3. Viscosity as a function of the CD concentration as calculated from Equation 1.

Complex constants, K, 3 or 44 mmolal -1 and concentrations of adsorption sites, B, 4.4 or 14.8

mmolal, respectively, have been used. These B-values equal the concentration of HM-PEG

hydrophobic tails at HM-PEG concentrations of 3 or 10 %w/w, respectively. The dashed line

is a linear extrapolation of the trend in the region 0 ≤ c CD ≤ 0.5 B for K= 44 mmolal -1 and B =

4.4 mmolal.

Results and Discussion

The full line in Figure 4 is obtained from Equation 1 with values of B=4.4 mmolal (the

concentration of polymer hydrophobic tails in a 3%w/w HM-PEG solution), and K =44

mmolal-1 (the binding constant that was obtained for a C14 aliphatic chain in combination with

M-α-CD in our previous investigation).24 However, the experimental data are very different

and it is obvious that initially CD influences the viscosity much stronger than expected from

these values of B and K . At low c CD only a small fraction of the hydrophobic tails can be

deactivated (from stoichiometrical considerations), but the effect on the viscosity is dramatic.

Obviously deactivation of the first few hydrophobic associations has a much stronger

influence on the viscosity than what could at first be expected. Actually, by changing the

value of B from 4.4 mmolal to 0.4 mmolal, a value that is suggested by the initial slope, and

by keeping K =44 mmolal-1 constant a much better representation of the experimental data is

obtained (dotted line). This is a quite surprising result since this means that it is enough toeliminate only about 10% of the hydrophobic tails to reduce the viscosity to a level virtually

-8-



corresponding to that at excess CD. As was explained in the Experimental section this is

based on the assumption that 1:1 complexes are formed. Since the hydrophobic tails are

relatively long we note that there is a possibility that higher complexes may form. Olson et al

have shown by NMR-measurements that two or even more α-CD molecules can bind to a

C12-hydrophobic group attached to a PEG chain.29 This tendency is likely to be more

important at high c CD. This can however not explain the behavior that was observed since

formation of higher complexes would actually mean that the 10% is an overestimation.

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5

cCD (mmolal)

0

K= 44 mmolal-1

B=4.4 mmolal

K= 44 mmolal-1

B=0.4 mmolal

6

Figure 4. Relative viscosity, η / η0 , as a function of the CD concentration in a solution with

3%w/w of HM-PEG. Filled circles represent the experimental data. The full line represents

the theoretical viscosity calculated from Equation 1 with K= 44 mmolal -1 and B = 4.4 mmolal,

or with B = 0.4 mmolal -1 (dashed line).

To be able to rationalize this observation a general discussion about structure in a HM-PEG

solution, and its concentration dependency, is needed.30 Micelle-like structures may appear

already at concentrations of about 10-3 % w/w.31,30 Since the triblock structure of the

HM-PEG chains results in an attraction between micelles,32 clusters that contain many

micelles are likely to form. While micelles probably have fairly well defined aggregation

numbers the clusters may appear in a wide range of sizes, and it seems reasonable that the

average cluster-size increases with concentration.33-35 Below the concentration where

clusters start to interact and connect to each other the viscosity of the solution is likely to be

low, while above this concentration the viscosity increases rapidly. At this stage the solution

is often referred to as containing a three-dimensional network that extends over macroscopic

distances. The structure as a function of the concentration that follows from the abovediscussion is schematically illustrated in Figure 5. Regions with higher concentration of

-9-



polymer correspond to clusters of micelles, and within these clusters inter-micellar links are

likely to be numerous, while polymers that connect micelles located in different clusters have

to span polymer depleted regions and are more rare. One indication of that the solution is

inhomogeneous is given by the phase behavior, Figure 6. A solution of the “corresponding

diblock polymer”, which has a chemical structure corresponding to half the triblock polymer,

has a phase behavior very much resembling that of an unmodified PEG polymer with a

phase separation at high temperatures. This is expected since the hydrophobic tails are

hidden in the core of micelles and it is only the PEG part that is exposed towards the

aqueous solution. Despite that similar micellar structures are expected to form with the

triblock polymer a solution based on HM-PEG has a much more pronounced tendency to

phase separate, and a phase concentrated in polymer is obtained in equilibrium with a phase

depleted in polymer. This situation appears already at slightly elevated temperatures, and

only a small change of the PEG/solvent interaction is needed to induce phase separation.

The very pronounced difference in phase behavior between the di- and tri-block polymers

can be traced to the attraction between micelles.36 The molecular picture that emerges is

that the solution is likely to be inhomogeneous with large concentration fluctuations, and at

intermediate concentrations a percolated network forms via relatively few bridges between

different clusters. This picture is valid in the concentration range that has been investigated.

Unimers Flower

Micelles

Clusters Network

increased cHM-Peg

Unimers Flower

Micelles

Clusters Network

increased cHM-Peg

Figure 5. Schematic representation of the self-aggregation of HM-PEG as function of

increasing c HM-PEG.

-10-



0 1 2 3 4 5 6 7 8 9 100

20

40

60

80

100

120

1

2

T c p

( ° C )

c (% w/w)

Figure 6 . Partial phase diagram for HM-PEG. The filled circles represent the triblock and the

open circles represent the diblock. Reproduced from 36

With this in mind we are now in a position to explain the fact that the viscosity was affected

much stronger than what was first expected from Equation 1. In order to reduce viscosity

strongly it is only needed to decouple associations between different clusters, and since

these are only expected to involve a small fraction of the total number of HM-PEG chains a

strong initial decrease in viscosity on addition of CD can be understood. This could be the

explanation to that a CD concentration corresponding to only about 10% of the hydrophobic

groups has to be added to the 3 wt% HM-PEG solution to give a viscosity that is virtually the

same as in a solution containing the unmodified PEG.

One reason for this behavior could be that bridges between clusters correspond to HM-PEG

chains that are more stretched than links between micelles within the clusters. This meansthat the former break and reform more frequently and are more likely to be presented to CD

molecules. Furthermore, the fact that the solution is depleted in polymer between clusters

may result in a relatively high CD concentration here, and termination of HM-PEG chains

located in this region increases also for this reason. A cartoon-picture of a HM-PEG solution

containing a low CD concentration could then, as illustrated in Figure 7, be viewed as

discrete clusters covered on the surface by CD. It should however be noted that we do not

believe that this is a static situation but rather Figure 7 should be seen as a snapshot. For the

sake of completeness we have to mention the possibility that the phenomenon (with a rapidly

decreasing viscosity) possibly could be shear-induced, and the network is being broken

-11-



down into fragments that orient in the flow. However, we find this unlikely since identical

results have been obtained with many different measuring geometries of the rheometer

(cone and plate of different diameters, plate-plate, and double gap), and special care was

paid to perform control measurements also at very low shear rates. The results were also not

affected by whether the samples had been subject to pre-shear or not, and also with the

rheometer in the oscillatory shear mode identical results were obtained.

cCD

0

cCD

0

Figure 7 .The figure shows a schematic representation of the binding of CD to HM-PEG

hydrophobic tails. Already at rather low concentrations of CD the percolated network

structure is eliminated because hydrophobic associations between clusters are inhibited. At

higher CD-concentrations also clusters and individual micelles are expected to be

disengaged by CD.

Measurements have also been performed at other polymer concentrations (5 and 10% w/w

polymer). As a matter of fact a similar result but even more pronounced deviation from the

expected value of B was found at the two other investigated concentrations, Table 1 and

Figure 8. The value of B obtained from the experimental data is virtually unaffected by an

increased HM-PEG concentration (in the investigated range). This means that the fraction

B/c hydrophobe decreases with increasing HM-PEG concentration, and at the highest HM-PEG

concentration this ratio is as low as 4%. This may mean that the number of polymer chains

that participate within one cluster grows with increasing polymer concentration (increasing

cluster size), which also has been suggested before.34

An increasing size of the “decoupled” clusters would be likely to give a contribution to the

viscosity of the solution. Indeed, in a closer look it can be seen that the experimental curvescan be divided into three different regions, instead of two, Figure 8. This behavior becomes

-12-



more pronounced with increasing HM-PEG concentration, and in the second region, located

at intermediate CD concentrations, the change in viscosity is less dramatic than the initial

steep decrease. We refer the decrease in this intermediate region to disengagement of

individual clusters and micelles by CD. While B was obtained by extrapolation to η / η0 = 0 at

low CD concentration, a similar extrapolation can be made in this intermediate region to

obtain B2 . B2 would then be connected to the actual HM-PEG concentration in the solution.

Indeed, B2 is in contrast to B dependent of the HM-PEG concentration, Table 1.

Table 1. Data from three different HM-PEG concentrations. B and B2 were obtained by

extrapolation to η / η0 =0 in the low c CD and intermediate c CD regions respectively.

Concentration HM-

PEG (%w/w)

Concentration

Hydrophobic tails,

c hydrophobe (mmolal)

B (mmolal) B/ c hydrophobe B2 (mmolal)

3 4.4 0.45 10% 2.42

5 7.4 0.42 6% 3.30

10 14.8 0.53 4% 4.39

-13-



Figure 8. Relative viscosity, η / η0 , as a function of CD concentration in solutions with 3% w/w,

5% w/w or 10% w/w HM-PEG, respectively . B-values were obtained by extrapolation to η / η0

= 0 from the behavior at low CD concentration (data represented as filled circles has been

used in the extrapolation). The B2 values were obtained by extrapolation to η / η0 = 0 from the

behavior at intermediate CD concentrations (open circles). Data represented by open

squares have not been used in the extrapolations.

-14-

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5

cCD (mmolal)

0

B 2

3%

B

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5

cCD (mmolal)

0

B

5%

B 2

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5

cCD (mmolal)

0

B

10%

B 2



In the structural picture that evolved above, clusters are connected with a rather small

number of bridges, and by adding CD these bridges are deactivated. This is related to the

situation for which the percolation theory has been developed.37 This was quite recently

used to rationalize rheological data in a related system, where microemulsion droplets wereconnected into a network with HM-PEG.38 At a certain concentration of bridges (here

between clusters) a percolated network is anticipated, and at this point the viscosity is

expected to increase rapidly (diverge) since an infinite network that extends over

macroscopic distances forms. Below the percolation threshold the viscosity can be seen as a

summation of contributions from all different cluster sizes and is given by:37

( ) ((7.0

0

−

−− −−−∝ CD PEG HM cCD PEG HM ccccη ))

)

)

(3)

In Equation 3 we have used the concentration variable ( since a HM-PEG

molecule that is terminated with CD at one end is expected to be incapable of forming a

bridge between clusters. At all three investigated HM-PEG concentrations (3, 5, and 10 %)

we expect a percolated network at c CD = 0. Following the discussion above, in the initial

stages addition of CD terminates hydrophobic tails of HM-PEG polymers that are important

for the percolated network (between clusters), leaving the clusters almost unaffected. Thus,

in the present view the percolation threshold, , becomes dependent on the

HM-PEG concentration, and the specificity of CD to preferentially deactivate bridges between

clusters implies that a pure HM-PEG solution should always have a higher viscosity than a

CD containing sample with the same effective concentration ( , Figure 9. In the

figure it can be seen that the percolation theory provides a good representation of CD-

containing solutions. It is interesting to see that the viscosity as a function of HM-PEG

concentration (without CD) has a rather different behavior, with a different functional form.

Obviously Equation 3 can not be used to represent the decrease in viscosity upon dilution of

a pure HM-PEG solution (in the investigated concentration range). This may be taken as an

evidence of that the structure and its dependency on the concentration variable in solutions

of HM-PEG is different if the solution also contains CD.

CD PEG HM cc −−

)cCD

CD PEG HM cc −−

( PEG HM

cc −−

-15-



0 1 2 3 4 5 6 7 80.01

0.1

1

10

100

( P a

s )

(cHM-PEG

- cCD

) (mmolal)

Figure 9. The figure shows the viscosity as a function of the effective concentration variable

(c HM-PEG – c CD ), see text. Three sets of data with varying c HM-PEG (3%w/w ( △ ), 5%w/w ( ○ ) and

10%w/w ( □ )) are shown. The full lines are best fits to Equation 3 for each HM-PEG

concentration, and dotted vertical lines represent the corresponding percolation thresholds

(c HM-PEG - c CD )c . The viscosity as function of c HM-PEG without CD is represented by ( ■ ).

Conclusions

The most surprising observation in the present investigation is that it is enough to terminate a

rather small fraction (about 10% in a 3% HM-PEG solution) of the HM-PEG hydrophobic tails

with CD molecules to reduce the viscosity to a level virtually corresponding to that at excess

CD. To rationalize this observation, advantage was taken of that concentration fluctuations

are likely to be substantial in a HM-PEG solution of this concentration and that viscosity, in

this view, becomes strongly dependent on HM-PEG chains that connect different clusters of

micelles. By deactivating these latter associations, the viscosity decreases rapidly.

Another interesting observation is that the fraction of HM-PEG chains that are active in

interconnecting different clusters seems to decrease with an increasing polymer

concentration (in the investigated range). This may be taken as an indication that sizes of the

clusters increase with an increasing HM-PEG concentration.

-16-



Acknowledgement

This investigation was sponsored by the Center for Amphiphilic Polymers (CAP).

References

(1) Glass, J. E. Polymers in Aqueous Media; American Chemical Society: Washington,DC, 1989; Vol. 223.

(2) Glass, J. E. Hydrophilic Polymers; Performance with Environmental Acceptability ; American Chemical Society: Washington, DC, 1996; Vol. 248.

(3) Strauss, U. P. Hydrohpobic polyelectrolytes. Polymers in aqueous media; AmericanChemical Society: Washington DC, 1989; Vol. 223; pp 317-324.

(4) Williams, P. A.; Meadows, J.; Phillips, G. O.; Senan, C. Cellulose: Sources and

Exploration 1990, 37 , 295-302.

(5) Landoll, L. M. J. Polym. Sci. 1982, 20 , 443-455.

(6) Tanaka, R.; Meadows, J.; Phillips, G. O.; Williams, P. A. Carbohydrate Polymers

1990, 12 , 443-459.

(7) Picton, L.; Muller, G. Macromol. Symp. 1997, 114, 133-138.

(8) Joabsson, F.; Rosen, O.; Thuresson, K.; Piculell, L.; Lindman, B. J. Phys. Chem. 1998, 102 , 2954-2959.

(9) Karlson, L.; Joabsson, F.; Thuresson, K. Carbohydrate Polymers 2000, 41, 25-35.

(10) Nilsson, S.; Thuresson, K.; Hansson, P.; Lindman, B. J. Phys. Chem. 1998, 102 ,7099-7105.

(11) Piculell, L.; Nilsson, S.; Sjöström, J.; Thuresson, K. Assosciatve polymers in aqueousmedia; American Chemical Society: Washington DC, 2000; Vol. 765; pp 317-335.

(12) Funasaki, N.; Yodo, H.; Hada, S.; Neya, S. Bull. Chem. Soc. Jpn. 1992, 65 , 1323-1330.

(13) Junquera, E.; Tardajos, G.; Aicart, E. Langmuir 1993, 9, 1213-1219.

(14) Ma, Z.; Glass, J. E. Polym. Matrl. Sci. Engin. 1993, 69, 494-495.

(15) Mwakibete, H.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1994, 10 , 3328-3331.

(16) Mwakibete, H.; Bloor, D. M.; Wyn-Jones, E.; Holzwarth, J. F. Langmuir 1995, 11, 57-60.

(17) Park, J. W.; Song, H. J. J. Phys. Chem. 1989, 93, 6454-6458.

(18) Wan Yunus, W. M. Z.; Taylor, J.; Bloor, D. M.; Wyn-Jones, E. J. Phys. Chem. 1992,96 , 8979-8982.

(19) Eisenhart, E. K.; Johnson, E. A. Method for improving thickeners for aqueoussystems. In U.S. Patent ; Rohm and Haas Company: United States, 1992.

(20) Lau, W.; Shah, V. M. Method for improving thickeners for aqueous systems. In U.S.Patent ; Rohm and Haas Company: United States, 1994.

(21) Zhang, H.; Hogen-Esch, T. E.; Boschet, F.; Margaillan, A. Langmuir 1998, 14, 4972-4977.

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-18-

(22) Akiyoshi, K.; Sasaki, Y.; Kuroda, K.; Sunamoto, J. Chemistry Letters 1998, 93-94.

(23) Islam, M. F.; Jenkins, R. D.; Bassett, D. L.; Lau, W.; Ou-Yang, H. D. Macromolecules

2000, 33, 2480-2485.

(24) Karlson, L.; Thuresson, K.; Lindman, B. Carbohydrate polymers 2002, 50 , 219-226.

(25) Karlson, L.; Nilsson, S.; Thuresson, K. Colloid Polym. Sci. 1999, 798-804.

(26) Green, M. S.; Tobolsky, A. V. J. Chem. Phys. 1946, 14, 80-89.


(28) Gregory, P.; Huglin, M. B. Makromol. Chem. 1986, 187 , 1745-1755.

(29) Olson, K.; Chen, Y.; Baker, G. L. J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 2731-2739.

(30) Winnik, M. A.; Yekta, A. Current Opinion in Colloid & Interface Science 1997, 2 , 424-436.

(31) Yaminsky, V. V.; Thuresson, K.; Ninham, B. W. Langmuir 1999, 15 , 3683-3688.

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Langmuir 1997, 13, 4204-4218.



Complex formed in the system hydrophobically modified polyethylene

glycol / methylated α-cyclodextrin / water. An NMR diffusometry study.

L. Karlson,

*,1

, C. Malmborg,

2

K. Thuresson

2

and O. Söderman

2

1 Akzo Nobel Surface Chemistry AB, SE-444 85 Stenungsund, Sweden

2 Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O.

Box 124, SE-221 00 Lund, Sweden.

* To whom correspondence should be addressed.

fax: +46 303 839 21

e-mail: [email protected]

Abstract

In aqueous solutions hydrophobically modified polyethylene glycol (HM-PEG) forms a

transient polymer network held together by intermolecular hydrophobic associations. In the

present investigation we have used NMR-diffusometry to study how the addition of

methylated α-cyclodextrin (M-α-CD) influences the polymer network. The addition of M-α-CD

resulted in an increased mean self-diffusion of HM-PEG, D HM-PEG, which is referred to adegradation of the polymer network when hydrophobic associations are disrupted due to

complex formation between the hydrophobic groups of HM-PEG and M-α-CD. Addition of

small amounts of M-α-CD results in a dramatic increase in D HM-PEG. Upon further addition of

M-α-CD the increase in D HM-PEG is less dramatic and at excess M-α-CD, D HM-PEG levels off and

equals the mean self diffusion coefficient for unmodified PEG with the same molecular

weight. The suggested interpretation is that the addition of the first molecules of M-α-CD

mainly reduces the probability for hydrophobic associations inter-connecting different clusters

of polymer micelles whereas at higher M-α-CD concentrations a disengagement of the

individual clusters into separate HM-PEG molecules becomes important.

- 1 -



Introduction

Associative polymers (APs) are commonly used as rheology modifiers in water based paint

formulations. Compared to conventional thickeners APs offer a flow behavior closely

resembling Newtonian flow over a wide range of shear rates. When applying the paint thisleads to improved leveling and a better hiding power of the paint. Another important

advantage offered by the use of APs compared to conventional thickeners is a dramatic

reduction of the roller spatter.

One class of APs frequently used is the hydrophobically modified urethanes (HEURs). The

HEUR thickeners used in technical applications are normally composed of diurethane linked

polyethylene glycol blocks with hydrophobic alkyl end-groups.1 The viscosity-enhancing

effect of HEUR originates from the formation of a transient polymer network held together by

physical linkages of assembled hydrophobic groups. A large number of investigations have

been carried out in order to obtain information about the association mechanisms of HEUR

polymers. The work is summarized in two recent review articles.1,2 The synthesis procedure

of technical HEUR thickeners results in a broad distribution in molecular weights.1 This

synthetic route and the obtained molecular weight distribution is attractive from a practical

point of view, while interpretation of results from experiments aimed at physical

characterization becomes more complicated. To simplify interpretations from fundamental

physico-chemical investigations of HEUR polymers, polymers with narrow molecular weight

distributions have been synthesized and used for this purpose.3,4 HEUR polymers with

narrow molecular weight distributions are often referred to as hydrophobically modified

polyethelen glycol (HM-PEG) or “Triblock” polymers. It is now commonly accepted that

HM-PEG form flower like micelles at low concentration.1,2

Upon increasing polymer concentration aggregated structures are formed. First micelles

form, which become connected into clusters, and at higher polymer concentrations a

percolated network is formed, which spans the entire sample. The three dimensional networkis held together by polymer molecules with one hydrophobic end in a micelle in one cluster

and the other end in a micelle in a neighboring cluster.5,3,4,2 (Figure 1)

One way to study the properties of the network is offered by NMR self-diffusion

measurements.6,7 With this technique, the molecular displacements of the species in the

samples may conveniently be followed. Such data convey information about polymer size

polydispersity, including possible effects of association.8,9 For instance, for an extensively

associated polymer solution, one expects slow diffusion and also a broad distribution in thevalues of the diffusion coefficients, provided that the life-time of the polymers in the

- 2 -



associated structures is long compared to the time-scale of the measurements.8 Any effects

that would decrease the degree of association then leads to a increase in the self-diffusion

coefficients as well as a to a narrower distribution of diffusion coefficients.

Unimers Flower

Micelles

Clusters Network

increased cHM-PEG

Unimers Flower

Micelles

Clusters Network

increased cHM-PEG

Figure 1. Schematic representation of the self-aggregation of HM-PEG as function of

increasing c HM-PEG.

A fruitful way to study association mechanisms of a polymer network in aqueous solution is

by gradually decoupling the network by the addition of a third component. An excess of

surfactant has been used for this purpose in many studies. An alternative route of decoupling

the polymer network is offered by cyclodextrin (CD). CD is a cyclic oligomer of glucose. The

CD molecules either consist of 6, 7 or 8 glucose units and they are denoted α-CD, β-CD or

γ -CD, respectively. As shown in Figure 2, the CD molecule has the shape of a truncated

cone. The exterior of the molecule is hydrophilic while the interior forms a lipophilic cavity. In

aqueous solutions CD molecules form inclusion complexes with lipophilic substances

provided that the lipophilic molecules have a shape that fits in the cavity. Examples of

complex formation constants found in the literature for cyclodextrin in combination with some

different surfactants are presented in table 1.

- 3 -



Table 1. Complex formation constant, K 1 (mM -1 ), for α -CD and β -CD in combination with

sodium dodecyl sulphate (SDS), tetradecyl sulphate (TDS), hexadecyl sulphate (HDS),

dodecyl trimethyl ammonium bromide (DTAB), tetradecyl trimethyl ammonium bromide

(TTAB) and cetyl trimethyl ammonium bromide (CTAB).

Surfactant K 1 Reference

-CD -CD

DTAB 23.7 10

TTAB 61.0 39.8 10

CTAB 99.2 67.7 10

SDS 25.6 11

TDS 48.2 11

HDS 53.3 11

Hydrophobic cavity

OO

OO

O

O

OH

O

O

O O

OOH

O

O

O

O

OH OH

O

O

O

OH

O

OO

OHOH

O

O

Hydrophobic cavity

OO

OO

O

O

OH

O

O

O O

OOH

O

O

O

O

OH OH

O

O

O

OH

O

OO

OHOH

O

O

Figure 2 . To the left is shown one example of the chemical structure of a methylated

α -cyclodextrin molecule and to the right a schematic representation of the geometry of the

same molecule.

The effect of adding CD to an AP-solution is a reduction of the viscosity, and the use of CD

to reduce the viscosity of AP-solutions was first described in two patents by the Rohm and

Haas Company.12,13 Later, more fundamental studies to explain the effect of CD on

HM-polymer solutions have been performed.14-16 The CD molecule is believed to form a

“nut and bolt” complex with a lipophilic group of the HM-polymer. This means that when a

polymer hydrophobic tail is hidden within the CD-cavity, this polymer is “deactivated” and

does not contribute to the connectivity. This process will eventually result in a partial orcomplete disruption of the three-dimensional polymer network.

- 4 -



In an earlier paper we have reported on the effect of methylated α-cyclodextrin (M-α-CD) on

aqueous solutions of HM-PEG in the polymer concentration range from 3 to 10% w/w. The

degradation of the polymer network was followed by viscosity measurements, and by

rationalizing the data in a simple model the concentration of binding sites could be extracted.

It was found that the number of polymer hydrophobic tails that needed to be blocked to

substantially reduce the viscosity only constituted a small fraction of the total number of

hydrophobic groups in the system. This result was in sharp contrast to what was earlier

found for the system containing hydrophobically modified ethyl (hydroxyethyl) cellulose

(HM-EHEC), where a good agreement between the calculated concentration of binding sites

and the total number of hydrophobic groups in the system was found.17 The findings were

interpreted in the following way. In a HM-PEG solution, the dynamics and the viscosity are

strongly dependent on the polymer linkages connecting different clusters, and by adding CD

the probability of forming these cluster-spanning linkages reduces dramatically.

In the present work, self-diffusion of the different components in the system has been studied

in order to confirm the suggested model. Both self-diffusion of HM-PEG and of M-α-CD as a

function of the CD-concentration have been followed for a constant amount of polymer. In

line with previous investigations the data have been used to obtain information about

polymer transport dynamics and association mechanism.18-23

Experimental

Materials

Hydrophobically end-modified polyethylene glycol (HM-PEG) with the structure C1618-EO140-

IPDU-EO140-C1618 was used in this study. C1618-EO140 denotes an ethoxylate of a mixture of

unsaturated alcohols (C16 to C18), and IPDU represents an isophorone diurethane group

connecting two ethoxylated alcohol molecules. HM-PEG was synthesized and characterized

according to procedures described elsewhere.24 The weight average molecular weight

( Mw =13500) and polydispersity index ( Mw/ Mn = 1.1) were determined by size exclusion

chromatography (SEC). Before use, the HM-PEG was purified from low molecular weight

impurities, such as salt and low molecular weight PEG, by dialysis. A 3% solution of the

polymer was dialyzed against water of Millipore quality for a week with repeated exchange of

the water. For the dialysis Spectra/Por® molecularporous membrane tubing with a molecular

weight cut off of 6-8000 was used. After the dialysis the polymer was recovered by freeze

drying.

- 5 -



The cyclodextrin used for this study was methylated alpha-cyclodextrin (M-α-CD) supplied by

Wacker-Chemie under the trade name Cyclodextrin Alpha W6 M1.8. The degree of

substitution of methyl groups was 1.6 – 1.9 according to the producer. The material was used

as supplied, without any further purification.

Polyethylene glycol with molecular weight of 20,000 (PEG 20,000) was obtained from Merck

and was used without further purification.

Samples

The NMR samples were prepared by using D2O (99.8%) supplied by Dr. Glaser AG Basel,

Switzerland, as the solvent. Three stock solutions were prepared with HM-PEG

concentration 3% w/w and M-α

-CD concentrations of 0, 0.2 (1.78 mmole/kg) and 3.35 % w/w(29.87 mmole/kg), respectively. The stock solutions were prepared in test tubes, sealed with

Teflon tightened caps, and then equilibrated for 24 h at room temperature. From the stock

solutions, samples with desired compositions were prepared by weight, directly in NMR

tubes that were flame sealed. The samples that were not measured immediately after the

preparation were stored at -18°C in order to minimize degradation. Before the measurements

the samples were left to equilibrate in room temperature for at least 24 h. Visual inspection of

the samples before the measurements showed that they were clear and homogenous.

Two stock solutions containing 1 % w/w PEG 20,000 were prepared, of which one also

contained 1 % w/w M-α-CD. From these two stock solutions the final samples containing

1 % w/w PEG 20,000 and M-α-CD to a concentration of 0, 0.25, 0.5, 0.75 and 1 % w/w,

respectively, were prepared directly in the NMR tubes, which were flame sealed.

Methods

Pulsed Field Gradient (PFG) NMR 1H experiments were performed on a 200 MHz Bruker

DMX spectrometer equipped with a Bruker DIFF-25 gradient probe driven by a Bruker

BAFPA-40 unit.25 The temperature was 25°C. In this study, we have performed Hahn spin-

echoes for measurements on PEG and used the stimulated-echo technique for

measurements on CD. Typical values used for δ have been 0.5 - 3 ms and for ∆ 20 - 100 ms

(for the meaning of δ and ∆ see next section).

- 6 -





Results and discussion

Figure 3 displays a semilogarithmic plot of the mean self-diffusion coefficient, DHM-PEG, for a

3% w/w solution of HM-PEG as a function of the concentration of methylated α-cyclodextrin

(cCD). The polymer concentration of 3% w/w is far above the onset of polymer aggregation

and the solution (without cyclodextrin) is expected to contain a percolated network of “flower”

micelles (compare Figure 1). Also given in Figure 3 are the widths of the obtained

distributions of polymer diffusion coefficients. In the pure HM-PEG system (without

cyclodextrin), DHM-PEG has a value of 2.5 x 10-13 m2/s while the value of σ implies a broad

distribution of diffusion coefficients. This implies that the life-time of the associated structures

is long compared to the time-scale of the experiments, which is of the order of 100 ms.

Moreover, the size of the associated structures varies considerably, from essentially infinite

for those who take part in the percolated network, to smaller numbers for those who are not

part of the sample-spanning network.

0 5 10 15 20 25 3010

-13

10-12

10-11

10-10

10-9

0

2

D PEG

for unmodified PEG

DCD

for free CD

D H M - P E G , D C D

( m

2 / s )

cCD

(mmolal)

Figure 3. Mean self-diffusion coefficients for HM-PEG, DHM-PEG; (open circles) and for

M-α -CD, DCD, (triangles) and the distribution in DHM-PEG, σ , (filled circles) as a function of CD

concentration in 3%w/w solution of HM-PEG. The lower dashed line represents the mean

self-diffusion coefficient for unmodified PEG (MW= 20000 g/mol) in 1% w/w solution of PEG.

The upper dashed line represents DCD when no HM-PEG or PEG is present.

An interesting observation that we will first discuss is the fact that the width of the distribution

of diffusion coefficients is considerably lower for the sample without any added CD. Upon

addition of small amounts of CD, σ first increases. We interpret this observation as follows.

- 8 -



There are two diffusion mechanisms for HM-PEG in these solutions. In one, an individual

polymer performs a random walk between neighboring polymer micelles, in which process

one of the hydrophobic end-caps leaves a micelle and enters an adjacent micelle. The

second end-cap subsequently transfers to the new micelle. Thus the polymer moves in much

the same fashion as a geometer. The second process involves diffusion flow of individual

micelles or clusters of micelles. The two diffusion mechanisms are independent and their

contributions to the observed polymer diffusion are therefore additive. While the first process

depends on the (average) distance between micelles and the solubility of end-caps in water,

the second depends on the size of the clusters. Thus, the second process is expected to give

rise to polydispersity effects, while the first process is expected to be characterized by a

single diffusion coefficient. Without CD present, a majority, if not all, polymer molecules take

part in the percolated network, and the diffusion process is dominated by the first of the two

proposed mechanisms. As CD is added, finite clusters are formed (see further discussion

below) and as a consequence the diffusion rate increases and the width of the diffusion

coefficient distribution increases.

Figure 3 shows that DHM-PEG increases dramatically with the addition of methylated

α-cyclodextrin (M-α-CD) to the HM-PEG solution. This increase is accompanied by a

substantial decrease in the width of the diffusion coefficient distributions. The change is most

pronounced at small additions of CD, below 1 mmole. This corresponds well with the results

of the viscosity measurements reported in a previous paper.17 The combined results of the

self-diffusion and the viscosity experiments at low concentrations of CD are presented in

Figure 4. At low cCD we expect each CD molecule to bind to one hydrophobic end-group of

the HM-PEG and that the hydrophobic group that has formed complex with a CD molecule is

no longer available for hydrophobe-hydrophobe associations. CD opposes the associative

behavior of HM-PEG, and the results suggest a disintegration of the polymer network since

the viscosity decreases and the diffusion increases. Since the total concentration of

hydrophobic groups (chydrophobe) is 4.4 mmolal in the solution, only about 20% or less of the

hydrophobic groups can be covered by a cyclodextrin molecule in this concentration range.

Supported by the viscosity measurements we interpret the initial steep increase of DHM-PEG as

a degradation of the polymer network into separate clusters with dangling PEG chains, each

decorated with a CD molecule, sticking out in the solution. This interpretation would imply,

that M-α-CD preferentially binds to hydrophobes that are part of HM-PEG that binds the

clusters together.

- 9 -



0 1 20.0

0.2

0.4

0.6

0.8

1.0

B

cCD

(mmolal)

10-13

10-12

10-11

D ( m 2 / s )

Figure 4. Mean self-diffusion coefficient for HM-PEG, DHM-PEG; and relative viscosity, η / η0 , as

a function of CD concentration in 3%w/w solution of HM-PEG. B was obtained by

extrapolation to η / η0=0 from the behavior at low CD concentration.17

At about 0.5 mmolal of added CD, D HM-PEG reaches a second region where the increase in

D HM-PEG is slower compared to the initial steep increase. (Figure 5) The CD concentration,

where the break-point between the two regions is found, corresponds well to what was found

in the viscosity measurements. Included in the figure is the concentration of binding sites,

B=0.45 mmolal, which was found to be needed to be inhibited in the rheological

measurements to disengage the network and reduce viscosity strongly.17 In the viscosity

measurements we also found a second region were the decrease in viscosity was more

moderate. This concentration range is located almost in the same interval as that of the

second region of the self-diffusion increase. We referred this more moderate viscosity

decrease to a disengagement of individual clusters into separate HM-PEG molecules, andfinally, virtually every hydrophobic group is covered by a cyclodextrin molecule. D HM-PEG

continues to increase, albeit very moderately, up to cCD = 10 mmolal which is more than twice

the total concentration of hydrophobic groups. One possible explanation is that the complex

formation is not quantitative and at this stage free cyclodextrin is present in the solution,

although we feel this is less likely, on account of the large binding constant for alkyl chains to

CD (table 1). Another, perhaps more likely reason is that more than one cyclodextrin

molecule bind to each hydrophobic group. Here it should be noted that the viscosity

measurements indicated a total disruption of the polymer network already at cCD about 2.5

- 10 -



mmolal. We note that the viscosity at cCD close to chydrophobe is almost equal to the viscosity of

the solvent and small variations in the viscosity are therefore difficult to detect.

At high cCD the polymer network is expected to be totally disconnected and it can be seen

from Figure 3 that DHM-PEG adopts a constant value of about 3 x 10-11 m2/s. In addition, the

widths of the distribution of diffusion coefficients are narrow here. This is in good agreement

with results obtained for the unmodified polyethylene glycol (PEG) with a molecular weight of

20,000 g/mole where we measured D to 3.3 x 10-11 m2/s. This also corresponds well to what

has been previously reported for the self diffusion of PEG.26,27

0 5 10 15

10-13

10-12

10-11

B

D H M - P E G

( m 2 / s )

cCD

(mmolal)

Figure 5 . Mean self-diffusion coefficient for HM-PEG, DHM-PEG; as a function of CD

concentration in 3%w/w solution of HM-PEG. The dashed line represents B obtained from

Figure 4.

As noted above, the distribution in self-diffusion coefficients, σ, decreases with the addition of

M-α-CD indicating a reduction of the size distribution of the polymer aggregates. This is in

line with the discussion above. At excess M-α-CD the reduction levels off at a constant value

which most likely reflects the diffusion coefficient of individual HM-PEG molecules that are

decorated at both ends with M-α-CD. Here the value of σ would then reflect the molecular

weight distribution of HM-PEG.

We have also measured the self-diffusion of M-α-CD ( DCD). (Figure 3) A single exponential

decay fits well to the measured decay of the intensity, I , as a function of k for CD also when

HM-PEG is present. This suggests that the exchange rate of a CD molecules bound to a

hydrophobic group is fast relative to the experimental time scale (around 100 ms). In the

- 11 -



investigated concentration regime, DCD varies moderately and reaches a plateau value at

around 10-15 mmolal CD. Furthermore, DCD at the plateau in the solution containing

HM-PEG is about 1 x 10-10 m2/s. This is low compared to what was found for a solution of

M-α-CD without HM-PEG where DCD was found to be 2.5 x 10-10 m2/s. The difference is

probably due to an obstruction to the movement of non-bound CD molecules exerted by the

HM-PEG chains despite the rather low volume fraction of the polymer. For hard spheres at

low volume fractions Φ, the obstruction effect is given by

Φ−=

210 D D , where D and D0

are the diffusion coefficients in the presence and absence of obstructing spheres,

respectively. However, in the present case, we have a situation where CD molecules have to

diffuse through a medium of essentially overlapping polymer chains. In addition, the polymer

network may be considerably inhomogeneous on the relevant length-scales. This causes areduction in the diffusion coefficient which is considerably higher than simple obstruction

theories would imply.9,28 This hypothesis is supported by measurements of DCD in the

presence of unmodified PEG (MW= 20,000 g/mol) that shows a visible obstruction effect

already at concentration of unmodified PEG (c PEG) of 1 %w/w, see Figure 6 (please note that

the concentration of PEG is lower for the data presented in Figure 6, compared to the data

presented for HM-PEG). We also stress the fact that DCD in 1%w/w PEG is almost

independent of cCD shows that the complex formation between unmodified PEG and CD is of

minor importance and that the interaction is mainly an obstruction effect.

0 5 10 15 20 25 30 35 40

4.0x10-11

8.0x10-11

1.2x10-10

1.6x10-10

2.0x10-10

2.4x10-10

cCD

(mmolal)

D H M - P E G ,

D C D

( m 2 / s )

Figure 6 . Mean self-diffusion coefficient for unmodified PEG (squares) and for M-α -CD

(triangles) as a function of CD concentration in 1%w/w solution of unmodified PEG.

- 12 -



We have calculated the fraction of bound CD to HM-PEG ( P b) at two different cCD according

to Equation 5 where DCD,obs is the observed self-diffusion of CD at the actual cCD and DCD,free

is the self-diffusion of un-associated CD (which we take from data in Figure 3 at excess CD).

DCD,obs = P b DHM-PEG + (1- P b ) DCD, free (5)

Unfortunately, measurements of DCD below cCD = 2 mmolal was not possible due to low

signal to noise ratio and therefore calculations could only be done above B. As expected the

fraction of bound CD decreases with increasing cCD (table 2). The calculation of bound CD

molecules divided by the total number of hydrophobic groups (CD/hydrophobe) at cCD = 10.7

mmolal give another indication that more than one CD molecule can bind to each

hydrophobic group, although one has to be careful with such a conclusion, since the value of

DCD,free is uncertain, as it will depend on the structure and inhomogenities in the polymernetwork, which properties presumably changes upon addition of CD.

Table 2 Data from two different concentrations of M-α -CD. The values of P b are obtained

from calculations using Equation (1).

cCD

(mmolal) DCD,obs (m2 /s)

P b CD/hydrophobe

2.7 3.0 10-11

0.73 0.4

10.7 5.8 10-11 0.58 1.4

- 13 -



Conclusions

In this investigation, methylated α-cyclodextrin (M-α-CD) has been added to an aqueous

solution of hydrophobically modified polyethylene glycol (HM-PEG). Addition of M-α-CD had

a large impact on the mean self-diffusion coefficient of HM-PEG ( DHM-PEG). When the

concentration of M-α-CD (c CD) increased, DHM-PEG first increased rapidly. At intermediate c CD

the increase in DHM-PEG was less dramatic and at excess CD DHM-PEG attained a constant

value. These results confirm the model suggested from the results of rheological

measurements in an earlier study on the same system.17 It is likely that DHM-PEG, as well as

the viscosity, is strongly dependent on HM-PEG chains that connect different clusters of

micelles and that the complex formation is primarily deactivating these associations at low

c CD. At higher c CD the complex formation results in a degradation of the clusters and micelles

into separate HM-PEG molecules where all hydrophobic end groups are hidden in

cyclodextrin molecules.

It was also possible to determine the mean self-diffusion coefficient of M-α-CD ( DCD) in the

system. The results have been used to establish the fraction of M-α-CD that is bound to

HM-PEG. The results indicate that more than one M-α-CD molecule bind to each

hydrophobic group at high concentration of M-α-CD.

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