Hydrophilic–oleophobic coatings on cellulosic materials by plasma assisted polymerization in...

ORIGINAL PAPER

Hydrophilic–oleophobic coatings on cellulosic materialsby plasma assisted polymerization in liquid phaseand fluorosurfactant complexation

Ricardo Molina • Miguel Gomez • Chi-Wai Kan •

Enric Bertran

Received: 16 October 2013 / Accepted: 27 November 2013 / Published online: 4 December 2013

� Springer Science+Business Media Dordrecht 2013

Abstract Materials with hydrophilic–oleophobic

properties are of relevance due to their application to

different fields such as self-cleaning coatings, liquid–

liquid separation membranes and functional textiles

for different technical applications. In this work,

hydrophilic–oleophobic coatings have been deposited

on cellulosic materials (filter paper and bleached

cotton) by means of plasma assisted polymerization of

acrylic acid solutions in water followed by cationic

fluorosurfactant complexation. Chemical composition

of the coatings on cellulosic materials was character-

ized by means of FTIR–ATR and XPS whereas their

morphology was studied by SEM. Hydrophilic–oleo-

phobic behavior was characterized by means of

contact angle and wetting time. Additionally wetting

properties of cationic, anionic and non-ionic surfactant

solutions on the hydrophilic–oleophobic coatings

were used to characterize the polyelectrolyte electro-

static forces upon the functionalized layer.

Keywords Hydrophilic–hydrophobic coating �Plasma polymerization in liquid � Cotton �Cellulose � Acrylic acid � Fluorosurfactant

complexation

Introduction

Surfaces that can simultaneously display hydrophilic-

ity and oleophobicity, based on a favorable interaction

with polar liquids and an unfavorable interaction with

nonpolar liquids, are of great interest in different

technological applications such as oil/water separation

membranes (Yang et al. 2012; Howarter and Young-

blood 2009; Wen et al. 2013) or self cleaning surfaces

(Howarter and Youngblood 2007, 2008). The design

of hydrophilic–oleophobic surfaces is often based on

the realization of stimuli-responsive materials com-

bining moieties of largely different surface tension,

such as fluoroalkylated oligomeric silanes containing

hydrophilic monomers (Sawada et al. 1996) or

segmented polyurethanes containing an assembly of

polyoxyethylene, polydimethylsiloxane, and perflu-

oropolyether. This soft polymer blocks can switch into

oleophobic, hydrophobic or hydrophilic surface

R. Molina (&) � M. Gomez

Chemical and Biomolecular Nanontechnology

Department, Institute of Advanced Chemistry of

Catalonia (IQAC), Consejo Superior de Investigaciones

Cientificas (CSIC), Jordi Girona 18-26, 08034 Barcelona,

Spain

e-mail: ricardo.molina@iqac.csic.es

C.-W. Kan

Institute of Textiles and Clothing, The Hong Kong

Polytechnic University, Hung Hom, Kowloon,

Hong Kong

E. Bertran

Applied Physics and Optics Department, Barcelona

University, c/Avenida Diagonal 647, Barcelona, Spain

123

Cellulose (2014) 21:729–739

DOI 10.1007/s10570-013-0131-0

behaviour in response of the changing polarity of the

contact liquid (Vaidya and Chaudhury 2002). Stimuli

responsive surfaces can also be obtained by complex-

ation of cationic fluorinated surfactants on negatively

charged films obtained by means of plasma assisted

polymerization of acrylic acid (Hutton et al. 2000) or

maleic anhydride (Lampitt et al. 2000). Different

methods are usually employed in order to promote

plasma polymerization of a monomer on a solid

substrate. The common method consists in the intro-

duction of the monomer with a carrier gas in the

plasma reactor and polymerization takes place as a

result of the free radicals generated during plasma

treatment. Plasma polymerized acrylic acid films have

been obtained at low (Gilbert and Johansson 1993) and

atmospheric pressure plasmas (Ward et al. 2003; Nisol

et al. 2013). Additionally, at atmospheric pressure,

plasma assisted polymerization can be obtained with

aerosol or atomised liquid droplets of monomer inside

a carrier gas (Tatoulian et al. 2007; Mix et al. 2012).

However, all these methods described above have the

drawback that the amount of monomer that has been

polymerized respect to non polymerized monomer

(polymerization yield) is very difficult to be

determined.

The use of atmospheric pressure dielectric barrier

discharge plasma in order to initiate the polymeriza-

tion of a monomer solution is an interesting novel

procedure in order to promote the deposition of

polymer films onto different solids substrates (Molina

et al. 2013a). Discharges in and in contact with liquids

generate UV radiation, shock waves, very active

molecules or atoms (OH, O, H2O2) and free radicals

than can promote polymerization, oxidation and

degradation processes of pollutants. However, in mild

experimental conditions, polymerization of mono-

mers prevails over oxidation and degradation pro-

cesses and the properties of the final polymer coating

resemble that obtained by conventional polymeriza-

tion methods.

This work is focused on deposition of functional

polymer films onto cellulosic substrates (bleached

cotton and filter paper) by means of plasma initiated

polymerization in liquid phase. In this way, initial

study of plasma assisted polymerization of acrylic acid

solutions over glass was carried out in order to study

polymerization processes in liquid phase and the

properties of the obtained films. Subsequently, plasma

polymerization of acrylic acid solutions at different

concentrations was carried out on cellulosic substrates.

Hydrophilic–olephobic behavior can be obtained after

cationic fluorosurfactant complexation on the plasma

polymerized acrylic acid films over cellulosic sub-

strates. Wetting properties with different liquids

(water, hydrocarbons and surfactant solutions) have

been investigated.

Experimental

Materials

Glass (plain microslides, Corning), bleached cotton

fabric (plain weave, bleached without optical bright-

ener with a grammage of 180 g/m2 (article 210) from

EMPA), and filter paper (filter paper reams H quality,

Rubicolor S.L.) were use as substrates. Acrylic acid

(Panreac, anhydrous 99 %) was used as monomer for

plasma assisted polymerization. Cationic fluorosurf-

actant solution (S-106A Chemguard) was applied on

plasma polymerized acrylic acid films on cellulosic

substrates in order to obtain hydrophilic–oleophobic

coatings. Buffered solution at pH 2 (citric acid/sodium

hydroxide/hydrogen chloride, Merck KGaa) was used

in swelling studies. Milli-Q water and hydrocarbons

with different chains lengths heptane (Merck, 99 %),

octane (Fluka, 99.5 %), decane (Merck, 99 %), dode-

cane (Sigma, 99 %) and hexadecane were used in

order to evaluate the wetting behavior of the hydro-

philic–oleophobic coatings. Cationic surfactant (Hex-

adecyltrimethylammonium bromide (CTAB), Merck

99 %, 27 mM), anionic (Dioctyl sodium sulfosucci-

nate (AOT), Fluka 99 %, 22 mM) and non-ionic

(polyoxyethylene (2) cetyl ether (Brij 52), Uniqema

23 mM) surfactant solutions above their critical

micelle concentration (&1, 2.6 and 19.9 mM respec-

tively) (Neugebauer 1990; Umlong and Ismail 2005;

Wang et al. 2013) were used to analyze the polyelec-

trolyte electrostatic forces upon the functionalized

layer. Surface tension of wetting liquids was measured

with a KSV Sigma 700 electrobalance.

Plasma polymerization

A dielectric barrier discharge (DBD) reactor operating

at atmospheric pressure was used in this work (Fig. 1).

Gas mass flow meter and controllers (Bronkhorst,

Ruurlo, The Netherlands) were used in order to

730 Cellulose (2014) 21:729–739

123

introduce helium gas (5 Ln min-1) in the reactor

chamber. A 100 kHz signal was generated with a GF-

855 function generator (Promax, L’Hospitalet de Llob-

regat, Spain) connected to a linear amplifier AG-1012

(T&C Power Conversion Inc., Rochester, NY, USA).

The incident power in the plasma reactor was kept

constant at 30 W. A matching network and two

transformers (HR-Diemen S.A., Sant Hipolit de Volt-

rega, Spain) were connected to the amplifier output in

order to increase the voltage up to &20 kV. The

distance between the two electrodes was kept constant at

approximately 5 mm.

Glass, bleached cotton fabric and filter paper

substrates were introduced inside the reactor and

100 ll of different concentrations (1, 10, 20 and

100 % in weight) of acrylic acid in water solution

where placed over the substrates. In situ plasma

polymerization process was carried out over a period

of time of 10 min to obtain completely dried macro-

scopic films.

Fluorosurfactant complexation

After plasma assisted polymerization of acrylic acid

solutions, samples were immediately immersed into a

diluted (1/100) cationic fluorosurfactant solution for

15 min, washed with distilled water and left dry at

room temperature for 24 h.

Equilibrium weight change in water of polyacrylic

acid films

In a water solution at neutral pH, polyacrylic acid

(PAA) is an anionic polymer, i.e. many of the side

chains of PAA will lose their protons and acquire a

negative charge. This makes PAAs polyelectrolytes,

with the ability to absorb and retain water and swell to

many times their original volume. However, at acid

pH PAA side chains are not deprotonated and swelling

behaviour and water retain decreases significantly.

Therefore, swelling behavior of polymerized acrylic

acid films were carried out by gravimetric method as a

function of immersion time in MilliQ water at neutral

pH and in a buffered solution at pH 2.

Quantitative evaluation of PAA crosslinking

efficiency

It is well know that the non-reacted monomer and

linear polymerized acrylic acid are water soluble

whereas the crosslinked one remains water insoluble.

Therefore, the qualitative crosslinking efficiency cor-

responding to the films obtained by means of plasma

initiated polymerization of acrylic acid solutions have

been evaluated by their solubilisation rate in water at

neutral pH. Therefore, polymerized films were

immersed for 24 h in MilliQ water left dry 1 day at

40 �C. Final weight of the dried film was compared

with the obtained after plasma initiated polymeriza-

tion in order to obtain the crosslinking efficiency (CE)

taking into account the following equation:

CE %ð Þ ¼ Wdd=WPAAð Þ � 100 ð1Þ

where WPAA is the weight of PAA obtained after

plasma initiated polymerization and Wdd is the weight

of dried PAA films previously immersed in water for

24 h.

FTIR

FTIR of the samples were performed in a Nicolet

AVATAR 360 in the range of 400–4000 cm-1.

Transmission mode was used in order to characterize

PAA films obtained after plasma initiated polymeri-

zation. KBr pellets were prepared with a powdered

dried plasma polymerized PAA films. A total of 32

Fig. 1 Glass reactor cell

(left) and liquid phase

plasma polymerization

scheme (right)

Cellulose (2014) 21:729–739 731

123

scans were collected for each measurement at a

resolution of 4 cm-1. ATR measurements were per-

formed in untreated and coated cellulosic samples

(filter paper and bleached cotton fabric) using a ZnSe

crystal at 45�. Spectra were obtained with an average

of 64 scans using a resolution of 4 cm-1. An advanced

ATR correction algorithm (OMNIC 7.3 from Thermo

Electron Corporation) was used in order to correct for

band intensity distortion, peak shifts and polarization

effects. Corrected ATR spectra are highly comparable

to their transmission equivalents (Molina et al. 2013b).

XPS

X-ray photoelectron spectroscopy was used for the

evaluation of surface chemical changes. Samples were

analyzed using a PHI Model 5500 Multitechnique

System with an Al Ka monochromatic X-ray source

operating at 350 W. The measurements were done at a

normal emission angle. Survey scans were taken in the

range 0–1100 eV, with pass energy of 187.85 eV. High

resolution scans were obtained on the C1s, O1s, N1s and

F1s photoelectron peaks, with pass energy of 23.5 eV.

Binding energies were referenced to the C1s photoelec-

tronpeak position for C–C and C–H species at 285.0 eV

or C–O species at 286.4 eV for bleached cotton (Gorjanc

et al. 2010). Surface composition has been estimated

after a linear background subtraction from the area of the

different photo-emission peaks modified by their corre-

sponding sensitivity factors (Briggs and Seah 1983).

Contact angle and absorption time

Contact angle was determined by drop shape analysis

method using a CAM 200 apparatus located at the

Applied Physics department of the University of

Barcelona. Liquid droplets (&5 ll) of water and

hexadecane were used to measure the evolution of the

contact angle of polar and non-polar liquids. Absorp-

tion time was determined by the time taken for a

cellulosic substrate (bleached cotton or filter paper) to

completely absorb a drop (&10 ll) of the different

test liquids used (Milli-Q water, hydrocarbons and

surfactant solutions).

SEM

The morphology of the plasma polymerized acrylic

acid films was studied by scanning electron

microscopy (model Hitachi S-3500 N). Samples were

previously coated with Au/Pd (thickness coating

*20 nm) in a sputtering device Polaron SC500.

Results and discussion

Exposure of pure and water solutions (1–20 %) of

acrylic acid monomer to an atmospheric plasma for a

period of time of 10 min results in the formation of a

dry polymerized film. Film polymerization yield

(Table 1) seems to be independent on acrylic acid

concentration obtaining a value close to 80 %. How-

ever, acrylic acid monomer is severely irritating and

corrosive to the skin and the respiratory tract. There-

fore, the use of acrylic acid water solutions reduces its

toxicity and the monomer solution and can be applied

in a more safety way during an atmospheric plasma

treatment. For this reason, films obtained after plasma

assisted polymerization of acrylic acid monomer in

water solutions have been characterized.

In order to evaluate the film swelling behaviour,

films obtained on the glass slides after plasma assisted

polymerization of acrylic acid solution (20 %), were

immersed in water solutions with different pH for

different periods of time (Fig. 2). A clear different

swelling behaviour is observed below and above pKa

(&4.5) of poly(acrylic acid) hydrogels (Jabbari and

Nozari 2000). At pH 7 the carboxyl group of the

polymer becomes deprotonated leading to strong

charge repulsion force and increasing the swelling

capabilities of the polymerized films. It is also

observed a decrease in weight change by increasing

immersion time as a consequence of dissolution of

unreacted monomer or linear polymerized acrylic

acid. However, at pH 2 no deprotonation of carboxylic

acid group occurs and initial weight change and

swelling behaviour is reduced. It is observed that no

Table 1 Evaluation of film polymerization yield of acrylic

acid (AA) aqueous solutions treated with atmospheric plasma

AA

(%)

Initial

monomer (mg)

Film weight

(mg)

Film polymerization

yield (%)

1 1.8 ± 0.1 1.4 ± 0.2 79 ± 10

10 12.3 ± 1.1 8.6 ± 1.0 70 ± 9

20 22.1 ± 1.0 18.2 ± 2.2 82 ± 8

100 48.5 ± 6.1 37.5 ± 7.4 76 ± 6

732 Cellulose (2014) 21:729–739

123

significant variation on weight change is observed

during 8 h indicating the stability of the plasma

polymerized film at pH below pKa of poly(acrylic

acid). Large experimental errors observed in weight

change are attributed to a combination of different

processes and effects such as, plasma polymerization

mechanisms, evaporation, irregularity in spreading of

solution droplets over the glass slide used as a

substrate and dissolution of monomer or linear poly-

mer occurring at the same time that swelling process

(Molina et al. 2013a).

FTIR characterization of acrylic acid plasma poly-

merized films was performed in order to identify

polymerization process and possible new chemical

groups incorporated during plasma polymerization in

water (Fig. 3). FTIR spectra corresponding to

untreated acrylic acid shows characteristic main bands

corresponding to vinyl groups (1622 and 966 cm-1),

C=O (1711 cm-1), CH2 deformation (1451 cm-1) and

the C–O stretching (1112 cm-1) (Kirwan et al. 2003;

Dong et al. 1997). During in situ plasma initiated

polymerization of acrylic acid the characteristic peaks

of vinyl groups (C=C, CH2 = stretching peaks) dis-

appeared. This results suggest that plasma polymeri-

zation take place most probably by a radical

polymerization mechanism through the path of vinyl

bonds (C=C) cleavage. The presence of the monomer

main absorption peaks (C=O at 1711 cm-1, CH2

deformation at 1451 cm-1 and the C–O stretching at

1112 cm-1) in the plasma polymerized acrylic acid

reveals the highly retention of the monomer chemical

composition. Therefore it is assumed that monomer

polymerization prevails over oxidation or degradation

processes.

Incorporation of acrylic acid and fluorosurfactant

complexation on cellulosic materials

In order to obtain functional cellulosic materials

acrylic acid solutions at different concentrations were

polymerized on bleached cotton and filter paper

substrates and then subsequently immersed in a

cationic fluorosurfactant solution. Figure 4 illustrates

the changes in the FTIR spectrum related to the

different steps of the functionalization process over

bleached cotton fabric. The fixation of the acrylic acid

plasma polymer in the bleached cotton substrate is

revealed by the presence of the absorption peak at

1711 cm-1 due to the C=O bonds and the C–O

0 2 4 6 80

500

1000

1500

2000

2500

3000

3500W

eigh

t cha

nge

(%)

Immersion time (hours)

pH 2 pH 7

Fig. 2 Swelling behavior corresponding to films obtained after

plasma assisted polymerization of acrylic acid solutions (20 %)

below and above pKa of poly(acrylic acid)

2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

Plasma polymerized AA 20%

Acrylic acid (AA)

Ab

sorb

ance

(a.

u.)

Wavenumber (cm-1)

C=O

CH2

C-O

C=C CH2 =CH2C-O

Fig. 3 FTIR spectra of the acrylic acid monomer and film

obtained plasma assisted polymerized acrylic acid solution

(20 %)

3500 3000 2500 2000 1500 10000,0

0,4

0,8

1,2

1,6

2,0

2,4

PPAA20-S106A

PPAA20

UT

Abs

orba

nce

(a.u

)

Wavenumber (cm-1)

Fig. 4 FTIR–ATR corresponding to bleached cotton (UT),

plasma assisted polymerized acrylic acid (20 %) over bleached

cotton (PPAA20) and subsequent cationic fluorosurfactant

complexation (PPAA20-S106A)

Cellulose (2014) 21:729–739 733

123

stretching band at 1112 cm-1. After cationic fluoro-

surfactant complexation, new peaks appear in the

region between 1450 and 870 related to the presence of

C–F bonds (Kharitonov 2008). It can be observed the

presence of a peak at 1636 cm-1 in parallel with a

saturated broad peak at 3200 cm-1 suggesting that

after fluorosurfactant complexation water is also

retained in the structure.

Figure 5 shows FTIR spectra, in the region of

interest (2000–800 cm-1), corresponding to plasma

polymerization of acrylic acid solutions at different

concentrations performed in both bleached cotton and

filter paper substrates. It can observed an increase in

absorbance signal attributed to the presence of acrylic

acid (C=O, 1711 cm-1) as a function of acrylic acid

solution concentration suggesting that the amount of

polymerized film on bleached cotton fabric and filter

paper increases as a function of acrylic acid solution.

In addition after cationic fluorosurfactant complexa-

tion, the peak located at 1636 cm-1 suggests the

presence of water independent of acrylic acid per-

centage. Whereas bleached cotton fabric have a high

signal to noise ratio due to the good contact between

sample and ATR crystal, the signal to noise ratio is low

in filter paper due to the poor contact and changes are

more difficult to be observed. However, same ten-

dency in filter paper and in bleached cotton substrates

seems to occur.

The formation of the acrylic acid film and fluorosurf-

actant complexation was also followed by means of XPS

(Table 2). No significant differences in the percentage of

carbon and oxygen atoms can be found after acrylic acid

film formation due to the similar chemical composition of

acrylic acid and bleached cotton fibers. However, after

fluorosurfactant complexation, it is observed a decrease

in carbon and oxygen atoms and an increase of fluor

atoms, confirming the presence of the fluorosurfactant on

the bleached cotton surface. Also sulfur atoms are present

and are attributed to impurities or other possible surfac-

tants present in the cationic fluorosurfactant solution.

High resolution scans for the C1s photoelectron

peak reveal the different carbon functionalities on the

bleached cotton surface for the different treatments

(Fig. 6). Bleached cotton spectra (Fig. 6a) is mainly

composed of a major peak corresponding to C–O

functionalities (&286.6 eV), as expected for a cellu-

losic fibre, a shoulder band at lower binding energies

attributed to aliphatic carbon atoms (&285.0 eV) and

a shoulder band at high binding energies (C=O at

&288.2 eV and O–C=O at &289.0 eV) (Topalovic

et al. 2007; Gorjanc et al. 2010).

Acrylic acid polymerization on bleached cotton

surface (Fig. 6b) results in a decrease in the C–O

groups and an increase in the C–C and O–C=O groups

as a consequence of the presence of the layer of

polymerized acrylic acid on the bleached cotton

surface. The signals attributed to bleached cotton

disappear indicating that the thickness of the film

deposited on bleached cotton substrate is greater than

the XPS penetration depth (10 nm). After cationic

0,0

0,4

0,8

1,2COTTON

20% PPAA-S106A

10% PPAA-S106A

1% PPAA- S106A

UT

Ab

sorb

ance

(a.

u.)

Wavenumber (cm-1)

2000 1800 1600 1400 1200 1000 800 2000 1800 1600 1400 1200 1000 8000,00

0,07

0,14

0,21

0,28FILTER PAPER

20%PPAA - S106A

10%PPAA - S106A

1%PPAA - S106A

UT

Ab

sorb

ance

(a.

u.)

Wavenumber (cm-1)

(a) (b)

Fig. 5 FTIR–ATR spectra corresponding plasma assisted polymerized acrylic acid solutions (PPAA) at different concentrations and

subsequent cationic fluorosurfactant (S106A) complexation over a bleached cotton and b filter paper substrates

Table 2 Elemental atomic composition corresponding to the

bleached cotton (UT), plasma assisted polymerized acrylic acid

solution (20 %) over cotton substrate (PPAA20) and after

cationic fluorosurfactant complexation (PPAA20-S106A)

C (%) O (%) N (%) S (%) F (%)

UT 62.64 36.83 0.52 0.00 0.00

PPAA20 67.55 32.17 0.28 0.00 0.00

PPAA20-S106A 58.76 14.94 1.75 1.72 22.83

734 Cellulose (2014) 21:729–739

123

fluorosurfactant complexation, different fluor func-

tionalities (CF2, CF3) (Lampitt et al. 2000) appears

whereas the signal associated to acrylic acid (O–C=O)

is reduced.

SEM observations corresponding to polymerized

acrylic acid surfactant functionalized films over bleached

cotton (Fig. 7) or filter paper (Fig. 8) substrates reveal

that the morphology of the films depends on acrylic acid

concentration.

At low acrylic acid concentrations, a thin film is

observed on individual cellulosic fibers forming both

the bleached cotton and filter paper substrates

(Figs. 7b, 8b). Cellulosic fibers preserve their identity

and a not completely homogeneous film is observed

-2

0

2

4

6

8

10

12

14

16

18

20

O-C

=O C=O

C-O

C-C

No

rmal

ized

inte

nsi

ty (

ato

m %

)

Binding energy (eV)

UT

296 294 292 290 288 286 284 282 280 296 294 292 290 288 286 284 282 280-2

0

2

4

6

8

10

12

14

16

18

20

C=O

No

rmal

ized

inte

nsi

ty (

ato

m%

)

Binding energy (eV)

UT PPAA20 PPAA20- S106AC

-C

C-O

O-C

=O

C-F

2

C-F

3

(a) (b)

Fig. 6 Deconvolution of carbon functionalities (a) of bleached

cotton (UT) and (b) comparison of carbon functionalities of

plasma assisted polymerized acrylic acid solution (20 %) over

bleached cotton substrate (PPAA20) and after cationic fluoro-

surfactant complexation (PPAA20-S106A) respect to bleached

cotton (UT)

(a) (b) (c)

Fig. 7 SEM observation corresponding to a bleached cotton and cationic fluorosurfactant complexation on plasma assisted

polymerized acrylic acid solutions over bleached cotton substrate at b low (1 %) and c high acrylic acid concentration (20 %)

Cellulose (2014) 21:729–739 735

123

that sometimes connect fibers between them. At high

acrylic acid concentration (Figs. 7c, 8c), both

bleached cotton and filter paper substrates fibers loss

their identity and are almost recovered by a homoge-

neous thin film. Therefore, the amount of acrylic acid

polymerized on a substrate can be modulated by the

acrylic acid monomer concentration in water solution.

Wetting properties of polymerized acrylic acid

surfactant functionalized samples on liquids with

different nature and characteristics were studied. In

order to confirm hydrophilic-oleophobic behavior,

contact angle was measured for water and hexadecane

for both paper and bleached cotton polymerized acrylic

acid surfactant functionalized samples. Figure 9 shows

contact angle values as a function of time for

hexadecane and water over bleached cotton and filter

paper after acrylic acid (20 %) plasma assisted poly-

merization and fluorosurfactant complexation.

Water contact angle on functionalized cotton

changes from 126� to 53� in 50 s and water absorption

(a) (b) (c)

Fig. 8 SEM observation corresponding to (a) untreated filter paper and cationic fluorosurfactant complexation on plasma assisted

polymerized acrylic acid solutions over filter paper substrate at b low (1 %) and c high acrylic acid concentration (20 %)

0

20

40

60

80

100

120 H2O

Hexadecane

Con

tact

ang

le (

º)

Time (s)

Filter paper

0 100 200 300 400 500 6000 10 20 30 40 50 60 70 80 90 100 110

30

40

50

60

70

80

90

100

110

120

130 H

2O

Hexadecane

Con

tact

ang

le (

º)

Time (s)

Cotton fabric

Fig. 9 Contact angle as a function of time for hexadecane and water over (left) bleached cotton and (right) filter paper after acrylic acid

(20 %) plasma assisted polymerization and fluorosurfactant complexation

736 Cellulose (2014) 21:729–739

123

time is estimated at &60 s. This value is significantly

higher than the water absorption time corresponding to

bleached cotton (\5 s) and evidences the presence of

the hydrophobic groups on bleached cotton surface.

Hexadecane contact angle on functionalized cotton

changed from 76� to 29� in 80 s and hexadecane

absorption time is 100 s whereas in bleached cotton

hexadecane absorption time is very low (\5 s).

Therefore, hexadecane absorption time is greater than

water absorption time, indicating a moderate oleo-

phobic-hydrophilic behavior of the cotton functional-

ized substrates. Water contact angle on functionalized

filter paper changes from 119� to 0� in 70 s (absorption

time of 70 s), whereas hexadecane contact angle value

still constant at 76� for more than 10 min. This result

evidences the hydrophilic–hydrophobic behavior in

the filter paper functionalized surface. It is suggested

that the different wetting behavior observed between

filter paper and cotton functionalized surfaces can be

attributed to the different surface roughness present on

both substrates and film homogeneity as observed by

SEM.

Absorption time of hydrocarbons with different

chain length was also measured in order to evaluate

the oleophobic behavior of cotton and filter paper

functionalized surfaces. It can be observed, that

absorption time on filter paper substrate is greater

than in cotton functionalized substrate (Fig. 10).

Wetting absorption time increases as a function of

hydrocarbon chain length or surface tension (heptane

(20.1 mN/m)\octane (21.6 mN/m)\decane

(23.8 mN/m)\dodecane (25.4 mN/m)) (Hough and

White 1980) and oleophobic behaviour seems to occur

for long hydrocarbon chain length. It is suggested that

for shorter hydrocarbons chains, hydrocarbon mole-

cules can penetrate easily between the fluorosurfactant

structures and reach the polyacrylic acid substrate.

Best oleophobic behavior is obtained for high acrylic

acid solution concentration (20 %) on filter paper

substrate (absorption time [1 day).

Finally, wetting properties of cationic, anionic and

non-ionic (CTAB, AOT and Brij 52 respectively)

surfactant solutions above critical micelle concentra-

tion (CMC) were used to analyze the polyelectrolyte

electrostatic forces upon the functionalized layer. The

different absorption times due to the surface charge

distribution are show in Fig. 11. It can be observed

that wetting time obtained for surfactant solutions is

higher than the obtained for water (60 s). This is

attributed to the direct micelle structure that forms

these surfactants in water where the hydrophobic

chain length is located in the outermost part of the

micelle structure. Cationic surfactant solution (CTAB)

have a great absorption time ([10 min) attributed to

an electrostatic repulsion with the cationic fluorosurf-

actant. On the other hand, non-ionic (Brij 52) and

anionic (AOT) surfactants solutions have similar

absorption times (&100 s) slightly higher than that

obtained in water.

Heptane Octane Decane Dodecane0

100

200

300

400

87000

Hydrocarbon

Ab

sorp

tio

n t

ime

(s)

PAPER PPAA10-S106A PAPER PPAA20-S106A COTTON PPAA10-S106A COTTON PPAA20-S106A

Fig. 10 Absorption time of different hydrocarbon drops for

paper and cotton samples functionalized at different acrylic acid

concentration (10 % (PPAA10) and 20 % (PPAA20)) and

fluorosurfactant complexation (S106A)

CTAB Brij52 AOT0

100

200

300

400

500

600

700

800

900

Ab

sorp

tio

n t

ime

(s)

PAPER 10PAA-S106A PAPER 20PAA-S106A COTTON 10PAA-S106A COTTON 20PAA-S106A

(35,1 mN/m) (30,0 mN/m)(28,1 mN/m)

Fig. 11 Absorption time of different surfactant solutions

(above CMC) for paper and cotton samples functionalized at

different acrylic acid concentration (10 % (PPAA10) and 20 %

(PPAA20)) and fluorosurfactant complexation

Cellulose (2014) 21:729–739 737

123

Conclusions

In this paper, plasma assisted polymerization of

acrylic acid solutions in water were carried out over

glass substrates and cellulosic materials (bleached

cotton and filter paper). Experimental results revealed

that polymerization yield seems to be independent of

acrylic acid concentration in water. About 70–80 %

polymerization yield could be obtained. FTIR and

swelling behavior seems to indicate that the plasma

polymerized acrylic acid films reveals a highly

retention of the monomer chemical composition.

SEM pictures showed clearly that acrylic acid film

was formed in the cellulosic material surface and that

the amount of polymerized film increases as a function

of acrylic acid concentration. The FTIR–ATR and

XPS surface analysis further concluded the formation

of acrylic acid film in the cellulosic material surface.

Hydrophilic–olephobic behavior can be obtained after

cationic fluorosurfactant complexation on the plasma

polymerized acrylic acid films. Wetting absorption

time increases as a function hydrocarbon chain length

or surface tension and oleophobic behavior seems to

occur for long hydrocarbon chain length (e.g. decane

or hexadecane). Additional wetting behavior studies

using surfactant solutions reveal that absorption times

are higher than the obtained for water. Whereas non-

ionic (Brij 52) and anionic (AOT) surfactant solutions

have similar wetting times, an increase in wetting time

is observed for cationic surfactant solution (CTAB).

Therefore, plasma assisted polymerization in liquids

seems a promising technique in order to promote the

deposition of functional polymer films onto textile and

porous substrates.

Acknowledgments The work was supported by a grant from

the CSIC/RGC Joint Research Scheme sponsored by the

Research Grants Council of Hong Kong and the Spanish

National Research Council (Reference No. S-HK007/12). We

are grateful to Chemguard for the generous gift of

fluorosurfactant solutions. The authors would also like to

thank J. Fortuno (Electron Microscopy Service, ICM/CSIC,

Barcelona, Spain) for his valuable help in SEM analysis.

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