Surface studies of radiation grafted sulfonic acid membranes: XPS and SEM analysis

Surface studies of radiation grafted sulfonic acid

membranes: XPS and SEM analysis

Mohamed Mahmoud Nasef *, Hamdani Saidi

Business and Advanced Technology Centre, Universiti Teknologi Malaysia, Jalan Semarak, 54100 Kuala Lumpur, Malaysia

Received 31 October 2004; accepted 10 May 2005

Available online 13 June 2005

www.elsevier.com/locate/apsusc

Applied Surface Science 252 (2006) 3073–3084

Abstract

PTFE-g-polystyrene sulfonic acid membranes prepared by radiation-induced graft copolymerization of styrene onto

poly(tetrafluoroethylene) (PTFE) films followed by sulfonation reactions were investigated with respect to their morphology

and surface chemical properties. The chemical composition of the membranes surfaces was monitored at various degrees of

grafting using X-ray photoelectron spectroscopy (XPS). Considerable differences in the concentration of the chemical

components of the surfaces were observed despite the predominance of all membranes surfaces by a hydrocarbon fraction

originated from the incorporated sulfonated polystyrene grafts. The distribution of the sulfonated polystyrene grafts in

membranes having various degrees of grafting was investigated by scanning electron microscopy (SEM). The membranes

achieved a homogenous distribution at degrees of grafting of 24% and above. The results of this work suggest that the

membranes have a chemically sensitive surfaces and this is most likely to have an impact on their interfacial properties and

chemical stability.

# 2005 Elsevier B.V. All rights reserved.

Keywords: XPS; Surface elemental analysis; SEM; Radiation grafted sulfonic acid membranes

1. Introduction

Polymer electrolyte membranes are class of materi-

als that is receiving an increasing attention in a wide

number of solid state and electrochemical device

applications including batteries, fuel cells, super

capacitors and chemical sensors [1,2]. The selection

of a polymer electrolyte membrane for an application

* Corresponding author. Tel.: +603 26911294;

‘fax: +603 26911294.

E-mail address: [email protected] (M.M. Nasef).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2005.05.013

critically depends on the chemical reactions taking

place in the process [3]. Sulfonic acid containing

membranes have been considered for conducting

protons in proton exchange membrane (PEM) fuel

cells where high power density and low resistance are

essential to maintain high efficiency [4]. Nafion is the

standard commercialmembrane thatmostwidely tested

in this type of fuel cells. However, the high cost of these

membranes and similar commercialmaterials (Aciplex,

Dow and Flemion) has been reported to be the main

cause behind the growing demand on the development

of alternative but less costly membranes [5,6]. The

.

M.M. Nasef, H. Saidi / Applied Surface Science 252 (2006) 3073–30843074

latest development on the alternative emerging mem-

branes for PEM and direct methanol fuel cells were

reviewed by Savadogo [7,8].

Among the potential alternatives, radiation grafted

sulfonic acid membranes have the advantage of easy

preparation, controllable composition and no shaping

problem as membrane preparation starts from a film

already in a foil form [9]. More importantly, these

membranes can be produced with desirable properties

at low cost (25 s/m2) [10]. Many studies have

reported the preparation of sulfonic acid membranes

by radiation-induced grafting of styrene or its

derivatives onto various fluorinated and hydrocarbon

polymers using simultaneous and preirradiation

techniques followed by sulfonation reactions. These

studies together with the recent advances in radiation

grafted sulfonic membranes and their applications

were reviewed by Nasef and Hegazy [11].

In our recent studies, we reported on the preparation

of sulfonic acid membranes by radiation-induced

grafting of styrene onto PTFE films using simultaneous

irradiation technique, followed by a sulfonation

reaction [12,13]. The membrane physical, chemical,

thermal and structural properties were found to be

strongly dependent on the degree of grafting [13–15].

The surface properties of the membranes were found to

be dependant not only on the preparation procedure

(grafting and subsequent sulfonation) but also on the

degree of grafting as revealed by XPS [16]. The

chemical analysis of a membrane based on the

copolymer of tetrafluoroethylene and perfluorovinyl

ether (PFA) after PEM fuel cell test showed that

membranes undergo substantial degradation particu-

larly at the surface layers under the influence of

oxidative chemical attack [17]. Although, it is obvious

that the structures and the composition of the

membranes at the top layers have a strong impact on

the membrane degradation during its use in PEM fuel

cell, very little detailed information is available on the

chemical composition changes at the surface layers

with the variation in the degree of grafting and their

impact on membrane properties. This information is of

high significance for understanding the surface nature

of these membranes and its possible impact on its

interfacial behavior as well as degradation sites.

The objective of this study is to report on detailed

surface analysis of PTFE-g-polystyrene sulfonic acid

membrane using XPS with the focus on structural

changes taking place in the surfaces with the variation

in the degree of grafting and their implication the

membrane properties. The morphologically-induced

changes taking place in the membranes at various

degrees of grafting are also monitored by scanning

electron microscopy (SEM).

2. Experimental

The membranes were prepared using a two-step

procedure similar to that reported earlier [12,13]. In

the first step, radiation-induced graft copolymeriza-

tion of styrene onto PTFE (90 mm) films (Bohlender,

Germany) was carried out by styrene diluted with

dichloromethane using simultaneous irradiation tech-

nique. The grafting mixture was placed in a tightly

sealed glass ampoule and irradiated to a total dose of

20 kGy using g-rays from a 60Co source at a dose rate

of 1.32 kGy/h under nitrogen atmosphere and at room

temperature. After grafting, the grafted films were

removed, thoroughly washed with toluene several

times and dried under vacuum until a constant weight

was obtained. The degree of grafting was calculated as

the percent of weight increase in the grafted film with

respect to the weight of the original PTFE film. In the

second step, the grafted films were sulfonated using a

mixture composed of chlorosulfonic acid (30 vol%)

diluted with 1,1,2,2-tetrachloroethane at a temperature

of 90 8C for 5 h. After sulfonation, the membranes

were washed with dichloromethane and ion free water

several times and eventually rinsed in ion free water.

2.1. SEM measurements

SEM measurements were carried out by a Philips

XL-40 microscope. Cross-sections of samples were

obtained by slicing membranes frozen in liquid

nitrogen with a microtome. The samples were dried,

mounted laterally on sample holder using double-

sided adhesion tape and then sputter-coated with thin

gold film before being observed by SEM.

2.2. XPS measurements

XPS measurements were conducted on vacuum

dried membrane samples using a Kratos XSAM-HS

surface micro-analyzer having a Mg Ka X-ray source

M.M. Nasef, H. Saidi / Applied Surface Science 252 (2006) 3073–3084 3075

Fig. 1. Variation of the degree of grafting with the monomer

concentration.

(1253.6 eV) in fixed analyzer transmission (FAT)

mode. The system was calibrated using pure silver

plate giving Ag 3d5/2 at 368.25 eVand DAg = 6.00 eV.

Low X-ray flux of the non-monochromatized Mg Ka

line normally operated at 10 mA and 12 kV was used,

while charge neutralizer was switched on to minimize

the charging effect. The vacuum system was kept at

4.0 � 10�9 Torr. Wide scans in the range of 50–

1150 eV were recorded at pass energy of 160 eV with

1 eV and dwell time of 0.1 s per step, respectively.

Narrow scans at higher resolution (at pass energy of

20 eV with a step size of 0.05 eV and dwell time of

0.1 s step) were performed for the C1s, F1s, O1s and

S2p regions. Binding energies of all photoelectron

effects were corrected by deducing the charging effect

values based on C1s at 285 eV for terminal hydro-

carbon (–CH). The Gaussian peak fitting parameter

with straight baseline was applied for peak analysis

using Vision software supplied by Kratos. The

elemental composition of the top layers of the

membrane samples was directly retrieved from XPS

scans of the surface, whereas that of the submerged

and bulk layers were obtained after mechanical

removing of top layers using sharp doctor knife.

The sample was first weighed, gradually scrapped in

all directions from one side until it lost approximately

40% of its original weight. The top surface of the

scraped sample is then considered as the core part of

the membrane before scraping.

3. Results and discussion

PTFE-g-polystyrene sulfonic acid membranes with

degrees of grafting in range of 8–36% were prepared

by radiation-induced grafting of styrene diluted with

dichloromethane under controlled parameters onto

PTFE films. The variation in the degree of grafting in

the membranes was obtained by keeping the grafting

parameters, i.e. irradiation dose, temperature, dose

rate, type of solvent and film thickness unvaried to

exclude their effects on the grafting reaction while

varying the monomer concentration in the range 25–

60 vol%. The variation in the degree of grafting with

the monomer concentration is shown in Fig. 1. The

degree of grafting was found to increase with the

increase in the monomer concentration within the

range applied in this study. This behavior is attributed

to the increase in the styrene diffusion and its

concentration in the grafting zone as a result of the

availability of more styrene in the grafting solution

leading to an enhancement in the grafting yield.

Exposing PTFE film to g-radiation in presence of

styrene monomer causes the formation of free radicals

in grafting mixture components. These radicals

initiate graft copolymerization reactions in which

graft growing chains propagate forming polystyrene

side chain grafts, which eventually undergo termina-

tion reaction in presence of the diluting solvent as

described in the plausible mechanism shown in Fig. 2.

Since PTFE scarcely swells in the grafting solution

(styrene and dichloromethane) and the grafting

temperature is far below the glass transition tempera-

ture of the PTFE film, the grafting mechanism in this

system follows front mechanism where grafting starts

at the layers close to the surface of the film forming

thin grafted layers, which swell in the grafting solution

allowing progressive diffusion of the monomer

towards the middle of the film [9–12]. The grafted

copolymer films were found to have structures

predominated by hydrocarbon fraction originated

from polystyrene at their surfaces as revealed by

XPS surface analysis [16]. Sulfonation of the grafted

PTFE films led to membranes having degrees of

sulfonation close to 100% [13]. A summary of the


Fig. 2. A plausible mechanism for simultaneous radiation-induced grafting of styrene onto PTFE films.

physico-chemical properties of PTFE-g-PSSA mem-

branes is presented in Table 1.

3.1. Morphology of the membranes

The morphological changes taking place in the

membranes with the variation in the degree of grafting

was monitored in cross-sections of the samples in

Table 1

The physico-chemical properties of PTFE-g-polystyrene sulfonic

acid membranes

Degree of

grafting (wt%)

Ion exchange

capacity

(mmol/g)

Water

uptake

Ionic

conductivity

(S/cm)

8 0.65 7.0 0.0002

13 1.00 16.3 0.0013

24 1.55 20.6 0.0220

36 2.01 25.4 0.0250

order to investigate the penetration depth of poly-

styrene grafts. Fig. 3 shows SEM micrographs of

cross-sectional view of PTFE-g-polystyrene sulfonic

acid membranes having various degrees of grafting.

The structure of the sulfonated membranes appears to

be uniform with two distinct regions: dark region in

the middle of samples (A and B) and clear region on

their edges representing the un-grafted and the grafted

components of the membranes, respectively. The un-

grafted part of the membrane, which can be seen

obviously in sample A starts to diminish with the

increase in the grafting level (as seen sample B) until it

disappears in the sample C (24% degree of grafting).

This indicates that a homogeneous graft distribution is

achieved at this degree of grafting and above (36%

degree of grafting, sample D). These results support

the data presented in Table 1 that shows the ionic

conductivity is drastically increasing with the increase

in the degree of grafting and such increase tends to be


Fig. 3. SEM micrographs of cross-sections of PTFE-g-polystyrene sulfonic acid membranes having various degrees of grafting: (A) 8%, (B)

13%, (C) 24% and (D) 36%.

insignificant upon achieving homogeneous graft

distribution in the membranes (at degrees of grafting

of 24% and above).

This behavior can be explained by taking the graft

distribution into account. At low degree of grafting

polystyrene sulfonic acid grafts with their hydration

domains are only distributed near the surface of the

membrane while its bulk remains un-grafted and

exerting high resistance. Consequently, the mobility of

H+ is hindered and the conductivity is compromised

by the segregation in the water domains. As the degree

of grafting increases and the polystyrene grafts

achieves a homogenous distribution, the un-grafted

hydrophobic fraction diminishes allowing the sulfonic

acid water domains to form a network across the

membranes. Therefore, the mobility of H+ is enhanced

and the conductivity reaches high values so that any

further increase in the degree of grafting no longer

brings significant changes to it. Thus, it can be

concluded that beside the ion exchange capacity and

the water uptake, the homogenous distribution of the

polystyrene sulfonic acid grafts across the membranes

is very essential to achieve high ionic conductivity

levels.

It is important to mention that no samples with a

degree of grafting between 13 and 24% was available

for graft distribution analysis by SEM. Such range of

grafting might be significant as other workers reported

the presence of homogenous graft distribution of

polystyrene sulfonic acid grafts hosted in poly(tetra-

fluoroethylne-co-hexafluoropropylene) films at degree

of grafting as low as 13.6% [17].

3.2. Chemical analysis of the surfaces of the

membranes

3.2.1. Effect of grafting on the elemental

composition

The composition of the surface layers of PTFE-g-

polystyrene sulfonic acid membranes was investigated

by XPS. Fig. 4 shows XPS wide scan spectra of PTFE-

g-polystyrene sulfonic acid membranes having var-


Fig. 4. XPSwide scan spectra of PTFE-g-polystyrene sulfonic acid membranes having various degrees of grafting: (a) 8%, (b) 13%, (c) 24% and

(d) 36%.

ious degrees of grafting. All membranes show four

primary characteristic peaks corresponding to C1s,

F1s, O1s and S2p at binding energies (BE) of 296.5,

693.6, 536.1 and 168.2 eV, respectively. The atomic

concentrations of these elements were determined

from the intensities of their respective peaks after

being corrected with respect to sensitivity factor and

presented in Table 2. The elemental atomic concen-

trations were found to vary with the rise in the degree

of grafting. For instance, the surface concentration of

fluorine decreases with the increase of the degree of

grafting unlike that of sulfur and carbon both of which

increase with the increase in the degree of grafting.

Such behavior can be attributed to the increase in the

consumption of C–F bonds (defluorination reaction)

of CF2 groups of PTFE matrix and its subsequent

Table 2

The elemental atomic concentrations of PTFE-g-polystyrene sulfo-

nic acid membranes with various degrees of grafting

Degree of grafting (%) Atomic composition (%)

F S C O

0 59 – 30 –

8 16.3 4.5 56.9 22.3

13 13.6 4.9 59.7 21.8

24 8.5 5.8 63.1 22.6

36 4.8 6.5 67.3 21.4

substitution with polystyrene grafts. Therefore, the

surfaces were enriched with polystyrene side chain

grafts, which was subsequently sulfonated giving rise

to the concentration of both carbon and sulfur at the

membranes surfaces. Unlike, other elemental compo-

nents, the content of oxygen seems to have an

inconsistent trend indicating that other source of

oxygen than sulfonic acid groups might be involved.

Similar surface structure was reported for other

radiation grafted polystyrene sulfonic acid membranes

based on copolymers of tetrafluoroethylene with

hexafluoropropylene (FEP) and tetrafluoroethylene

with perfluorovinyl ether (PFA), respectively [18,19].

To illustrate the causes of the variation of the

elemental concentration with the increase in the

degree of grafting, C1s was closely investigated in all

membranes. Fig. 5 shows a narrow scan of C1s spectra

of PTFE-g-polystyrene sulfonic acid membranes

having various degrees of grafting. A predominant

C1s peak of hydrocarbon (CH) at BE of 285.0 eV

(corrected) is observed together with a weaker peak

assigned for C1s of fluorocarbon (CF) separated by

around 7.1 eV, i.e. located at BE of 292.1 eV. As the

degree of grafting increased, the area of CF peak

decreased whereas that of CH peak increased causing

the relative intensity of CF to CH to vary downward

from 0.14 to 0.07 as shown in Table 3. Theoretically,


Fig. 5. XPS narrow scan spectra of C1s of PTFE-g-polystyrene

sulfonic acid membranes having various degrees of grafting: (A)

8%, (B) 13%, (C) 24% and (D) 36%.

the ratio of CF/CH in the PTFE-g-polystyrene sulfonic

acid membranes (8–36% grafted) prepared is this

study should be decreasing from 3.0 to 0.98 taking into

account their equivalent weight (EW) where four C2F4units (8CF) stand for one unit of polystyrene (8CH) and

assuming every styrene unit is monosulfonated [20].

These results indicate that the hydrocarbon fraction of

the membranes, i.e. polystyrene sulfonic acid grafts

predominate the membranes structure at the surfaces.

Removing surface layers by scarping is found to

change the peak ratio towards higher fluorocarbon

content by around 20–45% depending on the degree of

grafting. It can be concluded that the membranes have

hydrocarbon rich surfaces. This is most likely to have

a strong impact on the chemical stability of these

membranes and would create surface sensitivity on the

membrane part when it is assembled with electrodes in

PEM fuel cell and water electrolyzer applications.

This conclusion goes very well with the observation

reported in our previous study, which showed a similar

Table 3

The experimental and the theoretical CF/CH ratios in PTFE-g-

polystyrene sulfonic acid membranes with various degrees of graft-

ing

Degree of grafting (%) 8 13 24 36

EW (g/mmol) 1538 1000 667 495

CF/CH (theoretical) 3.00 2.00 1.30 0.98

CF/CH (experimental) 0.14 0.11 0.95 0.07

electrolyte membrane based on PFA film substantially

lost its polystyrene sulfonic acid-rich surface layers

after being tested in PEM fuel cell for few hundreds of

hours [21]. Commercial radiation grafted membranes

(Permion 4010, RAI Inc., Hauppauge, NY), which

have a sulfonated polystyrene grafted FEP structure

were found to behave in a similar manner after being

tested in water electrolyzer [20].

3.2.2. Effect of grafting on the elemental ratio

To further illustrate the changes taking place in the

surface elemental composition of PTFE-g-polystyrene

sulfonic acid membranes, the atomic ratios: C/F, C/O,

C/S and O/S were estimated from their respective peak

areas, and correlated with the degree of grafting as

shown in Fig. 6. Both C/F and C/O ratios increase with

the increase in the degree of grafting and such increase

is found to be drastic in the former and slight in the

latter. In contrast, C/S and O/S ratios, show decreasing

trends with the variation in the degree of grafting. The

drastic increase in C/F ratio at the membranes surfaces

indicates that the increase in the degree of grafting is

associated with an enhancement in the defluorination

reaction and the surface richness of the membranes

with hydrocarbon polystyrene is a function of the

degree of grafting. The slight increase in C/O ratio

suggests that the contribution of side reaction with

oxygen during grafting is very small. The decrease in

Fig. 6. Variation of the atomic ratios of basic elements in the

surfaces of PTFE-g-polystyrene sulfonic acid membranes with

the degree of grafting.


C/S and O/S ratios with the increase in the degree of

grafting of the membranes reflects the increase in the

concentration of the sulfur resulted from sulfonation.

It also indicates that the incorporation of more

polystyrene provides more hosting sites for sulfona-

tion products. These observations are in a complete

agreement with the results of membrane properties

present in Table 1, which show the increase in the ion

exchange capacity with the increase in the degree of

grafting.

However, a close investigation to O/S ratio in all

membranes shows that it is higher than three folds

referring to the structure of (–SO3�) groups. It

maintains values of 5.0, 4.5, 3.9 and 3.3 for

membranes having degrees of grafting of 8, 13, 24

and 36%, respectively. This confirms the existence of

other oxidation product species in the membrane

surfaces. This clearly means that the oxygen of the

surface layers is not only present in a form of S–O

bonds but also in other forms such as C–O bonds. It

can be noticed that the degree of oxidation (referring

to O/S ratio) is relatively high on the surfaces of the

membranes with lower grafting level and such trend

decreases with the increase in the degree of grafting.

Generally, the increase in the oxygen content in the

membranes surfaces, can be attributed to the undesired

reaction with the traces of oxygen remaining in the

reaction solution during the grafting process, which

mostly leads to the formation of CO groups. The

Fig. 7. C1s core level spectrum of 36% grafted P

content of CO varies depending not only on the

availability of oxygen in the grafting solution but also

on that of the free radicals in PTFEfilms during grafting

reaction. For instance, at low monomer concentration,

there is a high possibility for more radicals to react with

the oxygen existing in the grafting solution as a small

number of them takes part in the grafting reaction and

this explains the high concentration ofCO inmembrane

of low degree of grafting presented in Table 2. On

contrary, at high monomer concentration more radicals

contribute to the grafting reaction leaving less number

of radicals to react with oxygen and as a result the CO

content is reduced.

3.2.3. Effect of grafting on C1s group

representatives

Fig. 7 shows C1s core level spectrum of 36%

grafted PTFE-g-polystyrene sulfonic acid membrane.

The spectrum is deconvoluted into five component

peaks. The BE of all peaks were corrected for a

charging effect of 1.0 eV. The peaks components with

BE at 285.0 eV stands for C–H and that at 292.1 eV is

for CF2. The former represents the grafted polystyrene

moiety (aromatic ring) in the membrane whereas the

latter is assigned for the PTFEmatrix of the membrane

as discussed before. The component peak at 286.2 eV

is assigned for aliphatic C–H species of the

polystyrene chain grafts attached to PTFE matrix

[22]. The peak at 286.7 eV stands for CO and C–CFn

TFE-g-polystyrene sulfonic acid membrane.


Table 4

The percentage of the various types of chemical group representatives obtained from the curve fitting of C1s spectra of PTFE-g-polystyrene

sulfonic acid membrane with various degrees of grafting

Degree of

grafting (wt%)

C–H

(285 eV)

CH–CFn(polystyrene) (286.2 eV)

CO/C–CFn(286.7 eV)

C–S

(288.7 eV)

CF2(292.1 eV)

8 66.0 4.3 4.8 1.7 23.2

13 68.3 5.6 5.3 1.9 18.9

24 75.2 7.1 5.9 2.1 9.7

36 78.6 9.6 6.4 2.4 3.5

species in the PTFE matrix [23]. The peak with BE of

288.7 eV, is attributed to C–S species [24].

The content of the various types of chemical group

representatives obtained from the curve fitting of C1s

spectra of PTFE-g-polystyrene sulfonic acid mem-

branes with various degrees of grafting is shown in

Table 4. As can be seen, all concentrations of the groups

originated from the grafted polystyrene sulfonic acid

side chain risewith the increase in the degree of grafting

whereas that of the PTFE matrix representatives

decrease. This confirms the previous results of the

accumulation of many of the grafted polystyrene side

chains on the surface of the membranes forming

polystyrene sulfonic acid rich surfaces upon sulfona-

tion, despite the achievement of bulk grafting penetra-

tion inmembranesmatrix at a degree of grafting of 24%

as revealed by SEM analysis.

Fig. 8. O1s core level spectrum of 36% grafted P

These results can be explained taking into account

the mechanism of grafting and the availability of

monomer molecules in the grafting zone. During the

grafting a large amount of monomer accumulates on

the surface of film forming graft front. By the time the

graft front moves further inwards, the availability of

monomer in the grafting zone leads to further addition

of monomer molecules to the graft chains near the

surface than in the middle of the films causing the

surface region to be rich with grafts compared to the

middle of the film.

3.2.4. Effect of grafting on O1s group

representatives

Fig. 8 shows O1s core level spectrum of PTFE-g-

polystyrene sulfonic acid membrane (36% grafted).

The spectrum is deconvoluted into three peaks having

TFE-g-polystyrene sulfonic acid membrane.


Table 5

The percentage of various types of chemical group representatives

obtained from the curve fitting of O1s spectra of PTFE-g-polystyr-

ene sulfonic acid membranes with various degrees of grafting

Degree of

grafting (wt%)

O–H

(533.2 eV)

O S

(531.1 eV)

O–C

(528.6 eV)

8 9.2 76.7 14.1

13 10.4 77.8 11.8

24 11.2 79.2 9.6

36 11.9 81.4 6.7

corrected BE at 533.2, 531.1 and 528.6 eV (charging

effect = 1.8 eV), respectively. The first peak is

assigned for O–H of water molecules associated with

sulfonic acid groups, which is giving the membranes

its strongly hydrophilic nature. The remaining of

water traces in the analyzed membrane samples

despite drying under ultra vacuum conditions is

attributed to the existence of strong hydrogen bonding

effect between water molecules and sulfonic acid

groups. The second peak is assigned for O S of –

SO3� groups incorporated during the sulfonation of

the polystyrene grafted PTFE films, which is

responsible for imparting the hydrophilicity to the

membranes. The third peak is assigned for the oxygen

of O–C group caused by oxidation during grafting

reactions as discussed earlier. The percentages of

chemical groups for the three component peaks in O1s

for membranes having various degree of grafting are

shown in Table 5. The concentration of O–H of water

increases with the increase in the degree of grafting.

This observation is in a complete agreement with the

dependence of the swelling of the membranes on the

degree of grafting as resembled from Table 1. The

increase in the degree of grafting was found to cause

an increase in both water uptake and number of water

molecules per sulfonic acid groups (hydration

number) and that was attributed to the increase in

the number of sulfonic acid groups incorporated in the

membranes with the increase in the grafting level

giving rise to the imparted hydrophilicity [13]. This

remark agrees well with the increase in the

concentration of O S peaks with the increase in the

degree of grafting shown in Table 4. However, the

analysis of the membranes after scraping their

surfaces showed O S peaks indicating the presence

of S in the depth of the membranes having degrees of

grafting as high as 24% and above. This behavior can

be attributed to the diffusion of the sulfonating agent

during the sulfonation process, which is facilitated by

the increase the amount of polystyrene grafted in the

membranes. This observation is a complete agreement

with the finding by Walsby et al. [25], in which

sulfonation reaction of polystyrene grafted poly(vi-

nylidene fluoride) (PVDF) was found to follow front

mechanism similar to that of grafting reaction.

On the other hand, the concentration of the

oxidation product O–C is found to decrease with

the increase in the degree of grafting indicating the

enhancement in the grafting reaction with the

availability of more styrene monomer molecules at

the expense of the side oxidation reaction as the degree

of grafting was varied by using different concentration

of monomer in dichloromethane. It can be concluded

that the oxidation product originates from the

sulfonation predominates the membrane surfaces at

all degrees of grafting. This creates high sensitivity in

the surfaces of the membranes towards hydrolysis and

chemical transformation. Similar observation was

reported for radiation grafted polystyrene sulfonic

acid membranes based on polyethylene when they

were investigated with XPS [24].

3.2.5. Effect of grafting on S2p group

representatives

Fig. 9 shows S2p core level spectrum of PTFE-g-

polystyrene sulfonic acid membrane. The spectrum of

36% grafted membrane is deconvoluted into four

peaks with corrected BE at 168.1, 169.2, 167.3 and

168.4 eV (charging effect = 1.9). The major two peaks

at 168.1 and 169.2 eVare assigned to 2p3/2 and 2p1/2

of the sulfur originated from sulfur of high oxidation

state, i.e. sulfonic acid group (–SO3�) [24]. The minor

peaks at 167.3 and 168.4 eVare assigned for 2p3/2 and

2p1/2 of the sulfur of lower oxidation state such as

S O (SO2), S–Cl, S–S and S–C, which might be

formed during the sulfonation process [23]. The

calculated percentage of (–SO3�) is found to equal to

�86%. The percentages of chemical groups for the

two component peaks in S2p for PTFE-g-polystyrene

sulfonic acid membranes having various degrees of

grafting are presented in Table 6. The percentage of

the sole product of sulfonation (incorporation of –

SO3� groups) to the by-products does not vary despite

the increase in the degree of grafting. This indicates

that among all sulfonation products, sulfonic acid


Fig. 9. S2p core level spectrum of 36% grafted PTFE-g-polystyrene sulfonic acid membrane.

Table 6

The percentage of various types of chemical group representatives

obtained from the curve fitting of S2p spectra of PTFE-g-polystyr-

ene sulfonic acid membrane with various degrees of grafting

Degree of

grafting (wt%)

–SO3

(168.2 6 eV)

–SO2, S–Cl, S–C, S–S

(167.3 eV)

8 87 13

13 85 15

24 87 13

36 86 14

groups (–SO3�) dominate the surfaces in all mem-

branes. Therefore, the surfaces of these membranes

are very sensitive to chemical transformations and

their interfacial properties are expected to be affected.

4. Conclusions

Variation of the degree of grafting was found to

have a strong impact on the chemical and morpho-

logical changes taking place in PTFE-g-polystyrene

sulfonic acid membranes during their preparation by

radiation-induced grafting of styrene onto PTFE films

and subsequent sulfonation. SEM investigation

showed that the membranes have achieved a homo-

geneous graft distribution at a degree of grafting of

24% and above. The concentrations of the elemental

components (C, F, S and O) were found to be functions

of the degree grafting as revealed by XPS analysis.

The membranes were found to have polystyrene

sulfonic acid rich surfaces giving them high chemical

sensitivity towards hydrolysis and chemical transfor-

mation. The variation in the water uptake and the ion

exchange capacity with the degree of grafting in the

membrane was accompanied by structural changes in

the surfaces layers, which were found to be is a form of

predominance of sulfur originated from SO3� groups

in the surfaces of the membranes. The results of these

investigations suggest that the structure of top layers

plays an important role in affecting the interfacial

properties and the stability of these membranes.

Acknowledgement

The authors wish to gratefully acknowledge the

financial support from the Ministry of Science,

Technology and Innovation (Malaysia) under IRPA

program.

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Surface studies of radiation grafted sulfonic acid membranes: XPS and SEM analysis

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