Journal of New Materials for Electrochemical Systems, 3, 311-319 (2000)
© J. New Mat. Electrochem. Systems
Surface investigations of radiation grafted FEP-g-polystyrene sulfonic acid membranes using XPS
M. M. Nasefa,*, H. Saidia and M. A. Yarmob
aMembrane
Research Unit, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, Jalan Semarak, 54100
Kuala Lumpur, Malaysia
bDepartment of Chemistry, Faculty of Physical and Applied Science, Universiti Kebangsaan, Malaysia, 43600 Bangi, Selangor, Malaysia
(Received September 28, 1999; received in revised form December 20, 1999)
Abstract: X-ray photoelectron spectroscopy (XPS) investigations of FEP-g-polystyrene sulfonic acid membranes prepared by radiation-induced
graft copolymerization of styrene onto poly(tetrafluoroethylene-co-hexafluoropropylene), FEP films were conducted to monitor the morphological
changes accompanied the membrane two-step preparation procedure and variation of the degree of grafting. The spectra of XPS were analyzed
with the main focus on carbon, fluorine, sulfur, and oxygen spectra as they compose the basic elemental components of the membrane. The
original FEP film was found to undergo structural changes in terms of chemical composition under the influence of grafting and sulfonation. The
surface of the resulting membranes was found to have a nearly pure sulfonated hydrocarbon structure. The atomic ratio of F/C in the membranes
was found to be strongly dependent on the degree of grafting whereas, the binding energies of the elemental components of the membranes were
found to be independent of the degree of grafting. The results of this study suggest that surface structural properties play a signification part in
the degradation of radiation-grafted membranes.
Key words:
onto fluorinated polymers have shown the capability to provide an
alternative route to produce low cost perfluorinated sulfonic acid
membranes [11]. Most of radiation grafted, sulfonic acid
membranes have been prepared by grafting of styrene or its
derivatives onto various fluorinated polymers films followed by
sulfonation reaction [11-18]. For chemical stability reasons, the
polymer backbone has been confined to fluorinated polymers.
This is due to their outstanding chemical as well as thermal
stability and mechanical integrity. Among fluorinated polymers,
FEP is advantageous in terms of high radiation resistance and
therefore various ranges of doses can be applied.
1. INTRODUCTION
Perfluorinated sulfonic acid, cation exchange membranes have
shown the potential to be used as separators and electrolytes in
electrochemical applications such as chloro-alkali industry, water
electrolysis and solid polymer electrolyte (SPE) fuel cells. [1,2].
This is due to their high chemical and thermal stability as well as
mechanical strength. However, the high cost of these membranes
such as Nafion , Dow developmental membrane and Aciplex
has prevented SPE fuel cell to be commercially competitive [3, 4].
Therefore, many efforts have been devoted to develop alternative
membranes having a combination of high ionic conductivity,
stability and low cost [5-10].
Zhi-li et al. [19] carried out the preparation of cation exchange
membrane by grafting of styrene with divinylbenzene onto FEP
using preirradiation technique and subsequent sulfonation.
Scherer and co-workers [20-23] have reported the preparation of
similar membranes by grafting of styrene on FEP films followed
In the search for less expensive and better ionic conducting
membranes, radiation-induced grafting of functional monomers
*To
whom correspondence should be adressed. Fax: + 603 2914427, e-mail:
[email protected].
311
312
M. M. Nasef et al./J. New Mat. Electrochem. Systems 3, 311-319 (2000)
by sulfonation using both simultaneous and preirradiation
techniques. In their work divinylbenzene and triallyl cyanorate
were used as crosslinkers. The resulting crosslinked FEP-gpolystyrene sulfonic acid membranes were characterized by
measuring their physico-chemical properties such as water uptake,
ion exchange capacity and proton conductivity [23]. Other
membrane properties such as thermal stability [24, 25],
crsytallinity [26] and state of water [27] were also determined.
The FEP-g-polystyrene sulfonic acid membranes were found to
have very promising performance in SPE fuel cell (1400 h at
temperature up to 80 oC) [28]. The membrane properties were
found to be dependent mainly on the percentage of styrene grafted
thereon when subsequently sulfonated [20, 21, 23]. However, a
study showing the morphological changes taking place in the
surfaces of the membrane under the influence of membrane
preparation as well as variation of the degree of grafting and their
possible impacts on the membrane chemical stability have not
been reported yet. It is very interesting to study the structural
changes taking place in the surfaces of the membranes during the
preparation and their influence on the membrane stability.
We prepared similar FEP-polystyrene sulfonic acid membranes by
radiation grafting of styrene onto FEP films followed by
sulfonation using simultaneous irradiation [29]. The membranes
were found to have comparable physico-chemical properties
comparable with the commercially available membranes.
Moreover, they have a thermal stability up to a temperature of 300
oC in oxygen [30]. In the present work, we report on
morphological investigations of FEP-g-polystyrene sulfonic acid
membranes using XPS to clarify the structural changes induced in
the surface of FEP film by grafting of styrene and subsequent
sulfonation reaction as well as the variation of the degree of
grafting in the membranes. Measurement included original and
grafted FEP films as references.
2. EXPERIMENTAL
In the second step, the grafted FEP films were sulfonated using a
mixture of chlorosulfonic acid and 1,1,2,2-tetrachloroethane at 90
oC for 4 h. The sulfonated membranes were treated with 0.5 M
KOH solution and regenerated with 1 M HCl solution then
washed acid free using deionized water. Details of sulfonation and
physico-chemical properties of the resulted membranes were
reported elsewhere [30]. The degree of sulfonation was calculated
by taking the number of moles of sulfonic acid groups and the
number of grafted styrene molecules in the membranes into
account. The membranes were found to achieve a degree of
sulfonation close to 100 %.
2. 2. XPS Measurements
XPS measurements were conducted on dry samples (original,
grafted and sulfonated membranes in acid form) using Kratos
XSAM-HS surface micro analyzer using Mg Kα X-ray source
(1253.6 eV) in Fixed Analyzer Transmission (FAT) mode. Binding
energies of the instrument were calibrated using pure silver plate
and gives Ag 3d5/2 at 368.25 eV and ∆ Ag =6.00 eV. Low X-ray
flux of the non-monochromatized MgKα line normally operated at
10 mA and 12 kV, while charge neutralizer was switched on in
order to minimize the charging effect. The sample areas excited by
the X-ray spot had a size of 240 µm2. The vacuum system was
kept at 4.0 x 10-9 torr. Wide scans are carried out in the range of
50 to 1150 eV were recorded at pass energy of 160 eV with a step
size of 1 eV and dwell time of 0.1 s step. 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, S1s and
O1s regions. Each element scanning is repeated 5 times in order to
get reproducible results. Both surfaces of the samples were
investigated to establish the symmetrical structure of each
membrane. Binding energy of photoelectrons were corrected
based on C1s at 284.5 eV for terminal hydrocarbon (-Cα). The
Gaussian peak fitting parameter with straight baseline was applied
for peak analysis using Vision software supplied by Kratos.
2. 1. Membrane Preparation
2. 3. Chemical Stability Measurements
FEP-g-polystyrene sulfonic acid membranes were prepared by
two-step procedure. In the first step, styrene (Fluka, purity of ≥ 99
%) was grafted onto FEP film (Porghof, USA) using simultaneous
irradiation technique. A grafting mixture containing pieces (5 cm
x 5 cm) of FEP film immersed in styrene (20-60 vol %) diluted
with dichloromethane was irradiated in a glass ampoule using grays from a 60Co source to a total dose of 20 kGy under nitrogen
atmosphere at room temperature. The grafted films were
thoroughly washed with toluene and then dried under vacuum.
More details on grafting step and some selected properties of FEP
grafted films are presented elsewhere [29]. The amount of
polystyrene contained in grafted film was represented by degree
of grafting, which was determined as per the following equation:
Chemical stability measurements were carried out by immersing
vacuum dried membranes in acid form into a solution of 3 %
H2O2 (30 % J. T Bakers COMS electronic grade) containing 4
ppm Fe++ ions (Fe SO4. 7H2O, BDH ‘Analar’) at various
temperatures in the range of 40-70 oC for 5 hours. The membrane
were vacuum dried (1 torr, 24 h, 80oC) , weighed and the loss in
their weight was gravimetrally determined.
Degree of grafting (%) =
Wg − W0
W0
x100
Where, Wg and W0 are the weights of grafted and original FEP
films, respectively.
3. RESULTS AND DISCUSSION
Radiation-induced grafting of styrene onto FEP and subsequent
sulfonation of the grafted film resulted in FEP-g-polystyrene
sulfonic acid membranes having a degree of grafting ranging from
5 to 52 %. A generalized molecular structure of FEP-gpolystyrene sulfonic acid membrane is given in Fig. 1. The
membranes were found to have a good combination of water
uptake, hydration number and ion exchange capacity. Moreover,
they achieved ionic conductivity in the order of magnitude of 10-2
Ω-1 cm-1 (at a degree of grafting ≥ 16 %).
Surface investigations of radiation grafted FEP-g-polystyrene/J. New Mat. Electrochem. Systems 3, 311-319 (2000)
Fig. 1. A generalized molecular structure of FEP-g-polystyrene
sulfonic acid membrane: (a) FEP main chain (b) sulfonated
polystyrene side chain.
3. 1. Identification of the Original FEP Film
Survey wide scan of original FEP film is shown in Fig. 2. The
spectrum of FEP film consists of two major peaks having
corrected binding energies of 292.20 eV and at 689.10 eV beside
a very small peak at 529.70 eV. The peak at 292.20 eV is assigned
for C1s from C-F while the peak at 689.10 eV is assigned for F
1s. The shift in the binding energy of C-F compared that reported
in literature for C-H (284.50 eV) is due to the chemical shift
owing to the electron attraction towards fluorine atom, which
equal 7.70 eV. The value of such chemical shift is in a complete
agreement with the literature [31]. An amount of 2.7 % of oxygen
was detected as indicated by O1s peak at 529.70 eV. The detection
of such small amount of oxygen in FEP film surface could be
ascribed to the contamination by oxygen during the polymer
fabrication or the film extrusion.
Fig. 2. XPS survey wide scan spectrum of the original FEP film.
313
The narrow scan (core level spectrum) of C 1s is shown in Fig. 3.
From the curve fitting, the spectrum is deconvoluted into seven
component peaks having corrected binding energies at 297.2,
295.7, 294.2, 288.9, 287.7, 285.8 and 284.5 eV representing CF3;
CF2; CF; >C=O or CF-CF; C or C-CF; -Cβ- and Cα-, respectively.
Such results are in a good agreement with those reported by
Yasuda et al. (1994) [32] upon their investigation of
perfluorosulfonate cation exchange membrane prepared by plasma
polymerization. The shifts in the binding energies of CF3; CF2 and
CF peaks is most probably due to the contaminations of -CF3 by
some -CF2O, -CF2 by some of -CFO and -CF by some of >C=O.
The F/C ratio calculated from the spectrum is found to be 2.15,
which is slightly higher than the theoretical F/C value estimated
from the chemical structure of FEP monomer and was found to
equal 2. This indicates that more fluorine atoms are located at the
film surface. The relative content of CF2 is found to be 59 %
compare to 2.3 % and 4 % for CF and CF3, respectively.
Therefore, CF2 in FEP backbone apparently, dominates the
surface among the fluorine containing peaks. From these results, it
is evident that polymer film composed mainly of fluorine as well
as carbon.
3. 2. Effect of Grafting
Fig. 4 shows survey wide scan spectrum of FEP-g-polystyrene
film having a degree of grafting of 52 %. As can be seen, the
intensity of C1s increases while that for of C1F decreases
compared to the corresponding intensities in the spectrum of the
original FEP film. The atomic concentration of C1s increases
form 29.57 % to 68.45 % while that of F1s dropps from 63.6 % to
15.1 %. This behavior is due to the consumption of CF bonds near
the film surface in the formation of radicals, which initiate
grafting reaction in presence of styrene molecules and finally
allow polystyrene grafts to be attached to the main chain of FEP
film. The incorporation of polystyrene side chain grafts introduces
Fig. 3. Narrow scan of C1s spectrum of the original FEP film.
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M. M. Nasef et al./J. New Mat. Electrochem. Systems 3, 311-319 (2000)
spectrum is deconvoluted into seven component peaks having
corrected binding energies of 294.9, 293.8, 289.8, 289.1, 287.2,
285.6 and 284.50 eV and representing CF3; CF2; CF; >C=O or
CF-CF; CO or C-CF; -Cb- and Ca-, respectively. Unlike the
deconvulated C1s spectra of the original FEP film, the styrene
grafted FEP film shows various changes in terms of surface
structure. A considerable reduction in the intensities of the
fluorine-containing peaks takes place. The intensities of CF3, CF2
and CF peaks dropped from 6.9, 96.8 and 3.8 to 1.4, 4.1 and 1.0,
respectively. This reveals the dominance of emerged hydrogencontaining peaks in the surface of the grafted film. The difficulty
in the specific determination of, -Cβ- and Cα- of aliphatic (-CH2CH=) and aromatic (-C6H5) of the polystyrene incorporated in the
grafted film is due to the minor chemical shifts in these carbon
atoms [33,34]. It can be also seen that the contribution of -Cβ- is
higher than that of Cα-, while the contribution of >C=O and CO
groups are low compared to both of them.
Fig. 4. XPS survey wide scan spectrum of FEP-g-polystyrene film
having a degree of grafting of 52 %.
hydrocarbon components in the form of aliphatic (-CH2-CH=) and
aromatic (-C6H5) into the fluorinated structure of FEP film.
Consequently, F/C ratio sharply falls to 0.22. A considerable
amount of oxygen of 6.3 % was detected in the spectrum of the
grafted film. This is probably due to the reaction with the oxygen
remaining in the grafting mixture during the grafting reaction or/
and to the reaction with the atmospheric oxygen when the film is
exposed to air, leading to the formation of CO and/or >C=O
groups.
From the aforementioned results, it evident that grafting of styrene
onto FEP films induces considerable changes into the surface
structure of the FEP film. Such changes can be monitored not only
by the incorporation of hydrocarbon components and the
reduction of F/C ratio in the film surface but also by the little
shifting in the binding energies of CF3, CF2 and CF groups.
3. 3. Effect of Sulfonation
Fig. 5 shows the narrow scan spectrum and curve fitting of C 1s of
FEP-g-polystyrene film having a degree of grafting of 52 %. The
Fig. 6 shows survey wide scan spectrum of FEP-g-polystyrene
sulfonic acid membrane having a degree of grafting of 52 %.
Compared to the spectrum of the FEP-g-polystyrene film, an
additional peak appears at 168.4 eV (corrected) which can be
assigned for S2p. Moreover, the peak intensity of O1s that have
corrected binding energy at 531.90eV increases, while that of F1s
at 688.7 eV (corrected) remarkably decreases. This result is
evident from data given in Table 1, which shows the atomic
Fig. 5. Narrow scan of C1s spectrum of FEP-g-polystyrene film
having a degree of grafting of 52 %.
Fig. 6. XPS survey wide scan spectrum of FEP-g-polystyrene
sulfonic acid membrane having a degree of grafting of 52 %.
Surface investigations of radiation grafted FEP-g-polystyrene/J. New Mat. Electrochem. Systems 3, 311-319 (2000)
Table 1. Atomic concentration percent of C1s, F1s, O1s AND
S2p of FEP-g-polystyrene sulfnonic acid membrane compared to
original and grafted FEP films as obtained form XPS spectra.
Status of FEP film
Original
52 % Grafted
52 % Grafted
and sulfonated
C 1s
20.96
86.45
72.37
Atomic concentration (%)
F 1s
O 1s
76.34
2.70
7.25
6.30
0.88
23.70
S 2p
3.48
concentration percent of C1s, F1s, O1s and S2p of FEP-gpolystyrene sulfonic acid membrane compared to original and
grafted FEP films obtained from XPS spectra. It is found that the
F/C ratio decreased from 0.22 upon grafting to a value as low as
0.012. This is ascribed to the introduction of sulfonic acid groups
(-SO3-) to the surface of the grafted film via sulfonation of the
polystyrene side chains. Consequently, more disappearance of the
main-chain component (CF2) available near the surface of the film
takes place leading to an increase in the peak intensity of O 1s
accompanied by the emergence of new S 2p peak.
Fig. 7 shows the curve fitting of C 1s of the spectrum of FEP-gpolystyrene sulfonic acid membrane having a degree of grafting of
52 %. It is found that the deconvolution of the spectrum shows the
emergence of new peak at 286.8 eV (corrected) and disappearance
of two out of the three fluorine-containing peaks early found in
the original as well as grafted FEP film. The newly emerged peak
is assigned for C-S group introduced by sulfonation of benzene
ring of the polystyrene grafts. The disappeared peaks were
assigned for CF3 and CF groups that were found in the grafted
FEP film and the remaining small one at 292.3 eV is characteristic
for CF2 which represent the main component in FEP backbone. In
addition, the intensities of >C=O or CF-CF; CO or C-CF; -Cβ-
Fig. 7. Narrow scan of C1s spectrum of FEP-g-polystyrene
sulfonic acid membrane having a degree of grafting of 52 %.
315
and Cα- peaks having binding energies of 289.3, 288.7, 285.7 and
284.5 are shown to be decreased. This indicates that the
incorporation of (-SO3-) groups induces additional changes in the
surface of the grafted film and such changes are most likely to
occur at the expense of fluorine content.
Fig. 8 shows the curve fitting of S2p spectrum of FEP-gpolystyrene sulfonic acid membrane having a degree of grafting of
52 %. The spectrum is deconvoluted into four peaks at 170.1,
171.4, 169.3 and 170.5 eV (charging effect = 1.30). The major
two peaks at 170.1 and 171.4 eV are assigned for 2p3/2 and 2p1/2
of sulfur having higher oxidation state (-SO3-). Whereas, the
minor peaks at 169.3 and 170.5 eV are assigned for 2p3/2 and
2p1/2 of sulfur having lower oxidation state (-SO2-) might be
associated with the sulfonic acid groups. The atomic percentage of
sulfur of higher oxidation number (-SO3-) is found equal to 88 %.
Fig. 9 shows the curve fitting of the spectrum of O1s of FEP-gpolystyrene sulfonic acid membrane having a degree of grafting of
52 %. The spectrum is deconvoluted into three peaks at 533.0,
531.3 and 528.4. The major peak which has a corrected value of
531.4 eV is characteristic for the oxygen present in (-SO3-) group
while minor peaks having corrected values at 533.0 and 528.4 eV
can be assigned for H2O and CO group, respectively. The
appearance of H2O peak is probably due to the strong hygrosopic
nature of the sulfonated membranes. It can be concluded that
sulfonation of the grafted polystyrene film brings more structural
changes into the layer close to the surface in terms of chemical
composition as well as binding energy
3. 4. Effect of Variation of the Degree of Grafting
Fig. 10 shows XPS survey wide scan spectra of FEP-gpolystyrene sulfonic acid membrane having various degree of
grafting (5-52 %). It can be clearly seen that the intensities of C1s,
Fig. 8. Narrow scan of O1s spectrum of FEP-g-polystyrene
sulfonic acid membrane having a degree of grafting of 52 %.
316
M. M. Nasef et al./J. New Mat. Electrochem. Systems 3, 311-319 (2000)
Table 2 . Atomic concentration percent of C1s, F1s, O1s and S2p
of FEP-g-polystyrene sulfnonic acid membranes having various
degrees of grafting as obtained form XPS spectra.
Degree of
grafting (%)
5
22
31
40
52
C 1s
64.25
66.12
68.55
70.25
72.37
Atomic concentration (%)
F 1s
O 1s
10.60
22.30
7.70
21.30
5.50
20.50
2.70
23.40
0.88
23.70
S 2p
3.50
3.88
3.30
3.70
3.48
fluorine content of the main chain of the membrane with the
increase in the degree of grafting.
Fig. 9. Narrow scan of S2p spectrum of FEP-g-polystyrene
sulfonic acid membrane having a degree of grafting of 52 %.
F1s, O1s and S2p peaks vary with the increase in the degree of
grafting. The intensities of C1s, O1s and S2p peaks increase as the
degree of grafting increases while the intensity of F1s decreases.
These results are supported by the data given in Table 2, which
shows the atomic concentration percent of C1s, F1s, O1s and S2p
of FEP-g-polystyrene sulfonic acid membranes having various
degrees of grafting. The atomic concentration of C 1s increases
with the increase in the degree of grafting while that of F 1s
decreases. Such behavior can be attributed to the increase in the
content of sulfonated polystyrene side chains, at the expense of
To obtain better understanding of the composition changes taking
place in the surfaces of FEP-g-polystyrene sulfonic acid
membranes with the increase in the degree of grafting, the atomic
ratios of the elemental components of the membrane were
evaluated. Table 3 shows the changes in the atomic ratios of the
elemental components (F/C, S/C, O/C and O/S) of FEP-gpolystyrene sulfonic acid membranes having various degrees of
grafting. The atomic ratios were calculated from Table 2.
Elemental ratio (F/C) of the original film is included as a
reference. It can be noticed from the first column that radiationinduced grafting of styrene onto FEP film causes a sharp decrease
in F/C ratio compare to the original film. Moreover, the F/C ratio
continues to decrease with the increase in the degree of grafting.
The sharp decrease in F/C ratio upon grafting is due to the rupture
in C-F bonds available at the film surface under the effect of γradiation to form the radicals required for initiating the grafting
reaction in presence of styrene molecules. As the polystyrene
content in FEP films increases, more ruptures in C-F bonds take
place and therefore F/C ratio continue to decrease with the
increase in the degree of grafting.
The second and third columns in the Table 3 show that the S/C as
well as O/C ratio show no significant changes despite the
variation in the degree of grafting of the membranes. This can be
understood from the fact that a degree of sulfonation close to 100
% in the membranes was achieved at various degrees of grafting.
This means that the ratio of sulfonic acid groups to the benzene
rings of grafted polystyrene is ~1.
Table 3. Changes in the ratios of F/C, S/C, O/C and O/S OF FEPg-polystyrene sulfnonic acid membranes having various degrees
of grafting as obtained form XPS spectra.
Fig. 10. XPS survey wide scan spectrum of FEP-g-polystyrene
sulfonic acid membrane having various degrees of grafting: (A) 5
% (B) 22 % (C) 40 % (D) 52 %.
Degree of
grafting (%)
0
5
22
31
40
52
F/C
2.18
0.16
0.12
0.09
0.04
0.01
S/C
_
0.050
0.060
0.050
0.050
0.050
O/C
_
0.35
0.32
0.30
0.33
0.33
O/S
_
6.37
5.60
6.20
6.30
6.80
317
Surface investigations of radiation grafted FEP-g-polystyrene/J. New Mat. Electrochem. Systems 3, 311-319 (2000)
The changes in binding energies of C 1s, F 1s, O 1s and S 2p
under the influence of variation of the degree of grafting were
investigated. Table 4 shows the binding energies of C 1s, F 1s, O
1s and S 2p of FEP-g-polystyrene sulfonic acid membranes
having various degrees of grafting. It can be seen that the binding
energies of C1s, F1s, O1s and S2p peaks almost have no
significant shifts despite the increase in the degree of grafting
within the membrane. This indicates that there is no change in the
oxidation state of the elemental components of the membrane and
therefore, it can be concluded that the binding energies of the
elemental components of the membranes are independent of the
degree of grafting. Similar results were reported for PTFE-gpolystyrene and PFA-g-polystyrene membranes having various
degrees of grafting [35, 36].
60
40 oC
50 oC
60 oC
70 oC
50
40
Weight loss (%)
Finally, the last column in Table 3 shows that O/S ratio is nearly
constant for all membrane samples except for the membrane
having 22 % degree of grafting. However, it is higher than the
theoretical ratio, which equals 3 referring to the chemical
composition of (-SO3 -). This is due to the increase in the oxygen
content in the membranes as a result of contamination of the
sample by moisture and the oxygen already present in the original
and grafted FEP films. The significant drop in O/S ratio of 22 %
grafted membrane is most probably due to less oxygen
contamination.
30
20
10
0
0
10
20
30
40
50
60
Degree of grafting (%)
Fig. 11 - The relationship between the weight loss and the degree
of grafting of FEP-g-polystyrene sulfonic acid membranes after
treatment with 3 % H2O2 solution having 4 ppm ferrous ions for 5
hours at various temperatures.
3. 5. Chemical Stability
The chemical stability was investigated to monitor changes taking
place in the chemical resistance of the membranes with the
variation of the degree of grafting. The percentage of the weight
loss of the dry membrane was used to indicate the loss in the
membrane physico-chemical properties. Fig. 11 shows the
relationship between the weight loss and the degree of grafting of
FEP-g-polystyrene sulfonic acid membranes after treatment with
3 % H2O2 solution having 4 ppm ferrous ions for 5 hours at
various temperatures. It was found that all membranes record no
loss in the weight i.e. they remain stable at a temperature of 40 oC.
However, at a temperature of 50 oC all membranes started to
seriously degrade and the amount of weight loss was found to
increase with the increase in both the temperature (up to 70 oC)
and the degree of grafting.
These results can be explained by the taking the chemical
composition of the membranes into consideration. The
membranes are composed of highly degradation-resistant
fluorinated backbones and degradable sulfonated hydrocarbon
grafts. Therefore, it can be stated that the membrane degradation
is controlled by the amount of sulfonated polystyrene incorporated
therein and such degradation is most likely taking place ternary
hydrogens of a-carbon in the polystyrene grafts, which are
susceptible to chemical attack [37, 38].
At a temperature of 40 oC the polystyrene grafts show adequate
chemical stability supported by the inherent high chemical
resistance of FEP domain. However, as the temperature increases
in the range of 50-70 oC, the hydroxyl radicals generated in the
oxidizing solution increase in number as well as activity and
aggressively attack the ternary hydrogens at a-carbon of
polystyrene grafts. On the other hand, the increase in the weight
loss with the increase in the degree of grafting is due to the
increase in the degradable sulfonated polystyrene fraction in the
Table 4. Corrected binding energies of C1s, F1s, O1s AND S2p of FEP-g-polystyrene sulfonic acid membranes having various degrees of
grafting
Degree of
grafting (%)
CF2
C1s
5
22
31
40
52
292.5
292.4
292.4
292.4
292.3
C-S
289.5
289.7
289.6
289.7
289.3
-CO or
C-CF
288.7
288.5
288.7
288.4
288.7
F1s
>C=O or
CFn-CF
287.2
287.3
287.1
286.9
286.8
-C-
-C
285.9
285.7
285.7
285.7
285.7
284.5
284.5
284.5
284.5
284.5
688.3
689.2
689.1
688.9
688.8
H2O
O1s
SO3/=O
SO2
533.3
533.4
533.3
533.2
533.1
531.8
531.7
531.5
531.4
531.4
528.6
528.6
528.5
5288.5
528.4
(SO3)
2p1/2
170.4
170.3
170.3
170.1
170.9
S2p
(SO3) (SO2)
2p2/3 2p1/2
168.8 169.6
168.6 169.6
168.6 169.5
168.5 169.4
168.7 169.2
(SO2)
2p2/3
167.9
167.9
167.8
167.7
167.6
M. M. Nasef et al./J. New Mat. Electrochem. Systems 3, 311-319 (2000)
membranes at the expense of the highly stable FEP domain. This
leads to an increase in the oxidizing agent uptake and as a result
more degradation occurs with the increase in the degree of
grafting. These results are in complete agreement with XPS data
of the surface analysis of the membranes presented in Table 3,
which show a drastic decrease in the fluorine content in the
surface of the membranes with variation of the degree of grafting.
These results suggest that changes taking place in the surface
structure of the membrane can be used to pretend the stability of
the membranes and their possible degradation mechanism. Such
information is very important to take necessary precautions to
improve the membrane stability.
It is important to mention that the high value of weight loss at a
temperature above 50oC is mostly resulting not only from the
degradation of bound sulfonated polystyrene grafts, but also from
the disolution and the leaching of some of the sulfonated
polystyrene chains that might be chemically unbound to the FEP
backbone as well as the possible minor degradation taking place
as a result of presence of little amount of oxygen in the FEP
grafted films.
4. CONCLUSIONS
Structural changes induced by grafting of styrene onto FEP film
and subsequent sulfonation were determined using XPS. The
structural characteristics of a series of FEP-g-polystyrene sulfonic
acid membrane having various degrees of grafting were studied. A
qualitative as well as quantitative correlation were established
between the structural changes taking place in the surface of FEP
film upon grafting and sulfonation. The membranes were proved
to have structures composed of carbon, fluorine, sulfur and
oxygen. The amount of carbon and fluorine were found to be
strongly dependent on the amount of polystyrene grafts
incorporated in the membrane. The chemical stability of the
membranes was found to be influenced by the decrease in the
fluorine content coupled with the increase in the hydrocarbon
fraction in the membranes as a result of variation of degree of
grafting. Therefore, the conclusion that need to be emphasized is
that evaluation of the structural changes taking place in the
membrane surfaces during the preparation is very important for
pretending the stability and performace of the membrane in
electrochemical application.
ACKNOWLEDGEMENT
The authors would like to acknowledge the financial support by
the Ministry of Science, Environment and Technology, Malaysia.
Dr. Hussin Mohd Nor is thanked for his continuous interest.
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