A facile method for the preparation of poly(vinylidene fluoride) membranes filled with cross-linked sulfonated polystyrene

Article abstract of DOI:10.1016/j.reactfunctpolym.2015.12.006

Pores of poly(vinylidene fluoride) (PVDF) membranes were filled with tetrabutylammonium 4-vinylbenzene sulfonate (TVS), a cross-linking agent, and a radical initiator in dimethyl sulfoxide, followed by radical polymerization and subsequent replacement of

Full text of DOI:10.1016/j.reactfunctpolym.2015.12.006

Reactive and Functional Polymers 99 (2016) 42–48
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Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
A facile method for the preparation of poly(vinylidene fluoride)
membranes filled with cross-linked sulfonated polystyrene
Naeun Kang a, Junhwa Shin b, Taek Sung Hwang c, Youn-Sik Lee a,
⁎
a
Division of Chemical Engineering, Chonbuk National University, 567 Baekje-Daero, Deokjin-gu, Jeonju, Jeonbuk 561-756, South Korea
Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup-si, Jeollabuk-do, South Korea
Department of Chemical Engineering, College of Engineering, Chungnam National University, Daejeon 305-764, South Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 October 2015
Received in revised form 1 December 2015
Accepted 10 December 2015
Available online 12 December 2015
Pores of poly(vinylidene fluoride) (PVDF) membranes were filled with tetrabutylammonium 4-vinylbenzene
sulfonate (TVS), a cross-linking agent, and a radical initiator in dimethyl sulfoxide, followed by radical polymer-
ization and subsequent replacement of tetrabutylammonium ions by protons. This pore-filling method using an
organic-soluble sulfonate monomer was very efficient and convenient, as it avoids the highly acidic sulfonation
step. The dimensional changes in the length of the PVDF membranes filled with cross-linked sulfonated polysty-
rene were similar to Nafion 212, but their water uptakes were higher and they exhibited greatly improved me-
chanical moduli and elongations. Proton conductivities of the modified membranes were also higher than Nafion
212 under identical conditions.
Keywords:
Organic-soluble ion-pair monomer
Pore-filling polymerization
Pore-filled poly(vinylidene fluoride)
membrane
© 2015 Elsevier B.V. All rights reserved.
Sulfonated polystyrene
1. Introduction
property as well as hydrophilicity to porous PVDF membranes via
pore-filling polymerization [18,22]. PVDF membranes filled with
Perfluorosulfonic acid membranes, such as Nafion, have been widely
used in polymer electrolyte membrane fuel cells (PEMFCs) because of
their good mechanical properties, high thermal and chemical stability,
high proton conductivity, and long-term durability [1–3]. However,
those polymer membranes have some limitations such as high cost
and low conductivity at high temperatures due to decreased humidity
[4,5]. In order to overcome these limitations, alternative membranes
such as poly(ether sulfone), poly(ether ketone), poly(phenylene), poly-
imide, poly(ether ether ketone) and polybenzimidazole have been de-
veloped [6–11]. Aromatic hydrocarbon polymers have appreciable
thermal and chemical stability, and exhibit high proton conductivity
when highly sulfonated. However, in general, such highly sulfonated
polymer membranes exhibit severely decreased dimensional stability
and mechanical strength due to excessive water uptake [12,13]. In
order to overcome issues associated with excessive water uptake,
cross-linked membranes, inorganic/polymer nanocomposite mem-
branes, grafted membranes, and pore-filled membranes have been ac-
tively investigated [12–18].
sulfonated polystyrene have also been prepared by filling the pores
with styrene, irradiating with an electron beam, and sulfonating with
chlorosulfonic acid [23]. Dobrovolsky and coworkers reported that
PVDF membranes could be filled with a solution of styrene and
divinylbenzene in toluene, followed by radical polymerization and sub-
sequent sulfonation with sulfuric acid to prepare PVDF membranes
filled with sulfonated polystyrene [24]. In these studies, sulfonic acid
groups were incorporated into the PVDF membranes filled with poly-
styrene using sulfuric acid or chlorosulfonic acid. One problem associat-
ed with these pore-filling methods is degradation of the polymer chains
[25].
The highly acidic sulfonation step would be avoided if styrene was
replaced by sodium 4-vinylbenzene sulfonate. In other words, polymer-
ization of PVDF membranes filled with sodium 4-vinylbenzene sulfo-
nate would obviate the need of a sulfonation step. The ionic monomer,
however, is not compatible with the hydrophobic PVDF membrane
and does not efficiently fill the membrane pores. One possible way to
overcome this pore-filling problem is to replace the sodium counterion
with tetrabutylammonium ion, because the resulting ion-pair mono-
mer, tetrabutylammonium 4-vinylbenzene sulfonate (TVS), will be
organic-soluble.
Poly(vinylidene fluoride) (PVDF) membranes have been extensively
used for scientific research and industrial processes because of their
good mechanical properties and high thermal and chemical stability
[19–21]. Yamaguchi et al. successfully imparted a proton-conducting
In this study, sodium 4-vinylbenzene sulfonate was treated with
tetrabutylammonium hydroxide to synthesize organic-soluble TVS.
PVDF membranes were filled with TVS, N,N′-methylenebisacrylamide
(cross-linking agent), and 2,2′-azobisisobutyronitrile (AIBN, initiator)
⁎
Corresponding author.
E-mail address: yosklear@jbnu.ac.kr (Y.-S. Lee).
http://dx.doi.org/10.1016/j.reactfunctpolym.2015.12.006
1381-5148/© 2015 Elsevier B.V. All rights reserved.

N. Kang et al. / Reactive and Functional Polymers 99 (2016) 42–48
43
in dimethyl sulfoxide (DMSO), followed by radical polymerization
2.3. Preparation of PVDF–CSPS
and subsequent washing with dilute HCl solution to replace the
tetrabutylammonium ions with protons and to remove any unreacted
cross-linking agent together with the monomer (Scheme 1). To the
best of our knowledge, this is the first study of its kind where PVDF
membranes filled with cross-linked sulfonated polystyrene (CSPS)
were directly prepared using an organic-soluble sulfonate monomer
without the highly acidic sulfonation step. In this study, the preparation
of PVDF membranes filled with CSPS is described, together with their
properties such as ion- exchange capacity (IEC), dimensional stability,
thermal stability, mechanical properties, oxidative stability, and proton
conductivity.
PVDF membranes were immersed in a solution of TVS, N,N′-
methylenebisacrylamide (cross-linking agent), and AIBN (radical initia-
tor, 0.5 wt.% with respect to the total amount of TVS and cross-linking
agent) in DMSO for 3 h. The following three weight ratios of TVS to
N,N′-methylenebisacrylamide were used: 90:10, 85:15, and 80:20. The
membranes were sealed between two glass plates, polymerized at
70 °C for 12 h in an oven, and then washed with methylene chloride
to remove any unreacted compounds. The resulting membranes were
immersed in a 1.0 N HCl solution and kept for 24 h to replace
tetrabutylammonium ions with protons, washed with water, and
dried in a vacuum oven. The resulting membranes filled with CSPS
were designated PVDF–CSPS-10, PVDF–CSPS-15, and PVDF–CSPS-20,
respectively, where the numbers indicate the loading of cross-linking
agent with respect to the TVS used.
2. Experimental
2.1. Materials
2.4. Instruments
PVDF membranes (thickness: 90 μm, porosity: 80%) were provided
by Amogreentech, South Korea. Sodium 4-vinylbenzene sulfonate,
DMSO, and N,N′-methylenebisacrylamide were purchased from
Sigma-Aldrich Chemical Co. Tetrabutylammonium bromide was pur-
chased from Sejinci Co. Hydrochloric acid (35–37%), 1.0 N hydrochloric
acid solution, magnesium sulfate, sodium chloride, sodium hydroxide,
methylene chloride, and acetone were purchased from Samchun Pure
Chemical Co., Ltd. AIBN, aqueous phenolphthalein (1%), and hydrogen
peroxide (35%) solutions were purchased from Dae-Jung Chemicals &
Metals Co., Ltd. All the aforementioned chemical reagents and solvents
were used as received.
Fourier-transform infrared (FTIR) spectra were recorded on a JASCO
4100E FTIR spectrometer under ambient conditions over the wave
number range of 4000–600 cm−1 1H NMR spectra were recorded on
.
a JEOL FT-NMR (400 MHz) spectrometer in CDCl3. Sample membranes
were dried in a vacuum oven at 80 °C for 24 h before measurement,
and the contact angles were measured directly after dropping water
on the membrane surface (Surface Electro Optics Phoenix 150). Thermal
stability of the sample membranes was measured on a SDT Q600 Ther-
mogravimetric Analyzer at a heating rate of 10 °C/min under nitrogen
atmosphere. Mechanical properties of the membranes (60 × 5 mm)
were measured on a universal testing machine (UTM, LR5K Plus, Lloyd
Instruments). All values were calculated thrice and their averages
were measured. Membrane surface images were recorded on a field
emission scanning electron microscope (FESEM, Hitachi SU-70).
2.2. Synthesis of TVS
Sodium 4-vinylbenzene sulfonate (1.0 g, 4.85 mmol) and
tetrabutylammonium bromide (1.55 g, 4.85 mmol) were dissolved in
water (6 mL) and acetone (5 mL), respectively, and the two solutions
were mixed in a two-neck flask. The solution was left undisturbed at
room temperature for 6 h under an argon atmosphere. Then, excess ac-
etone was added to the reaction mixture to precipitate sodium bromide,
and unreacted acetone was evaporated using a rotary evaporator. The
residue was extracted with methylene chloride, and the mixture of or-
ganic solutions was dried over magnesium sulfate and concentrated
using a rotary evaporator to yield a yellow gel-like product (2.0 g,
95%). 1H nuclear magnetic resonance (NMR) (400 MHz, CDCl3, ppm):
7.8–7.83 (d, 2H), 7.24–7.33 (d, 2H), 6.62–6.69 (dd, 1H), 5.68–5.73 (d,
1H), 5.19–5.27 (d, 1H), 3.19–3.24 (t, 8H), 1.53–1.61 (m, 8H),
1.32–1.41 (m, 8H), 0.92–0.95 (t, 12H). Elemental analysis: calculated
for C24H43NO3S: C 67.72%; H 10.18%; N 3.29% and S 7.53%. Found: C
67.10%; H 10.87%; N 3.46% and S 6.99%.
2.5. Measurements
2.5.1. Ion exchange capacity
The PVDF–CSPS membranes were immersed in a 2.0 N HCl solution
and kept for 24 h. After washing several times with water, the samples
were placed in an aqueous 3.0 M NaCl solution for 24 h, and then re-
moved. The resulting solution was titrated with 0.01 N NaOH, and the
IEC values were calculated from the following equation:
IEC ðmmol=gÞ ¼ ½CNaOH ꢀ VNaOHꢁ=Wdry;
where CNaOH is the concentration of NaOH solution, VNaOH is the volume
of NaOH solution, and Wdry is the weight of sample membranes.
Scheme 1. Experimental procedure for the preparation of PVDF membranes filled with cross-linked sulfonated polystyrene using organic-soluble TVS.

44
N. Kang et al. / Reactive and Functional Polymers 99 (2016) 42–48
2.5.2. Dimensional stability and water uptake
the water and DMSO solutions were 27 and 50 wt.%, respectively.
Weight ratio of the monomers to the cross-linking agent was 90:10.
The PVDF membranes filled with monomers and cross-linking agent
were polymerized and purified using a method similar to the prepara-
tion of PVDF–CSPS membranes. As expected, TVS filled the PVDF
membrane pores more efficiently than sodium 4-vinylbenzene sulfo-
nate (42 vs. 0% in water; 62% vs. 29% in DMSO), because it is more
organic-soluble than the latter. Weight increase of the membranes
was higher in DMSO, because the monomers and cross-linking
agent could be more highly concentrated in DMSO (27 wt.% in water
vs. 50 wt.% in DMSO) due to its higher solubility, and the PVDF mem-
branes are more wetted by DMSO.
The sample membranes were dried in a vacuum oven at 80 °C for
24 h, and then immersed in deionized water at 25 °C or 80 °C and
kept for 1 day. Dimensional stability and water uptake of the mem-
branes were calculated from the following equations:
Â
Ã
ΔLð%Þ ¼ Lwet−Ldry =Ldry ꢀ 100
Â
Ã
Water uptakeð%Þ ¼ Wwet−Wdry =Wdry ꢀ 100
where Lwet and Ldry are the lengths of the wet and dry membranes, and
Wwet and Wdry are the weights of the wet and dry membranes,
respectively.
Fig. 2 presents the weight increase of the membranes after pore-
filling polymerization but before ion exchange with aqueous HCl solu-
tion. Initially, the weight of the membranes increased from 21% to
105% as the polymerization time extended from 2 to 8 h, and no consid-
erable change was observed with further extension of polymerization
time, up to 12 h. Thus, polymerization time was fixed at 12 h for this
experiment.
2.5.3. Oxidative stability
The membranes were immersed in Fenton's reagent (3-wt.% H2O2
aqueous solution containing 2-ppm FeSO4) at 80 °C and room tempera-
ture. Oxidative stability was calculated using the following equation:
Oxidative stability ð%Þ ¼ ½W=Woꢁ ꢀ 100;
The increase of weight due to CSPS with tetrabutylammonium ions
filled in the membranes ranged from 97% to 105%. In order to replace
tetrabutylammonium ions with protons, the membranes were im-
mersed in 1.0 N HCl solution and kept for 1 day, washed with water,
and dried. This decreased the weight of the membranes by 20–22%
(calculated weight reduction: 22–26%), and the net weight increase
was 57–62%. Dobrovolsky and coworkers reported that the weight in-
crease of PVDF membranes (thickness: 90 μm, porosity: not provided)
after polymerization of styrene and divinylbenzene for 12 h was ap-
proximately 40%, corresponding to a weight increase of about 50%
after 100% sulfonation [24].
where Wo and W are the weights of the initial and remaining mem-
brane, respectively.
2.5.4. Proton conductivity
Proton conductivity experiment was conducted on a SI-1260
(Solartron Company, USA). Impedance measurements were conducted
on a four-electrode system over a frequency range of 0.01–100 kHz.
The formula to calculate proton conductivity is as follows:
σ ðS=cmÞ ¼ L=AR;
where L is the distance between the two electrodes (cm), A is the cross-
sectional area of the membranes (cm2), and R is the electrical resistance
of the membranes (Ω).
SEM images of the membranes indicated that the highly porous
PVDF membrane was almost completely filled by CSPS (Fig. 3). After
treatment with HCl solution, the membranes appear to be slightly less
completely filled due to replacement of the large tetrabutylammonium
ions with protons. In order to achieve complete filling of the mem-
branes, the pore-filling procedure (pore-filling, polymerization, and
treatment with HCl solution) was repeated for PVDF–CSPS-10. The
weight increase of the twice pore-filled membrane was 81% with re-
spect to the original PVDF. A SEM image indicated that the twice-
polymerized membrane surface appears to be more completely filled
even after treatment with HCl solution. However, the increment in
water uptake was about 85%, indicating that the repeated pore-filling
polymerization process significantly increased water uptake.
3. Results and discussion
TVS was prepared from sodium 4-vinylbenzene sulfonate and
tetrabutylammonium bromide via ion exchange. The NMR spectrum
(Fig. 1) and elemental analysis data confirmed that TVS was successfully
synthesized with high purity. The obtained TVS was dissolved well in
organic solvents such as methylene chloride, toluene, and DMSO.
In order to determine the pore-filling efficiency of TVS on the
PVDF membranes, they were immersed in a solution containing
TVS or sodium 4-vinylbenzene sulfonate together with N,N′-
methylenebisacrylamide in water or DMSO at room temperature and
kept for 3 h. The contents of the monomer and cross-linking agent in
Fig. 2. Weight increase of PVDF–CSPS-10 membranes from the original PVDF membrane at
different polymerization times (before replacement of tetrabutylammonium ions with
protons).
Fig. 1. 1H NMR spectrum of TVS in CDCl3.

N. Kang et al. / Reactive and Functional Polymers 99 (2016) 42–48
45
Fig. 3. SEM images of membrane surfaces: (a) original PVDF, (b) initial PVDF–CSPS-10, (c) PVDF–CSPS-10 after washing with 1.0 N HCl solution, and (d) PVDF–CSPS-10 after the second
pore-filling polymerization and subsequent treatment with 1.0 N HCl solution.
Furthermore, the membranes became brittle. These results are probably
due to the increased CSPS content in the PVDF membrane (62% vs. 81%).
Thence, twofold pore-filling polymerization was not performed.
FTIR spectra of PVDF and PVDF–CSPS-10 membranes were recorded
in attenuated total reflection (ATR) mode, and the results are shown in
Fig. 4. The peaks due to C–F stretching vibrations of the PVDF membrane
are observed at 1171 and 1121 cm−1 [26]. These membranes showed
characteristic peaks for the sulfonic acid group at 1034 and
1006 cm−1, an sp2 C–H stretching peak at 3000 cm−1, and aromatic
C_C stretching peaks in the range of 1450–1600 cm−1. A broad absorp-
tion peak in the range of 1700–1590 cm−1 may be due to amide carbon-
yl groups derived from the cross-linking agent. After replacing
tetrabutylammonium ions with protons, the intensities of the sp3 C–H
stretching peaks decreased significantly, indicating the efficient remov-
al of the former by HCl treatment.
Experiments to determine thermal degradation behavior of PVDF
and PVDF–CSPS membranes were conducted under nitrogen atmo-
sphere, and the results are shown in Fig. 5. The weight of the PVDF
membrane remained unchanged up to 400 °C and decreased rapidly
by approximately 45% above 430 °C. On the contrary, the PVDF–CSPS
membranes showed a two-step degradation behavior: (1) weight loss
at 200–430 °C due to elimination of CSPS and (2) weight loss above
430 °C due to decomposition of the PVDF membranes. The degradation
onset temperatures of the PVDF–CSPS membranes were not consider-
ably affected by the content of cross-linking agent.
The IEC values of PVDF–CSPS-10, PVDF–CSPS-15, and PVDF–CSPS-20
were determined to be 2.1, 2.0, and 1.7 mmol/g, respectively (Table 1).
The theoretical IEC value decreased from 2.9 to 2.3 mmol/g as the load-
ing of the cross-linking agent was increased from 10 to 20 wt%, because
Fig. 4. FTIR spectra: (a) original PVDF and PVDF–CSPS-10 membranes (b) before and
Fig. 5. TGA curves of the original PVDF and PVDF–CSPS membranes under nitrogen atmo-
(c) after replacement of tetrabutylammonium ions with protons.
sphere (heating rate: 10 °C/min).

46
N. Kang et al. / Reactive and Functional Polymers 99 (2016) 42–48
Table 1
because the cross-linking agents are less hydrophilic than the sulfonat-
ed styrenes in the CSPS. The dimensional changes in length and water
uptake of the membranes were measured at 25 °C and 80 °C in water,
and the results are summarized in Table 1. It is evident from the table
that the length increment and water uptake of the pore-filled PVDF
membranes decreased with increased loading of cross-linking agent,
because of increased cross-linking density. The dimensional changes
in length of the membranes were similar to Nafion 212, but their
water uptakes were relatively higher.
Mechanical properties of the PVDF, pore-filled PVDF, and Nafion 212
membranes were measured and listed in Table 2. The pore-filling poly-
merization process improved the tensile strength, but more significant-
ly Young's modulus of the PVDF membrane. The improved tensile
strength and Young's modulus of the pore-filled membranes can be at-
tributed to the cross-linked network of the membranes. The elongations
at break of the pore-filled PVDF membranes were also much higher than
the original PVDF membranes. This might be due to moisture captured
by sulfonic acid groups under experimental conditions. The elongation
gradually decreased with an increase of cross-linking agent content, be-
cause of the increased cross-linking network and more restricted chain
mobility between the polymer chains [27].
Oxidative stabilities of the membranes were measured using
Fenton's reagent (2-ppm FeSO4 + 3-wt.% H2O2) at 80 °C (Fig. 6a). The
weight losses of PVDF–CSPS-10, PVDF–CSPS-15, and PVDF–CSPS-20
after 4 h were 28%, 15% and 4%, respectively. The increase of stability
with cross-linking agent content is due to the more compact structure
that restricts the diffusion of hydroxyl radical into the membrane.
However, irrespective of the cross-linking agent content, the CSPS
chains were almost completely lost from the PVDF membranes after
1 day. This indicates that the sulfonated polystyrene chains are
decomposed and removed from the membranes under such conditions.
Polystyrene-grafted poly(ethylene-co-tetrafluoroethylene) membranes
were reported to be decomposed in 3-wt.% H2O2 + Fe2+ at 60 °C after
several hours, and their oxidative stabilities were tested in 3-wt.%
IEC values, contact angles, dimensional stability, and water uptake of original PVDF, PVDF–
CSPS, and Nafion 212 membranes.
Sample
PVDF
PVDF–CSPS-10 2.9
PVDF–CSPS-15 2.6
PVDF–CSPS-20 2.3
IEC (mmol/g)
Contact ΔL (%)
Water
uptake (%)
angle
(°)
Calculated Measured
25 °C 80 °C 25 °C 80 °C
–
–
98
41
50
60
–
–
–
–
–
2.1
2.0
1.7
0.9
12
10
9
24
16
12
20
45
33
27
16
67
54
36
20
Nafion 212
–
5
of the decreased content of sulfonic acid groups at higher loadings of
cross-linking agent. The experimental values of IEC were smaller than
the theoretical counterparts. This is probably due to incomplete ion ex-
change between the sulfonic acid groups within the cross-linked mem-
branes and NaCl [27]. Trace amounts of water present in the membranes
even after drying in a vacuum oven may also contribute to the discrep-
ancy. Dobrovolsky and coworkers reported IEC values of 2–2.4 mmol/g
when the weight increment of cross-linked polystyrene was 40%, which
is close to our IEC values [24]. However, it is difficult to directly compare
our IEC values with those reported by the Dobrovolsky group because
the loading of divinylbenzene and degree of sulfonation of polystyrene
were not reported in their study.
The water contact angle was 98° on the original PVDF membrane
surface (Table 1), but it decreased significantly on the pore-filled mem-
branes. This indicates that the hydrophobic PVDF membranes became
more hydrophilic after the pore-filling process, because the ionic poly-
mer (CSPS) filled in the membranes is hydrophilic. As the content of
the cross-linking agent increased from 10 to 20 wt.%, the water contact
angle increased from 41° to 60°. This is probably because of the fact that
hydrophobicity of the pore-filled PVDF membranes increases with an
increase of the cross-linking agent content. This can be understood, as
Table 2
Mechanical properties and proton conductivities of PVDF, PVDF–CSPS and Nafion 212 membranes.
Sample
Mechanical property
Proton conductivity (S/cm)
Tensile strength (MPa)
Young's modulus (MPa)
Elongation at break (%)
90% RH
30 °C
30% RH
75 °C
45 °C
60 °C
75 °C
90 °C
PVDF
41
48
55
54
–
32
170
271
268
253
–
–
0.13
0.1
0.06
0.05
–
–
–
–
–
PVDF–CSPS-10
PVDF–CSPS-15
PVDF–CSPS-20
Nafion 212
121
152
157
–
0.14
0.12
0.1
0.18
0.15
0.14
0.11
0.21
0.19
0.17
0.13
0.22
0.21
0.19
0.14
0.0104
0.0078
0.0034
0.0068
0.09
Fig. 6. Changes in the weight of PVDF–CSPS membranes with Fenton's reagent (2-ppm Fe2+ + 3-wt.% H2O2) at (a) 80 °C and (b) room temperature.

N. Kang et al. / Reactive and Functional Polymers 99 (2016) 42–48
47
preparation of polymer membranes filled with various types of ionic
polymers.
Acknowledgments
This study was supported by the Radiation Technology R&D program
administered through the National Research Foundation of Korea
funded by the Ministry of Science, ICT and Future Planning (no.
1501001882).
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