A facile approach for the preparation of cross-linked sulfonated polyimide membranes for fuel cell application

Article abstract of DOI:10.1039/b613561g

A facile approach has been successfully developed for the preparation of a series of cross-linked sulfonated polyimide (SPI) membranes via the condensation reaction between the sulfonic acid groups and the activated hydrogen atoms of SPIs in the presence of phosphorous pentoxide: methanesulfonic acid in the ratio of 1: 10 by weight (PPMA, method 1) or phosphorous pentoxide only (method 2). The resulting sulfonyl linkages are very stable and the cross-linked SPI membranes showed greatly improved water stability in comparison with the uncross-linked ones while high proton conductivity was maintained. The Royal Society of Chemistry.

Full text of DOI:10.1039/b613561g

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
A facile approach for the preparation of cross-linked sulfonated polyimide
membranes for fuel cell application{
Jianhua Fang,*a Fengxia Zhai,a Xiaoxia Guo,a Hongjie Xua and Ken-ichi Okamotob
Received 18th September 2006, Accepted 29th November 2006
First published as an Advance Article on the web 22nd December 2006
DOI: 10.1039/b613561g
A facile approach has been successfully developed for the preparation of a series of cross-linked
sulfonated polyimide (SPI) membranes via the condensation reaction between the sulfonic acid
groups and the activated hydrogen atoms of SPIs in the presence of phosphorous pentoxide :
methanesulfonic acid in the ratio of 1 : 10 by weight (PPMA, method 1) or phosphorous
pentoxide only (method 2). The resulting sulfonyl linkages are very stable and the cross-linked SPI
membranes showed greatly improved water stability in comparison with the uncross-linked ones
while high proton conductivity was maintained.
stability is 1000–2500 h at 100 uC.14–17 On the other hand,
cross-linking is a common and effective method to enhance the
Introduction
Polymer electrolyte membrane (PEM) is the key component of
mechanical properties, to suppress membrane swelling degree
a fuel cell system. Current state-of-the-art PEMs used in
and to improve the membrane durability. There are many
practical systems are sulfonated perfluoropolymers, typically
reports on the preparation of cross-linked sulfonated polymer
DuPont’s Nafion, which have high proton conductivity, good
membranes in the literature. Kerres and coworkers extensively
mechanical properties, and high thermal, chemical and
investigated the preparation of various ionically cross-linked
electrochemical stability. However, some drawbacks such as
acid–base blends and ionomer membranes [acid: sulfonated
the high cost, low conductivity at high temperature, and high
poly(ether sulfone), sulfonated poly(ether ether ketone), sulfo-
nated poly(phenylene oxide); base: polybenzimidazole],22–25
methanol permeability of the perfluoropolymers seriously limit
their industrial application. In the past decade many efforts
covalently cross-linked membranes through the reaction
have been made on the development of low cost and high
between sulfinate groups of sulfonated polymers and alkyl or
aryl dihalides (cross-linkers),26,27 and covalent–ionically cross-
performance sulfonated hydrocarbon polymers as the alter-
linked membranes.28–30 However, as pointed out by Kerres in
native membrane materials for fuel cell application. Many
the review article,31 the ionically cross-linked acid–base (blend)
sulfonated hydrocarbon polymer membranes such as sulfo-
nated polystyrene and derivatives,1–3 sulfonated polysul-
membranes showed unacceptable swelling in water at the
fones,4–6 sulfonated poly(ether ketone)s,7,8 sulfonated
temperatures above 70–90 uC leading to possible destruction of
the membranes in fuel cell operation. The covalently cross-
polyphenylenes,9,10 and six-membered ring sulfonated poly-
imides (SPIs)11–21 have been reported to have comparable or
linked (blend) membranes are stable but have the disadvantage
even higher proton conductivities than Nafion when fully
that much effort is involved in the synthesis of the polymers
containing both sulfonate and sulfinate groups.31 Pintauro and
hydrated. However, a major problem associated with most of
the sulfonated hydrocarbon polymer membranes is their poor
coworkers reported on a photo-cross-linking method which is
water stability, i.e., the membranes highly swell or even
based on the photochemical reaction between the a-methyl
dissolve in water and thus lose mechanical properties especially
groups of the sulfonated polyphosphazene and benzophenone
(photoinitiator) under the exposure of UV.32 However, it is
when their ion exchange capacities (IECs) are at high levels
(e.g. IEC . 2.0 meq g21) which are generally essential to
well known that a-methyl group-containing polymers such as
provide high proton conductivity. To enhance the water
sulfonated polystyrene are unstable toward radical oxidation.
stability while maintaining high proton conductivity, one
Guiver and coworkers prepared a series of cross-linked
approach is to optimize the chemical structure and/or to
sulfonated poly(ether ether ketone) membranes by using
control the morphology of membranes. Successful examples
polyatomic alcohols as the cross-linker and the cross-linking
have been reported with SPI membranes of which proton
is based on the condensation reaction between sulfonic acid
conductivities are around 0.20 S cm21 (in water) and the water
groups of the polymer and polyatomic alcohols.33 This method
seems, in principle, applicable to all kinds of sulfonated
polymers, but the water stability of the resulting membranes
was not investigated. Theoretically the resulting sulfonic acid
ester linkages can still undergo hydrolysis and therefore they
are expected not to be very stable. Han and coworkers
prepared a series of cross-linked SPI membranes by self-cross-
linking of maleimido-end-capped SPI oligomers and by
reaction of the oligomers with a diacrylate compound, but
aSchool of Chemistry and Chemical Technology, Shanghai Jiao Tong
University, 800 Dongchuan Road, Shanghai 200240, China.
E-mail: jhfang@sjtu.edu.cn
bDepartment of Advanced Materials & Engineering, Faculty of
Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi
755-8611, Japan
{ Electronic supplementary information (ESI) available: Synthesis of
model compounds and FT-IR and 1H NMR spectra of BNIPF and
BMPS–BNIPF. See DOI: 10.1039/b613561g
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these cross-linked membranes displayed rather poor water
stability (around 200 h at 80 uC) probably because of the very
poor hydrolytic stability of maleimido rings.34 Okamoto and
coworkers reported on a series of interesting branched/cross-
linked SPI membranes which were prepared through the
sulfuric acid was completely added, and the reaction mixture
was stirred at room temperature for half an hour followed by
heating at 80 uC for 6 h. After cooling to room temperature,
the solution mixture was poured into 100 g of crushed ice.
Sodium chloride was added and the mixture was stirred
overnight. The mixture was filtered, and the solid was washed
with saturated sodium chloride solution and then dried in
vacuo at 80 uC for 20 h. The obtained solid was added to
200 mL of DMSO with stirring. The mixture was filtered, and
the filtrate was distilled under reduced pressure. The obtained
solid was washed with acetone, and dried in vacuo at 100 uC
for 20 h. 28.77 g of white product was obtained. Yield: 96.9%.
IR (KBr, cm21): 3448, 2358, 1598, 1502, 1216, 1134, 1048, 830,
reaction of anhydride-end-capped SPI oligomers and
a
triamine monomer. These SPI membranes showed good water
stability as well as high proton conductivity,17 but the
application of this method to other kinds of sulfonated
polymers such as sulfonated polysulfones and sulfonated
poly(ether ether ketone)s is not easy. Very recently Lee et al.
reported on the preparation of a series of cross-linked SPI
membranes based on the chemical reaction between the
carboxyl groups from a non-sulfonated diamine moiety and
the hydroxyl groups of N,N-bis(2-hydroxyethyl)-2-aminotha-
nesulfonic acid (cross-linker).35 It seems that their cross-linked
SPI membranes are still organo-soluble judging from the fact
that they got 1H NMR spectra of the cross-linked samples.
They reported that their cross-linked SPI membranes had
water stability of around 12–42 days at 80 uC but no informa-
tion is available for the stability test under severer conditions
such as in boiling water. In fact, the ester cross-links they have
can still undergo hydrolysis. The development of a facile
approach to produce stable cross-linking is strongly desired. In
this paper, we report on a novel and facile approach for
preparation of cross-linked and highly stable SPI membranes
and their water stability and proton conductivities are also
described.
1
696, 568. H NMR spectrum (in DMDO-d6), d (ppm): 7.48–
7.52 (2H), 7.08–7.14 (2H).
Synthesis of 3,39-bis(4-sulfophenoxy)benzidine (BSPOB)
To a dry 200 mL 3-neck flask were added under nitrogen flow
3.244 g (15 mmol) of 3,39-dihydroxybenzidine, 15 mL of NMP,
6.237 g (31.5 mmol) of 4-fluorobenzenesulfonic acid sodium
salt, 10 mL of toluene and 6.0 g of anhydrous potassium
carbonate. The mixture was magnetically stirred at room
temperature for 0.5 h, and then heated at 140 uC for 4 h and
170 uC for 24 h. The produced water was evaporated together
with toluene as an azeotrope and collected in a Dean–Stark
trap. After cooling to room temperature, the reaction mixture
was added to methanol with stirring. It was filtered and the
solid was dissolved in water followed by acidification with
concentrated hydrochloric acid. The resulting precipitate
was filtered, washed with water and ethanol successively, and
dried in vacuo. The crude product was de-colored with
activated carbon in methanol in the presence of an excess
amount of Et3N and re-acidified with concentrated hydro-
chloric acid. The precipitate was washed with methanol and
water and dried in vacuo at 120 uC for 15 h. 5.2 g of off-white
solid was obtained. Yield: 65%. IR (KBr, cm21): 3478, 2920,
2632, 1638, 1596, 1530, 1496, 1388, 1252, 1214, 1166, 1128,
1030, 1006, 838, 734, 690, 604, 572, 490. 1H NMR spectrum (in
DMDO-d6 in the presence of a drop of Et3N), d (ppm): 7.51–
7.54 (m, 4H), 7.11 (d, 2H, J = 8.0 Hz), 6.99 (s, 2H), 6.78–6.83
(m, 6 H), 4.89 (4 H, NH2).
Experimental section
Materials
1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA), 1,8-
naphthalenedicarboxylic anhydride (NDA) and 2,29-benzidi-
nedisulfonic acid (BDSA) were purchased from TCI. NTDA
was vacuum sublimed before use. BDSA was purified by de-
colorization with activated carbon in methanol in the presence
of a slight access of triethylamine (Et3N) under nitrogen flow
followed by acidification, thoroughly washing the precipitate
with methanol and de-ionized water successively and finally
drying at 120 uC in vacuo for 20 h. 9,9-bis(4-aminophenyl)
fluorene (BAPF) was synthesized according to a literature
method.36 The syntheses of sulfonated diamines, 4,49-diami-
nodiphenyl ether-2,29-disulfonic acid (ODADS) and 4,49-bis(4-
aminophenoxy)biphenyl-3,39-disulfonic acid (BAPBDS) have
been previously reported.12,15 The synthesis of 3,39-bis(4-
sulfophenoxy)benzidine (BSPOB) is described as follows.
Other compounds were purchased from SCRC and used as
received except that 1-methyl-2-pyrrolidinone (NMP) and
triethylamine (Et3N) were distilled under reduced (for NMP)
Synthesis of 9,9-bis[4-(1,5-naphthylimido)phenyl]fluorene
(BNIPF)
To a dry 100 mL 3-neck flask were added under nitrogen
flow with stirring 1.1890 g (6.0 mmol) of 1,8-naphthalenedi-
carboxylic anhydride (NDA), 1.044 g (3.0 mmol) of BAPF,
20 mL of m-cresol, 0.732 g (6.0 mmol) of benzoic acid and
0.775 g (6.0 mmol) of isoquinoline. The reaction mixture
was slowly heated to 150 uC and kept at this temperature
for 10 h. Subsequently the reaction temperature was raised
to 180 uC and maintained for 10 h. After cooling to room
temperature, the reaction mixture was poured into 250 mL
of isopropanol. The resulting precipitate was filtered off,
washed with isopropanol a couple of times, and dried in vacuo.
1.85 g yellow–brownish solid was obtained. Yield: 87%. The
˚
or normal (for Et3N) pressure and dried with 4 A molecular
sieve prior to use.
Synthesis of 4-fluorobenzenesulfonic acid sodium salt (FBSNa)
14.42 g (150 mmol) of fluorobenzene was added to a 300 mL
3-neck flask which was pre-cooled in an ice-bath. 22.8 mL of
fuming sulfuric acid (SO3, y165 mmol) was added with
magnetic stirring. The ice-bath was removed after the fuming
1
FT-IR and H NMR spectra are shown in Fig. S1 and S2{,
respectively.
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Synthesis of 2,7-bis(4-methylphenylsulfonyl)-9,9-bis[4-(1,5-
naphthylimido)phenyl]fluorene (BMPS–BNIPF)
80 uC for 1 h, 120 uC for 2 h, and 170 uC for 10 h, successively.
The membranes were thoroughly rinsed with deionized water
till the water was neutral and then dried in vacuo at 100 uC
for 20 h.
To a dry 100 mL 3-neck flask were added under nitrogen flow
with stirring 0.708 g of BNIPF (1.0 mmol), 0.418 g (2.2 mmol)
of p-toluenesulfonic acid monohydrate and 15 mL of PPMA.
The reaction mixture was heated at 80 uC for 4 h. After cooling
to room temperature, the mixture was poured into ice water.
The resulting precipitate was filtered off, thoroughly washed
with deionized water and dried in vacuo at 100 uC. 0.96 g white
Measurements
Infrared (IR) spectra were recorded on a Perkin-Elmer
Paragon 1000PC spectrometer. 1H NMR spectra were
recorded on a Varian Mercury Plus 400 MHz instrument.
Proton conductivity in the membrane planar direction was
measured by an ac impedance method with two platinum
electrodes using a Hioki 3552 Hitester instrument over the
frequency range from 100 Hz to 100 KHz as has been reported
in previous papers.12,13 Proton conductivity s was calculated
from the following equation:
1
product was obtained. Yield: 95%. The FT-IR and H NMR
spectra are shown in Figs. S3 and S4{, respectively.
Polymerization
The experimental procedures for sulfonated polyimides are
described as follow using NTDA–BSPOB/BAPF (9/1) copoly-
imide as an example.
s = D/(LBR)
(1)
To a dry 100 mL 3-neck flask were added under nitrogen
flow with stirring 1.901 g (3.6 mmol) of BSPOB, 0.1392 g
(0.4 mmol) of BAPF, 25 mL of m-cresol and 1.22 mL of
Et3N. After the diamines were completely dissolved, 1.072 g
(4.0 mmol) of NTDA and 0.693 g of benzoic acid were added.
The reaction mixture was stirred at room temperature for half
an hour, and then heated at 80 uC for 4 h and 180 uC for 20 h.
After cooling to around 120 uC, the highly viscous mixture was
diluted with an additional 20 mL of m-cresol and then slowly
poured into 250 mL of acetone. The fiber-like precipitate was
filtered off, washed with acetone a couple of times, and dried
in vacuo.
where D is the distance between the two electrodes, L and B
are the thickness and width of membrane (measured from the
samples stored at ambient atmosphere when the conductivity
measurements were performed at relative humidities below
90% or from the hydrated samples when the conductivity
measurements were performed at relative humidities above
90% or in liquid water), respectively, and R is the resistance
measured.
Water sorption experiments were carried out by immersing
three sheets of film (20–30 mg per sheet) of a polyimide into
water at 100 uC for 5 h. Then the films were taken out, wiped
with tissue paper, and quickly weighed on a microbalance.
Water uptake of the films, S, was calculated from
Film formation and proton exchange
Films were prepared by casting the sulfonated polyimide
(in their triethylammonium salt form) solutions in DMSO
(BSPOB, ODADS and BDSA-based SPIs) or m-cresol
(BAPBDS-based SPI) onto glass plates and dried at 80 uC
(DMSO solutions) or 120 uC (m-cresol solution) for 10 h. The
as-cast films were soaked in methanol at 60 uC for 6 h. Proton
exchange was performed by immersing the films in 1.0 M
hydrochloric acid at room temperature for two days. The
proton exchanged films were washed with deionized water and
then dried in vacuo at 100 uC for 20 h.
S = (Ws 2 Wd)/Wd 6 100 (%)
(2)
where Wd and Ws are the weight of the dry and corresponding
water-swollen film sheets, respectively. Water uptake of a
polyimide was estimated from the average value of S of
each sheet.
Tensile measurements were performed with an Instron 4456
instrument at room temperature in an ambient atmosphere
(y80% relative humidity) at a crosshead speed of 1 mm min21
.
Sulfonated polyimide membranes were soaked in distilled
water at 100 uC for one month and subjected to tensile
measurement as soon as they were taken out. The thickness of
the hydrated membranes was used for tensile strength
calculation.
Cross-linking treatment
Two methods were used for the preparation of cross-linked
membranes and the experimental details are described as
follows:
Method 1: dry SPI membranes in their proton form were
immersed into phosphorus pentoxide : methanesulfonic acid
(1 : 10 by weight, PPMA) in a glass vessel at 80 uC for 3–48 h
under nitrogen atmosphere. The membranes were taken out,
thoroughly rinsed with deionized water till the water was
neutral and dried in vacuo 100 uC for 20 h.
Method 2: dry SPIs in their proton form were dissolved in
DMSO containing 5 wt% phosphorus pentoxide at room
temperature. The weight ratio between SPI and phosphorus
pentoxide was controlled at 1 : 1. The solution mixture was
cast onto glass plates and dried at 80 uC for 10 h in an air oven.
The glass plates were moved to a vacuum oven and dried at
Results and discussion
Polymer synthesis and cross-linking
A series of SPIs were synthesized by one-step condensation
polymerization of NTDA, sulfonated diamines and BAPF in
m-cresol in the presence of triethylamine (Et3N) and benzoic
acid at 180 uC for 20 h (Scheme 1). This is a literature
method which was first reported by Mercier.37 Four kinds of
sulfonated diamines, BSPOB, BAPBDS, ODADS and BDSA,
were employed for polymerization. The side chain-type
sulfonated diamine, BSPOB, was synthesized via the reaction
1104 | J. Mater. Chem., 2007, 17, 1102–1108
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The preparation of cross-linked SPI membranes was
performed in two ways. Method 1 is that the dry SPI
membranes (in their proton form) were immersed into
PPMA at 80 uC for a given time. Method 2 is that the SPI
(in proton form) solutions in DMSO containing 5 wt%
phosphorus pentoxide were cast into films which were
subsequently thermally treated at high temperature (170 uC)
in vacuo for 10 h. For both methods, the cross-linking reaction
of SPIs is based on the chemical reaction between the sulfonic
acid groups of a sulfonated diamine moiety and the activated
hydrogen atoms of a non-sulfonated diamine moiety (BAPF)
in the presence of a condensation agent (PPMA or phosphorus
pentoxide), and the resulting cross-linking bonds are the very
stable sulfonyl groups (Scheme 3). Ueda et al. have found that
many diaryl sulfones could be readily synthesized by con-
densation reactions of arenesulfonic acids and activated
(electron-rich) aromatic hydrocarbons in PPMA, and sodium
4-phenoxybenzenesulfonate could even undergo self-condensa-
tion polymerization in PPMA to form high molecular weight
poly(ether sulfone).38,39 Based on the discovery by Ueda et al.
we expect that sulfonated polymer membranes might also be
readily cross-linked by just treating them with PPMA or
phosphorus pentoxide without introducing additional cross-
linkers. To meet the requirement of reactivity of phenyl rings
for the cross-linking reaction with sulfonic acid groups, BAPF
was used as the diamine co-monomer because the 2,7-positions
of this compound are highly reactive. Table 1 lists the cross-
linking treatment conditions and results for a series of SPI
membranes. Although all the SPI membranes are insoluble in
PPMA, cross-linking reactions could still occur by immersing
them in PPMA at 80 uC for a given time. The formation of
cross-links is judged from the insolubility of the membranes in
organic solvents (DMSO and m-cresol) in which the mem-
branes were soluble before treating with PPMA. The thickness
of the membranes for cross-linking treatment was in the range
of 20–60 mm, and it was observed that the sorption of PPMA
in SPI membranes was very fast and the cross-linking rate was
almost identical for the same kind of membranes with different
thickness. The cross-linking rate is mainly determined by the
structure of the sulfonated diamine moiety. As shown in
Table 1, with method 1 the cross-linking rate is in the order:
NTDA–BSPOB/BAPF (9/1) & NTDA–BAPBDS/BAPF (3/1)
. NTDA–BDSA/BAPF (3/1), NTDA–ODADS/BAPF (3/1)
and NTDA–ODADS/BAPF (5/1). BSPOB is a side chain-type
diamine with pendent sulfonic acid groups and there are no
Scheme 1
Scheme 2
between 3,39-dihydroxybenzidine and 4-fluorobenzenesulfonic
acid sodium salt in the presence of anhydrous potassium
carbonate in NMP at 180 uC for 24 h (Scheme 2). To achieve
high IECs (.2.0 meq g21) and thus high proton conductivities
of SPIs, the molar fraction of the non-sulfonated diamine,
BAPF, in the diamine monomers was controlled to be no
higher than 25%.
Scheme 3
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Table 1 Solubility changes of SPI membranes before and after
immersing in PPMA at 80 uC
Solubilitya
Cross-
linking
Method treatment m-Cresol DMSO
Membrane
NTDA–BSPOB/BAPF (9/1)
No
3 h
10 h
No
10 h
24 h
No
+
2
2
+
+2
2
+
+
2
2
2
+2
2
+
1
2
NTDA–BAPBDS/BAPF (9/1)
NTDA–ODADS/BAPF (5/1)
1
1
1
1
2
24 h
48 h
10 h
No
+2
2
2
+2
2
2
NTDA–ODADS/BAPF (3/1)
+
+
1
1
2
24 h
48 h
10 h
No
24 h
48 h
10 h
+2
2
2
+
+2
2
+2
2
2
+
+2
2
Fig. 1 FT-IR spectra of NTDA–BDSA/BAPF (3/1) membranes: (a)
NTDA–BDSA/BAPF (3/1)
cross-linked, (b) uncross-linked.
1
1
2
2
2
FT-IR measurement. As shown in Figs. S3 and S4{, both
spectra suggest formation of the desired product, 2,7-
bis(4-methylphenylsulfonyl)-9,9-bis[4-(1,5-naphthylimido)phe-
nyl]fluorene (BMPS–BNIPF).
a
+, soluble; +2, partially soluble; 2, insoluble on heating.
substituents in the ortho positions of the sulfonic acid groups
leading to less steric effects in the cross-linking reaction than
the rest of the main chain-type ones. This is the main reason
that NTDA–BSPOB/BAPF (9/1) had a much faster cross-
linking rate than the other ones. Another structural factor
affecting cross-linking rate is the reactivity of the sulfonic
acid groups. Ueda et al. have investigated that with the same
aromatic hydrocarbons electron-rich phenyl rings to which
sulfonic acid groups are attached are favorable for the
formation of diaryl sulfones.38 This is also true for the present
cross-linking reactions of SPI membranes. NTDA–BAPBDS/
BAPF (3/1), for example, underwent a cross-linking reaction
significantly faster than NTDA–ODADS/BAPF (3/1). This
should be attributed to the relatively higher reactivity of the
sulfonic acid groups of the BAPBDS moiety because the steric
effect is similar. Polymer chain flexibility has little effect on
cross-linking rate judging from the fact that NTDA–ODADS/
BAPF (3/1) and NTDA–BDSA/BAPF (3/1) displayed the
same cross-linking rate although ODADS is flexible whereas
BDSA is highly rigid. In addition, the cross-linking rate hardly
depends on the molar fraction of BAPF because the same
cross-linking rate was observed for NTDA–ODADS/BAPF
(3/1) and NTDA–ODADS/BAPF (5/1). In the case of method
2, all the SPIs except NTDA–BAPBDS/BAPF (3/1) could be
readily cross-linked under the same conditions. The inapplic-
ability of method 2 to NTDA–BAPBDS/BAPF (3/1) is because
of its insolubility in DMSO.
Fig. 1 shows the FT-IR spectra of NTDA–BDSA/BAPF
(3/1) before and after cross-linking treatment. No significant
change was observed between these two spectra except that the
relative intensity of the absorption band at 1030 cm21 which is
assigned to the stretch vibration of the sulfonic acid group
decreased after cross-linking. This is because some of the
sulfonic acid acid groups were consumed (formation of
sulfonyl groups) during the cross-linking process. Similar
phenomena were observed with other SPI membranes. The
consumption of sulfonic acid groups due to the cross-linking
reaction was also reflected in the reduction of IEC which was
determined by a titration method. As shown in Table 2, for
both NTDA–BAPBDS/BAPF (3/1) and NTDA–BDSA/BAPF
(3/1) membranes, cross-linking led to a reduction in IEC by a
factor of about 20% which is close to the theoretical maximum
value (content of BAPF moiety, 25% by mole).
Mechanical properties
The mechanical properties of the cross-linked and uncross-
linked SPI membranes in both the dry state and the wet state
were investigated and the results are listed in Table 2. It can be
seen that for NTDA-BSPOB/BAPF(9/1) the cross-linked
membrane displayed both higher maximum stress (MS) and
larger elongation at break (EB) than the uncross-linked
membrane indicating improved mechanical properties due to
cross-linking. Unlike NTDA–BSPOB/BAPF (9/1), in dry state
other SPI membranes displayed significantly lower MS than
the corresponding uncross-linked ones. However, in wet state
for all the SPI membranes cross-linking led to significant
improvement in mechanical properties, i.e., cross-linking is
more favorable for maintaining mechanical properties of the
wet membranes. Moreover, all the cross-linked membranes
showed reasonably good toughness judging from the fact that
their EB values are larger than 5%. More studies are needed to
elucidate the effect of cross-linking density and polymer
structure on membrane mechanical properties.
To confirm if the cross-linking reaction indeed occurred
between sulfonic acid groups and the fluorenylidene groups,
a
model compound, 9,9-bis[4-(1,5-naphthylimido)phenyl]
fluorene (BNIPF), was synthesized via the reaction of 1,5-
naphthalenedicarboxylic anhydride (NDA) and 9,9-bis(4-
aminophenyl)fluorene (BAPF) at 190 uC in m-cresol in the
presence of benzoic acid and isoquinoline (see ESI{). Then
BNIPF and p-toluenesulfonic acid were dissolved in PPMA
and the mixture were heated at 80 uC for 4 h. The resulting
crude product (yield: 94%) was directly used for 1H NMR and
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Table 2 IEC, water uptake (WU) at 100 uC, maximum stress (MS), elongation at break (EB), proton conductivity (s) at 100% relative humidity
and 60 uC and water stability of cross-linked and uncross-linked SPI membranes
Membrane
Cross-linking treatment
IEC/meq g21
WU/wt%
MSc/MPa
EBc (%)
s/S cm21
Water stability/h
NTDA-BSPOB/BAPF(9/1)
NTDA-BAPBDS/BAPF(3/1)
NTDA-ODADS/BAPF(5/1)
NTDA-BDSA/BAPF(3/1)
No
Yes (method 1)
No
Yes (method 1)
No
Yes (method 1)
No
Yes (method 2)
2.02
1.96
2.16
1.80
2.82a
2.08
2.55
2.11
130
60
114 (44)
128 (62)
59 (19)
44 (24)
70 (8.0)
33 (11)
97 (18)
77 (27)
5.5 (10)
13 (13)
14 (5.0)
21 (9.5)
15 (1.8)
22 (5.2)
11 (8.7)
8.3 (14)
d
0.21
0.16
0.16
0.12
0.22
0.13
NMd
0.15
150
.720
150
.720
0.5
.720
10 min
400
110
90
120b
85
200b
120
a
b
c
Theoretical value. Measured at room temperature. The data in parentheses refer to wet membranes. NM: not measured.
Proton conductivity and water stability
conductivity dropped drastically as temperature increased
above 120 uC.
The water uptake, proton conductivity and water stability of
the cross-linked and uncross-linked SPI membranes are shown
in Table 2. It can be seen that cross-linking caused significant
suppression of membrane swelling and lower water uptake was
obtained with the cross-linked membranes. The proton
conductivities of the cross-linked membranes also slightly
decreased but were still at a high level [.0.1 S cm21 at 100%
relative humidity (RH) and 60 uC]. Fig. 2 shows the variation
in proton conductivity of the cross-linked and uncross-linked
SPI membranes as a function of relative humidity (RH). For
both cross-linked and uncross-linked membranes, the proton
conductivity decreased drastically as RH decreased which is a
common phenomenon observed with many other sulfonated
polymer membranes. Moreover, in the whole range of RH the
cross-linked membranes generally displayed lower proton
conductivities than the corresponding uncross-linked ones.
It’s a big challenge to enhance the proton conductivity at low
humidities from the viewpoint of practical use. The variation
of proton conductivity of various cross-linked SPI membranes
as a function of temperature is shown in Fig. 3. For all the
membranes the conductivity increased with an increase in
temperature which is superior to Nafion in which proton
The water stability test for cross-linked and uncross-linked
SPI membranes was performed by immersing them in
deionized water at 100 uC and characterized by the time
elapsed when the hydrated membranes began to lose mechani-
cal strength. The criterion for the judgment of the loss of
mechanical properties is that the membranes were broken
when lightly bent. As shown in Table 2, among the uncross-
linked membranes, BSPOB and BAPBDS-based SPIs
displayed much better water stability than ODADS and
BDSA-based ones. This is because BSPOB and BAPBDS
have higher basicity than ODADS and BDSA as we have
previously reported.13–16 Cross-linking caused great improve-
ment in water stability for all the SPI membranes. NTDA–
BSPOB/BAPF (9/1) membrane in its uncross-linked state, for
example, started to lose mechanical strength after being soaked
in deionized water at 100 uC for 150 h. However, the cross-
linked membrane could maintain mechanical strength at the
same soaking temperature for more than one month indicating
much improved water stability. In fact, after this soaking
treatment the cross-linked membrane was still tough and had a
tensile strength of about 10 MPa in fully hydrated state which
Fig. 2 Variation of proton conductivity of cross-linked (C, by
method 1) and uncross-linked (UC) SPI membranes as a function of
relative humidity at 60 uC.
Fig. 3 Variation of proton conductivity of cross-linked (C, by
method 1) SPI membranes as a function of temperature at 100 uC
relative humidity.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 1102–1108 | 1107

View Article Online
5 F. Wang, M. Hickner, Y. Kim, T. Zawodzinski and J. McGrath,
J. Membr. Sci., 2002, 197, 231.
6 D. Poppe, H. Frey, K. Kreuer, A. Heinzel and R. Mulhaupt,
Macromolecules, 2002, 35, 7936.
7 P. Xing, G. Robertson, M. Guiver, S. Mikhailenko and
S. Kaliaguine, Macromolecules, 2004, 37, 7960.
8 X. Li, C. Liu, H. Lu, C. Zhao, Z. Wang, W. Xing and H. Na,
J. Membr. Sci., 2005, 255, 149.
9 H. Ghassemi and J. McGrath, Polymer, 2004, 45, 5847.
10 T. Kobayashi, M. Rikukawa, K. Sanui and N. Ogata, Solid State
Ionics, 1998, 106, 219.
11 H. Kim and M. Litt, Polym. Prepr. (Am. Chem. Soc., Div. Polym.
Chem.), 2001, 42, 486.
12 J. Fang, X. Guo, S. Harada, T. Watari, K. Tanaka, H. Kita and
K. Okamoto, Macromolecules, 2002, 35, 9022.
13 X. Guo, J. Fang, T. Watari, K. Tanaka, H. Kita and K. Okamoto,
Macromolecules, 2002, 35, 6707.
should be high enough for fuel cell use. BDSA is
a
commercially available sulfonated diamine which has been
widely used for preparation of various SPIs. However, the
poor water stability of BDSA-based SPI membranes is a very
serious problem which made them hard to use. In this study,
NTDA–BDSA/BAPF (3/1), although insoluble in hot water,
became highly brittle after being soaked in deionized water at
100 uC for only ten minutes indicating very poor water
stability. After cross-linking, however, it could maintain
reasonable high mechanical strength (y10 MPa) after being
soaked in deionized water at 100 uC for 400 h, i.e., the stability
is about three orders longer than that of the uncross-linked
membrane.
It should be noted that the cross-linking method developed
in this study is also applicable to many other sulfonated poly-
mers such as sulfonated polystyrenes, sulfonated poly(ether
sulfone)s, sulfonated poly(ether ketone)s, sulfonated poly-
phenylenes and sulfonated polybenzimidazoles. Sulfonated
polystyrene of a degree of sulfonation of 42%, for example,
was water soluble at room temperature. However, it could
be readily cross-linked by immersing the membrane into
PPMA at 80 uC for half an hour. The resulting membrane
was insoluble and its dimensions were stable even in
boiling deionized water. The detailed studies will be reported
elsewhere.
14 Y. Yin, J. Fang, H. Kita and K. Okamoto, Chem. Lett., 2003, 32,
328.
15 T. Watari, J. Fang, K. Tanaka, H. Kita, K. Okamoto and
T. Hirano, J. Membr. Sci., 2004, 230, 111.
16 Y. Yin, J. Fang, T. Watari, K. Tanaka, H. Kita and K. Okamoto,
J. Mater. Chem., 2004, 14, 1062.
17 Y. Yin, S. Hayashi, O. Yamada, H. Kita and K. Okamoto,
Macromol. Rapid Commun., 2005, 26, 696.
18 K. Miyatake, H. Zhou, H. Uchida and M. Watanabe, Chem.
Commun., 2003, 3, 368.
19 K. Miyatake, H. Zhou and M. Watanabe, Macromolecules, 2004,
37, 4956.
20 K. Miyatake, H. Zhou, T. Matsuo, H. Uchida and M. Watanabe,
Macromolecules, 2004, 37, 4961.
21 B. Einsla, Y. Hong, Y. Kim, F. Wang, N. Gunduz and J. McGrath,
J. Polym. Sci., Part A: Polym. Chem., 2004, 42, 862.
22 M. Walker, K.-M. Baumgartner, M. Kaiser, J. Kerres, A. Ullrich
and E. Rauchle, J. Appl. Polym. Sci., 1999, 74, 67.
23 J. Kerres, A. Ullrich, T. Haring, M. Baldauf, U. Gebhardt and
W. Preidel, J. New Mater. Electrochem. Syst., 2000, 3, 229.
24 L. Jorissen, V. Gogel, J. Kerres and J. Garche, J. Power Sources,
2002, 105, 267.
25 C. M. Tang, W. Zhang and J. Kerres, J. New Mater. Electrochem.
Syst., 2004, 7, 287.
26 J. Kerres, W. Cui, R. Disson and W. Neubrand, J. Membr. Sci.,
1998, 139, 211.
27 J. Kerres, W. Cui and M. Junginger, J. Membr. Sci., 1998, 139,
227.
28 J. Kerres, W. Zhang, A. Ullrich, C.-M. Tang, M. Hein, V. Gogel,
T. Frey and L. Jorissen, Desalination, 2002, 147, 173.
29 J. Kerres, W. Zhang and T. Haering, J. New Mater. Electrochem.
Syst., 2004, 7, 299.
30 W. Zhang, V. Gogel, K. A. Friedrich and J. Kerres, J. Power
Sources, 2006, 155, 3.
31 J. Kerres, Fuel Cells, 2005, 5, 230.
32 Q. Guo, P. N. Pintauro, H. Tang and S. O’Connor, J. Membr. Sci.,
1999, 154, 175.
Conclusions
(1) A new and facile approach has been successfully developed
for preparation of cross-linked SPI membranes via a con-
densation reaction between the sulfonic acid groups and the
activated hydrogen atoms of SPIs in the presence of a
condensation agent (PPMA or phosphorus pentoxide).
(2) The resulting cross-linked SPI membranes showed greatly
improved water stability in comparison with the uncross-
linked ones while high proton conductivity was maintained.
Acknowledgements
This work was supported by National Natural Science
Foundation of China (No. 20474037). J.F. first developed
the synthetic method for BSPOB as he worked in Professor
Morton Litt’s lab of Department of Macromolecular Science
and Engineering, Case Western Reserve University, USA.
33 S. Mikhailenko, K. Wang, S. Kaliaguine, P. Xing, G. Robertson
and M. Guiver, J. Membr. Sci., 2004, 233, 93.
34 S. Yang, W. Jang, C. Lee, Y. Shul and H. Han, J. Polym. Sci.,
Part B: Polym. Phys., 2005, 43, 1455.
35 C. H. Lee, H. B. Park, Y. S. Chung, Y. M. Lee and B. D. Freeman,
Macromolecules, 2006, 39, 755.
References
1 Z. Shi and S. Holdcroft, Macromolecules, 2005, 38, 4193.
2 N. Carretta, V. Tricoli and F. Picchioni, J. Membr. Sci., 2000, 166,
189.
3 M. Lopez, V. Compan, J. Garrido, E. Riande and J. Acosta,
J. Electrochem. Soc., 2001, 148, E372.
4 H. Ghassemi, G. Ndip and J. McGrath, Polymer, 2004, 45, 5855.
36 M. Langsam and W. Burgoyne, J. Polym. Sci., Part A: Polym.
Chem., 1993, 31, 909.
37 C. Genies, R. Mercier, B. Sillion, N. Cornet, G. Gebel and
M. Pineri, Polymer, 2001, 42, 359.
38 M. Ueda, K. Uchiyama and T. Kano, Synthesis, 1984, 323.
39 M. Ueda, Makromol. Chem., Rapid Commun., 1985, 6, 271.
1108 | J. Mater. Chem., 2007, 17, 1102–1108
This journal is ß The Royal Society of Chemistry 2007