Synthesis and characterization of a novel poly(arylene ether sulfone) containing pendent imidazole groups for high temperature proton exchange membranes

Article abstract of DOI:10.1039/c2jm34417c

A series of poly(arylene ether sulfone) containing pendent imidazole groups (PSf-Im-x) have been successfully synthesized based on a novel monomer 2,2′-bis-(2-methyl-imidazol-1-yl-methyl)-biphenyl-4,4′-diol (MIPO). The pendent imidazole groups along the polymer chain were expected to provide functional sites for the acid-base interaction with the doping phosphoric acid (PA) when they are used as polymer electrolyte membranes for high temperature fuel cell applications. The PA content of the linear PSf-Im-x membranes is about 172.3-235.8% in 85 wt% H₃PO₄ at room temperature. The volume swelling of these membranes is 114.4-194.0%, lower than that of polybenzimidazole (PBI) with similar PA content. The proton conductivities of the membranes are 0.021-0.053 S cm^-1 at 140 °C under absolutely dehydrated state. The low volume swelling and good proton conductivity may be attributed to the side-chain-type structures of pendent imidazole groups, which facilitate ion transport. To obtain higher acid doping while maintaining mechanical properties, cross-linked membranes were prepared by the reaction of the imidazole group of the polymer and p-xylene dichloride. The PA content of the membranes with 20% cross-linking is 313.2% in 85 wt% H ₃PO₄ at 80 °C. The stress at breaking and the proton conductivity of the membrane is 3.2 MPa at room temperature and 0.063 S cm ^-1 at 140 °C in an absolutely dehydrated state.

Full text of DOI:10.1039/c2jm34417c

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PAPER
Synthesis and characterization of a novel poly(arylene ether sulfone)
containing pendent imidazole groups for high temperature proton exchange
membranes
a
Jing Wang,ab Jifu Zheng,a Zhuo Zhaoab and Suobo Zhang
*
Received 7th July 2012, Accepted 7th September 2012
DOI: 10.1039/c2jm34417c
A series of poly(arylene ether sulfone) containing pendent imidazole groups (PSf-Im-x) have been
successfully synthesized based on a novel monomer 2,20-bis-(2-methyl-imidazol-1-yl-methyl)-biphenyl-
4,40-diol (MIPO). The pendent imidazole groups along the polymer chain were expected to provide
functional sites for the acid–base interaction with the doping phosphoric acid (PA) when they are used
as polymer electrolyte membranes for high temperature fuel cell applications. The PA content of the
linear PSf-Im-x membranes is about 172.3–235.8% in 85 wt% H3PO4 at room temperature. The volume
swelling of these membranes is 114.4–194.0%, lower than that of polybenzimidazole (PBI)ꢁwith similar
PA content. The proton conductivities of the membranes are 0.021–0.053 S cmꢀ1 at 140 C under
absolutely dehydrated state. The low volume swelling and good proton conductivity may be attributed
to the ‘‘side-chain-type’’ structures of pendent imidazole groups, which facilitate ion transport. To
obtain higher acid doping while maintaining mechanical properties, cross-linked membranes were
prepared by the reaction of the imidazole group of the polymer and p-xylene dichloride. The PA
ꢁ
content of the membranes with 20% cross-linking is 313.2% in 85 wt% H3PO4 at 80 C. The stress at
breaking and the proton conductivity of the membrane is 3.2 MPa at room temperature and 0.063 S
cmꢀ1 at 140 ꢁC in an absolutely dehydrated state.
cellsꢁ (PEMFCs) arising at operational temperatures below
Introduction
100 C.
Proton exchange membrane fuel cells (PEMFCs), which have
Acid–base polymer membranes, typically a basic polymer
been recognized as one of the most promising power generation
doped with a non-volatile inorganic acid or blended with a poly-
technologies, could provide clean and efficient energy for
meric acid, have been considered for use as PEMs in elevated
stationary, transportation and portable electronics.1,2 Proton
temperature fuel cell applications.5 In 1995, Wainright et al.
exchange membranes (PEM), key components for PEMFCs,
successfully developed a high temperature polymer electrolyte by
have undergone much research in the past decades.3 Current
doping H3PO4 (PA) into polybenzimidazole (PBI), thereby
commercial PEMs based on perfluorosulfonic acid (PSFA), such
creating polymers that function as PEMFCs at high temperatures
as Nafion, have good mechanical properties, excellent chemical
and low humidity.6 Since that time, comprehensive investigations
stability and exhibit high proton conductivities only when
of the acid doped PBI for use in PEMFCs have been reported.7–10
hydrated (about 0.09 S cmꢀ1 at 25 ꢁC), therefore, they are limited
To optimize membrane properties, high molecular weight PBIs
to operating temperatures up to around 80 ꢁC.4 Several chal-
are desired to achieve mechanically stable membranes with high
lenges for the PEMFC power technology are associated with low
PA content and therefore high proton conductivities. However,
operating temperatures, including CO poisoning of the electrode
high molecular weight PBI has poor solubility and processability
Pt/catalyst, complexity of the heat and water management
for membrane casting, and, the 3,30,4,40-tetraaminobiphenyl
systems and limiting reaction kinetics.4 Development of high
(TAB) monomer commonly used for the synthesis of PBI is toxic.5
temperature PEMs would be an effective approach to overcome
Alternative polymeric materials for high temperature PEM
the technical challenges of the proton exchange membrane fuel
include polymers bearing basic sites such as aromatic polyether
containing main chain pyridine,11 aromatic poly(ether sulfone)s
containing triazine groups,12 poly(arylene ether ketone) contain-
ing pendent quaternary ammonium groups13 and polysulfone
containing imidazolium groups.14
aKey Laboratory of Polymer Ecomaterials, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, China.
E-mail: sbzhang@ciac.jl.cn; Fax: +86 431 85262117; Tel: +86 431
85262118
In the PA doped membranes, a higher PA content leads to
higher proton conductivities. The PA content can be increased at
bGraduate School of the Chinese Academy of Sciences, Beijing 100039,
China
22706 | J. Mater. Chem., 2012, 22, 22706–22712
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elevated temperatures to obtain membranes with a higher
amount of PA. However, under high PA content, although the
proton conductivities of the PA doped membranes are high, the
mechanical stability deteriorates because the membranes swell in
H3PO4. As found in the PEM with sulfonic acid groups and
AEM (anion exchange membrane) with quaternary ammonium
groups, if the polymer structure contains flexible pendent side
chains linking the polymer main chain and the sulfonic acid
groups or quaternary ammonium groups, nanophase separation
between hydrophilic and hydrophobic domains may be
improved, which would enhance conductivity and decrease
swelling in water.15–18 The strategy of utilizing a ‘‘side-chain-
type’’ structure is also expected to be useful in the PA doped high
temperature PEMs.
Measurements
1H NMR spectra were measured at 300 MHz on an AV300
spectrometer. The inherent viscosities were determined with an
Ubbelohde capillary viscometer at 30 ꢂ 0.1 ꢁC on 0.5 g dLꢀ1
concentrations of polymer in CHCl3 or DMAc. Molecular
weights were also determined by gel permeation chromatography
(GPC) using a Waters 515 HPLC pump, coupled with a Waters
410 differential refractometer detector and a Waters 996 photo-
diode array detector operating at a wavelength of 260 nm. DMF
was used as the eluent. Tensile measurements were carried out
with an Instron-1211 mechanical testing instrument at a speed of
1 mm minꢀ1
.
Synthesis of monomer 2,20-bis-(2-methyl-imidazol-1-yl-methyl)-
biphenyl-4,40-diol (MIPO)
The covalent cross-linking of membranes may be another
reasonable method for improving dimensional stability without
impairing proton conductivity. Covalent cross-linking of PBIs
has been achieved by thermal treatment,19 chemical reaction
between PBI and the cross-linker p-xylene chloride,20 p-xylene
dibromide21 or epoxy resin.22 Lee et al.23,24 recently reported
development of new H3PO4-doped cross-linked benzoxazine–
benzimidazole copolymer membranes with interpenetrating
network structures for use in fuel cells at elevated temperatures.
These cross-linked membranes show enhanced mechanical
properties and very high proton conductivity. Kallitsis et al.25,26
also improved the performance of the polyether containing main
chain pyridine by covalent cross-linking.
To a 250 mL round-bottomed flask, 2,20-bis-dimethylamino-
methyl-biphenyl-4,40-diol (30.0 g, 0.10 mol) was dissolved in
dimethylbenzene and 2-methylimidazole (0.30 mol, 24.0 g) was
added. The solution was stirred at 140 ꢁC with a nitrogen inlet for
12 h. The precipitated solid was collected by filtration, washed
with ethanol several times and was dried under vacuum at 80 ꢁC
for 24 h to produce the final product. The yield was 90%; the
1
ꢁ
melting point was 298 C. H NMR (DMSO-d6, ppm): d 7.20–
7.24 (2H, dd), 7.05–7.06 (2H, d), 6.89 (2H, s), 6.86 (2H, s), 6.74–
6.75 (2H, d), 5.05 (4H, s) and 3.42 (6H, s). 13C NMR (DMSO-
d6): d 153.5, 143.9, 131.1, 126.4, 126.2, 126.1, 123.8, 120.0, 115.4,
44.0, 12.9. Anal. calcd for C22H22N4O2 (374.4): C, 70.57%; H,
5.92%; N, 14.96%. Found: C, 70.62%; H, 5.89%; N, 14.91%.
In this paper, we report the synthesis of poly(arylene ether
sulfone) containing pendent imidazole units (PSf-Im) based on a
novel
monomer
2,20-bis-(2-methyl-imidazol-1-yl-methyl)-
biphenyl-4,40-diol (MIPO). The pendent imidazole groups along
the polymer chain were expected to provide functional sites for
the acid–base interaction with the doping phosphoric acid. Use
of this novel monomer affords precise control of the imidazole
concentration in the final polymer, which leads to highly repro-
ducible materials. The PSf-Im copolymer is a thermally stable
polymer and with excellent solubility in polar solvents. The linear
membranes are directly cast from the PSf-Im polymer. The cross-
linked membranes are obtained by using p-xylene chloride as a
cross-linker that reacts with the imidazole groups of PSf-Im. The
solubility, swelling in the PA solution, mechanical properties,
proton conductivity and oxidation stability of both linear and
cross-linked membranes were studied.
Synthesis of poly(arylene ether sulfone) containing pendent
imidazole groups (PSf-Im-x)
The DF values of the copolymers, where DF represents the
degree of functionalization (the number of imidazole groups/
repeat unit), were controlled by adjusting the molar ratio of
MIPO to BP (4,40-biphenol monomer). A typical synthesis
procedure of PSf-Im-60, where 60 refers to the feed percent of
MIPO, was as follows. A flame-dried 100 mL three-necked flask
equipped with a nitrogen inlet and overhead stirrer was charged
with MIPO (2.2464 g, 6 mmol), BP (0.7448 g, 4 mmol), 4,40-
bifluorodiphenylsulfone (2.5425 g, 10 mmol), K2CO3 (2.90 g, 21
mmol) and dry DMAc (50 mL; 10% solids). The mixture was
kept at room temperature for a few minutes and then slowly
heated at 120 ꢁC for 12 h. After cooling to room temperature, an
additional 30 mL of DMAc was added so as to dilute the highly
viscous solution, after which the solution was filtered and
dropped into stirred deionized water. The fiber-like precipitate
was filtered off and washed with hot water three times prior to
being dried under vacuum to produce the final product.
Experimental section
Materials
2,20-Bimethylaminemethylene-4,40-biphenol was synthesized as
reported in our previous paper.27 4,40-Biphenol (BP), 4,40-
bifluorodiphenylsulfone, 2-methylimidazole and p-xylylene
dichloride were purchased from Aldrich. Polybenzimidazole
(PBI) powder was kindly provided by Polymics ITP Co. Ltd.,
China. Formaldehyde solution (37%), dimethylamine solution
(33%), potassium carbonate (K2CO3), N,N-dimethylacetamide
(DMAc), dimethylbenzene and LiCl were purchased from
Sinopharm Group Chemical Reagent Co., Ltd. All other
reagents were obtained from commercial sources and used as
received.
Membrane casting of linear PSf-Im-x and cross-linked C-PSf-
Im-70-n
For linear membrane casting, the polymer PSf-Im-x was dis-
ꢁ
solved in DMAc at 80 C under stirring to form a 5% solution.
The resulting solution was then filtered and degassed and
transferred to a preheated glass dish where the cross-linked
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reaction occurred, followed by solvent evaporation yielding the
clear, thin membrane.
The membranes were sandwiched between two pairs of gold-
plate electrodes. The membranes and the electrodes were set in a
Teflon cell and the distance between the reference electrodes was
1 cm. 1 cm ꢃ 5 cm membranes were introduced to the conduc-
The following represents a typical procedure for the synthesis
of C-PSf-Im-70-30. PSf-Im-70 (1.10 g, 2 mmol) was dissolved in
20 mL of dimethylacetamide (DMAc), and then 0.42 mmol p-
xylylene dichloride was added to the solution. After 10 min of
vigorous stirring, the homogeneous solution was degassed and
transferred to a preheated glass dish where the cross-linking
reaction occurred, followed by solvent evaporation yielding the
clear, thin membrane.
ꢁ
tivity cell and heated to 140 C and held for 60 min. Measure-
ments were taken as the cell then cooled to 100 ꢁC in 10 ꢁC steps.
Fenton test
To test membrane degradation by oxidative radicals the
produced membranes were immersed in a Fenton solution con-
taining 3% H2O2 and 4 ppm Fe2+ (added as FeSO4). In this
solution hydroxyl radicals were produced resulting in oxidative
attack of the membranes, mimicking possible operating condi-
tions during use in fuel cells. The membrane samples were
immersed in the Fenton solution at 80 ꢁC. The stability was
evaluated by the time when the membrane broke into pieces after
being shaken vigorously.
Membrane casting of PBI
The PBI membrane was obtained by dissolving PBI powder (5 wt
%) and LiCl (1 wt%) in dimethylacetamide at 120 ꢁC and casting
the solution at 120 ꢁC for 12 h. Acid-doped PBI was obtained by
immersion of the PBI membrane in phosphoric acid (85 wt%) at
room temperature for 48 h, blotting with filter paper and then
ꢁ
drying at 110 C for 24 h.
Results and discussion
Acid doping
Synthesis of monomers and copolymers containing pendent
Thethoroughly dried membranewas cut into pieces of 4 cm ꢃ 5 cm
and immersed into 85 wt% H3PO4 solutions at room, or elevated
temperatures, for 48 hin orderto obtain the desiredPAcontent (wt
%). The PA content (wt%) of a membrane is defined as the mass
changes of the membrane sample before and after doping:
imidazole groups (PSf-Im-x)
Scheme 1 shows the two-step synthesis of 2,20-bis-(2-methyl-
imidazol-1-yl-methyl)-biphenyl-4,40-diol (MIPO). In step one,
2,20-bis-dimethylaminomethyl-biphenyl-4,40-diol is synthesized
according to the method we reported previously.27 In step two,
DABP, which decomposes to an o-quinone methide intermediate
at 140 ꢁC,28,29 reacts with 2-methylimidazole to yield the product,
MIPO, as shown in Scheme 2. The structure of MIPO was
PA content (wt%) ¼ (mA ꢀ mB)/mB ꢃ 100
(1)
where mA is the mass of the membrane after the doping and
blotting with filter paper and then drying at 110 ꢁC for 24 h, and
mB is the mass of the membrane before the doping, respectively.
The area and volume swellings of a membrane sample in an
acid solution were determined by measuring the membrane
dimension before (DB) and after (DA) immersion of the sample
for 48 h and drying with blotting paper and then drying at 110 ꢁC
for 24 h, and calculated using eqn (2).
1
confirmed by the H NMR and 13C NMR spectrum (Fig. 1).
The poly(arylene ether sulfone) with pendent imidazole groups
(PSf-Im-x) was obtained from MIPO, BP and 4,40-bifluor-
odiphenylsulfone by nucleophilic substitution polycondensation
(Scheme 1). The use of a monomer containing pendent imidazole
groups eliminates the need for post polymerization modifications
and ensures reproducibility of the imidazole content. The imid-
azole content of the copolymers was readily controlled through
the feed ratio of the monomers MIPO and BP. The
Swelling (%) ¼ (DA ꢀ DB)/DB ꢃ 100
(2)
Solubility test
The cross-linking was characterized by measuring the solubility
of the membranes in DMSO. 0.1 g of membrane was immersed in
20 mL DMSO 80 ꢁC under stirring for 48 h. Then the remaining,
not dissolved membrane samples were collected, dried and
weighed. The solubility of the membranes was then obtained
from the weight losses observed.
Proton conductivity
The proton conductivities (s, S cmꢀ1) of the membranes were
obtained using s ¼ d/LsWsR (d is the distance between reference
electrodes, and Ls and Ws are the thickness and width of the
membrane, respectively). Here, ohmic resistance (R) was
measured by four-point probe alternating current (ac) impedance
spectroscopy using an electrode system connected to an imped-
ance/gain-phase analyzer (Solatron 1260) and an electrochemical
interface (Solatron 1287, Farnborough Hampshire, ONR, UK).
Scheme
1
Sythentic procedure of 2,20-bis-(2-methyl-imidazol-1-yl-
methyl)-biphenyl-4,40-diol (MIPO) and copolymer poly(arylene ether
sulfone) containing pendent imidazole groups (PSf-Im-x). Reagents and
conditions: (i) formaldehyde solution (37%), dimethylamine solution
(33%), 24 h, 25 ꢁC; (ii) 2-methylimidazole, dimethylbenzene, 12 h, 140 ꢁC.
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Scheme 2 Reaction mechanism of preparing 2,20-bis-(2-methyl-imida-
zol-1-yl-methyl)-biphenyl-4,40-diol (MIPO).
Fig. 1 1H NMR and 13C NMR spectra of 2,20-bis-(2-methyl-imidazol-1-
yl-methyl)-biphenyl-4,40-diol (MIPO).
Fig. 2 1H NMR spectra of PSf-Im-70, -100 in CDCl3.
polyꢁcondensation reaction was completed in a few hours (12 h) at
120 C in the presence of 2.1 equivalents of K2CO3. These mild
reaction conditions yielded high molecular weight polymers as
shown by their inherent viscosities and GPC (Table 1). Typical
average molecular weights and polydispersities ranged from
74 000 to 86 000 and 2.1 to 2.3, respectively. Fig. 2 shows the 1H
NMR spectrum of PSf-Im-70, -100 in CDCl3. The synthesized
polymer exhibited good solubility in CDCl3 and polar aprotic
solvents such as DMF, DMAc, DMSO and NMP. Fig. 3 shows
the TGA (thermo-gravimetric analysis) curves for the PSf-Im-70
membranes, which ꢁwere recorded under a nitrogen atmosphere
The membranes with a higher imidazole content absorbed more
phosphoric acid. The PA content of the PSf-Im-70 membrane
was 235.8%, which is similar to that found for PBI membranes
(239.7% in 85 wt% PA at room temperature). The swelling of the
PSf-Im-70 membranes in 85 wt% PA, however, was less than that
of PBI membranes. For example, the swelling of the PSf-Im-70
membrane in H3PO4 was 110.0% in area and 194.0% in volume,
whereas the swelling of the PBI membrane in H3PO4 was 146.7%
in area and 237.5% in volume (Table 2). When the membranes
are doped in H3PO4, the imidazole groups are protonated, which
then react with the phosphoric acid anion to create sites on the
polymer chain in which free phosphoric acid molecules can
immobilize. Separating the imidazole groups from the polymer
main chain, e.g., localizing the imidazole groups on side chains
grafted onto the polymer main chain, decreases the interaction
between H3PO4 and the polymer main chain, and decreases
membrane swelling in H3PO4. The imidazole groups for PBI, are
located in the polymer main chain, while for PSf-Im-x they are
located on side chains grafted onto the polymer main chain.
ꢁ
from 40 C to 800 C. Two decomposition steps were observed
for PSf-Im-70. The first decomposition step, at a temperature
range from 270 to 450 ꢁC, was attributed to the loss of imidazole
groups, whereas the second step corresponded to the degradation
of the PSf backbone.
PA content, mechanical properties and conductivity of linear
PSf-Im-x membranes
Fig. 4 shows the variations of PA content of linear PSf-Im-x
membranes as a function of time immersing the membranes in
85 wt% PA solutions at 25 ꢁC. The PA content of the membranes
increased during the first 10 h, and then levelled off with time.
Table 1 Inherent viscosity, molecular weight and DF values of PSf-Im-x
a
hinh
(dL gꢀ1
Mn
DFb (104 g molꢀ1
Mw
(104 g molꢀ1
Sample
)
)
)
PDIc
PSf-Im-50
PSf-Im-60
PSf-Im-70
PSf-Im-80
0.43
0.42
0.40
0.41
1.20 8.2
1.40 8.3
1.59 8.6
1.81 7.9
2.00 7.4
17.6
16.0
17.7
16.8
16.5
2.1
1.9
2.1
2.1
2.2
PSf-Im-100 0.42
a
Inherent viscosity measured at a concentration of 0.5 dL gꢀ1 in DMAc
b
at 30 ꢁC. Degree of functionalization ¼ (number of the imidazole
groups/repeat unit). c Polydispersity.
Fig. 3 TGA curves for PSf-Im-70 and C-PSf-Im-70-40.
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Fig. 4 PA content of PSf-Im-x membranes as a function of time
Fig. 5 Conductivities of acid doped PSf-Im-x membranes as a function
immersed in 85 wt% phosphoric acid at room temperature.
of temperature.
The mechanical properties of acid doped PSf-Im-x membranes
are shown in Table 2. The doping PA molecules reduced the
tensile strength but increased the elongation of the membranes, a
common observation in the acid doped PBI.30,31 Before doping,
the PSf-Im-50, 60, and 70 membranes had tensile strengths of
40.2–70.5 MPa and elongated by 20–70%. After doping, the
tensile strength of the acid doped PSf-Im-50, 60, 70 membranes
decreased to 11.1, 7.8, and 4.2 MPa, and elongation increased to
56.0, 89.2, and 180.0%, respectively.
conductivities are high, the mechanical stabilities of the PA
doped membranes deteriorate. For example, the PSf-Im-70
membrane swells to yield a gel with 85 wt% PA at 80 ꢁC. To
overcome this problem, we used p-xylylene dichloride to make a
cross-linked membrane. The chemical structure of the cross-
linked polymer is shown in Scheme 3. Assuming complete reac-
tion of each mole of the p-xylylene dichloride with 2 equivalents
of PSf-Im-70 imidazole groups, the extent of cross-linking would
be 10, 20, 30 and 40%, of the total imidazole groups, respectively.
Virgin PSf-Im-70 is thermally stable at temperatures up to 270 ꢁC
while cross-linked PSf-Im-70-40 (C-PSf-Im-70-40) could be
thermally stable up to 340 ꢁC, as shown in Fig. 3. The weight loss
observed for C-PSf-Im-70-40 at temperatures below 340 ꢁC is
due to removal of the residual water bound with the hydrophilic
imidazolium moieties. Membranes with 10–40% cross-linking are
Fig. 5 shows the proton conductivity of PA doped PSf-Im-x
membranes as a function of temperature under anhydrous
conditions. Conductivities of these membranes were all above
0.01 S cmꢀ1. Conductivity increased linearly with increasing
temperature. In addition, conductivities increased with increased
PA content at the same temperature. The PSf-Im-70 membranes
with PA content of 235.8% displayed a conductivity of 0.048 S
cmꢀ1 at 130 ꢁC, whereas the PBI membrane with a PA content of
239.7% exhibited a conductivity of 0.041 S cmꢀ1 at 130 ꢁC as seen
in Table 2. The higher conductivity of PSf-Im-70 was due to the
‘‘side-chain-type’’ structures that facilitate ion transport.
PA content, mechanical properties and conductivity of a cross-
linked C-PSf-Im-70-n membrane
In the PA doped membranes, higher doping temperatures
enhanced PA doping levels, which in turn lead to higher proton
conductivities. However, at a high PA content, although proton
Scheme 3 Chemical structures of the cross-linked polymers.
Table 2 Variations and mechanical properties of linear and cross-linked membranes in 85 wt% of phosphoric acid solutions at 25 ꢁC and conductivities
at 130 ꢁC
Tensile strength
(MPa)
Tensile modulus
(MPa)
Elongation at
break (%)
Conductivity
(S cmꢀ1
)
Sample
Mass (%)
Area (%)
Volume (%)
PSf-Im-50
PSf-Im-60
PSf-Im-70
C-PSf-Im-70-10
C-PSf-Im-70-20
C-PSf-Im-70-30
C-PSf-Im-70-40
PBI
172.3
202.4
235.8
213.5
199.2
170.4
148.6
239.7
50.0
72.3
110.0
36.5
30.9
23.8
15.5
146.7
114.4
141.2
194.0
118.4
102.8
86.8
11.1
7.8
4.2
5.9
6.6
8.2
10.9
4.8
12.9
7.19
2.49
28.5
23.8
16.0
64.8
3.14
56
89
180
120
100
71
0.019
0.034
0.048
0.043
0.034
0.019
0.009
0.041
65.0
237.5
44
110
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example, after 2 days of immersion in the 85 wt% PA solution at
25 ꢁC, the linear PSf-Im-70 membrane showed a volume swelling
of 194.0%, while the C-PSf-Im-70-10, 20, 30, 40 membranes
showed lower volume swellings: 118.4, 102.8, 86.8, 65.0%,
respectively.
In Fig. 6 is depicted the temperature dependence of the ionic
conductivity of the cross-linked membranes, and the linear PSf-
Im-70 membrane. The conductivity of membranes is highly
dependent on the PA content. Compared with the linear PSf-Im-
70 membrane, immersing the cross-linked membranes in phos-
phoric acid solution at 25 ꢁC gave a lower PA content. As a
result, the conductivity of the cross-linked membrane is lower
compared to the neat polymer, and decreased with the increasing
extent of cross-linking.
Increasing the doping temperature can enhance the mobility of
polymer chains and the free volume for the acid doping, which
leads to the increased PA content of the membranes. Thus, the
cross-linked membranes were doped in 85% H3PO4 at tempera-
tures up to 80 ꢁC in order to achieve high PA content. By
immersing the membrane samples in 85% phosphoric acid at
ꢁ
80 C for 48 h, as an example, the linear PSf-Im-70 membrane
swelled to a gel. As a comparison, the cross-linked membranes
had a PA content of 385.2, 313.2, 209.1 and 178.7%, respectively,
corresponding to volume swellings of 170.8, 150.0, 105.2 and
84.8% for cross-linking degrees of 10, 20, 30 and 40%, respec-
tively, as shown in Table 3.
The mechanical properties of the linear and cross-linked
membranes are listed in Tables 2 and 3 for different PA contents.
When cross-linked, the membranes show a much improved
mechanical strength. As seen from Table 3, the cross-linked
membrane C-PSf-Im-70-20 with PA content of 313.2% exhibited
a similar tensile strength and young modulus as those for linear
PSf-Im-70 with PA content of 235.8%. It should be noticed that
the two membranes have similar mechanical properties and the
measured conductivity was very different, as shown in Fig. 6
(bottom). A higher conductivity is observed for the cross-linked
membranes with a PA content of 313.2%. As expected, higher PA
contents are effective in achieving higher conductivity. The cross-
linked membranes make this possible to some extent without
deteriorating the mechanical strength.
Fig. 6 Conductivities of C-PSf-Im-70-n membranes doping in 85 wt%
H3PO4 at 25 ꢁC (top) or 80 ꢁC (bottom) as a function of temperature.
homogeneous, transparent membranes with a light yellow color.
The solubility of these membranes in DMSO was measured at 80
ꢁC by determining their residual weight after 48 h of immersion.
Membrane samples of linear PSf-Im-70 with no cross-linker
dissolved in DMSO at 80 ꢁC within one-half hour. The 10–40%
cross-linked membranes were more stable since only 3–5 wt%
was soluble during 48 h of immersion in DMSO.
The doping ability of the cross-linked membranes in phos-
phoric acid at 25 ꢁC is depicted in Table 2 and for comparison the
doping ability of the linear polymers is also presented. The cross-
linked membrane C-PSf-Im-70-n possessed a PA content of
148.4–213.5%, which was lower than that of linear PSf-Im-70
under the same conditions. This is because cross-linking leads to
more compact chemical structures that decrease the doping
ability. The volume swelling of the cross-linked membranes is
also lower than that of the linear PSf-Im-70 membrane. For
Oxidation stability
Fenton test treatment was performed in order to evaluate
whether the membranes were able to withstand a strong
oxidizing environment during the fuel cell operation. The resis-
tance to oxidation of the membranes was examined by observing
the dissolving behavior. The PSf-Im-70 started to break into
pieces after 36 h, while the PSf-Im-50 kept its shape for 240 h.
Table 3 Variations and mechanical properties of cross-linked membranes in 85 wt% of phosphoric acid solutions at 80 ꢁC and conductivities at 130 ꢁC
Tensile strength
(MPa)
Tensile strength
(MPa)
Elongation at
breaking (%)
Conductivity
(S cmꢀ1
)
Sample
Mass (%)
Area (%)
Volume (%)
C-PSf-Im-70-10
C-PSf-Im-70-20
C-PSf-Im-70-30
C-PSf-Im-70-40
385.2
313.2
209.1
178.7
62.5
35.0
28.3
23.2
170.8
150.0
105.2
84.8
1.7
3.2
5.4
6.0
3.2
3.7
4.6
10.9
100
46
33
31
0.075
0.056
0.031
0.023
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 22706–22712 | 22711

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For the covalently cross-linked membranes, the cross-linking
structure indeed enhanced the free radical oxidative stability. The
cross-linked membranes with 10–40% cross-linking degree could
keep its shape for more than 360 h.
Conclusions
High molecular weight poly(arylene ether sulfone) with pendent
imidazole groups (PSf-Im-x) was successfully synthesized using a
novel
monomer
2,20-bis-(2-methyl-imidazol-1-yl-methyl)-
biphenyl-4,40-diol (MIPO) and its ability to absorb phosphoric
acid was characterized. With a PA content of 172.3% to 235.8%,
the membranes showed a volume swelling of 114.4–194.0%, a
proton conductivity of 0.021–0.053 S cmꢀ1 at 140 ꢁC, and a
tensile strength of 4.2–11.1 MPa. The low volume swelling and
good proton conductivity are attributed to the ‘‘side-chain-type’’
structures of the pendent imidazole groups. Using p-xylylene
dichloride, cross-linked membranes PSf-Im-70 membranes (C-
PSf-Im-70-n) were prepared. Higher doping temperatures
increased the PA of the cross-linked membranes. C-PSf-Im-70-20
membrane had a PA content of 313.2% in 85 wt% H3PO4 solu-
tion at 80 ꢁC and exhibited a proton conductivity of 0.063 S cmꢀ1
at 140 ꢁC. Compared to linear membranes, cross-linked
membranes result in increased chemical stability towards
oxidation by peroxide radicals in the Fenton test.
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22712 | J. Mater. Chem., 2012, 22, 22706–22712
This journal is ª The Royal Society of Chemistry 2012