Polymer electrolyte membranes based on poly(arylene ether)s with penta-sulfonated pendent groups

Article abstract of DOI:10.1039/c2ta00363e

The preparation and characterization of new polymeric ionomers based on a fully aromatic poly(arylene ether) backbone with locally pentasulfonated pendent groups for applications as proton exchange membranes is reported. The high molecular weight sulfonated polymers were obtained by the polycondensation of new (2,6-difluorophenyl) (4-(1,2,3,4,5-pentaphenylbenzene)phenyl)methanone, 4,4′-difluorodiphenyl methanone, and 4,4′-dihydroxydiphenylsulfone, followed by sulfonation using sulfuric acid in high yields. The polymers produced tough, flexible, and transparent membranes by solvent casting. Membranes with ion exchange capacities between 0.8 and 1.7 mEq g^-1 showed high proton conductivities and low methanol permeabilities. Compared to Nafion 117, these sulfonated membranes exhibited better microphase separation morphologies. The fully humidified membranes also exhibited considerably good mechanical properties, with tensile strengths from 35 to 45 MPa and elongations at break from 23 to 49%. This investigation demonstrates a controllable high density sulfonated side group of a poly(arylene ether sulfone) membrane with tunable and balanced properties, which is promising for proton exchange membrane fuel cell technology. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2ta00363e

Journal of
Materials Chemistry A
View Article Online
View Journal | View Issue
PAPER
Polymer electrolyte membranes based on poly(arylene
ether)s with penta-sulfonated pendent groups
Cite this: J. Mater. Chem. A, 2013, 1,
1465
*
Jinhui Pang, Kunzhi Shen, Dianfu Ren, Sinan Feng, Yang Wang and Zhenhua Jiang
The preparation and characterization of new polymeric ionomers based on a fully aromatic poly(arylene
ether) backbone with locally pentasulfonated pendent groups for applications as proton exchange
membranes is reported. The high molecular weight sulfonated polymers were obtained by the
polycondensation of new (2,6-difluorophenyl) (4-(1,2,3,4,5-pentaphenylbenzene)phenyl)methanone,
4,40-difluorodiphenyl methanone, and 4,40-dihydroxydiphenylsulfone, followed by sulfonation using
sulfuric acid in high yields. The polymers produced tough, flexible, and transparent membranes by
solvent casting. Membranes with ion exchange capacities between 0.8 and 1.7 mEq g-1 showed high
proton conductivities and low methanol permeabilities. Compared to Nafion 117, these sulfonated
membranes exhibited better microphase separation morphologies. The fully humidified membranes also
exhibited considerably good mechanical properties, with tensile strengths from 35 to 45 MPa and
elongations at break from 23 to 49%. This investigation demonstrates a controllable high density
sulfonated side group of a poly(arylene ether sulfone) membrane with tunable and balanced properties,
which is promising for proton exchange membrane fuel cell technology.
Received 18th September 2012
Accepted 9th November 2012
DOI: 10.1039/c2ta00363e
www.rsc.org/MaterialsA
change as well as decreased mechanical strength of the
lm.14–16 As a strategy to improve the proton conductivity and
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs) are under
consideration as future power sources for transport, stationary
and portable applications.1–3 One of the key materials in a
PEMFC is a proton exchange membrane (PEM), which allows
for proton transport from the anode to the cathode. The current
state-of-the-art PEM materials are peruorinated polymers such
as Naon or Flemion because of their good physical and
chemical stabilities along with their high proton conductivities
over a wide range of relative humidities at moderate operation
temperatures. However, these membranes still have a number
of drawbacks especially for large-scale applications, such as
high cost, high methanol permeability, and poor performance
at temperatures above 80 ꢀC, which calls for the development of
new membrane materials.4,5
Aromatic hydrocarbon polymers are alternative PEM mate-
rials, and a number of sulfonated aromatic polymers such as
poly(ether ether ketone)s,6–10 polyimides,11,12 and poly-
benzimidazole have been developed as suitable candidates.13
Generally, membranes based on these polymers require high
ion exchange capacities (IECs) to achieve proton conductivities
comparable to those of peruorinated polymers. Although a
high IEC value results in a high conductivity for a polymer,
excessive water uptake may result in exorbitant dimensional
mechanical properties, sulfonated side chain and sulfonated
multiblock copolymers have been extensively studied.17–20 The
technique with sulfonated side chain PEMs is to design a
polymer architecture that distinctly separates the hydrophilic
sulfonic acid groups from the hydrophobic polymer main
chain by locating the sulfonic acid groups on exible side
chains, which may induce improved nanophase separation
between the hydrophilic and hydrophobic domains. For
example, Guiver and co-workers reported PEMs prepared by
attaching exible pendent sulfonated aromatic side chains to
poly(arylene ether sulfone), which showed a superior proton
conductivity of 147 mS cmꢁ1 at 80 ꢀC with a slightly higher
water content than Naon.21 Our group also reported PEMs
based on poly(arylene ether) with aromatic or aliphatic
sulfonated side chains,22–24 which exhibited high proton
conductivities and excellent dimensional stabilities. Another
approach to induce nanophase separation is through
sulfonated multiblock copolymer architecture, whereby
sulfonic acid groups are concentrated in blocks along the
polymer chain. McGrath et al.25 prepared multiblock
sulfonated poly(arylene ether)s with promising morphological
structures by two-step polycondensation. The fuel cell perfor-
mance of a single cell using this multiblock copolymer at 40%
RH (relative humidity) was comparable to that of Naon, and it
performed much better than the alternating and random
polymers. More recently, random copolymers designed with
densely localized concentrations of sulfonated units in the
Alan G. MacDiarmid Institute, Department of Chemistry, Jilin University, No. 2699
Qianjin street, Changchun, P.R. China. E-mail: pangjinhui@jlu.edu.cn; Fax: +86 431
85168199; Tel: +86 431 85168199
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. A, 2013, 1, 1465–1474 | 1465

View Article Online
Journal of Materials Chemistry A
Paper
main chains have attracted considerable attention because of added to a 250 mL, three-necked, round-bottom ask equipped
the high contrast in polarity between the hydrophilic and with a mechanical stirrer, a nitrogen inlet/outlet, and a reux
hydrophobic units; this promotes the formation of hydro- condenser. Triphenylphosphine (PPh3; 0.1 g), PdCl2(PPh3)2
philic–hydrophobic phase-separated structures. It has been (0.05 g), and cuprous iodide (CuI; 0.05 g) were added and
found that the morphological structures of the copolymers washed down with 50 mL of triethylamine. The mixture was
were comparable to that of Naon 117, which explains their slowly heated to 60 ꢀC in an oil bath, and phenylacetylene
high proton conductivities.26,27 On the other hand, Hay et al. (5.5 mL, 0.05 mol) was added with another 40 mL of triethyl-
reported poly(arylene ether)s with high density sulfonated amine. The reaction temperature was raised and maintained at
endcaps, in order to enhance the difference between the 80 ꢀC for 4 h. Aer the mixture had cooled to room temperature,
hydrophilic and hydrophobic units.28,29 Their membranes the reaction subsequently proceeded for about 16 h. The
showed signicantly phase-separated morphologies. However, mixture was ltered to provide a white, solid lter cake, which
their structures were limited in the number of sulfonated units was washed with distilled water twice to remove the inorganic
because the number of terminated groups was restricted. salts that formed, and the crude product was recrystallized from
Furthermore, hydrophilic units at the periphery increase the acetone. A dried white crystal (11.9 g) was obtained in a yield of
water solubility of polymers, which prevents an increase in the 75.1%. Mp: 130 ꢀC. dH (500 MHz; CDCl3; Me4Si): 6.99–7.05 (2H,
IEC values.
m, FPh), 7.36–7.39 (3H, m, Ph), 7.41–7.53 (1H, m, FPh), 7.54–
In this paper, we attempt to attach high density sulfonated 7.58 (2H, m, C^CPh), 7.61–7.65 (2H, m, C^CPh), 7.84–7.87
groups into the side chains of poly(arylene ether) and prepare (2H, m, COPh). Mass spectrum: m/z 317.5.
poly(arylene ether) containing higher density pentasulfonated
(2,6-DIFLUOROPHENYL)
(4-(1,2,3,4,5-PENTAPHENYLBENZENE)
side chains. We investigate the relationship between the PHENYL)METHANONE(3). To a three neck round-bottomed ask
structure, morphology and properties.
equipped with a mechanical stirrer and an argon in–out
channel, compound 2 (3.18 g, 10 mmol), tetraphenylcyclo-
pentadienone (3.84 g, 10ꢀ mmol), and sulfolane (5 mL) were
added and heated at 250 C for 3 h under argon. Aer cooling
the mixture was poured into methanol and the precipitate was
puried by recrystallization from toluene to give 6.1 g of 3 as a
light yellow powder in a yield of 90%. Mass spectrum: m/z 675.4,
698.6 (Na+), 714.9 (K+).
Materials
2,6-Diuorobenzoyl chloride, phenylacetylene, tetraphenylcy-
clopentadienone, PPh3, CuI, and PdCl2(PPh3)2 were purchased
from Sigma-Aldrich Ltd. Sulfolane, bromobenzene, TEA (tri-
ethylamine), DMAc (dimethylacetamide), and toluene were
purchased from Shanghai Chemical Reagent Co. Ltd. 4,40-
Dihydroxydiphenylsulfone, 4,40-diuorodiphenyl methanone,
and K2CO3 were purchased from Sinopharm Chemical Reagent
Beijing Co. Ltd. Other commercially available materials and
solvents were used without further purication.
Synthesis and sulfonation of the poly(arylene ether)
copolymer
SYNTHESIS OF NONSULFONATED POLY(ARYLENE ETHER)S (P-X). A
typical polycondensation procedure was conducted as follows
(Scheme 2): to a round-bottomed ask equipped with a Dean–
Stark trap, 4,40-diuorodiphenyl methanone (0.9819 g, 4.5
Synthesis of monomer
(4-BROMOPHENYL)(2,6-DIFLUOROPHENYL)METHANONE (1).
A
250 mL three-neck ask equipped with a mechanical stirrer, a mmol), 3 (0.3374 g, 0.5 mmol), 4,40-dihydroxydiphenylsulfone
dropping funnel, and a nitrogen inlet was charged with 100 mL (1.2514 g, 5.0 mmol), and K2CO3 (0.759 g, 5.05 mmol) were
of bromobenzene. The solution was cooled to 0 ꢀC, and added. Then, sulfolane (8 mL) and toluene (3 mL) were added
anhydrous AlCl3 (14.6 g, 0.11 mol) was added in several into the ask under argon. The reaction mixture was stirred at
portions. Aer the solution had been stirred for 10 min, 2,6- 140 ꢀC for 3 h. Aer the removal of toluene, the reaction
diuorobenzoyl chloride (17.6 g, 0.1 mol) was added dropwise temperature was increased to 220 ꢀC, and the reaction was
over 10 min. Then the temperature was raised to 90 ꢀC and continued for 6 h. Aer cooling to room temperature, the
maintained there for 8 h. The mixture solution was poured mixture was poured into deionized water. The resulting ber
into 100 mL of 1 M hydrochloric acid ice-water solution aer was ltered and washed with hot deionized water and hot
the reaction solution had cooled to room temperature. The ethanol. The polymer was dried under vacuum at 120 ꢀC for 12 h
organic phase was separated and washed with 10% NaOH (1 ꢂ to give pure nonsulfonated poly(arylene ether) P-10 (x ¼ 10). The
100 mL) and H2O (4 ꢂ 100 mL). The solution was dried with yield was 98%.
anhydrous MgSO4, and the excess bromobenzene was evapo-
SYNTHESIS OF THE SULFONATED POLY(ARYLENE ETHER)S (SP-X).
rated under vacuum. Aer being recrystallized in petroleum P-x (1 g) and 10 mL H2SO4 (95–98%) were put into a 100 mL
(60–90 ꢀC) and dried, 26.7 g of white crystals was obtained. single neck round-bottomed ask. The reaction mixture was
Yield: 90%. Mp (melting point): 86 ꢀC. dH (500 MHz; CDCl3; stirred at room temperature for 12 h. Then the yellow mixture
Me4Si): 6.98–7.05 (2H, m, FPh), 7.42–7.52 (1H, m, FPh), 7.61– solution was poured into ice-water, and a exible polymer ber
7.65 (2H, m, BrPh), 7.71–7.75 (2H, m, COPh). Mass spectrum: was obtained. The ber was ltered and washed with hot
m/z 299 (H+), 336.3 (K+).
deionized water and hot ethanol. The polymer was dried under
ꢀ
(2,6-DIFLUOROPHENYL)(4-(PHENYLETHYNYL)PHENYL)METHANONE vacuum at 120 C for 12 h to give pure sulfonated poly(arylene
(2). 1 (14.85 g, 0.05 mol) and 150 mL of triethylamine were ether)s SP-x. The yield remained above 97%.
1466 | J. Mater. Chem. A, 2013, 1, 1465–1474
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry A
Wwet ꢁ Wdry
Analysis and measurements of the copolymers
Water uptake ¼
ꢂ 100%
Wdry
The viscosities of the obtained copolymers were determined by
using an Ubbelohde viscometer in a thermostatic container
with a polymer concentration of 0.5 g dLꢁ1 in DMAc at 25 ꢀC. FT-
IR spectra (lm) were measured on a Nicolet Impact 410 Four-
ier-transform infrared spectrometer. 1H NMR experiments were
carried out on a Bruker 510 spectrometer (1H, 500 MHz) using
DMSO-d6 and CDCl3 as solvents.
where Wdry and Wwet are the weights of the dried and wet
samples, respectively. The swelling ratio was calculated from
the change of the lm length by
Lwet ꢁ Ldry
Swelling ratio ¼
ꢂ 100%
Ldry
where Lwet and Ldry are the lengths of the wet and dry
membranes, respectively.
Film formation and proton exchange
SP-x copolymer (1.0 g) was dissolved in DMAc (10 mL) over-
night. Aerwards, the solution was ltered with a ne glass
fritted lter funnel and cast directly onto clean glass plates.
Aer being carefully dried at 60 ꢀC for 10 h and vacuum-dried at
120 ꢀC for 24 h, tough and exible lms of SP-x were obtained.
The membranes were transformed to the acid forms by proton
exchange in 1 M H2SO4 for 24 h at room temperature. Then, the
membranes were soaked and washed thoroughly with deion-
ized water. The thicknesses of the membranes were in the range
of 70–90 mm.
Oxidative and hydrolytic stability
A small piece of the membrane sample was soaked in Fenton's
reagent (3% H2O2 aqueous solution containing 2 ppm FeSO4)
ꢀ
for 1 h at 80 C. The oxidative stability was evaluated from the
change in weight and appearance of the test sample.
Proton conductivity and methanol permeability
Fully hydrated membranes (4 cm ꢂ 1 cm) were measured by a
four-electrode AC impedance method from 0.1 Hz to 100 kHz
with 10 mV AC perturbation and 0.0 V DC rest voltage using a
Princeton Applied Research Model 273A Potentiostat (Model
5210 frequency response detector, EG&G PARC, Princeton, NJ).
The measurements were carried out with the cells immersed in
the constant-temperature water, and the proton conductivity
was determined by the equation:
Thermal properties of the membranes
Differential scanning calorimetry (DSC) measurements were
performed on a Mettler Toledo DSC821e instrument at a heat-
ing rate of 10 ꢀC minꢁ1 from 50 to 300 ꢀC under nitrogen.
Thermogravimetric analysis (TGA) on a Perkin Elmer Pyris 1
thermal analyzer system was employed to assess the thermal
stability of the membranes. Before the analysis, the membranes
were dried and kept in the TGA furnace at 120 ꢀC under air for
30 min to remove water. The samples were evaluated over a
temperature range of 100–800 ꢀC at a heating rate of 10 ꢀC
minꢁ1 under N2.
L
s ¼
RA
where s is the proton conductivity, L is the distance between the
electrodes used to measure the potential (L ¼ 1 cm), R is the
membrane resistance, and A is the membrane area.
A stainless steel diffusion cell was used to measure the
methanol permeability as described in the literature.32 The cell
consisted of two reservoirs that were separated by a membrane.
A 10 M methanol solution and deionized water were placed on
each side.
Magnetic stirrers were used in each compartment to ensure
uniformity. The concentration of the methanol in the water
reservoir was determined by using a SHIMADZU GC-8A chro-
matograph. The methanol permeability was calculated as
follows:
Ion exchange capacity
The ion exchange capacity (IEC) values of the membranes were
determined by acid–base titration. The dried membrane was
weighed and immersed in 1.0 M H2SO4 solution for 24 h to
protonize the sulfonic acid groups and was then washed thor-
oughly with deionized water until the pH was neutral. The
membrane was then immersed in a 2.0 M solution of NaCl for
12 h to replace the protons of the sulfonic acid groups with
sodium ions. The solutions were titrated using 0.02 M NaOH
solution, with phenolphthalein as the indicator. The IEC values
were calculated from the titration results as the ratio of the
amount of NaOH consumed (mmol) to the weight of the dried
membrane samples (g).
A DK
CBðtÞ ¼
CAðt ꢁ t0Þ
VB
L
where A (in cm2), L (in cm), and VB (in mL) are the effective area,
the thickness of the membrane, and the volume of the perme-
ated reservoirs, respectively. CA and CB (in mol Lꢁ1) are the
methanol concentration in the feed and in the permeate,
respectively. t and t0 (in s) represents time and the time lag,
Water uptake and swelling ratio measurements
respectively. DK (in cm2 sꢁ1
)
denotes the methanol
The membranes were soaked in deionized water to reach permeability.
equilibrium at the desired temperature. The membranes were
The methanol uptake and the swelling ratio in high
then dried at 120 ꢀC for 24 h. The weights and lengths of the dry concentration methanol solution (10 mol Lꢁ1) were measured
and wet membranes were measured. The water uptake content in the same way as they were measured in water, which is
was calculated by
described above.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. A, 2013, 1, 1465–1474 | 1467

View Article Online
Journal of Materials Chemistry A
Paper
3 was conrmed using 1H NMR spectroscopy. The signals at
7.3–7.4 ppm were assigned to the protons in the benzophenone
structure of monomer 3, while those at 6.8–7.0 ppm were
assigned to the electron-rich phenyl groups.
Morphology
SAXS measurements were carried out on membranes ion-
exchanged to the Pb2+ form by immersion in a 1 wt% aqueous
solution of lead acetate. The scattering experiments were per-
formed on a SAXSess camera equipped with a CCD detector.
CuKa radiation with a wave length (l) of 0.1542 nm was
generated by a Rigaku D/max-2550 X-ray generator operating at
50 kV and 200 mA. Dry membranes were analyzed in a solid
sample holder at 25 ꢀC. The wave vector (q) was calculated
according to:
Synthesis of poly(arylene ether) copolymer
A series of poly(arylene ether) polymers with pendent penta-
sulfonation sites were successfully synthesized via a standard
aromatic nucleophilic substitution reaction of monomer 3 with
4,40-diuorobenzophenone and 4,40-dihydroxydiphenylsulfone,
and were named P-x (as shown in Table 1 and Scheme 2, x is
the ratio of monomer 3 in the copolymer). A sulfone and
carbonyl-rich main chain was selected because of its high
adaptability for PEM application. The content of the diuoride
monomer 3 in the copolymerization procedure can control the
IEC aer sulfonation. All the polymers (P-x) were soluble in
chloroform, THF and polar aprotic solvents. The structure of
the polymers P-x were conrmed by 1H NMR spectroscopy. The
1H NMR spectrum of the polymer P-20 is shown in Fig. 2.
Several specic protons were clearly assigned, and the expected
copolymer structure was conrmed. The integral ratio of the
peaks from 6.5 to 7.1 ppm which were assigned to the protons
in the monomer 3 moieties and all proton peaks was calcu-
lated to determine the composition of the polymer. The
calculated result (x ¼ 20) matched well with its initial
composition.
q ¼ 4plꢁ1sinq
where 2q is the scattering angle. The characteristic separation
length (d), i.e. the Bragg spacing, was calculated as:
d ¼ 2pqꢁ1
Mechanical properties
The mechanical properties of the dry/wet membranes were
measured at room temperature on a SHIMADIU AG-I 1KN at a
strain rate of 2 mm minꢁ1. The size of the samples was 20 mm
ꢀ
ꢂ 4 mm. The membranes were dried at 120 C under vacuum
for 12 h. The samples in the wet state were obtained by
immersing them in water for 12 h.
Results and discussion
Synthesis of monomer
As a new diuoride monomer having ve sulfonation positions,
(3) was designed and prepared as shown in Scheme 1. Monomer
3 was prepared by a three-stage synthetic procedure starting
with the Friedel–Cras reaction of 2,6-diuorobenzoyl chloride
and bromobenzene to give compound 1. Then compound 2 was
obtained by a palladium-catalyzed Sonogashira coupling reac-
tion of compound 1 and phenylacetylene.30 Finally, as described
similarly in the literature,28,31 the multi-phenyl compound 3 was
synthesized through a Diels-Alder reaction. The structures of
these compounds were conrmed by FT-IR, LC-MS and 1H NMR
spectroscopy. As shown in Fig. 1, the structure of the monomer
Fig. 1 1H NMR spectrum of the monomer 3.
Scheme 1 Synthesis of the difluoride monomer. a: AlCl3, 100 ꢀC, 6 h. b: Phenylacetylene, TEA, PdCl2(PPh3)2, PPh3, CuI. c: Tetraphenylcyclopentadienone, 250 ꢀC, 3 h.
1468 | J. Mater. Chem. A, 2013, 1, 1465–1474
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry A
Table 1 Intrinsic viscosities, ion exchange capacities (IECs), and thermal properties of sulfonated polymers SP-x
IECa (mEq g-1)
IECb (mEq g-1)
[h] (dL gꢁ1
)
Tg (ꢀC)
c
Td (ꢀC)
d
Td (ꢀC)
Polymer
x
SP-20
SP-18
SP-16
SP-14
SP-12
SP-10
SP-8
20
18
16
14
12
10
8
1.67
1.55
1.42
1.28
1.13
0.97
0.80
1.53
1.46
1.35
1.20
1.01
0.92
0.80
1.20
1.33
1.08
0.91
0.87
0.77
0.61
222
214
210
205
203
200
199
238
239
240
242
246
248
251
360
365
372
378
386
393
404
a
Theoretical value. b Calculated by titration. c Onset weight loss temperature. d 5% weight loss temperature.
4,40-dihydroxydiphenylsulfone as the other two monomers in
the copolymerization because of their strongly electron-with-
drawing groups, ketone or sulfone. According to literature
Sulfonation of the copolymers
The copolymers were sulfonated using sulfuric acid (95–98%) at
room temperature. We chose 4,40-diuorobenzophenone and
Scheme 2 Synthesis of penta-sulfonated poly(arylene ether)s.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. A, 2013, 1, 1465–1474 | 1469

View Article Online
Journal of Materials Chemistry A
Paper
Fig. 4 1H NMR spectrum of the sulfonated poly(arylene ether) copolymer (SP-
20).
Fig. 2 1H NMR spectrum of the poly(arylene ether) copolymer (P-20).
reports,28,33 polymer main chains containing diphenylsulfone
and benzophenone moieties have excellent stabilities even
under the harsh sulfonation conditions so that neither sulfo-
nation nor chain cleavage occurs. Therefore, the sulfonated
reaction occurred on the electron-rich aromatic pendant
phenyls in the side chain. In order to achieve complete sulfo-
nation to P-x, an excess amount of sulfonic acid was used. Aer
sulfonation the reaction mixture was put into deionized water to
remove excess sulfuric acid, and the product was dried to give
SP-x. The SP-x was readily soluble in polar aprotic solvents such
as DMF, DMAc, DMSO and NMP. Tough, exible and trans-
parent membranes of SP-x were obtained by a solution casting
method. The structures of SP-x were conrmed by FT-IR and 1H
NMR spectroscopy. As shown in Fig. 3, the characteristic IR
absorption of the sulfonic acid group (nS]O) and diphe-
compared to the theoretical value which is calculated by
considering a proton of each sulfonic acid group in the repeat
unit (Table 1). The viscosity values of the resulting SP-x were in
the range 0.6–1.3 dL gꢁ1, indicating their high molecular
weights.
Thermal properties
The thermal stabilities of P-x and SP-x were evaluated using a
TGA instrument under a nitrogen atmosphere. The P-x exhibi-
ted high thermal degradation temperatures. The onset weight
loss temperature is higher than 450 ꢀC and the 5% weight loss
temperatures are above 500 ꢀC, indicating their high thermal
stabilities. Compared to P-x, the SP-x have lower onset weight
loss temperatures (Table 1 and Fig. 5). However it is higher than
ꢀ
nylcarbonyl (nC]O) were observed at 1038 and 1658 cmꢁ1
,
240 C, meeting a requirement of PEMFCs. All the SP-x exhibi-
ted a two-step degradation prole. As shown in Fig. 5, the SP-8
and SP-20 degradation processes were observed clearly. The rst
weight loss at about 250 ꢀC was attributed to the degradation of
the sulfonic acid groups, while the second weight loss peak at
about 400 ꢀC was due to the degradation of the main chains of
the sulfonated polymers. The Tg (glass transition temperature)
respectively. These results conrmed the successful introduc-
tion of the sulfonic acid groups onto the polymer side chains.
Fig. 4 shows the 1H NMR spectra of P-20 and SP-20. In
comparing the spectra before and aer sulfonation, the peak
(H11) at 6.65 ppm has disappeared completely aer sulfonation,
which indicates that complete sulfonation occurred on each
pendent phenyl ring of the SP-20.
ꢀ
values of the P-x were over 190 C, and those of the SP-x were
observed before degradation occurred. The Tg values of the
sulfonated copolymers are much higher than those of the non-
sulfonated ones, since chain rigidity is increased through
hydrogen bonding interactions of the sulfonic acid groups.
The elementary characterization data for SP-x are summa-
rized in Table 1. Experimental ion exchange capacities (IECs) of
SP-x were determined by titration of the membranes in acid
form with sodium hydroxide. The results show IEC values
Fig. 3 FT-IR of the copolymers.
Fig. 5 TGA curves of the copolymers.
1470 | J. Mater. Chem. A, 2013, 1, 1465–1474
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry A
Water uptake, swelling ratio and oxidative stability
Water uptake is very closely related to the proton conductivities
and the mechanical properties of PEM materials. Appropriate
water absorption helps form connected hydrophilic channels
which serve as a proton transport medium. However, excess
water in the polymer may destroy the hydrophobic phase
leading to a loss of mechanical properties, and may even result
in the polymer lm being soluble in water. Several methods
have been probed to study the morphology effects on the water
absorption behavior and conductivity of copolymer, to investi-
gate the relationship between the polymer structure and proton
conductivity and dimension stability. We focus on the intro-
duction of high density sulfonic acid on the end of the side
chains of the poly(arylene ether) copolymers to improve the
proton conductivity and control water absorption for
membranes. The water absorption of SP-x membranes was
investigated in detail (Table 2). Upon increasing the IEC of SP-x
from 0.8 to 1.67 mEq g-1, the water uptake increased from 4.3%
to 30.5% at room temperature and from 7.0% to 46.8% at 80 ꢀC,
which indicates that all the membranes exhibit a lower water
uptake than many random copolymers and single sulfonated
side chain copolymers.1,17 Additionally, SP-x membranes
exhibited good dimension stability and the largest swelling
ꢀ
ratio is lower than 24% at 80 C, similar to that of Naon 117.
Fig. 6 shows the temperature effect on the water uptake and
swelling ratios of SP-x copolymers. The water uptake and
swelling ratio appear to exhibit similar trends, increasing as the
temperature increases. Below 80 ꢀC,ꢀ both increased slowly.
When the temperature went above 80 C, the membranes with
high amounts of sulfonated content exhibited a sharp increase
in water sorption and swelling ratio, which is attributed to the
formation of large and continuous ion networks in the
sulfonated polymers.
The oxidative stabilities of the SP-x lms were evaluated in
Fenton's reagent at 80 ꢀC for 1 h. Naon 117, which is a typical
proton exchange membrane, was used for comparison. The
results are summarized in Table 2. For all membranes, no
obvious weight loss was observed, and their properties such as
exibility and transparency were maintained aer the test.
Fig. 6 Water uptake and swelling ratios of the SP-x and Nafion films as a
function of temperature.
Proton conductivity
The proton conductivities of SP-x increased as the IEC values
increased (listed in Table 3). Compared to Naon membranes
with similar IEC values, SP-10 membranes exhibited slightly
lower proton conductivities, however they were higher than
those of many random copolymers. For example, the proton
Table 2 Some properties of sulfonated poly(arylene ether)s
Water uptake
(wt%)
Dimensional
stability (L%)
IEC
Polymer
(mEq g-1)
RT
80 ꢀ
C
RT
80 ꢀC
Oxidative stabilitya (wt%)
Tensile strengthb (MPa)
Elongation at breakb (%)
SP-20
SP-18
SP-16
SP-14
SP-12
SP-10
SP-8
1.67
1.55
1.42
1.28
1.13
0.97
0.80
0.91
30.5
25.7
20.1
17.0
14.4
11.6
4.3
46.8
40.6
29.2
26.4
21.1
15.6
7.0
14.7
13.6
11.3
8.8
7.8
7.1
23.7
22.2
16.8
13.1
10.5
8.8
>99
>99
>99
>99
>99
>99
>99
>97
39 (35)
41 (36)
43 (38)
46 (39)
46 (42)
48 (44)
51 (45)
38 (28)
16.9 (49)
13.4 (42)
12.5 (38)
10.4 (39)
9.7 (26)
9.4 (22)
8.2 (23)
205 (230)
6.4
13.1
8.1
20.2
Naon 117
19.2
29.4
a
Residue weight aer treatment in Fenton's reagent (3% H2O2 containing 2 ppm FeSO4) at 80 ꢀC for 1 h. b Measurements of dry membranes, values
in parentheses represent properties of wet membranes.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. A, 2013, 1, 1465–1474 | 1471

View Article Online
Journal of Materials Chemistry A
Paper
Table 3 Proton conductivity and methanol permeability of the SP-x membranes
s (mS cmꢁ1
)
IEC
Methanol permeability
(cm2 sꢁ1
Polymer
(mEq g-1)
RT
80 ꢀ
C
)
Methanol uptakeb (%)
Dimensional changeb (%)
SP-20
SP-18
SP-16
SP-14
SP-12
SP-10
SP-8
1.67
1.55
1.42
1.28
1.13
0.97
0.80
1.81
1.52
0.91
57
55
40
35
26
11
7
—
—
80
125
122
87
75
55
31
17
75
34
3.79 ꢂ 10ꢁ7
3.39 ꢂ 10ꢁ7
2.27 ꢂ 10ꢁ7
1.90 ꢂ 10ꢁ7
1.36 ꢂ 10ꢁ7
4.82 ꢂ 10ꢁ8
2.52 ꢂ 10ꢁ8
3.28 ꢂ 10ꢁ7
1.63 ꢂ 10ꢁ7
2.94 ꢂ 10ꢁ6
59.7
53.4
39.8
40.9
24.4
13.5
15.6
—
18.7
19.6
15.0
16.2
7.3
5.1
5.4
—
SC-SPAE-100a
SC-SPAE-80a
Naon 117
—
—
—
—
118
a
Data from ref. 34. b Measured in 10 M methanol aqueous solution at 20 ꢀC.
conductivities of SPAEK-6F-30 (IEC ¼ 1.06 mEq g-1) at room Lꢁ1 methanol at room temperature were in the range 2.52 ꢂ
temperature and 80 ꢀC are 4.5 ꢂ 10ꢁ3 S cmꢁ1 and 1.1 ꢂ 10ꢁ2
S
10ꢁ8 to 3.79 ꢂ 10ꢁ7 cm2 sꢁ1 which is several times lower than
cmꢁ1, respectively,34 which are much lower than those of SP-10 the value of 2.94 ꢂ 10ꢁ6 cm2 sꢁ1 for Naon 117. The dimen-
(IEC ¼ 0.97 mEq g-1). Fig. 7 displays proton conductivities as a sional stabilities in high concentration methanol solution were
function of temperature. The proton conductivities of the SP-x also measured (Table 3), and slightly higher values than those
membranes increased with increasing temperature. The high- in water were observed. In order to evaluate the combined effect
est proton conductivity of 162 mS cmꢁ1 was observed for SP-20 of the proton conductivity and methanol crossover, a trade-off
with an IEC of 1.67 mEq g-1 at 100 ꢀC. In addition, the plot containing both methanol permeability and proton
conductivities of SP-20 and SP-18 were higher than that of conductivity at room temperature is shown in Fig. 8. It is clear
Naon 117 under the same conditions. Compared to the SC- that the preferred membranes, which have higher proton
SPAE-X polymer (our previous work),17 the SP-x exhibited higher conductivities and lower methanol permeabilities, are situated
proton conductivities (Table 3) at similar IEC levels. For at the top right corner of the plot. The relationship between the
example, the proton conductivity of SP-18 (IEC ¼ 1.55 mEq g-1) methanol permeability, proton conductivity and IEC were
is 122 mS cmꢁ1, which is much higher than that of SC-SPAE-80 investigated using a plot of selectivity versus IEC (Fig. 9). The
(IEC ¼ 1.52 mEq g-1) which has a proton conductivity of 34 mS selectivity, which is the ratio of the proton conductivity to the
cmꢁ1. This is attributed to the high density sulfonic acid groups methanol permeability, is oen used to evaluate the potential
present in every side group, resulting in better hydrophilic– performance of DMFC membranes. It is a useful predictive
hydrophobic microphase separation morphology in the SP-x parameter of performance, providing the proton conductivity is
polymers.
sufficiently high.35 The selectivity of SP-x membranes decreased
as the IEC value increased. In addition, the SP-x membranes
displayed much higher selectivities than the Naon membranes
due to their low methanol permeabilities. The SP-x (IEC > 1.1
mEq g-1) membranes exhibited high proton conductivities and
Methanol permeability and selectivity
The SP-x copolymers exhibited lower methanol permeabilities
than Naon 117. The methanol permeability values for 10 mol
Fig. 8 Proton conductivity versus methanol permeability of the SP-x and Nafion
117 films at room temperature.
Fig. 7 Proton conductivity of SP films as a function of temperature.
1472 | J. Mater. Chem. A, 2013, 1, 1465–1474
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry A
Fig. 10 The SAXS profiles of SP-20 and Nafion 117.
Fig. 9 Performance trade-off plot of IEC value versus selectivity.
formation.38 As a consequence, a longer pendent group will be
introduced into this series of polymer and further investigation
is on going.
low methanol permeabilities and could be promising materials
for DMFC applications.
Mechanical properties
Conclusion
It is essential for PEMs to possess adequate mechanical
strength under dry and humid conditions. Here, tensile tests of
both dry and wet membranes were conducted at room
temperature, and the results are summarized in Table 2. The
strengths of the membranes of SP-x are higher than that of
Naon 117. These data indicate that these membranes are
strong and tough.
A novel diuoride monomer containing ve electron-rich
phenyl rings has been successfully synthesized. Based on this
monomer, the poly(arylene ether sulfone) copolymer with a
penta-sulfonic acid pendent was obtained by poly-
1
condensation, followed by sulfonation in high yields. H NMR
spectroscopy conrmed the structures of monomer and the
corresponding copolymers. Tough, exible and transparent
sulfonated membranes (SP-x) were obtained by a solution
casting method, and showed excellent thermal stabilities and
suitable mechanical properties. The hydrophilic microphase in
the polymer signicantly affected the proton transport, water
absorption and dimensional stability. SP-x polymers with
higher local concentrations of penta-sulfonic acid pendents
exhibited lower water uptake, good dimensional stabilities and
high proton conductivities. With low amounts of sulfonic acid
groups (IEC ¼ 0.92 mEq g-1), SP-10 exhibited a proton
conductivity of 1.1 ꢂ 10ꢁ2 S cmꢁ1 at room temperature. This
performance was better than that of many sulfonated random
polymers, and it achieved a standard of proton conductivity
(>10ꢁ2 S cmꢁ1) suitable for aromatic PEMs. SP-20 (IEC ¼
1.67 mEq g-1) exhibited similar properties to Naon 117,
including proton conductivity, water uptake and swelling ratio.
Remarkably, the methanol permeability of SP-20 was almost
just one-tenth that of Naon 117. All these features can be
attributed to the special structure of SP-x copolymers, leading
to good morphology in the polymer. The combination of good
thermal stability, high proton conductivity, and low methanol
permeability makes SP-x attractive as PEMs for fuel cell
applications.
Morphology
The morphological features of the completely amorphous
ionomer membranes were studied by small-angle X-ray scat-
tering (SAXS) measurements on membranes having the
sulfonated domains selectively stained with Pb2+ to enhance
the contrast.36,37 In general, the extremely hydrophobic poly-
mer backbones and the superacidity and hydrophilicity of the
–CF2–SO3H units in Naon give rise to a distinct phase sepa-
ration and a quite well-dened morphology. The hydrocarbon
backbone is far less hydrophobic and the arylene sulfonic acid
units far less acidic, which leads to a morphology with a less
pronounced phase separation. The morphological differences
between Naon membranes and conventionally sulfonated
hydrocarbon aromatic membranes have previously been
extensively discussed by Kreuer and Jannasch on the basis of
SAXS data.15,37 As seen in Fig. 10, although the SP-20 possessed
a sulfonated poly(arylene ether) feature, the ionomer peak of
the membrane was shied towards a lower q-value (qmax
¼
0.75 nmꢁ1), as compared to the prole of the Naon 117
membrane (qmax ¼ 2.00 nmꢁ1). The values of qmax correspond
to d ¼ 8.4 and 3.14 nm, respectively, for the two ionomers. This
result was attributed to a high local concentration of sulfonic
acid groups, and their separation from the polymer main
chain, which promoted a high degree of phase separation.
Acknowledgements
Meanwhile a weak ionomer peak appeared at around 3.7 nmꢁ1
,
The authors would like to thank the China Natural Science
and this may be attributed to some sulfonic acid groups close Foundation (Grant no. 51103060) for nancial support of
to main chain with decreased mobility during the membrane this work.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. A, 2013, 1, 1465–1474 | 1473

View Article Online
Journal of Materials Chemistry A
Paper
20 X. Li, F. Paoloni, E. A. Weiber, Z. Jiang and P. Jannasch,
Macromolecules, 2012, 45, 1447–1459.
References
1 M. A. Hickner, H. Ghassemi, Y. S. Kim, B. R. Einsla and
J. E. McGrath, Chem. Rev., 2004, 104, 4587–4612.
2 H. Zhang and P. K. Shen, Chem. Rev., 2012, 112, 2780–
2832.
21 D. S. Kim, G. P. Robertson and M. D. Guiver, Macromolecules,
2008, 41, 2126–2134.
22 J. Pang, H. Zhang, X. Li, B. Liu and Z. Jiang, J. Power Sources,
2008, 184, 1–8.
3 A. K. Mishra, S. Bose, T. Kuila, N. H. Kim and J. H. Lee, Prog.
Polym. Sci., 2012, 37, 842–869.
23 J. Pang, H. Zhang, X. Li, L. Wang and Z. Jiang, J. Membr. Sci.,
2008, 318, 271–279.
4 C. H. Park, C. H. Lee, M. D. Guiver and Y. M. Lee, Prog. Polym.
Sci., 2011, 36, 1443–1498.
24 J. Pang, H. Zhang, X. Li, D. Ren and Z. Jiang, Macromol. Rapid
Commun., 2007, 28, 2332–2338.
5 S. Bose, T. Kuila, T. X. H. Nguyen, N. H. Kim, K. Lau and
J. H. Lee, Prog. Polym. Sci., 2011, 36, 813–843.
6 G. Alberti, M. Casciola, L. Massinelli and B. Bauer, J. Membr.
Sci., 2001, 185, 73–81.
7 S. Kaliaguine, S. D. Mikhailenko, K. Wang, P. Xing,
G. P. Robertson and M. D. Guiver, Catal. Today, 2003, 82,
213–222.
25 M. L. Einsla, Y. S. Kim, M. Hawley, H. S. Lee, J. E. McGrath,
B. Liu, M. D. Guiver and B. S. Pivovar, Chem. Mater., 2008, 20,
5636–5642.
26 T. Higashihara, K. Matsumoto and M. Ueda, Polymer, 2009,
50, 5341–5357.
27 K. Matsumoto, T. Higashihara and M. Ueda, Macromolecules,
2009, 42, 1161–1166.
8 J. Kerres, A. Ullrich, F. Meier and T. Haring, Solid State Ionics,
1999, 125, 243–249.
9 G. Ye, N. Janzen and G. R. Goward, Macromolecules, 2006, 39,
3283–3290.
10 X. Guo, J. Fang, T. Watari, K. Tanaka, H. Kita and
K. Okamoto, Macromolecules, 2002, 35, 6707–6713.
11 K. Miyatake, H. Zhou, H. Uchida and M. Watanabe, Chem.
Commun., 2003, 368.
28 S. Matsumura, A. R. Hlil, C. Lepiller, J. Gaudet, D. Guay and
A. S. Hay, Macromolecules, 2008, 41, 277–280.
29 S. Matsumura, A. R. Hlil, C. Lepiller, J. Gaudet, D. Guay,
Z. Shi, S. Holdcro and A. S. Hay, Macromolecules, 2008,
41, 281–284.
30 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron lett.,
1975, 50, 4467–4470.
31 L. F. Fieser, Org. Synth., 1973, 5, 604.
12 K. Miyatake, H. Zhou and M. Watanabe, Macromolecules,
2004, 37, 4956–4960.
32 B. Jung, B. Kim and J. M. Yang, J. Membr. Sci., 2004, 245,
61–69.
`
13 D. J. Jones and J. Roziere, J. Membr. Sci., 2001, 185, 41–58.
33 B. Liu, G. P. Robertson, D. S. Kim, M. D. Guiver, W. Hu and
Z. Jiang, Macromolecules, 2007, 40, 1934–1944.
34 P. Xing, G. P. Robertson, M. D. Guiver, D. S. Mikhailenko and
S. Kaliaguine, Macromolecules, 2004, 37, 7960–7967.
35 N. Li, D. S. Hwang, S. Y. Lee, Y. Liu, Y. M. Lee and
M. D. Guiver, Macromolecules, 2011, 44, 4901–4910.
36 N. Li, C. Wang, S. Y. Lee, C. H. Park, Y. M. Lee and
M. D. Guiver, Angew. Chem., 2011, 123, 9324–9327.
37 B. Latte and P. Jannasch, Adv. Funct. Mater., 2007, 17, 2823–
2834.
¨
14 R. Nolte, K. Ledjeff, M. Bauer and R. Muelhaupt, J. Membr.
Sci., 1993, 83, 211–220.
15 K. D. Kreuer, J. Membr. Sci., 2001, 185, 29–39.
16 K. D. Kreuer, S. J. Paddison, E. Spohr and M. Schuster, Chem.
Rev., 2004, 104, 4637–4678.
17 J. Pang, H. Zhang, X. Li and Z. Jiang, Macromolecules, 2007,
40, 9435–9442.
18 Y. Chen, R. L. Guo, C. H. Lee, M. Lee and J. E. McGrath, Int. J.
Hydrogen Energy, 2011, 37, 6132–6136.
19 O. Celebi, C. H. Lee, Y. Lin, J. E. McGrath and J. S. Riffle,
Polymer, 2011, 52, 4718–4726.
38 E. P. Jutemar and P. Jannasch, J. Membr. Sci., 2010, 1–2,
87–95.
1474 | J. Mater. Chem. A, 2013, 1, 1465–1474
This journal is ª The Royal Society of Chemistry 2013