Polymer electrolyte membranes derived from new sulfone monomers with pendent sulfonic acid groups

Article abstract of DOI:10.1021/ma102107a

New monomers containing two or four pendent phenyl groups were synthesized by bromination of bis(4-fluorophenyl) sulfone, followed by Suzuki coupling with benzeneboronic acid. The resulting monomers were converted to the corresponding sulfonated monomers

Full text of DOI:10.1021/ma102107a

9810 Macromolecules 2010, 43, 9810–9820
DOI: 10.1021/ma102107a
Polymer Electrolyte Membranes Derived from New Sulfone Monomers
with Pendent Sulfonic Acid Groups†
Nanwen Li,‡ Dong Won Shin,§ Doo Sung Hwang,‡ Young Moo Lee,*,‡,§ and
Michael D. Guiver*,‡,^
‡WCU Department of Energy Engineering, College of Engineering, Hanyang University, Seoul 133-791,
Republic of Korea, §School of Chemical Engineering, College of Engineering, Hanyang University,
Seoul 133-791, Republic of Korea, and ^Institute for Chemical Process and Environmental Technology,
National Research Council Canada, Ottawa, Ont. K1A 0R6, Canada
Received September 11, 2010; Revised Manuscript Received October 18, 2010
ABSTRACT: New monomers containing two or four pendent phenyl groups were synthesized by
bromination of bis(4-fluorophenyl) sulfone, followed by Suzuki coupling with benzeneboronic acid. The
resulting monomers were converted to the corresponding sulfonated monomers having two or four pendent
sulfonic acid groups, predominately at the p-phenyl position. Aromatic nucleophilic substitution (SNAr)
polycondensation using the di- and tetrasulfonated monomers provided sulfonated poly(arylene ether sulfone)
copolymers S2-PAES-xx and S4-PAES-xx, respectively, where xx refers to the molar ratio of the sulfonated to
non-sulfonated pendent phenyl monomer. Copoly(arylene ether sulfone)s based on the corresponding non-
sulfonated monomers were also synthesized for a parallel study on postpolymerization sulfonation of these
copolymers. Postsulfonation occurred predominately at the para-pendent phenyl site, and the reactions were
complete within a short time (about 30 min), without evidence of chain degradation. Flexible and tough
membranes having high mechanical strength were obtained by solution casting of all four series of copolymers.
The copolymers with two or four pendent sulfonic acid groups had high proton conductivities in the range of
44-142 mS/cm for S2-PAES-xx and 51-158 mS/cm for S4-PAES-xx at room temperature, respectively. The
methanol permeabilities of these copolymers were in the range of 0.8 ꢀ 10-8-15.0 ꢀ 10-7 cm2/s, which is lower
than Nafion (16.7 ꢀ 10-7 cm2/s). The S4-PAES-xx membranes displayed better properties (lower water uptake
and higher proton conductivities) than the S2-PAES-xx membranes, which can be attributed to the more blocky
architecture of the sulfonic acid groups in the S4 membranes. A combination of high proton conductivities, low
water uptake, and low methanol permeabilities for some of the obtained copolymers indicated that they are good
candidate materials for proton exchange membrane in fuel cell applications.
Introduction
dramatic loss of mechanical properties due to dimensional swelling
that render them unsuitable for practical PEM applications. The
Polymer electrolyte membrane fuel cells (PEMFC) have at-
tracted considerable attention as candidates for alternative power
sources due to their high power density, good energy conversion
efficiency, and zero emissions levels.1-3 Perfluorosulfonic acid
(PFSA) polymers such as Nafion (DuPont) are the most promis-
ing and state-of-the-art polymer electrolyte membranes (PEM)
for PEMFC. However, the material has several shortcomings that
limit its utility and performance, such as reduced proton conduc-
tivity at elevated temperatures (>80 ꢀC), high methanol/gas diffu-
sion, environmental incompatibility (poor recyclability), and sig-
nificant manufacturing costs.4-6 Consequently, much progress has
been made to develop novel hydrocarbon PEMs based on sulfo-
nated aromatic polymers, which have good physical properties and
are inexpensive. The most widely investigated PEMs include sulfo-
nated derivatives of poly(arylene ether sulfone)s (SPAES),7,8 poly-
(arylene ether ether ketone)s (SPEEK),9-11 poly(arylene sulfide
sulfone)s (SPSS),12,13 poly(arylene ether nitrile)s (SPAEEN),14-16
polyimides (SPI),17-21 and polyphenylenes (PP).22-24 Many hy-
drocarbon polymers show sufficiently high conductivities only at
high ion-exchange capacities (IEC)s, which causes extensive water
uptake above a critical temperature (percolation threshold), or a
dimensional stability and proton conductivity of aromatic iono-
mers are crucial issues that require improvement through careful
structural design.25
Proton conductivity of PEMs is closely related to several
parameters such as acidity, number and position of ionic groups,
main chain and/or side chain structures, composition and sequence
of hydrophilic and hydrophobic components, and membrane
morphology.26-28 Among these, acidity of ionic groups and mem-
brane morphology appear to be crucial, and they are inter-related.
Kreuer et al.29 reported that typical sulfonated aromatic polymers
are unable to form defined hydrophilic domains, as the rigid
aromatic backbone prevents the formation of continuous conduct-
ing channels and ionic clustering from occurring. Thus, various
strategies have been pursued to circumvent this and to obtain
efficient ionic networks for enhancing the proton conductivity.
One promising approach to enhance PEM properties and perfor-
mance is to induce distinct phase separation between the hydrophilic
sulfonic acid containing regions and the hydrophobic polymer main
chain by positioning the sulfonic acid groups on side chains grafted
onto the polymer main chain.30 If the polymer structure contains
flexible pendent side chains linking the polymer main chain and the
sulfonic acid groups, nanophase separation between hydrophilic
and hydrophobic domains may be improved.31,32 For example,
Jannasch and co-workers reported PEMs prepared by attaching
† NRCC Publication No. 52238.
*To whom correspondence should be addressed. E-mail: ymlee@
hanyang.ac.kr (Y.M.L.); michael.guiver@nrc-cnrc.gc.ca (M.D.G.).
pubs.acs.org/Macromolecules
Published on Web 11/04/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 23, 2010 9811
flexible pendent sulfonated aromatic side chains to polysulfone,
which showed proton conductivities of 11-32 mS/cm at 120 ꢀC.31
Another approach to induce nanophase separation is through block
copolymer architecture, whereby sulfonic acid groups are concen-
trated in blocks along the polymer chain. McGrath et al.33 prepared
multiblock sulfonated poly(arylene ether)s with a promising mor-
phological structure by a two-step polycondensation. Membranes
having a relatively low IEC of 0.95 mequiv/g had water uptake of
40%, but high proton conductivity of 80 mS/cm. More recently,
random copolymers designed with dense localized concentrations
of sulfonated units have been attracting considerable attention
because of the high contrast in polarity between hydrophilic and
hydrophobic units; this promotes the formation of hydrophilic-
hydrophobic phase-separated structures. It has been found that the
morphological structure of the copolymers was comparable to that
of Nafion 117, which explains their high proton conductivity.34,35
Another approach to enhance PEM performance is with high
IEC values, but to design the highly proton conducting PEM to
have less susceptibility to water swelling or solubility. Here, the
selection of a suitable mechanically stable polymer as a platform is
very important. The current attempts have already led to the deve-
lopment of some highly sulfonated polymers (with a high value of
more than 2.7 mequiv/g) with good water stability, such as sulfo-
nated poly( p-phenylene)s,24 sulfonated poly(phenylene sulfide)s,13
and sulfonated poly[bis(benzimidazobenzisoquinolinones)].21 More
recently, Kreuer et al. developed a new class of sulfonated polymers
with an extremely electron-poor poly(phenylene sulfone) back-
bone.36,37 These ionomers contain only strongly electron-withdraw-
ing units (-SO2-) connecting the phenyl rings. This new class of
polymers shows very high thermal, thermo-oxidative and hydro-
lytic stability, and distinct methanol rejection properties. Mem-
branes with ion exchange capacities ranging from 2.32 to 2.78
mequiv/g remain insoluble in water and have proton conductivities
higher than that of Nafion. Their lower solubility and degree of
swelling in water compared to other sulfonated poly(arylene)s allow
for the preparation of membranes with high ion exchange capacity
and high proton conductivity.
DBDFDPS, 12.1 g (100 mmol) of benzeneboronic acid, and 300 mL of
toluene were charged. The solid was completely dissolved at 50 ꢀC
with stirring. 150 mL of 10 wt % aqueous sodium carbonate
solution and 1.86 g (1.6 mmol) of tetrakis(triphenylphosphine)-
palladium(0) were carefully added into the solution. The reaction
mixture was heated at 110 ꢀC for 20 h, and then the solvent was
evaporated to obtain a solid. The resulting solid was dissolved in
dichloromethane/water mixture, and the suspended catalyst was
removed by filtration. The organic phase was washed with water
until neutral. The DPDFDPS monomer was obtained by evapo-
rating the solvent and purified by recrystallization from toluene
twice. Yield 84%; mp 180-181 ꢀC (DSC in N2). 1H NMR (300
MHz, DMSO-d6; ppm): 7.48-7.62 (m, 12H), 8.12-8.20 (m, 4H).
Bis[4-fluoro-3-(4-sulfophenyl)phenyl] Sulfone, Disodium Salt
(SDPDFDPS). To a 100 mL three-necked flask equipped with a
magnetic stirring device and nitrogen inlet was charged 10 g of
DPDFDPS. The flask was cooled in an ice bath, and then 15 mL
of concentrated sulfuric acid was slowly added with stirring.
After DPDFDPS was completely dissolved, 15 mL of fuming
sulfuric acid (SO3, 30%) was slowly added to the flask. The reaction
mixture was stirred at 0 ꢀC for 0.5 h and then slowly heated to 60 ꢀC
and maintained at this temperature for an additional 2 h. After
cooling to room temperature, the solution was carefully poured into
100 mL of ice-water. NaCl (33 g) was added, which produced a
white precipitate identified as the disodium salt of SDPDFDPS.
The powder was filtered and redissolved in 100 mL of water, and
then the pH was increased to 6-7 by the addition of aqueous 2 N
NaOH. An excess of NaCl was added to salt out the sodium form of
disulfonated monomer. The crude product was recrystallized from
water/ethanol (2:1) to give 12.5 g (yield: 89%) of white product. 1H
NMR (300 MHz, DMSO-d6; ppm): 7.56-7.58 (d, 8H), 7.59 (2H),
8.16-8.19 (m, 4H).
3,30,5,50-Tetrabromo-4,40-difluorodiphenyl Sulfone (TBDFDPS).
Bis(4-fluorophenyl) sulfone (12.7 g, 0.05 mol) was dissolved in
concentrated H2SO4 (250 mL) at room temperature. To this was
added NBS (37.4 g, 0.21 mmol, 4.2 equiv) divided into six portions
over a period of 1 h. The mixture was stirred vigorously at room
temperature for 2 h, and then the temperature was increased to
60 ꢀC and maintained at that temperature for 12 h. The mixture was
poured into crushed ice (∼500 g) to precipitate the solids. The
precipitated solids were filtered, washed with water several times,
followed by 100 mL of n-hexane, and finally dried to obtain
3,30,5,50-tetrabromo-4,40-difluorodiphenyl sulfone (TBDFDPS)
Herein, we report on the synthesis of two novel bis(fluorophenyl)
sulfone monomers containing two or four pendent phenyl or
phenylsulfonic acid groups. These monomers were utilized to
prepare series of sulfonated poly(arylene ether sulfone)s or poly-
(phenylene sulfone)s. Selected PEM properties such as thermal
and chemical stability, mechanical strength, water uptake behavior,
and proton conductivity were investigated in detail.
1
(40.5 g, 92%); mp 180-182 ꢀC. H NMR (300 MHz, CDCl3;
ppm): 8.06-8.08 (d, 4H).
3,30,5,50-Tetraphenyl-4,40-difluorodiphenyl Sulfone (TPDFDPS).
Using similar methodology to the diphenyl analogue, the monomer
with four pendent phenyl groups 3,30,5,50-tetraphenyl-4,40-difluoro-
diphenyl sulfone was prepared from 3,30,5,50-tetrabromo-4,
40-difluorodiphenyl sulfone (TBDFDPS) by reaction with 5 mol
equiv of benzeneboronic acid at 115 ꢀC for 24 h. The purified
product was obtained by recrystallization from toluene twice.
Yield 82%; mp 271-273 ꢀC. 1H NMR (300 MHz, CDCl3; ppm):
7.45-7.55 (m, 12H), 7.46-7.58 (d, 8H), 8.02-8.04 (d, 4H).
Bis[4-fluoro-3,5-bis(4-sulfophenyl)phenyl] Sulfone, Tetrasodium
Salt (STPDFDPS). TPDFDPS was sulfonated using fuming sul-
furic acid as before at 60 ꢀC for 2 h. The crude product was recrys-
tallized from water/ethanol (3:1) to give the STPDFDPS (yield:
85%) of white monomer. 1H NMR (300 MHz, DMSO-d6; ppm):
7.61-7.64 (d, 8H), 7.73-7.76 (d, 8H), 8.26-8.29 (d, 4H).
Experimental Section
Materials. Bis(4-fluorophenyl) sulfone (DFDPS), N-bromo-
succinimide (NBS), benzeneboronic acid, and tetrakis(triphenyl-
phosphine)palladium(0) were purchased from Sigma-Aldrich Ltd.
4,40-(Hexafluoroisopropylidene)diphenol (6F-BPA) was purchased
from Aldrich and recrystallized twice from toluene. All other
solvents and reagents (obtained from Aldrich) were reagent grade
and were used as received.
3,30-Dibromo-4,40-difluorodiphenyl Sulfone (DBDFDPS). Bis-
(4-fluorophenyl) sulfone (25.4 g, 0.1 mol) was dissolved in con-
centrated H2SO4 (150 mL) at room temperature. To this was added
NBS (14.13 g, 0.22 mmol, 2.2 equiv), divided into three portions and
added at 15 min intervals. The mixture was stirred vigorously at
room temperature for a further 6 h and then poured into crushed ice
(∼500 g) to precipitate the solids. The precipitated solids were
filtered, washed with water (600 mL) and then with 100 mL of
n-hexane, and purified by recrystallization from toluene to obtain
3,30-dibromo-4,40-difluorodiphenyl sulfone (DBDFDPS) (39.5 g,
89%); mp 158-160 ꢀC (DSC in N2). 1H NMR (300 MHz, DMSO-
d6; ppm): 7.61-7.66 (t, 2H), 8.10-8.14 (m, 2H), 8.66 (s, 1H),
8.43-8.46 (t, 2 H).
Synthesis of Copoly(arylene ether sulfone) (2-PAES-60). To a
round-bottomed flask equipped with a Dean-Stark trap,
DFDPS (0.2034 g, 0.8 mmol), DPDFDPS (0.4877 g, 1.2 mmol),
6F-BPA (0.6726 g, 2 mmol), and K2CO3 (0.3312 g, 2.4 mmol)
were charged. Then, 1-methyl-2-pyrrolidinone (NMP) (6 mL) and
toluene (4 mL) were added into the flask under nitrogen. The
reaction mixture was stirred at 140 ꢀC for 2 h. After removal of
toluene, the reaction temperature was increased to 165 ꢀC, and the
reaction was continued for 16 h. After cooling to room tempera-
ture, the mixture was poured into methanol. The resulting fiber
3,30-Diphenyl-4,40-difluorodiphenyl Sulfone (DPDFDPS). To
a 500 mL flask equipped with an Ar inlet/outlet, 16.5 g (40 mmol) of

9812 Macromolecules, Vol. 43, No. 23, 2010
Li et al.
was filtered and washed with water and hot methanol. The
polymer was dried in vacuo at 100 ꢀC for 8 h to give 2-PAES-60.
The yield was 0.96 g (95%).
The copolymer 4-PAES-20 was prepared by a similar meth-
odology using 3,30,5,50-tetraphenyl-4,40-difluorodiphenyl sulfone
(TPDFDPS), DFDPS, and 6F-BPA at 165 ꢀC for 20 h.
placed in a thermo-controlled chamber in liquid water for mea-
surement. Conductivity measurements under fully hydrated con-
ditions were carried out with the cell immersed in liquid water.
The methanol permeability was determined by using a cell
consisting of two half-cells separated by the membrane, which
was fixed between two rubber rings. Methanol (2 M) was placed
on one side of the diffusion cell, and water was placed on the
other side. Magnetic stirrers were used on each compartment to
ensure uniformity. The concentration of the methanol was
measured by using a Shimadzu GC-1020A series gas chromato-
graph. Peak areas were converted into methanol concentration
with a calibration curve. The methanol permeability was calcu-
lated by the following equation:
Synthesis of Sulfonated Poly(arylene ether sulfone). A series of
sulfonated poly(arylene ether sulfone)s were synthesized with dif-
ferent molecular structures and sulfonic acid content (SC). As an
example, the synthesis of S4-PAES-20 is as follows: 0.6726 g
(2 mmol) of 6F-BPA, 0.4068 g (1.6 mmol) of DFDPS, 0.3516 g
(0.4 mmol) of STPDFDPS, and 0.3312 g (2.4 mmol) of potassium
carbonate were charged to a three-necked 100 mL flask equipped
with a condenser, a Dean-Stark trap, a nitrogen inlet, and a
mechanical stirrer. Then, distilled DMSO (5 mL) and toluene
(3 mL) were added to the flask, and the reaction mixture was
heated at 145 ꢀC with stirring. The solution was allowed to reflux at
145 ꢀC while the toluene azeotropically removed the water in the
system. After 4 h, the toluene was removed from the reaction by
slowly increasing the temperature to 168 ꢀC. The reaction was
allowed to proceed for another 20 h. The resulting viscous solution
was cooled to room temperature and poured into isopropanol
(IPA). The resulting fiber was filtered and washed with IPA and
water. The copolymer was dried at 120 ꢀC in vacuo for at least 24 h.
Postpolymerization Sulfonation of Polymer. To a round-bot-
tomed flask equipped with a dropping funnel, 1 g of 2-PAES-60
or 4-PAES-20 was charged. Then, dry dichloromethane (20 mL)
was added into the flask, and the mixture was cooled to 5-10 ꢀC.
To the mixture was added dropwise a solution of chlorosulfonic
acid (0.6 mL, 3 mmol) in dry dichloromethane (20 mL) at 5 ꢀC,
and the mixture was stirred at this temperature for 30 min. After
the reaction, the mixture was poured into hexane. The resulting
polymer was washed with water. The polymer was dried in
vacuo at 100 ꢀC for 10 h to give PS2-PAES-60 or PS4-PAES-20.
Polymer Electrolyte Membrane Preparation. Since the copol-
ymers in the acid form were more amenable to casting films than
those in the salt form, they were first acidified. Copolymers
(powder) in the sodium sulfonate form were treated with 2.0 N
sulfuric acid at room temperature for 2 days for proton exchange.
The proton-exchanged copolymers were thoroughly washed with
deionized water and then dried in vacuum at 120 ꢀC for 24 h. The
acidified copolymers could be readily dissolved as 5-8 wt % solu-
tions in DMSO at 60 ꢀC. The solutions at 60 ꢀC were filtered and
cast onto glass plates and left to dry at 80 ꢀC for 20 h. Tough, ductile
ionomer membranes were obtained with a controlled thickness in
the range of 30-50 μm. The PEMs were dried in a vacuum oven at
120 ꢀC for 24 h.
A DK
CBðtÞ ¼
CAðt - t0Þ
ð1Þ
VB
L
where CA and CB are the methanol concentration of feed side
and permeated through the membrane, respectively. A, L, and
VB are the effective area, the thickness of membrane, and the
volume of permeated compartment, respectively. DK is defined
as the methanol permeability. t0 is the time lag.
Characterization Methods. The density of membrane was
measured from a known membrane dimension and weight after
drying at 100 ꢀC for 24 h. Water uptake was measured after drying
the membrane in acid form at 100 ꢀC under vacuum overnight. The
dried membrane was immersed in water at 20 ꢀC and periodically
weighed on an analytical balance until a constant water uptake
weight was obtained. From this, the volume-based water uptake
(WU) was obtained. A volume-based IEC (IECv) was obtained by
multiplying the membrane density by the IECw values, which were
estimated from the copolymer structure. This calculation resulted in
IECv (dry) based on the dry membrane density. An IECv (wet) was
then calculated based on membrane water uptake.38
From the conductivity and density data, proton diffusion
coefficient (Dσ) was calculated using the Nernst-Einstein equa-
tion where R is gas constant, T is the absolute temperature (K), F
is the Faraday constant, and c(Hþ) is the concentration of
proton charge carrier (mol/L).
RT
F2
σ
Dσ
¼
ð2Þ
cðHþÞ
Results and Discussion
Synthesis and Characterization of the Monomers. Two new
types of difluorodiphenyl sulfone monomers having two or four
pendent phenyl groups were prepared as shown in Scheme 1.
They could be polymerized either in the non-sulfonated form as
3,30-diphenyl-4,40-difluorodiphenyl sulfone (DPDFDPS) and
3,30,5,50-tetraphenyl-4,40-difluorodiphenyl sulfone (TPDFDPS)
or in the sulfonated form. The monomers were synthesized by
bromination of 4,40-difluorodiphenyl sulfone using NBS. The
reactions were conducted in concentrated sulfuric acid as solvent
and catalyst and at different temperatures, giving dibrominated
and tetrabrominated compounds DBDFDPS and TBDFDPS,
respectively.
Measurements. 1H NMR spectra were measured on a 300 MHz
Bruker AV 300 spectrometer using DMSO-d6 or CDCl3 as solvent.
The inherent viscosities were determined from 0.5 g dL-1 solutions
of polymer in NMP with an Ubbelohde capillary viscometer at
30.0 ( 0.1 ꢀC. The thermogravimetric analyses (TGA) were ob-
tained in nitrogen with a Perkin-Elmer TGA-2 thermogravimetric
analyzer at a heating rate of 10 ꢀC/min. The glass-transition tem-
perature (Tg) was determined on a Seiko 220 DSC instrument at a
heating rate of 20 ꢀC/min under nitrogen protection. Tg is reported
as the temperature at the middle of the thermal transition from the
second heating scan.
The proton conductivity (σ, S/cm) of each membrane film
(size: 1 cm ꢀ 4 cm) was obtained using σ = d/LsWsR (d: distance
between reference electrodes, and Ls and Ws are the thickness and
width of the membrane, respectively). The resistance value (R) was
measured over the frequency range from 100 mHz to 100 kHz by
four-point probe alternating current (ac) impedance spectroscopy
using an electrode system connected with an impedance/gain-
phase analyzer (Solartron 1260) and an electrochemical interface
(Solartron 1287, Farnborough, Hampshire, UK). 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. The cell was
The reactions proceeded cleanly and in high yield, and the
degree of bromination was readily controlled by adjusting
the temperature and the amount of NBS. Following this, Suzuki
coupling reactions of DBDFDPS or TBDFDPS with benzene-
boronic acid produced 3,30-diphenyl-4,40-difluorodiphenyl sul-
fone (DPDFDPS) or 3,30,5,50-tetraphenyl-4,40-difluorodiphenyl
sulfone (TPDFDPS), respectively, both in high yields (>84%).
The structures were confirmed by 1H NMR analysis, as shown
in Figure 1. Subsequently, the DPDFDPS and TPDFDPS were
readily sulfonated with fuming sulfuric acid at 60 ꢀC for 2 h
to give sulfonated monomers SDPDFDPS and STPDFDPS,

Article
Macromolecules, Vol. 43, No. 23, 2010 9813
Scheme 1. Synthetic Route To Prepare SDPDFDS and STPDFDPS Monomers
respectively. Under these conditions, the sulfonate substitution
reaction occurred predominately at the para-position to the
pendent phenyl groups because electronic and steric hindrance
to the fluorine atom because of the bulky ortho-substituted
pendent phenyl groups.
Instead, sulfonated poly(arylene ether sulfone)s containing
two or four pendent sulfonic acid groups (S2-PAES-xx or S4-
PAES-xx, where xx is the molar percentage of sulfonated
monomers in the total amount of difluorodiphenyl sulfone
monomers) were synthesized successfully by SNAr polycon-
densation in DMSO. Copolymers having different molar per-
centages of pendent group were obtained by adjusting the feed
ratios of SDPDFDPS or STPDFDPS to DFDPS (Scheme 3).
Characterization results are listed in Table 1. Two non-
sulfonated copoly(arylene ether sulfone)s (2-PAES-60 and
4-PAES-20) were also synthesized in NMP for a comparative
study of alternative route by postpolymerization sulfonation
(Scheme 4).
All the polymerization reactions by both routes proceeded
smoothly, with no evident cross-linking, even after prolonged
polymerization times at temperatures in the range of 160-
200 ꢀC. The sulfonated copolymers in the acid form were readily
soluble in polar aprotic solvents such as DMF, DMAc, NMP,
and DMSO. It should be pointed out that the sulfonated
copolymers in the salt form showed poor solubility in common
organic solvents, resulting in opaque solutions and membranes.
However, tough, flexible, and transparent membranes of copol-
ymers were obtained by solvent casting the sulfonated copoly-
mer in the sulfonic acid form.
1
effects render this position more reactive than other sites. H
NMR characterization confirmed the structures of SDPDFDPS
and STPDFDPS, as shown in Figure 1. The signals at about 7.54
and 7.76 ppm were assigned to the pendent phenyl group of
STBDFDPS, while those at 8.26 ppm were assigned to the
protons in the phenyl sulfone ring.
Synthesis of Sulfonated and Non-sulfonated Poly(phenyl
sulfone)s and Copoly(arylene ether sulfone)s. The original
intention for the preparation of these monomers was for
the preparation of poly(phenyl sulfones) having pendent
phenylsulfonic acid units. Kreuer and co-workers reported
the synthesis of sulfonated poly(phenyl sulfone)s having high
proton conductivity, chemical and oxidative stability, and
low water uptake,36,37 as shown in Scheme 2. Although the
poly(phenyl sulfone) PEMs exhibit some attractive proper-
ties such as high conductivity and oxidative stability, the
structural repeat unit leads to IEC values that are excessively
high, resulting in films that have poor mechanical properties.
Using the same polymerization methodology, it was in-
tended to use non-sulfonated pendent-phenyl sulfone mono-
mers to produce pendent-phenyl poly(phenyl sulfone), which
could be subsequently sulfonated and which would have a
lower IEC value and potentially a higher molecular weight
(Scheme 2). In spite of numerous polymerization attempts,
the precursor poly(sulfide sulfone) had low viscosity and low
molecular weight and thus could not be fabricated into
flexible and tough membranes. The inability to obtain high
molecular weight was attributed to steric hindrance effects,
whereby the larger thiolate nucleophile has hindered access
Postpolymerization Sulfonation of Copoly(arylene ether
sulfone)s. Electrophilic sulfonation using reagents such as
chlorosulfonic acid preferentially introduces sulfonic acid
groups on electron-rich sites of aromatic rings. However, if
vigorous sulfonation conditions are employed, such as high
reaction temperatures and extended reaction times, polymer

9814 Macromolecules, Vol. 43, No. 23, 2010
Li et al.
chain degradation may occur, resulting in a loss of mechan-
ical strength. Therefore, it is very important to carefully
control the reaction conditions for introducing sulfonic acid
group onto phenyl rings to avoid adverse side reactions. Our
earlier work has shown that pendent phenyl rings attached
to electron-donating polymer chain sites can be preferen-
tially sulfonated at the para position, without obvious chain
degradation occurring during sulfonation.39-43 In the pre-
sent study, the copolymers 2-PAES-60 and 4-PAES-20,
having pendent phenyl groups on the electron-withdrawing
site of the polymer chain were sulfonated on the para-phenyl
position with chlorosulfonic acid to produce PS2-PAES-60
and PS4-PAES-20, respectively (“postsulfonation” is distin-
guished by PS). On the basis of experimental IECw values, a
5-fold molar excess of chlorosulfonic acid was necessary for
complete sulfonation. The reaction proceeded smoothly in
dichloromethane solution at 5-10 ꢀC, and most of the pro-
duct precipitated out of the mixture within 10 min. However,
the reaction was continued for an additional 20 min to ensure
completion. The sulfonated copolymers were isolated as
white powders and were soluble in DMSO, NMP, DMF,
and DMAc. There was no evidence of chain degradation
occurring under these conditions, as indicated by viscosity
measurements and the mechanical properties of the resulting
sulfonated polymer membranes. Figure 2 shows the 1H
NMR spectra of PS2-PAES-60 and PS4-PAES-20 in the
sulfonic acid forms. In comparison with the spectra of the
parent copolymers 2-PAES-60 and 4-PAES-20, new signals
H-5 assigned to the protons adjacent to the sulfonic acid
group appeared at about 7.68 and 7.56 ppm, respectively.
The comparative signal integration of H-5 indicated com-
plete sulfonation (100% degree of sulfonation) on each
pendent phenyl ring. In the 1H NMR spectrum of polymers
with four pendent phenyl groups, H-7 and H-70, H-8 and
H-80 are not magnetically equivalent. This is because the
steric hindrance of the bulky phenyl groups prevents the
benzene ring of the neighboring phenoxy moiety from
rotating freely. Moreover, H-70 appeared farthest upfield at
6.61 ppm (Figure 2b) due to the ring current effect caused by
the neighboring benzene ring. The spectra of the sulfonated
Figure 1. 1H NMR spectra of non-sulfonated and sulfonated mono-
mers in DMSO-d6 (TPDFDPS in CDCl3).
Scheme 2. Synthetic Route to Sulfonated Poly(phenylene sulfone) Reported by Kreuer et al.36,37 and the Originally Intended Sulfonated Poly(phenylene
sulfone) with Pendent Phenylsulfonic Acid Groups of the Present Work

Article
Macromolecules, Vol. 43, No. 23, 2010 9815
Scheme 3. Synthesis of S2-PAES-xx and S4-PAES-xx by Copolymerization with Sulfonated Monomers (SDPDFDPS and TDPDFDPS)
Table 1. Characterization of Copolymers
IEC
tensile strength
(MPa)
elongation
at break (%)
methanol permeability
(10-6 cm2/S)
copolymer
η
inh (dL/g)a
calc
NMR
Tg
Td (5%)c
S2-PAES-40
2-PAES-60
0.62
0.48b
0.65
0.58
0.69
0.71
0.46b
0.56
0.57
0.62
0.58
0.59
1.19
0
1.16
0
240
174
241
246
258
274
176
239
239
260
272
286
349
446
350
342
335
332
447
351
352
349
334
332
60.7
73.2
57.8
58.7
54.9
51.2
55.2
49.6
50.2
45.9
46.7
42.2
12.5
10.6
11.2
9.8
11.8
9.5
12.5
10.2
14.5
12.3
9.6
0.10
PS2-PAES-60
S2-PAES-60
S2-PAES-70
S2-PAES-80
4-PAES-20
PS4-PAES-20
S4-PAES-20
S4-PAES-30
S4-PAES-35
S4-PAES-40
1.64
1.64
1.84
2.02
0
1.18
1.18
1.63
1.82
2.00
1.59
1.62
1.86
1.98
0
1.21
1.16
1.59
1.78
2.01
0.28
0.62
1.50
0.08
0.22
0.51
0.98
8.8
a 0.5 g dL-1 in DMSO at 30 ꢀC. b 0.5 g dL-1 in NMP at 30 ꢀC. c 5% weight loss temperature in N2 gas (acid form membrane).
polymers, whether derived from sulfonated monomers or by
postsulfonation, were essentially equivalent.
S2-PAES-xx and S4-PAES-xx were above 250 ꢀC (Table 1) but
lower than the decomposition temperature. The high thermal
stability presents the possibility of preparing membrane elec-
trode assemblies (MEA) by hot pressing. The Tgs of sulfonated
copolymers are much higher than those of non-sulfonated ones,
since chain rigidity is increased through hydrogen bond inter-
actions of sulfonic acid groups. The Tgs of S2-PAES-xx or S4-
PAES-xx increase with increasing proportions of sulfonated
monomers in the copolymers. At the same sulfonic acid content,
S4-PAES-xx polymers with four pendent phenyl groups had
higher Tg values than those of corresponding S2-PAES-xx
analogues because the presence of two bulky phenyl substituents
around the ether linkage further hinders polymer chain rotation
and thus increases chain rigidity. Supporting evidence for
restricted chain rotation is shown by the nonequivalence of
ortho-ether protons in the 1H NMR spectrum (Figure 2b).
Good mechanical properties of the membranes are one of
the necessary requirements for their effective use in DMFC
or PEMFC applications. Films in the dry state had tensile
stress at maximum load of 42.2-73.2 MPa and elongation at
Thermal and Mechanical Properties. The TGA curves of
non-sulfonated (2-PAES-60 and 4-PAES-20) and sulfonated
copolymers membranes are shown in Figure 3. The 5%
weight loss temperatures of 2-PAES-60 and 4-PAES-20
membranes are above 430 ꢀC and followed similar degrada-
tion profiles. A two-step degradation profile was observed
for all sulfonated copolymers in their acid form (Figure 3).
There was no weight loss up to 200 ꢀC because all the samples
were preheated at 150 ꢀC for 20 min to remove absorbed
water. The first weight loss occurred above 300 ꢀC, which is
associated with the degradation of the sulfonic acid groups,
and this initial weight loss increased with increasing DS. The
high decomposition temperature suggests that sulfonic
groups attached to pendent phenyls have high thermal
stability. The main weight loss at around 500-600 ꢀC is
related to the degradation of the polymer chain.
The Tgs of the non-sulfonated copolymers 2-PAES-60 and
4-PAES-20 were around 170 ꢀC, and the sulfonated copolymers

9816 Macromolecules, Vol. 43, No. 23, 2010
Li et al.
Scheme 4. Synthesis of 2-PAES-60 and 4-PAES-20 Copolymers and Corresponding PS2-PAES-60 and PS4-PAES-20 by Postpolymerization
Sulfonation
break of 8.8-14.5%, as shown in Table 1. Compared with
the data of Nafion, having tensile stress of 32 MPa and
elongation at break of 301.5% in the dry state, the SPAES
materials showed higher tensile strength and the lower
elongation than Nafion. The mechanical properties indicate
that the copolymer films were strong and flexible, suitable
for DMFC or PEMFC.
Water Uptake. Water uptake (weight and volume based) of
PEMs is an important parameter for IEC, proton conductivity,
dimensional stability, mechanical strength, and membrane-
electrode compatibility of the membrane. For a more realistic
comparison of the water uptake among the different mem-
branes, volumetric IEC (IECv, mequiv/cm3), which is defined
as molar concentration of sulfonic acid groups per unit volume
containing absorbed water, was calculated. Table 2 compares
the density, IEC, and water uptake (weight and volume based) of
the copolymer membranes and Nafion. The water uptake of
SPAES increased with IECw and IECv (dry), due to the increased
hydrophilicity, as shown in Figure 4. The highest water uptake
(wt %) was 62.6% and 50.8% at 20 ꢀC for the highest IECw
membranes S2-PAES-80 (2.02 mequiv/g) and S4-PAES-40 (2.0
mequiv/g), respectively, which was 3 times higher than that of
Nafion membrane. Moreover, the S4-PAES-xx membranes
showed a slightly lower water uptake trend than the S2-PAES-
xx membranes of the same SC. This effect can be attributed to
the higher local concentrations of hydrophilic sulfonic acid
groups of S4-PAES-xx membranes because of the blocky nature
of these units.
The IECv (wet) reflects the concentration of ions within the
polymer matrix under hydrated conditions, without distin-
guishing between those protons that are mostly associated with
the sulfonic acid groups and those that are fully dissociated.
The IECv (wet) value itself is affected by changes in acid and
water uptake and whether ion concentration remains constant

Article
Macromolecules, Vol. 43, No. 23, 2010 9817
equilibration with water, while hydration of S2-PAES-70 and
S2-PAES-80 resulted in excessive swelling and dilution of the
ion concentration. This is observed in Figure 5, whereby the
slope of the S2-PAES-xx curves at 20 ꢀC reverses direction due
to high water uptake (vol %) and reduced IECv (wet). At 80 ꢀC,
both series of membranes showed a reverse direction due to ion
dilution caused by swelling of membranes.
The IECv is plotted as a function of temperature in Figure 6.
Similar to Nafion, the IECv values decrease with increasing
temperature due to increased water volume within the polymer
matrix. The S2-PAES-40 membrane showed approximately the
same trend in IECv values as Nafion over the temperature
range, since the differences in gravimetric IEC were counter-
balanced by the differences in density (1.98 g/cm3 for Nafion
and 1.39 g/cm3 for S2-PAES-40). The other membranes dis-
played higher IECv than the Nafion membranes when the
temperature was lower than 60 ꢀC. The IECv (wet) of SPAES
membranes with high IECw (>1.8 mequiv/g) decreased sharply
at elevated temperatures, to values lower than Nafion, which
indicated that these membranes swelled excessively in water.
The S2-PAES-xx and S4-PAES-xx membranes shared a similar
dependency of the IECv upon temperature, but the latter ones
exhibited slightly higher IECv values, which is the result of their
relatively lower water uptake.
Proton Conductivity. The higher IECw membranes have
higher proton conductivity at all temperatures investigated, as
shown in Figure 7. Compared to Nafion membranes with
similar IECw values, SPAES membranes showed predictably
lower proton conductivities, in common with most aromatic
ionomers, which derives from a combination of lower proton
acidity of arylsulfonic acid and less effective phase separation.44
Similar to Figure 5, proton conductivities of copolymers de-
crease when expressed in terms of IECv, reflecting the phenom-
enon of ion dilution in the swelled polymer matrix (Figure 7).
As shown in Table 2, with increasing IECw values for S4-
PAES-xx from 1.18 to 2.0 mequiv/g, proton conductivities
increased from 51 to 158 mS/cm at 20 ꢀC and from 106 to 311
mS/cm at 80 ꢀC. These values are slightly higher than the
corresponding copolymer S2-PAES-xx with the same SC
values, which indicates that the more blocky sulfonic acid-
containing segments in the polymer chain are more effective
in proton conduction. Therefore, the S4-PAES-xx mem-
branes exhibit higher relative proton conductivity and lower
relative water uptake (vol %) than the S2-PAES-xx mem-
branes, as shown in Figure 8.
Figure 2. Comparative 1H NMR spectra of non-sulfonated and sulfo-
nated copolymers in DMSO-d6.
Figure 9 displays proton conductivities as a function of
temperature. The proton conductivities of the SPAES mem-
branes increased with increasing temperature. The highest
proton conductivity of 360 mS/cm was obtained for S4-
PAES-40 with IECw of 2.0 mequiv/g at 100 ꢀC, which is
much higher than that of Nafion 112 (184 mS/cm) measured
under the same conditions.
To further elucidate the proton-conducting properties of
the SPAES membranes, the proton diffusion coefficients
(Dσ) through the membranes were estimated from the proton
Figure 3. TGA curves for the non-sulfonated and sulfonated PAES
powders from measurements run at 10 ꢀC/min in N2.
conductivity and the IECv in hydrated membranes.45,46
A
or whether it varies. The IECv (wet) of S4-PAES-xx, measured
at 20 ꢀC, increased from 1.43 to 1.69 mequiv/cm3, correspond-
ing to IECw values increasing from 1.18 to 2.0 mequiv/g. The
IECv (wet) of S2-PAES-xx also increased from 1.32 to 1.59
mequiv/cm3 as IECw increased from 1.18 to 2.0 mequiv/g.
However, for highly sulfonated copolymers, the IECv (wet)
values of S2-PAES-70 and S2-PAES-80 were lower than those
of the less sulfonated S2-PAES-40 and S2-PAES-60 mem-
branes. For S2-PAES-40, S2-PAES-60, and all the S4-PAES-
xx membranes measured at 20 ꢀC, the increased sulfonic acid
group concentration of the dry polymer was retained after
general trend of the dependency of Dσ on the temperature
was obtained, as shown in Figure 10. At lower temperatures,
SPAES membranes showed lower or similar Dσ values
compared to Nafion, in spite of the higher IECv of SPAES.
These results are consistent with the fact that the SPAES
membranes having higher IECw showed lower proton con-
ductivity. However, membranes with lower the IECv values
that resulted from excessive matrix swelling and high water
uptake have much higher Dσ values at higher temperatures
compared with Nafion. The high Dσ could be attributed to
increased water content at higher temperatures, leading to

9818 Macromolecules, Vol. 43, No. 23, 2010
Li et al.
Table 2. Properties of the S2-PAES-xx and S4-PAES-xx Series and Nafion Membranes
water uptake
wt %b vol %c
20 ꢀC 80 ꢀC 20 ꢀC 80 ꢀC
IECva (mequiv/cm3)
proton conductivity (mS/cm)
copolymer
density (g/cm3) IECW (mequiv/g)
dry
wet
20 ꢀC
80 ꢀC
S2-PAES-40
S2-PAES-60
PS2-PAES-60
S2-PAES-70
S2-PAES-80
PS4-PAES-20
S4-PAES-20
S4-PAES-30
S4-PAES-35
S4-PAES-40
Nafion 112
1.39
1.41
1.40
1.43
1.45
1.39
1.40
1.42
1.43
1.45
1.98
1.19
1.64
1.60
1.84
2.02
1.20
1.18
1.63
1.82
2.00
0.90
1.65
2.31
2.25
2.63
2.93
1.67
1.65
2.31
2.60
2.90
1.78
1.32
1.59
1.58
1.57
1.54
1.43
1.43
1.65
1.67
1.69
1.29
18.2
32.2
30.2
47.3
62.6
12.1
11.4
28.4
39.1
50.8
19.0
24.5
43.1
39.5
76.8
134.2
14.9
14.0
37.1
64.8
115.2
28.6
25.3
45.1
42.3
67.6
90.8
16.8
15.1
40.3
55.9
73.8
37.6
34.0
60.8
55.3
109.8
194.6
20.7
19.4
52.7
44
75
78
95
142
49
51
83
82
153
157
195
298
108
106
164
209
311
174
92.7
167.0
56.6
112
158
90
a Based on volume of dry and/or wet membranes (IECv(wet)=IECv(dry)/(1 þ 0.01WU)). b WU (wt %)=(Wwet - Wdry)/Wdry ꢀ 100. c WU
(vol %) = ((Wwet - Wdry)/δw)/(Wdry/δm) ꢀ 100 (Wwet and Wdry are the weights of the wet and dry membranes, respectively; δw is the density of water
(l g/cm3), and δm is the membrane density in the dry state).
Figure 5. Water uptake dependence of volumetric IEC values (IECv
(wet)) of copolymer membranes in the wet state.
Figure 4. Water uptake dependence of weight IEC (IECw) and volu-
metric IEC (IECv) values of copolymer membranes.
increased dissociation of the protons from the SO3- groups.
In addition, the Dσ values obtained for S4-PAES-xx were
Figure 6. Volumetric IECv of SPAES and Nafion 112 in water as a
similar to those of the S2-PAES-xx membranes in spite of the
former’s higher IECv. The results are not contradictory to
the above-mentioned conductivity data. It is assumed that
the blocky and pendent sulfonic acid groups change the size
and shape of the hydrophilic ionic domains through which
proton transport occurs, which results in the high Dσ values
of S4-PAES-xx membranes. This result reveals that the more
blocky S4-PAES-xx membranes do indeed provide a less
encumbered proton conducting pathway, from which it can
function of temperature.
be inferred that the ionic domains are very dense and well
distributed.
Methanol Permeability and Selectivity. Polymer electrolyte
membranes intended for DMFC must both possess high proton
conductivity and be an effective barrier for methanol crossover
from the anode to the cathode compartment. The methanol
permeability through the polymer electrolyte membranes often

Article
Macromolecules, Vol. 43, No. 23, 2010 9819
Figure 9. Proton conductivity of SPAES membranes in water as a
function of temperature.
Figure 7. IECw (a) and IECv (b) dependence of proton conductivity of
the SPAES membranes.
Figure 10. Proton diffusion coefficient of SPAES and Nafion 112
membranes as a function of volumetric IECv in the water.
Figure 8. Relative proton conductivity vs relative volume water uptake.
follows a similar relationship to proton conductivity. That is, the
proton conductivity has a strong trade-off in its relationship
with the methanol permeability. Nafion not only has good
proton conductivity due to strongly interconnected ionic do-
mains structure but also has high methanol permeability. The
S2-PAES and S4-PAES copolymers exhibited comparatively
lower methanol permeability to Nafion. The methanol perme-
ability values for 10% methanol concentration at room tem-
perature were in the range of 0.08 ꢀ 10-6-1.5 ꢀ 10-6 cm2/s,
which is lower than the value of 1.67 ꢀ 10-6 cm2/s for Nafion.
Although S2-PAES-40 and S4-PAES-20 have lower ex situ
proton conductivities than Nafion 112, it is sufficient to achieve
Figure 11. Performance trade-off plot of the IECw values vs the
selectivity.
improved DMFC performance through its low methanol
permeability (i.e., better selectivity) (Figure 11). Selectivity,
which is the ratio of proton conductivity to methanol perme-
ability, is often used to evaluate the potential performance
of DMFC membranes. It is a useful predictive parameter of
performance, providing the proton conductivity is sufficiently
high. The selectivity is plotted as a function of IECw in Figure 11.
The selectivity of SPAES membranes decreases rapidly as the
IECw values increase. The S4-PAES-xx membranes displayed

9820 Macromolecules, Vol. 43, No. 23, 2010
Li et al.
higher selectivity than the S2-PAES-xx membranes due to the
higher proton conductivity and lower methanol permeability.
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Novel difluorodiphenyl sulfone monomers containing two or
four pendent phenyl groups have been successfully synthesized in
high yields by sequential bromination and Suzuki coupling. The
non-sulfonated monomer could either be polymerized and the
resulting polymer sulfonated or the monomer couldbesulfonated
and then polymerized. 1H NMR spectroscopy confirmed that the
pendent phenyl groups in either the monomers or the polymers
could be completely sulfonated predominately at the para-phenyl
site in a short time. The polymer electrolyte membranes showed
excellent thermal stability and mechanical properties. The nature
of the hydrophilic segments in the polymer electrolyte mem-
branes significantly affected the proton transport and other
properties. The S4-PAES-xx membranes, comprising a more
blockyhigher local concentration of sulfonic acid groups, showed
lower water uptake and low methanol permeability but higher
proton conductivities than their S2-PAES-xx counterparts hav-
ing the same sulfonic acid content. This infers that a local and
high density of sulfonic acid groups in the pendent phenyl is
important for optimum percolation. For example, S2-PAES-60
and S4-PAES-30 with similar IECw values exhibited proton
conductivity of 190 and 210 mS/cm, respectively, at 100 ꢀC,
higher than that of Nafion 117. The highest conductivity, i.e., 360
mS/cm, was obtained for S4-PAES-40 at 100 ꢀC. The methanol
permeability values of S2-PAES-60 and S4-PAES-30 were 2.8 ꢀ
10-7 and 2.2 ꢀ 10-7 cm2/s, respectively, which is several times
lower than Nafion. The combination of high thermal stability,
good relative proton conductivity, and low methanol transport
makes S2-PAES-60 and S4-PAES-30 attractive as PEM materials
for fuel cells applications.
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