Enhanced thermo-oxidative stability of sulfophenylated poly(ether sulfone)s

Article abstract of DOI:10.1016/j.polymer.2009.12.014

Monophenylated poly(ether sulfone)s (Ph-PES) and diphenylated poly(ether sulfone)s (DiPh-PES), were synthesized as starting materials for the preparation of sulfonated polymers with well-defined chemical structure. Mild post-polymerization sulfonation conditions led to sulfonated Ph-PES (Ph-SPES) bearing acid groups on both the pendant phenyl group and the backbone, and sulfonated DiPh-PES (DiPh-SPES) bearing acid groups only on the two pendant phenyl groups. Both series of polymers had excellent mechanical properties, high glass transition temperatures, good thermal and oxidative stability, as well as good dimensional stability. It is interesting to note that exclusively pendant-phenyl-sulfonated (bis-sulfophenylated) DiPh-SPES copolymers possessed obviously better thermal and oxidative stability compared with the corresponding pendant-phenyl-sulfonated/main-chain-sulfonated Ph-SPES copolymers. The methanol permeability values of the membranes were in the range of 7.0?×?10^-7-9.4?×?10^-8?cm²/s at 30?°C, which is several times lower than that of Nafion 117. DiPh-SPES-50 and Ph-SPES-40 also exhibited high proton conductivity (approximately 0.13?S/cm at 100?°C).

Full text of DOI:10.1016/j.polymer.2009.12.014

Polymer 51 (2010) 403–413
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Enhanced thermo-oxidative stability of sulfophenylated poly(ether sulfone)s
,
b
b
c
Baijun Liu a , Gilles P. Robertson , Dae-Sik Kim , Xiaohong Sun ,
*
Zhenhua Jiang a, Michael D. Guiver b
a Alan G. MacDiarmid Institute, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China
b Institute for Chemical Process and Environmental Technology, National Research Council, Ottawa, Ontario K1A 0R6, Canada
c Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Monophenylated poly(ether sulfone)s (Ph-PES) and diphenylated poly(ether sulfone)s (DiPh-PES), were
synthesized as starting materials for the preparation of sulfonated polymers with well-defined chemical
structure. Mild post-polymerization sulfonation conditions led to sulfonated Ph-PES (Ph-SPES) bearing acid
groups on both the pendant phenyl group and the backbone, and sulfonated DiPh-PES (DiPh-SPES) bearing
acid groups only on the two pendant phenyl groups. Both series of polymers had excellent mechanical
properties, high glass transition temperatures, good thermal and oxidative stability, as well as good
dimensional stability. It is interesting to note that exclusively pendant-phenyl-sulfonated (bis-
sulfophenylated) DiPh-SPES copolymers possessed obviously better thermal and oxidative stability
compared with the corresponding pendant-phenyl-sulfonated/main-chain-sulfonated Ph-SPES copoly-
mers. The methanol permeability values of the membranes were in the range of 7.0 ꢀ 10ꢁ7–9.4 ꢀ 10ꢁ8 cm2/s
at 30 ꢂC, which is several times lower than that of Nafion 117. DiPh-SPES-50 and Ph-SPES-40 also exhibited
high proton conductivity (approximately 0.13 S/cm at 100 ꢂC).
Received 18 March 2009
Received in revised form
8 November 2009
Accepted 11 December 2009
Available online 21 December 2009
Keywords:
Poly(ether sulfone)s
Proton exchange membranes
Sulfonation
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
made, there are still challenges to be met, such as complete property
evaluation of PEMs, optimization and simplification of sulfonation
Fuel cells are electrochemical devices for transforming
chemical energy directly into electricity with high efficiency, and
are one of the promising clean future power sources. Extensive
efforts are being made to develop proton exchange membranes
(PEM)s for use in proton exchange membrane fuel cell (PEMFC)
and direct methanol fuel cell (DMFC) systems[1,2]. One challenge
is to find high-performance PEMs alternative to NafionÒ, which is
a commercial perfluorinated sulfonic acid copolymer used as the
polymeric electrolyte in PEMFC systems. Sulfonated hydrocarbon-
based PEMs are being widely investigated in an attempt to over-
come the difficulties derived from high cost, limited operation
temperature (ꢃ80 ꢂC), high methanol crossover, and environ-
mental recycling uncertainties of the present commercial per-
fluorinated membranes [3,4].
reactions, controllability of DS (degree of sulfonation) and the site of
sulfonation, and performance enhancement and microstructure
improvement of PEMs by designing polymer chemical structures
[12,13].
Most of polyarylether-type PEMs are based on post-sulfonation
of existing polymers or on copolymers produced from sulfonated
monomers [3]. The post-polymerization sulfonation approach is
well known as a relatively simple reaction procedure and one that
does not necessarily result in chain degradation when sulfonation
conditions are appropriate. However, difficulties may exist in the
precise control of the site of sulfonation and the DS [14]. On the
other hand, PEM materials prepared by copolymerization of
sulfonated and non-sulfonated monomers provides random or
block copolymers with the potential for better control of sulfonic
acid content (SC) and defined polymer structures [15]. However,
a limited number of sulfonated monomers is available and the
preparation of new sulfonated monomers is not trivial. The present
work addresses these issues by using a strategy of simple and mild
post-sulfonation on precursor polymers having sites especially
amenable to sulfonation; hence, the site of sulfonation and DS is
controlled in a similar way to polymers prepared by the direct
copolymerization method.
The outstanding thermal and chemical stability of poly(arylene
ether)s, and their sulfonated derivatives such as polyether sulfone
(SPES) [5,6], polyetherketone (SPEK) [7,8], polyarylether (SPAE) [9,10]
and polyethernitrile (SPEN) [11] are the basis for investigation as
potential PEMs. Although a great variety of structures has been
* Corresponding author. Tel.: þ86 431 8516 8022; fax: þ86 431 8516 8868.
E-mail address: liubj@jlu.edu.cn (B. Liu).
0032-3861/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2009.12.014

404
B. Liu et al. / Polymer 51 (2010) 403–413
Miyatake and Hay et al. recently reported several sulfonated
target pendant-phenyl-sulfonated (bis-sulfophenylated) polymers
were successfully prepared, and the properties relevant to PEM
materials for FC applications were also evaluated.
polymers obtained by regioselective sulfonation with ClSO3H. [16]
It was suggested that polymers with the sulfonic groups attached
to pendant side groups are more stable to heat, hydrolysis and
oxidation. Jannasch et al. reported several SPES polymers obtained
by a grafting method based on the lithiation reaction [17].
However, there are still no incontrovertible data showing that
pendant-phenyl-sulfonated polymers possess better thermal and
oxidative stability than their main-chain-sulfonated analogues. In
addition, few pendant-phenyl-sulfonated PES polymers were
synthesized by the post-sulfonation approach under mild reaction
conditions [18].
Most recently, we reported that poly(aryl ether ketone)s bearing
one pendant phenyl could be readily sulfonated with H2SO4 within
several hours to form 4-phenylsulfonic acid substituted polymers.
The pendant phenyl ring is highly activated towards sulfonation at
the 4-position, and substitution occurs rapidly. However, it was
noted that the main-chain benzene ring could be sulfonated much
more slowly at the site para- to the pendant substituent when the
reaction time was prolonged, to yield pendant-phenyl-sulfonated/
main-chain-sulfonated polymers. [7] In order to prepare exclu-
sively pendant-phenyl-sulfonated polymers having well-defined
chemical structure and good performance, new polymeric precur-
sors having an improved design were developed, as shown in
Scheme 1. By blocking the weakly active site for sulfonation on the
main chain and replacing it with an additional pendant phenyl
substituent having a strongly active site for sulfonation, it is
possible to prepare well-defined fully disulfonated polymers within
a short reaction time.
2. Experimental
2.1. Chemicals and materials
4,40-Sulfonyldiphenol (98%) and phenylhydroquinone (97%)
were obtained from Sigma–Aldrich Ltd., and recrystallized from
acetone/water and toluene, respectively. Sulfuric acid (reagent
grade, 95–98%) was obtained from Sigma–Aldrich Ltd. 4-Fluo-
rophenylsulfone (99%, Sigma–Aldrich Ltd.), 2,5-diphenylquinone
(96%, Alfa Aesar) and all the other chemicals were obtained from
commercial sources, and used without further purification.
2.2. Synthesis of 2,5-diphenylhydroquinone
Zinc powder (0.4 mol), 2,5-diphenylquinone (0.01 mol) and
water (200 mL) were placed into a 1 L three-necked flask equipped
with a magnetic stirrer, a dropping funnel and a condenser.
Hydrochloric acid (34 mL) was added dropwise gradually into the
stirred mixture at reflux over a 6 h period. The reaction system was
allowed to reflux for another 4 h at reflux. The zinc powder
was removed by filtration when hot. The oil layer in the filtrate was
washed with cold water several times until a solid appeared and
then dried in a vacuum oven. Yellowish crystals were obtained after
recrystallisation from the toluene. Yield: 85%, m.p. 226 ꢂC (DSC).
Elem. Anal. Calcd. for C18H14O2 (262.30 g/mol): C, 82.42%; H, 5.38%.
Found: C, 82.31%; H, 5.42%.
In the present study, PES copolymers consisting of two types of
segments for the control of DS and ion exchange capacity (IEC)
were synthesized: one part of the segments containing pendant
phenyl that are readily sulfonated (Scheme 1), and the other part of
the segments that do not contain activated sites. With the aim of
preparing PEM materials with well-defined structure by site-
specific mild post-polymerization sulfonation, phenylated and
diphenylated SPES polymers were investigated and compared. The
1H NMR (DMSO-d6)
H5; 7.41, t (8.4 Hz), 4H, H6; 7.29, t (8.4 Hz), 2H, H7; 6.87, s, 2H, H2.
13C NMR (DMSO-d6)
in ppm: 146.95 (C1), 138.33 (C3), 128.84
(C6), 127.91 (C5), 127.31 (C4), 126.47 (C7), 117.39 (C2).
d inppm: 8.95, s, 2H, –OH; 7.57, d (8.4 Hz), 4H,
d
a
strongly active site for sulfonation
SO3H
SO3H
SO3H
SO3H
O
C
H2SO4
O
O
PAEK
R.T.
backbone
n
weakly active site for sulfonation
SO3H
SO3H
SO3H
SO3H
strongly active site for sulfonation
b
SO3H
SO3H
O
S
O
H2SO4
R.T.
PAES
O
O
backbone
n
SO3H
SO3H
SO3H
SO3H
strongly active site for sulfonation
Scheme 1. Representations of sulfonation reactions based on: a) Mono-phenylated PAEKs in previous study (ref. 7); b) Diphenylated PES in this study.

B. Liu et al. / Polymer 51 (2010) 403–413
405
4H. 13C NMR (DMSO-d6)
d
in ppm: 163.04 and 161.93, m; 148.56,
2.3. Synthesis of starting polymers
s; 148.31, s; 146.60, s; 140.57, s; 137.00, s; 136.80, s; 136.14–
134.99, m; 130.71 and 130.26, s; 129.56, s; 126.44, s; 125.79, s;
122.37, s; 118.60, s.
As an example of the general synthetic procedure for polymeri-
zation, the synthesis of the copolymer DiPh-PES-50 is as follows. 2,5-
Diphenylhydroquinone (2.623 g, 0.01 mol), 4,40-sulfonyldiphenol
(2.502 g, 0.01 mol), 4-fluorophenylsulfone (5.085 g, 0.02 mol),
anhydrous K2CO3 (1.794 g, 0.013 mol), DMAc (40 mL) and toluene
(20 mL) were added into a 100 mL three-necked flask equipped with
a magnetic stirrer, a nitrogen inlet, and a Dean–Stark trap with
a condenser. The system was allowed to reflux at about 150 ꢂC for
4 h, and then the toluene was removed. The reaction mixture was
heated to 180 ꢂC. After 12 h, the polymerization was complete and
then the resulting viscous polymer solution was precipitated into
250 mL of ethanol. The isolated polymer was refluxed in deionized
water and ethanol several times to remove the salts and solvents,
and dried at 120 ꢂC for 24 h. The white DiPh-PES-50 copolymer fiber
was obtained. The suffix 50 refers to the molar percentage of
diphenylated polymer repeat units.
2.5. Characterization and instruments
1H, 13C and 2D NMR spectra were obtained on a Varian Unity
Inova NMR spectrometer operating at frequencies of 399.95 MHz
for 1H and 100.575 MHz for 13C. Deuterated dimethylsulfoxide
(DMSO-d6) and deuterated chloroform (CDCl3) were selected as the
solvents.
Thermal stability of the polymers was evaluated using a TA
Instruments thermogravimetric analyzer (TGA) instrument model
2950. Polymer samples for TGA analysis were preheated at 150 ꢂC
for 30 min to remove moisture, and then heated at 10 ꢂC/min from
50 ꢂC to 800 ꢂC under air atmosphere. The obtained TGA curves
were adjusted such that the weight values of onset points were
chosen as 100%. A TA Instruments differential scanning calorimeter
(DSC) model 2920 was used for measuring glass transition
temperature (Tg). Samples for DSC analysis were initially heated at
a rate of 20 ꢂC/min to a temperature of 10 ꢂC below their decom-
position temperature, followed by quenching them to room
temperature. The samples were then heated at a rate of 10 ꢂC/min
under nitrogen atmosphere to evaluate Tg.
The proton conductivities of the membranes were calculated
from AC impedance spectroscopy data, obtained over a frequency
range of 1–107 Hz with oscillating voltage of 100 mV, using a Solar-
tron 1260 gain phase analyzer. Specimens in the form of 20 ꢀ 10 mm
strips were soaked in deionized water at room temperature for 48 h
prior to the test. Specimens were clamped in a frame between two
platinum electrodes, and then placed in a temperature controlled
cell open to the air by a pinhole, which was equilibrated at 100% RH
at ambient pressure. Measurements were carried out in four-point
All the other starting DiPh-PES and Ph-PES polymers were
prepared using the same technique.
DiPh-PES-50: 1H NMR (CDCl3)
d in ppm: 7.96–7.68, m, 12H;
7.45–7.15, m, 12H; 7.12–6.90, m, 12H.
DiPh-PES-100: insoluble in common deuterated solvents.
Ph-PES-50: 1H NMR (CDCl3)
6.79, m, 20H.
d
in ppm: 7.95–7.62, m, 12H; 7.39–
Ph-PES-100: 1H NMR (CDCl3)
d in ppm: 7.93–7.65, m, 4H; 7.35,
d (8 Hz), 2H; 7.27–7.16, m, 3H; 7.13, d (3 Hz), 1H; 7.09–7.02, m,
3H; 6.99, dd (8 Hz, 3 Hz), 1H; 6.91–6.80, m, 2H. 13C NMR (CDCl3)
d
in ppm: 161.83–161.54, m; 152.32, s; 148.33, s; 136.55, s; 135.96,
s; 136.06 and 135.17, m; 129.95–129.60, m; 128.77, s; 128.37, s;
128.03, s; 123.20, s; 122.90, s; 120.49, s; 117.83 and 117.04, s.
2.4. Preparation of sulfonated polymers and their membranes
mode. The conductivity (s) of the samples in the longitudinal
DiPh-PEK-50 (5 g) and concentrated sulfuric acid (100 mL) were
added into a 250 mL flask. After stirring at room temperature for 1
week, the resulting homogeneous viscous solution was poured into
a mixture of water and ice to get a silk-like solid. The solid was
thoroughly washed with water until the washing water had
a neutral pH. The sulfonated DiPh-PES-50 (DiPh-SPES-50) was
dried in a vacuum oven at 100 ꢂC for 24 h.
An amount of 0.5 g of the above dried polymer, DiPh-SPES-50,
was dissolved in 12 mL of DMAc and the solution was filtered
through a filter paper. The filtered solution was poured onto a glass
plate and dried in an oven at 50 ꢂC under a constant slow purge of
nitrogen for one week. The resulting flexible membrane was dried
in a vacuum oven at 120 ꢂC for 24 h. The thickness of all membrane
direction was calculated, using the relationship
s
¼ L/(R ꢀ d ꢀ W)
where L is the distance between the electrodes, d and W are the
thickness and width of the sample stripe respectively. R was derived
from the low intersect of the high frequency semi-circle on
a complex impedance plane with the Re (Z) axis.
Mechanical properties of the thin films were evaluated at room
temperature on an Instron 5565 instrument at a strain rate of
10 mm/min, and a 500 N load cell was used. The samples were
immersed in water for 48 h, and cut into a dumbbell shape
(DIN-53504-S3A).
2.6. Inherent viscosities, water uptake and swelling ratio
measurements
films was in the range of 80–120 mm.
All the other sulfonated polymers were prepared using the same
procedure, and 1H NMR spectroscopy was used to confirm their
structure.
Inherent viscosities (hinh) were measured using an Ubbelohde
viscometer at a polymer concentration of 0.5 g/dL in DMAc solu-
tions at 30 ꢂC.
DiPh-SPES-50: 1H NMR (DMSO-d6)
d in ppm: 8.05–7.70, m, 12H;
7.62–7.35, m, 10H; 7.35–7.08, m, 12H.
6
7
DiPh-SPES-100: 1H NMR (DMSO-d6)
d
in ppm: 7.75, d (8 Hz),
5
4H; 7.57, d (8 Hz), 4H; 7.46, d (8 Hz), 4H; 7.35, s, 2H; 7.11,
d (8 Hz), 4H. 13C NMR (DMSO-d6)
d
in ppm: 161.58, s; 148.45, s;
4
3
2
Zn, HCl
reflux
1
146.75, s; 136.27, s; 134.97, s; 134.62, s; 130.03, s; 128.73, s;
125.88, s; 124.58, s; 117.45, s.
O
O
HO
OH
Ph-SPES-50: 1H NMR (DMSO-d6)
d in ppm: 8.05–7.80, m, 12H;
7.62–7.36, m, 5H; 7.36–7.02 m, 13H.
Ph-SPES-100: 1H NMR (DMSO-d6)
d
in ppm: 7.88–7.75, m, 4H;
7.56, d (8 Hz), 2H; 7.49–7.37, m, 3H; 7.21, m, 1H; 7.13–7.02, m,
Scheme 2. Synthesis of 3,5-diphenylhydroquinone.

406
B. Liu et al. / Polymer 51 (2010) 403–413
O
S
O
S
HO
OH
HO
OH
F
F
O
O
K2CO3, DMAc
160-190 °C
O
S
O
O
S
O
O
S
O
O
O
O
O
O
O
O
O
x
x
1-x
1-x
95-98% H2SO4
25 °C
SO3H
O
S
O
O
S
O
O
S
O
HO3S
x = 1.0, 0.6, 0.5, 0.4, 0.3
Scheme 3. Synthesis of poly(ether sulfone)s bearing acid groups on both the main-chain and pendant-phenyl substituent.
Before testing the water uptake and swelling ratio of the
Where udry and uwet are the weights of dried and wet samples
respectively.
samples, the membranes were dried at 100 ꢂC overnight prior to
the measurements. After measuring the lengths and weights of dry
membranes, the sample films were soaked in deionized water for
24 h at predetermined temperatures. Before measuring the lengths
and weights of hydrated membranes, the water was removed from
the membrane surface by blotting with a paper towel.
The swelling ratio was calculated by:
lwet ꢁ l
Swelling ratioð%Þ ¼
dry ꢀ 100%:
ldry
The water uptake content was calculated by:
uwet
ꢁ
udry
Where ldry and lwet are the lengths of dry and wet samples
respectively.
Water uptake ðwt:%Þ ¼
ꢀ 100%:
udry
O
S
O
S
HO
OH
HO
OH
F
F
O
O
K2CO3, DMAc
160-190 °C
O
S
O
O
S
O
O
S
O
O
O
O
O
O
O
O
x
1-x
1-x
95-98% H 2SO4
25 °C
SO3H
O
S
O
S
O
S
O
O
O
O
x
HO3S
x = 1.0, 0.6, 0.5, 0.4
Scheme 4. Synthesis of poly(ether sulfone)s bearing acid groups on pendant-phenyl substituents.

B. Liu et al. / Polymer 51 (2010) 403–413
407
2.7. Oxidative stability, ion exchange capacity (IEC), and methanol
permeability
0.2% v/v (0.022 M) 1-butanol in aqueous solution. Compartment B
was filled with 100 mL of 0.2% v/v 1-butanol solution. The diffusion
cell was placed in a water bath held at 30 ꢂC and each compartment
was stirred by a separate stir plate to ensure uniform stirring.
Oxidative stability of the membranes was tested by immersing
the films into Fenton’s reagent (3% H2O2 containing 2 ppm FeSO4) at
80 ꢂC. The dissolution time (t) of polymer membranes into the
reagent was used to evaluate oxidative resistance. IEC of the
sulfonated polymers was measured using a typical titration method.
The films in acid form were equilibrated with excess 1 M NaCl for
24 h. The amount of the Hþ released from the membranes was
determined by titration of 0.1 M NaOH aqueous solution and
phenolphthalein as an indicator. The IEC was calculated according to
the equation: [IEC (meq gꢁ1) ¼ (VNaOH ꢀ CNaOH)/Ws], where Ws is the
dry weight (mg) of the sample and VNaOH and CNaOH are the volume
(mL) and molar concentration of NaOH solution, respectively.
Methanol permeability was measured using a simple two-
compartment glass diffusion cell. A membrane (2 cm ꢀ 2 cm) was
placed between two silicone rubber gaskets and with the two
compartments clamped together around the gaskets. The active area
of the membrane was 1.757 cm2. Compartment A was filled with
100 mL of 10% v/v (2.47 M) methanol with an internal standard of
Samples (4 mL each) were removed from compartment B at intervals
of approximately 15 min each. Methanol concentrations were
determined by 1H NMR spectroscopy. The method to calculate the
methanol permeability is reported in a previous study [19].
3. Results and discussion
Sulfonation is an electrophilic reaction, and the reaction rate and
site of substitution are influenced by the position and type of
substituents on the benzene rings. Although the sulfonation
conditions for polymers are often more vigorous than those used for
simple low-MW compounds, several series of polymers have been
be sulfonated under mild sulfonation conditions [20,21]. The mono-
phenylated polymers [7] wherein sulfonated sites occur both on the
side chain and main chain may be less desirable in terms of PEM
properties such as chemical stabilityand ability to go some degree of
phase separation between hydrophilic and hydrophobic regions. In
14
13
H-3,3'
H-12
11
5
12
O
9 10
O
SO2
H-13,14
H-9
8
4
1
n
7
6
3
2
2' 3'
H-6
H-2,2'
H-7
SO3H
13
14
11 12
9 10
H-3,3'
O
O
5
SO2
8
4
1
n
3
2
2' 3'
7
6
H-12
H-6
H3OS
H-9
H-2,2'
H-13
SO3H
13
12
9
O
O
SO2
O
SO2
O
SO2
1-x
6
x
H3OS
H-9 and all meta -SO2-
(6.5H)
all ortho -SO2-
(6H)
H-6,12,13
(2.5H)
8.0
7.5
7.0
Chemical Shift (ppm)
Fig. 1. From top to bottom, 1H NMR spectra of Ph-PES-100 (CDCl3), Ph-SPES-100 (DMSO-d6) and Ph-SPES-50 (x ¼ 0.5) (DMSO-d6).

408
B. Liu et al. / Polymer 51 (2010) 403–413
order to provide exclusively pendant-phenyl-sulfonated poly-
(arylene ether)s via post-polymerization sulfonation of phenylated
polymers, a series of diphenylated aromatic poly(ether sulfone)s
was prepared. The starting poly(ether sulfone) copolymers were
based on a diphenylated bisphenol monomer (Scheme 2), composed
of sulfonatable segments (activated by diphenylated moieties) and
non-sulfonatable segments (deactivated by the sulfone linkage). For
comparative purposes, mono-phenylated poly(ether sulfone)
analogues were prepared to study the sulfonation reaction and
properties of the resulting copolymers.
In order to obtain PEM materials with well-defined properties
by the post-polymerization sulfonation approach, it is necessary to
prepare polymers with specific sites that are strongly activated to
sulfonation and preferably ones that require only mild sulfonation
conditions to circumvent any chain degradation. In the present
study, it is notable that the sulfonation reaction rate was faster than
that of Victrex PEEK. Under the same reaction conditions, about one
month was required to achieve fully sulfonated PEEK (DSw1) [22].
Both the mono-phenylated and diphenylated PAES polymers
showed good controllability of reaction sites. For the mono-phe-
nylated SPAES series, one acid substituent was found at the
4-position of the pendant-phenyl groups, and another acid
substituent occurred on the main-chain. NMR spectra and titration
tests for IEC showed DS values of 2.0 under these reaction condi-
tions. Different from the mono-phenylated PAES polymers, which
led to pendant-phenyl-sulfonated/main-chain-sulfonated poly-
mers, the sulfonation of diphenylated polymers led exclusively to
pendant-phenyl-sulfonated polymers (Schemes
confirmed by NMR.
3 and 4), as
Figs. 1 and 2 show the aromatic region for the 1H NMR spectra of
unsulfonated and sulfonated polymers. The assignment of every
proton frequency was carried out with the help of various 1D and
2D homo-nuclear (1H–1H) and hetero-nuclear (1H–13C) NMR
experiments. The information obtained from the peak intensities
was also very helpful in the characterization of the more complex
copolymer spectra. It was first necessary to understand where the
sulfonation had occurred on the polymer backbone. The determi-
nation of the sulfonation sites was done using the information
supplied by the peak integration values from the 1H NMR and by
the 2D pulse sequences. NMR of the sulfonated homopolymers
confirmed what is normally expected from this electrophilic
aromatic substitution reaction: the sulfonation occurred at the para
SO3H
11
10
8
5
9
O
O
SO2
1
4
H-6
n
7
6
2
3
H-10
H-9
H-3
H-2
H3OS
11
10
9
O
O
SO2
O
SO2
O
SO2
1-x
x
6
all ortho -SO2-
(6H)
all meta -SO2-
(6H)
H-6,9,10,11
(6H)
SO3H
11
10
9
O
O
SO2
O
SO2
O
SO2
1-x
x
6
H3OS
all ortho -SO2- (6H)
H-6,9,10 (5H)
all meta -SO2- (6H)
8.0
7.5
7.0
Chemical Shift (ppm)
Fig. 2. From top to bottom, 1H NMR spectra of Di-Ph-SPES-100 (DMSO-d6), Di-Ph-PES-50 (CDCl3) and Di-Ph-SPES-50 (x ¼ 0.5) (DMSO-d6).

B. Liu et al. / Polymer 51 (2010) 403–413
409
position of the pendant phenyl group for both Ph-SPES and DiPh-
SPES polymers and it also occurred at the ortho-oxygen sites of the
Ph-PES polymers as illustrated in Fig. 1. A 2D ROESY spectrum was
required to unambiguously distinguish H-6 from H-9 in the
Ph-SPES-100 polymer. H-9 showed coupling through space with
H-12 while H-6 showed coupling with H-3. The lack of symmetry
and random distribution on position 9 or 10 of the pendant phenyl
groups in the repeat units of the Ph-(S)PES polymers explains the
presence of multiple peaks in both 1H and 13C NMR. For example,
H-2,20 and H-3,30 of Ph-PES-100 and Ph-SPES-100 appear at
different frequencies depending on whether they are spatially
located near the phenyl group (phenyl at the 10 position as illus-
trated in Fig. 1) or away from them (phenyl at the 9 position). This is
particularly apparent in the Ph-PES-100 spectrum of Fig. 1. Strong
hydrogen bonding between the –SO3H of Ph-SPES is possibly also
causing H-6 and H-9 to appear as multiple peaks due to different
conformation of the polymer in DMSO-d6. Those peaks appear to be
spin-coupled doublets, although in fact they are non-coupled
multiplets. On the other hand, the symmetry and the distance from
the polymer main chain of the –SO3H groups in DiPh-SPES-100
resulted in a series of single peaks for every proton as illustrated by
the simple 1H NMR pattern in Fig. 2. In addition to the peak
14
13
12
13
11 12
9 10
O
O
5
SO2
8
4
1
3
2
n
3
2
7
6
14
11
10
6
9
7
8
5
4
1
SO3H
13
12
13
3
14
11 12
9 10
7 6
O
O
5
SO2
8
4
1
2
n
3
2
H3OS
14
11
7
10
4
5
9
8
6
1
9
10
SO3H
10
11
8
5
9
O
O
SO2
3
4
3
1
2
n
7 6
2
4
11
5
8
7
1
6
H3OS
160
155
150
145
140
135
130
125
120 115
Chemical Shift (ppm)
Fig. 3. From top to bottom, 13C NMR spectra of Ph-PES-100 (CDCl3), Ph-SPES-100 (DMSO-d6) and DiPh-SPES-100 (DMSO-d6).

410
B. Liu et al. / Polymer 51 (2010) 403–413
assignments, the peak intensities also confirmed the di-substitu-
tion of the Ph-SPES and DiPh-SPES homopolymers. Fig.1 also shows
the copolymer Ph-SPES-50 and the integration values for selected
areas. Knowing which proton signal is present in each area makes
the verification of the copolymer ratios a simple task. For example,
the spectrum of Ph-SPES-50 in Fig. 1 shows an experimental proton
ratio of 6H:2.5H:6.5H which is expected from the structure dis-
played above the spectrum where x ¼ 0.5. Similarly, Fig. 2 shows
the spectra of DiPh-SPES-50 before and after sulfonation. Once
again, the integration values of selected areas helped in assessing
the copolymer ratio (x:1–x) and the DS on the pendant phenyl
groups. Finally, Fig. 3 shows the 13C NMR spectra of unsulfonated
Ph-PES-100 and sulfonated Ph-SPES-100 and DiPh-SPES-100; DiPh-
PES-100 is not shown due to its poor solubility properties in the
NMR solvents. As seen before in 1H NMR, symmetry in the DiPh-
SPES-100 results in fewer peaks compared with the Ph-(S)PES
polymers.
100
80
60
40
20
0
Ph-PES-100
Ph-SPES-30
Ph-SPES-40
Ph-SPES-50
Nafion 117
0
200
400
600
800
Ion exchange capacity (IEC) is another important method to
evaluate the degree of sulfonation reaction, which is highly rele-
vant with the exchangeable ions of polymer membranes. The IEC
data from titration test were in the range of 1.91–1.13 meq/g, which
were close to the expected values. The DS was readily controlled by
adjusting the monomer feed ratios in the copolymers. This
synthetic strategy has a considerable advantage over the normal
method of controlling DS by reaction time and temperature [23].
Glass transition temperature (Tg) is an important parameter for
applications of amorphous polymers. High Tgs are usually required
for PEM materials to operate at higher temperature [24]. For all the
samples, only one transition temperature was observed in the DSC
curve before the decomposition temperature, which indicated their
amorphous nature. As listed in Table 1, the non-sulfonated starting
polymers had high Tg values ranging from 190 ꢂC to 228 ꢂC. For both
phenylated and diphenylated series, Tg values increased with an
increase of the sulfone linkage contents. This could be attributed to
enhanced rigidity and polarity of the sulfone linkages. As expected,
at the same level of sulfone linkages, DiPh-PES polymers always
with more rigid and bulkier pendant groups had higher Tg values
than those of corresponding Ph-PES analogues. In general,
sulfonated polymers have higher Tg values than their non-
sulfonated analogues, because the polarity of the sulfonic acid
groups may enhance molecular chain interaction. The Tg values of
Ph-SPES and DiPh-SPES were as high as 263–285 ꢂC. For each of two
series, the sulfonated polymers with higher –SO3H content showed
higher Tg values.
Temperature (°C)
100
80
60
40
20
0
DiPh-PES-100
DiPh-SPES-40
DiPh-SPES-50
DiPh-SPES-60
Nafion 117
0
200
400
600
800
Temperature (°C)
Fig. 4. TGA curves of the polymers in air.
weigh loss temperatures in air were above 505 ꢂC. After sulfonation,
the thermal decomposition temperatures decreased to a range of
334–354 ꢂC, because of the lower decomposition temperature
associated with sulfonic acid groups. It is very interesting to note that
the DiPh-SPES series had better thermal stability than the Ph-SPES
series. An obvious difference of 14–17 ꢂC between DiPh-SPES and
Ph-SPES copolymers is observed in Fig. 4 and Table 1. To our
knowledge, this may be the first direct evidence that pendant-
phenyl-sulfonated polymers have better thermal stability than the
As shown by the TGA curves, all the phenylated starting polymers
possessed excellent thermal stability due to their fully aromatic
structure. No weight loss below 400 ꢂC was observed and the 5%
Table 1
Viscosity and thermal properties of the polymers.
Polymer
h
(dL/g)a
Tg (ꢂC)b
non-S
TD (ꢂC)c
non-S
S
S
non-S
S
Table 2
Ph-PES-100
Ph-PES-60
Ph-PES-50
Ph-PES-40
Ph-PES-30
DiPh-PES-100
DiPh-PES-60
DiPh-PES-50
DiPh-PES-40
–
–
190
209
211
220
225
217
219
225
228
–
529
516
516
517
515
524
522
521
526
–
Mechanical properties of the sulfonated polymers in the wet state.
0.52
0.50
0.49
0.61
–
0.45
0.51
0.48
1.6
1.5
1.5
1.8
–
1.4
1.6
1.4
284
280
278
263
–
285
280
269
336
334
344
340
–
351
351
354
Membrane
Tensile strength
(MPa)
Young’s
modulus
(GPa)
Elongation
at break (%)
Oxidative
stability (h)
Ph-SPES-50
Ph-SPES-40
Ph-SPES-30
DiPh-SPES-60
DiPh-SPES-50
DiPh-SPES-40
Nafion 117
24.5
39.8
38.9
21.6
35.6
40.9
24.5
0.37
0.72
0.54
0.36
0.40
1.00
0.16
54
116
52
139
83
0.6
2.5
>5.0
1.2
4.5
>5.0
>5.0
a
Inherent viscosity of non-sulfonated and sulfonated polymers.
Glass transition temperature. S refers to the sulfonated polymer, and non-S
b
60
316
referred to the corresponding starting material.
c
Onset temperature of decomposition.

B. Liu et al. / Polymer 51 (2010) 403–413
411
membranes and Nafion 117 were tested in the wet state and are
listed in Table 2. In comparison with Nafion 117, most of present
aromatic PEM membranes possessed higher tensile strength in the
range of 21.6–40.9 MPa and Young’s moduli of 0.36–1.00 GPa. It was
obvious that humidified Ph-SPES-60 and DiPh-SPES-60 membranes
having high –SO3H content exhibited lowest tensile strength and
moduli. Elongations at break were up to 52–139%, which suggested
they were flexible materials. Several typical tensile curves are
shown in Fig. 5, and the tensile strength results showed they were
strong membrane materials. By comparison, the unsulfonated
polymers had tensile stress in the range of 70–98 MPa and Young’s
moduli of 1.8–2.2 GPa, with elongations at break of 16–35%.
Water absorption, which is highly related to proton conductivity
and dimensional stability of the membranes, is one of the typical
characteristics of sulfonated polymers. To maintain high proton
conductivity, a sufficient amount of water associated within the
membrane is indispensable. However, the lower acidity of aromatic
sulfonic acid compared with the fluoroalkyl sulfonic acid groups in
Nafion leads to a higher DS requirement in order to obtain highly
conductive hydrocarbon-based PEM materials. Depending on
polymer design, an unfavorable trade-off effect of high DS or
sulfonic acid content may be excessive water uptake, leading to
unacceptable dimensional change or loss of dimensional integrity
[3,10]. The preparation of highly proton conductive alternative
hydrocarbon-based PEMs with high DS or sulfonic acid content, but
having small dimensional change is one of the important topics of
research for this class of materials. The water absorption and
dimensional change in water at 30 ꢂC and 80 ꢂC were measured, and
the results are listed in Table 3. For each of the series, both water
uptake and swelling ratio increased with an increase of sulfonic acid
ionic contents at given temperature. At room temperature, all the
membranes exhibited very good water absorption and swelling
ratio (swelling ratio ꢃ20%) characteristics. At 80 ꢂC, Ph-SPES-50 and
DiPh-SPES-60 membranes, both having high IEC values, did not
retain dimensional shape. However, DiPh-SPES-50, DiPh-SPES-40
and Ph-SPES-30 exhibited acceptable dimensional change (swelling
ratio ꢃ30%), even at elevated temperature.
60
40
20
0
Ph-SPES-40
DiPh-SPES-40
DiPh-SPES-50
0
20
40
60
80
100
120
Strain (%)
Fig. 5. Stress vs. strain curves of the membranes in the wet state.
corresponding main-chain-sulfonated (or a mixture of main-chain-
sulfonated/pendant-phenyl-sulfonated) analogues. It was also
observed these sulfonated polymers showed improved thermal
stability over Nafion for the sulfonic acid degradation under the
same testing conditions.
One of the key requirements for PEMs is good oxidative-hydro-
lytic stability under fuel cell operating conditions [3]. Oxidative
attack by HOꢄ and HOOꢄ radicals mainly occurs in the hydrophilic
domains, resulting in degradation of the polymer chains. Pendant-
phenyl-sulfonated polymers, wherein sulfonic acid groups are
attached to pendant groups distant from the polymer chain, are
expected to have improved oxidative resistance over main-chain-
sulfonated ones. The results evaluated in Fenton’s reagent at 80 ꢂC
are shown in Table 2. It is generally the case that polymers having
lower DS always exhibit better oxidative stability by this test [21]. It
was interesting to note that the diphenylated series exhibited
improved oxidative resistance over the mono-phenylated series,
when compared at similar DS. For example, DiPh-SPES-50
(IEC w 1.70 meq/g) maintained film integrity after a 3 h treatment in
Fenton’s reagent, whereas Ph-SPES-40 (IEC w 1.60 meq/g) had
already dissolved into Fenton’s reagent after 2.5 h treatment.
Especially, DiPh-SPES-40 (IEC w 1.50 meq/g) maintained film
integrity after 4 h treatment, which suggests excellent oxidative
resistance.
The proton conductivities of the membranes were estimated
from AC impedance spectroscopy data and the results at different
temperatures are presented in Fig. 6. Generally, the conductivities of
all the samples increase with temperature in the range of evaluated
temperatures. Besides this, for both DiPh-SPES and Ph-SPES series,
the membrane with higher IEC value had higher proton conduc-
tivity. All the membranes possessed room temperature conductiv-
ities higher than 0.01 S/cm, which is conventionally regarded as
a minimum value requirement for practical PEM application in FC.
DiPh-SPES-60 exhibited higher proton conductivities than that of
Nafion 117 throughout the temperature range of 30 ꢂC–70 ꢂC,
Good mechanical properties are another important factor for
PEM applications. The tensile properties of the present copolymer
Table 3
Water uptake and proton conductivity of the membranes.
Polymer
IEC (meq/g)a
Calc.
WU (%)b
30 ꢂC
SR (%)c
30 ꢂC
s
(S/cm)d
MP (cm2/s)e
30 ꢂC
Exp.
80 ꢂC
80 ꢂC
30 ꢂC
80 ꢂC
Ph-PES-50 (w82
Ph-PES-40 (w86
Ph-PES-30 (w92
DiPh-PES-60 (w83
DiPh-PES-50 (w87
DiPh-PES-40 (w95
Nafion 117 (w180
m
m
m
m)
m)
m)
1.95
1.59
1.21
2.11
1.81
1.50
0.90
1.72
1.52
1.13
1.91
1.63
1.35
0.90
50.6
32.8
12.5
55.4
41.5
14.8
22
–
19.2
10.6
3.8
20.0
13.8
8.7
–
59
29
10
83
46
18
57
0.141
0.086
0.036
0.147
0.098
0.057
0.112
5.1 ꢀ 10ꢁ7
2.8 ꢀ 10ꢁ7
9.4 ꢀ 10ꢁ8
7.0 ꢀ 10ꢁ7
4.6 ꢀ 10ꢁ7
1.4 ꢀ 10ꢁ7
1.5 ꢀ 10ꢁ6
400
22.0
–
85.0
26.6
30
55
4.6
–
30.0
11.5
20
mm)
mm)
mm)
m
m)
14
a
Ion exchange capability.
Water uptake.
Swelling ratio in plane direction.
Proton conductivity.
b
c
d
e
Methanol permeability.

412
B. Liu et al. / Polymer 51 (2010) 403–413
conductivity [26]. As shown in Fig. 7, all the data is situated at the
right top corner of the plot.
10-1
10-2
10-3
Combined with all the other physical properties, DiPh-SPES-50
appears to be the most promising material of the present series for
DMFC application.
4. Conclusion
A bisphenol monomer, 2,5-diphenylhydroquinone, was synthe-
sized via a one-step route. Derived from phenylhydroquinone and
2,5-diphenylhydroquinone, two series of starting materials,
mono-phenylated poly(ether sulfone)s (Ph-PES) and diphenylated
poly(ether sulfone)s (DiPh-PES), were synthesized via a typical
nucleophilic substitution polycondensation. These polymers were
sulfonated for extended reaction times, to observe the maximum DS
achievable, and the site of sulfonation. Under the same mild sulfo-
nation conditions using commercial concentrated sulfuric acid at
room temperature, Ph-PES polymers and DiPh-PES polymers
exhibited different behavior. NMR analysis revealed that fully
sulfonated di-phenylated-acid SPES polymers could be prepared
based on 2,5-diphenylated polymers. It was noted that the SPES
containing di(4-sulfonic acid)phenyl pendants (DiPh-SPES) exhibi-
ted extremely good thermo-oxidative stability. For example, from
TGA results, an obvious difference of 14–17 ꢂC between DiPh-SPES
and Ph-SPES copolymers was observed. Meanwhile, both DiPh-SPES
and Ph-SPES showed improved thermal stability over Nafion under
the same testing conditions. The sulfophenylated polymer was also
shown to have better oxidative stability than the one containing
main-chain sulfonation, as shown by the Fenton test. In comparison
with Nafion 117, these membranes possessed low methanol
permeability in the range of 7.0 ꢀ 10ꢁ7 to 9.4 ꢀ 10ꢁ8 cm2/s at 30 ꢂC.
DiPh-SPES-50 and Ph-SPES-40, with IEC values of 1.63 and
1.52 meq/g respectively, also exhibited high proton conductivity of
about 130 mS/cm and 117 mS/cm at 100 ꢂC, which are comparable
values to Nafion 117.
Nafion 117
Ph-SPES-30
Ph-SPES-40
Ph-SPES-50
DiPh-SPES-40
DiPh-SPES-50
DiPh-SPES-60
40
60
80
100
Temperature(°C)
Fig. 6. Proton conductivities of the membranes and Nafion 117.
although it did not retain dimensional shape at elevated tempera-
ture. At 100 ꢂC, proton conductivities of DiPh-SPES-50 and Ph-SPES-
40 were as high as 0.130 and 0.117 S/cm, respectively, which are
comparable to the value of 0.138 S/cm for Nafion 117.After making
comparisons between water absorption, proton conductivity and
methanol permeability, it was shown that the membranes having
higher water uptake always showed higher proton conductivity and
lower methanol permeability. Certainly, this is highly relevant with
the content of ionic groups.
One significant drawback for Nafion in DMFC application is its
high methanol crossover. This limitation is associated with the
microstructure of Nafion, whereby interconnected ionic domains
strongly contribute to its high proton conductivity, but at the same
time contribute to fast methanol diffusion [25]. As shown in Table 3,
the methanol permeability values of the membranes were in the
range of 7.0 ꢀ 10ꢁ7–9.4 ꢀ10ꢁ8 cm2/s at 30 ꢂC, which is several times
lower than the value of Nafion 117 of 1.55 ꢀ 10ꢁ6 cm2/s. The
membranes having a combination of high proton conductivity and
low methanol crossover are being pursued for DMFC system. It is
facile to evaluate the membranes through drawing a performance
trade-off plot containing both methanol permeability and proton
Combined with its good mechanical properties and low
dimensional change in hot water, a diphenylated polymer, DiPh-
SPES-50, is a promising PEM material for PEMFC and DMFC
applications.
Acknowledgement
Financial support for this project was provided by the National
Natural Science Foundation of China (No.: 50973040) and the
Science and Technology Development Plan of Jilin Province, China
(No.: 20090322).
1.E+00
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