Synthesis of highly sulfonated poly(arylene ether sulfone)s with sulfonated triptycene pendants for proton exchange membranes

Article abstract of DOI:10.1016/j.polymer.2011.02.033

A series of poly(aryl ether sulfone)s containing triptycene groups PES-x-TPD (x refers to molar percentage of TPD) were firstly synthesized through nucleophilic aromatic substitution polycondensation by using 2,5-triptycenediol (TPD), bis(4-hydroxyphenyl) sulfone (BHPS) and 4,4′-difluorodiphenyl sulfone (DFDPS). The sulfonation of copolymers was conducted at room temperature by using a mild sulfonating reagent (98% H₂SO₄), and the degree of sulfonation was readily and accurately controlled by adjusting the ratio of TPD and BHPS. The structures of PES-x-TPD and SPES-x-TPD were characterized by IR, ¹H NMR and ¹³C NMR spectra. These ionomers generally showed high thermal stability and mechanical strength at low humidity regardless of high IEC value. Meanwhile, it is noteworthy that these novel SPES-x-TPD membranes with high IEC value achieved high proton conductivity in a wide range of humidity at 80 °C. For example, SPES-60-TPD with the highest IEC value 2.86 mmol/g displays the conductivity of 2.5 × 10 ^-1 S/cm which is much higher than that of the perfluorinated Nafion membrane (1.1 × 10^-1 S/cm) at 80 °C and 94% RH. At 80 °C and 34% RH, SPES-60-TPD displays the conductivity of 4.5 × 10 ^-3 S/cm which is also higher than that of the Nafion membrane (3.0 × 10^-3 S/cm). Microscopic analyses revealed that well-dened phase separated structures and uniform ionic pathway was formed for SPES-45-TPD membrane with the IEC of 2.29 mmol/g. Moreover, a H₂/O₂ fuel cell using the SPES-55-TPD (IEC = 2.68 mmol/g) also showed better performance than that of Nafion 117 at 40 °C and 30% RH.

Full text of DOI:10.1016/j.polymer.2011.02.033

Polymer 52 (2011) 1738e1747
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of highly sulfonated poly(arylene ether sulfone)s with sulfonated
triptycene pendants for proton exchange membranes
,
,
,
,
c
Feixiang Gong a b, Hongchao Mao a b, Yuwei Zhang b c, Suobo Zhang a , Wei Xing
*
a Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
b Graduate School of Chinese Academy of Sciences, Beijing 100049, China
c State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of poly(aryl ether sulfone)s containing triptycene groups PES-x-TPD (x refers to molar
percentage of TPD) were firstly synthesized through nucleophilic aromatic substitution poly-
condensation by using 2,5-triptycenediol (TPD), bis(4-hydroxyphenyl) sulfone (BHPS) and 4,40-difluor-
odiphenyl sulfone (DFDPS). The sulfonation of copolymers was conducted at room temperature by using
a mild sulfonating reagent (98% H2SO4), and the degree of sulfonation was readily and accurately
controlled by adjusting the ratio of TPD and BHPS. The structures of PES-x-TPD and SPES-x-TPD were
characterized by IR, 1H NMR and 13C NMR spectra. These ionomers generally showed high thermal
stability and mechanical strength at low humidity regardless of high IEC value. Meanwhile, it is note-
worthy that these novel SPES-x-TPD membranes with high IEC value achieved high proton conductivity
in a wide range of humidity at 80 ꢀC. For example, SPES-60-TPD with the highest IEC value 2.86 mmol/g
displays the conductivity of 2.5 ꢁ 10ꢂ1 S/cm which is much higher than that of the perfluorinated Nafion
membrane (1.1 ꢁ 10ꢂ1 S/cm) at 80 ꢀC and 94% RH. At 80 ꢀC and 34% RH, SPES-60-TPD displays the
conductivity of 4.5 ꢁ 10ꢂ3 S/cm which is also higher than that of the Nafion membrane (3.0 ꢁ 10ꢂ3 S/cm).
Microscopic analyses revealed that well-defined phase separated structures and uniform ionic pathway
was formed for SPES-45-TPD membrane with the IEC of 2.29 mmol/g. Moreover, a H2/O2 fuel cell using
the SPES-55-TPD (IEC ¼ 2.68 mmol/g) also showed better performance than that of Nafion 117 at 40 ꢀC
and 30% RH.
Received 10 October 2010
Received in revised form
6 January 2011
Accepted 22 February 2011
Available online 1 March 2011
Keywords:
Sulfonated triptycene groups
Proton exchange membrane
Low humidity
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
conductivity under low relative humidity, is still inferior to that of
perfluorinated PEM materials. Nevertheless, the proton conduc-
Proton exchange membrane fuel cells (PEMFCs) are promising
new power sources for vehicles and portable devices [1e4]. As one
of the key components of PEMFCs, the proton exchange membrane
(PEM) is a proton conducting, electronically insulating polymer
material. Perfluorosulfonic acid PEMs, such as Nafion, are typically
used as the polymer electrolytes in PEMFCs because of their
excellent chemical and mechanical stabilities as well as high proton
conductivity [5,6]. But their disadvantages of high cost, low oper-
ation temperatures and high fuel permeability limit their wide
application [7,8]. Hence, in the past decade, a variety of proton
conductive materials, especially the nonfluorinated aromatic
hydrocarbon ionomers, have been proposed as alternative
membranes [9e17]. However, in spite of numerous efforts, the
performance of hydrocarbon PEM materials, especially their proton
tivity at low humidity would be an important issue especially in
fuel cells for electric vehicles and therefore, needs to be improved
[18].
In past years, Watanabe et al. reported that poly(arylene ether
sulfone)s containing sulfofluorenyl groups are highly proton
conductive and durable for 5000 h in operating fuel cells [19].
Meanwhile, the corresponding ionomer membranes with the
highest IEC value (IEC ¼ 3.26 mmol/g) showed comparable
proton conductivity to that of the perfluorinated ionomer
membrane (Nafion 117) under
a wide range of conditions
(80e120 ꢀC and 20e93% relative humidity). Ueda et al. reported
that poly(ether sulfone)s containing sulfonated binaphthyl units
also showed low water uptake, swelling ratio and water insolu-
bility even with a high IEC value of 3.19 mmol/g compared to
a typical sulfonated poly(ether ether sulfone) [20]. They attrib-
uted this excellent water stability under high IEC value to their
rigidity, bulkiness and the high hydrophobicity of the binaphthyl
structures. Moreover, due to the high IEC value, the membrane
* Corresponding author. Tel.: þ86 431 5605139.
E-mail address: sbzhang@ciac.jl.cn (S. Zhang).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.02.033

F. Gong et al. / Polymer 52 (2011) 1738e1747
1739
(IEC ¼ 3.19 mmol/g) exhibited high proton conductivity which
2.3. Synthesis of poly(aryl ether sulfone)s containing triptycene
groups
was comparable to or even higher than that of Nafion 117 over
a range of 30e95% relative humidity. According to the literature
above, sulfonic acid groups situated on rigidity, bulkiness and
high hydrophobicity groups may be helpful for high conductivity
at low humidity.
The synthesis of the poly(aryl ether sulfone)s containing trip-
tycene groups was accomplished by nucleophilic aromatic
substitution polycondensation.
A typical synthetic procedure,
The structural rigidity, nonplanarity, bulkiness, and
p
-electron
illustrated by the preparation of PES-30-TPD copolymer (30 refers
to molar percentage of TPD), is described as follows. 0.8590 g
(3 mmol) TPD, 1.7519 g (7 mmol) BHPS, 2.5425 g (10 mmol) DFDPS
and 4.1463 g (30 mmol) K2CO3 were added into a 100 mL three-
neck flask equipped with a magnetic stirrer, a DeaneStark trap
and a nitrogen inlet. Then, 13 mL NMP and 20 mL toluene were
charged into the reaction flask under a nitrogen atmosphere. The
reaction mixture was refluxed at 140 ꢀC for 4 h until water was
removed from the reaction mixture by azeotropic distillation, and
then excess toluene was distilled off. Then the mixture was stirred
at this temperature for another 10 h to give a viscous solution. The
mixture was cooled to 120 ꢀC and diluted with NMP. The solution
was filtered and poured into water to give white flakes of the
product, which were washed with hot water several times. The
resulting product was dried under vacuum at 120 ꢀC for 12 h.
Yield: 98%.
richness of iptycenes, particularly the basic triptycene scaffold,
have found versatile usages in constructing molecules of both
fundamental and practical importance [21]. Polymers containing
triptycene groups have been shown to possess excellent thermal
stability, chemical stability, low water absorption behavior and
mechanical enhancements properties [21,22], which are required
properties for the skeleton of PEMs. Meanwhile, combined
with their rigid, bulky, high hydrophobic structure and high
degrees of internal free volume, polymers containing sulfonated
triptycene groups may be also rendered good water-holding
capability and high proton conductivity under low humidity
conditions.
Therefore, in this article, a series of poly(aryl ether sulfone)s
containing triptycene groups (PES-x-TPD) were synthesized
through nucleophilic aromatic substitution polycondensation by
using 2,5-triptycenediol (TPD), bis(4-hydroxyphenyl) sulfone
(BHPS) and 4,40-difluorodiphenyl sulfone (DFDPS). The sulfonation
of copolymers above was conducted at room temperature by using
a mild sulfonating regent (95e98% H2SO4) and the degree of
sulfonation could be readily and accurately controlled by adjusting
the ratio of TPD and BHPS. The properties of corresponding novel
sulfonated copolymer films, such as water uptake, water swelling
ratio, proton conductivity, thermal properties, mechanical proper-
ties, morphology, oxidative stability and PEMFC performance were
also investigated in detail.
2.4. Sulfonation
The sulfonation reaction was conducted according to the liter-
ature method [24]. A typical reaction was shown as follows: To
a 100 mL of flask, 1 g of PES-30-TPD and 20 mL of concentrated
sulfuric acid (98%) were added. After stirring at room temperature
for 6 h, the homogeneous viscous solution was poured into
a mixture of water and ice to get a silk-like solid. The solid was
washed with water until the water reached neutral. Then the
sulfonated copolymer was dried in a vacuum oven at 100 ꢀC for
12 h.
2. Experimental
2.1. Materials
2.5. Membrane preparation
Anthracene, hydrobromic acid (40%) and quinone were
purchased from China National Pharmaceutical Group Corporation
and used as received. Bis(4-hydroxyphenyl) sulfone (BHPS) (Alfa
Aesar), 4,40-difluorodiphenyl sulfone (DFDPS) (Alfa Aesar) and
anhydrous potassium carbonate (Beijing Multi. Technology Co.,
Ltd.) were used as received without further purification. N-meth-
ylpyrrolidone (NMP) was distilled over CaH2 under reduced pres-
sure. All other reagents were obtained from commercial sources
and used as received.
The sulfonated copolymers were cast onto glass plate from their
DMAc solution (7e9wt%) after filtration and dried at 90 ꢀC for 7 h.
Then the ionomer membrane was dried in a vacuum oven at 60 ꢀC
for 5 h. The as-cast membranes were treated with 1.0 N sulfuric acid
at room temperature for 2 days for proton exchange. The obtained
membranes were thoroughly washed with deionized water. Tough,
ductile ionomer membranes were prepared with a controlled
thickness of 30e60 mm.
2.6. Measurements
2.2. Synthesis of 2,5-triptycenediol (TPD)
1H NMR spectra were measured at 300 MHz on a Bruker AV300
spectrometer (Germany). FT-IR spectra of PES-60-TPD and SPES-60-
TPD were obtained with a Bio-Rad digilab Division FTS-80 FT-IR
spectrometer (Cambridge, MA). The inherent viscosities were
determined on 0.5 g/dL concentration of polymer in NMP with an
Ubbelohde capillary viscometer at 30 ꢃ 0.1 ꢀC. Molecular weight
and molecular weight distributions were measured by GPC using
Ultrastyragel columns and THF as the eluent at a flow rate of 1 mL/
min. The values obtained were determined by comparison with
a series of polystyrene standards. The thermogravimetric analyses
(TGA) were obtained in nitrogen with a Perkin-Elemer TGA-2
thermogravimetric analyzer (Inspiratech 2000 Ltd., UK) at a heating
rate of 10 ꢀC/min. Tensile measurement were performed with
a mechanical tester Instron-1211 instrument (Instron Co., USA) at
a speed of 2 mm/min at 60% relative humidity.
2,5-Triptycenediol (TPD) was synthesized according to the
literature [23]. Anthracene (35.6 g) and 21.6 g of quinone in 200 mL
of toluene were heated under reflux for 6 h and then cooled to
room temperature. The precipitated solid was collected on a filter,
thoroughly washed with toluene and dried at 80 ꢀC for 6 h. The
product 1 was obtained in a yield of 91% (52.1 g). To the system of
compound 1 (52.1 g) in 300 mL of glacial acetic acid at the boiling
point, 1 mL of 40% hydrobromic acid were slowly added. The
solution took on an orange color which gradually faded as a fine
white solid precipitated out. After another half hour at the boiling
point, the reaction mixture was cooled, filtered and dried at 100 ꢀC
for 5 h. The product 2 was obtained in a yield of 93% (48.5 g). 1H
NMR (DMSO-d6): 8.8e8.7 (2H, s), 7.3e7.2 (4H, m), 6.9e6.8 (4H, m),
6.3e6.2 (2H, s), 5.8e5.7 (2H, s).

1740
F. Gong et al. / Polymer 52 (2011) 1738e1747
2.7. Ion exchange Capacity
which was placed on copper grids. Images were taken on an
ultrahigh-resolution transmission electron microscope (JEOLJEM-
2010FEF) using an accelerating voltage of 200 kV.
IEC of all ionomers were determined by 1H NMR spectra and
titration. In the 1H NMR method, changes in the integration ratio
for the aromatic protons were taken. In the titration method, the
membranes in Hþ form were immersed in a 1 M NaCl solution for
48 h to liberate the Hþ ions and replace them with Naþ ions. The
Hþ ions in solution were then titrated with 0.01 M NaOH aqueous
solution and phenolphthalein as the indicator.
2.11. Membrane electrode assemblies (MEAs)
Gas diffusion electrodes with three-layer structure were
prepared by the following steps, i.e., wet-proofing treatment of
a carbon paper (CP), coating of a gas diffusion layer (GDL) onto the
CP, and coating of a catalyst layer (CL) onto the GDL. The GDL was
fabricated by coating a slurry consisting of carbon black and PTFE
dispersions on CP. The loading amount of the GDL was 0.95 mg/cm2.
Pt-loaded carbon black, 5 wt% Nafion ionomer solution, 2-propanol,
and pure water were mixed in a planetary ball mill. The weight
ratio of Nafion to the CB was 0.7. The obtained catalyst ink was
uniformly spread over the GDL. The loading amount of Pt was
0.41 mg/cm2 for both the cathode and anode. The electrodes were
treated with 1 N HNO3 before assembling with the electrolyte
membrane. Membrane SPES-55-TPD (IEC ¼ 2.68 meq/g) or Nafion
(DuPont Nafion 117) was sandwiched between two gas diffusion
electrodes (geometric active area 9 cm2) and hot-pressed at 70 ꢀC
for 3 min at 1.0 MPa.
2.8. Water uptake and dimensional change
The humidity dependence of water uptake was measured by
placing the membrane in a thermocontrolled humidity chamber for
4 h. Then the membrane was taken out and quickly weighed on
a microbalance. Water uptake was calculated from
WU ¼ ðWs ꢂ WdÞ=Wd ꢁ 100 wt%
(1)
Where Ws and Wd are the weights of wet and dried membrane,
respectively.
Dimensional change of the hydrated membrane was investi-
gated by plating the membrane in water at 20 ꢀC or 80 ꢀC for 4 h,
and the changes of length were calculated from:
2.12. Fuel cell operation
D
Lc ¼ ðL ꢂ L Þ L
(2)
=
S
S
All cells were operated at a cell temperature of 40 ꢀC under
ambient pressure. H2 was fed to the anode at 300 mL/min with the
humidity of 30% RH. O2 was fed to the cathode at 2000 mL/min with
the humidity of 30% RH. Current voltage (IeV) curves were
measured under steady-state conditions.
Where Ls are the diameter of the dry membrane, L refers to those of
the hydrated membrane.
2.9. Proton conductivity
The proton conductivity (
1 cm ꢁ 4 cm) was obtained using
s
, S/cm) of each membrane (size:
¼d/Ls Ws R (d: distance
s
3. Results and discussion
between reference electrodes, and Ls and Ws are the thickness and
width of the membrane respectively). Here, ohmic resistance (R)
was measured by four-point probe alternating current (ac)
impedance spectroscopy using an electrode system connected
with an impedance/gain-phase analyzer (Solatron 1260) and an
electrochemical interface (Solatron 1287, Farnborough Hampshire,
ONR, 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 placed under a thermo-
controlled humid chamber. All samples were equilibrated at
the set temperature and humidity for at least 1 h before the
measurement.
3.1. Polymers synthesis
2,5-Triptycenediol (TPD) was easily synthesized through two-
step reactions as shown in Scheme 1. The preparation of PES-x-TPD
(x refers to molar percentage of TPD) copolymers were carried out
through nucleophilic aromatic substitution polycondensation from
the corresponding biphenol and difluoride monomers as shown in
Scheme 2. BHPS was added for adjusting the TPD ratio in copoly-
mers. SDPES was also synthesized for comparison. All of the PES-x-
TPD copolymers were obtained with high molecular weight as list
in Table 1. It can be seen from Table 1 that the molecular weight of
copolymers was decreased when the molar percentage of BHPS
increased, it could be easily explained by the reactivity of mono-
mers in process of this polycondensation. As we know, the reac-
tivity of monomers played an important role in obtaining high
molecular weight polymers. For nucleophilic aromatic substitution
reaction, BHPS was a low reactivity monomer due to the strong
electron-withdrawing sulfone group in its structure, so when the
molar percentage of BHPS increased, the molecular weight of
copolymers decreased. In order to avoid the side reactions and
2.10. Transmission electron microscopy
The membrane samples were stained with lead ions by ion
exchange of the sulfonic acid groups in 0.5 M lead acetate aqueous
solution, rinsed with deionized water, and dried in vacuum oven for
12 h. Then the dyed sample was embedded in epoxy resin and
sectioned using a microtome to yield a 50 nm thick sample
Scheme 1. Synthesis of 2,5-triptycenediol (TPD).

F. Gong et al. / Polymer 52 (2011) 1738e1747
1741
O
S
O
S
1-n
n
F
F
HO
OH
HO
R
OH
+
+
O
O
K2CO3
NMP
O
S
O
S
O
O
R
O
O
O
S
1-n
n
O
O
O
H2SO4
O
S
O
S
O
S
O
R
O
O
O
1-n
n
O
O
O
(SO3H)x
R:
SDPES
SPES-x-TPD
Scheme 2. Synthesis of SPES-x-TPD copolymers.
make sure a homogeneous system in sulfonation reaction, the
sulfonation of copolymers was conducted at room temperature by
using a mild sulfonating regent (95e98% H2SO4) according to the
literature. It was found that all of the sulfonation reaction
completed in 6 h and only three substitution site on the triptycene
group for per repeat unit were sulfonated from 1H NMR spectros-
copy results. The degree of sulfonation of the copolymer was
readily controlled through the monomer feed ratios of TPD and
BHPS as confirmed by titration (Table 2), so that a series of copol-
ymers with different IEC values were obtained. The sulfonated
copolymers as synthesized were denoted as SPES-x-TPD. The
solubility behavior of PES-x-TPD and SPES-x-TPD in common
organic solvents was listed in Table 2. All of the sulfonated copol-
ymers are soluble in polar aprotic solvents such as DMAc, DMF, and
DMSO but insoluble in water and methanol even with a high IEC
value (2.86 mmol/g). Flexible, tough and ductile membranes could
be obtained from these sulfonated copolymers by solvent-casting
method. The obtained membranes were treated with 1.0 M sulfuric
acid at room temperature for 2 days and then washed with
deionized water.
3.2. Characterization of copolymers and sulfonated copolymers
The structure of these novel copolymers were characterized by
FT-IR, 1H NMR, and 13C NMR spectra. Take SPES-60-TPD as an
example, Fig. 1 showed the FT-IR spectra of PES-60-TPD and SPES-
60-TPD. From the difference between the two curves, it was obvious
that the symmetric and asymmetric vibrations of the sulfonic acid
group appeared at 1168 and 1029 cmꢂ1, respectively. It was sug-
gested that the sulfonated of PES-x-TPD was successfully conducted.
Fig. 2 showed the 1H NMR (a) and 13C NMR (b) spectra of PES-60-TPD
in DMSO-d6. All of aromatic protons and carbons were well assigned
to the supposed chemical structure. Moreover, the integration ratio
of the peaks corresponded to the polymer composition expected
from the feed monomer. The multi-peaks assigned to proton 10 and
11 in Fig. 2(a) should attribute to conformation isomerism of trip-
tycene groups in space. The 1H NMR (a) and 13C NMR (b) spectra of
SPES-60-TPD fully assigned in DMSO-d6 were displayed in Fig. 3. By
compared with PES-60-TPD, it was easy to assign the peaks of SPES-
60-TPD. Aromatic protons located at the electron-rich ortho-ether
position (excepted the protons located at triptycene groups) were
strongly shielded, and their signals appeared at high field
(7.4e6.8 ppm) area, aromatic protons located at the electron-defi-
cient ortho-sulfonate groups positionwere strongly deshielded, and
their signals appeared at low field (8.1e7.8 ppm) area. Compared
with PES-60-TPD, there was nearly no changes of these signals.
While the protons located at sulfonated triptycene groups were
deshielded due to the strongly electron-withdrawing effects
of eSO3H, and their signals transferred into low field area. Mean-
while, due to the isomerism of sulfonated triptycene groups as
shown in Fig. 3(a), the peaks consistent with the protons in
sulfonated triptycene groups all split into multi-peaks. Combined
with the results of 13C NMR spectra, the sites of eSO3H were readily
assigned and the structure of SPES-60-TPD was fixed as shown in
Fig. 3. No partial sulfonated copolymer peaks could be observed
both in 1H NMR and 13C NMR spectra also suggested that the
sulfonation reaction was completely. For all copolymers, the IEC
values were calculated from the peak area ratio 3 þ 30þ300 to
10 þ 7þ6, and the obtained experimental IEC were in agreement
with the expected values from the copolymer composition.
Table 1
Molecular weight, inherent viscosity and mechanical properties of PES-x-TPD and
SPES-x-TPD membranes.
a
Polymer
Molecular
weight
(kDa)
hinh (dL/g) Maximum Elongation Young’s
of stress
at break
(%)
modulus
(MPa)
(MPa)b
Mn PDI
SDPES
97 1.96 0.95
73 1.58 0.75
75 1.78 0.79
81 1.73 0.82
81 1.66 0.86
85 1.83 0.89
93 1.56 0.93
99 1.31 0.99
42.8
45.9
40.5
39.7
38.4
35.1
32.1
27.6
21.8
60
10
12
17
21
27
29
32
38
650
983
809
755
694
416
411
358
336
SPES-25-TPD
SPES-30-TPD
SPES-35-TPD
SPES-40-TPD
SPES-45-TPD
SPES-50-TPD
SPES-55-TPD
SPES-60-TPD 105 1.49 1.08
a
Viscosity after sulfonation; 0.5 g/dL in NMP solution at 30 ꢀC.
Measured at room temperature and 60% RH.
b

1742
F. Gong et al. / Polymer 52 (2011) 1738e1747
Table 2
IEC, solubility behaviors of SPES-x-TPD copolymers.
Polymer
IEC (mmol/g)
IECCAL
Solvents
a
b
IECN
IECT
Methanol
NMP
DMSO
DMAc
DMF
CHCl3
H2O
Acetone
c
SPES-25-TPD
SPES-30-TPD
SPES-35-TPD
SPES-40-TPD
SPES-45-TPD
SPES-50-TPD
SPES-55-TPD
SPES-60-TPD
1.41
1.64
1.87
2.09
2.29
2.49
2.68
2.86
1.40
1.63
1.85
2.09
2.27
2.46
2.68
2.85
1.39
1.64
1.86
2.08
2.27
2.49
2.67
2.83
ed
ed
e
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
þþ
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
c
þþ
þþ
þþ
þþ
þþ
þþ
þþ
e
e
e
e
e
a
IEC from 1H NMR spectra.
IEC from titration.
þþ: Soluble at room temperature.
b
c
d
e: Insoluble.
3.3. Thermal and mechanical properties
3.4. Morphologies of membranes
Fig. 4 shows the thermal stability of the SDPES and SPES-x-TPD
copolymers investigated by thermogravimetric analysis (TGA) in
a nitrogen atmosphere. It can be seen that the SPES-x-TPD
membranes exhibited a typical three-step degradation pattern. The
first weight loss up to 150 ꢀC was ascribed to the loss of water
molecules, absorbed by the highly hygroscopic eSO3H groups. The
second weight loss around 300 ꢀC was due to the cleavage of the
sulfonic acid groups. The third stage weight loss of SPES-x-TPD
around 530 ꢀC was assigned the decomposition of polymer main
chain. From the results above, it is concluded that the SPES-x-TPD
are thermally stable to satisfy the requirement of thermal stability
for the use in PEMFCs.
In order to investigate hydrophilic-hydrophobic microphase
separation and the proton transport pathway of the SPES-x-TPD
membranes, transmission electron microscopic observations were
carried out as shown in Fig. 8. The dark areas stained with lead
ions represent the hydrophilic (ionic) domain and the brighter
areas represent the hydrophobic domain. For SPES-25-TPD
membrane with the lowest IEC of 1.41 mmol/g (Fig. 8(d)), only
Good mechanical properties of the PEMs are one of the demands
for PEMFC applications. The mechanical properties of the SDPES
and SPES-x-TPD membranes were measured at room temperature
and 60% RH. The stress-strain data are shown in Table 1. The
membranes have tensile stress at maximum load of 21.8e45.9 MPa
and elongation at break of 10e38% with the general trend of lower
maximum stress for the higher IEC membranes. The tension results
undoubtedly showed they were strong and flexible enough
membranes for fuel cell application even at high sulfonation
degree.
Fig. 1. The IR spectra of PES-60-TPD and SPES-60-TPD.
Fig. 2. 1H (a) and 13C NMR (b) spectra of PES-60-TPD in DMSO-d6.

F. Gong et al. / Polymer 52 (2011) 1738e1747
1743
2.29 mmol/g, the well-defined phase separated structures were
formed. The uniform ionic pathways and channels (Fig. 8(b))
which were favorable for proton transport also became obvious.
The image of SPES-55-TPD ionomer with highest IEC value
(2.68 mmol/g) in TEM experiment was shown in Fig. 8(a).
Compared with SPES-45-TPD, the hydrophilic domain of SPES-55-
TPD was much larger and the connectivity of hydrophilic domain
increased with increasing sulfonation degree as shown in Fig. 8. It
was reasonable that the hydrophilic aggregates became larger and
the connectivity of hydrophilic domain increased as increasing
IEC due to increased concentration of sulfonic acid groups [25].
Such microstructure allows the effective proton transportation via
the membrane, therefore, the membranes showed high proton
conductivity in a wide range of humidity.
3.5. Water uptake and dimensional change
The water content has a great effect on the properties of PEMs.
For most proton conductive polymers, water acts as the carrier for
proton transportation through the polymer membrane. Adequate
hydration of electrolyte membranes is crucial for high proton
conductivity. However, water swelling ratio should be also
considered because extreme swelling will decrease the dimen-
sional stability and mechanical properties of the polymer films. The
water uptake and swelling ratio data of SPES-x-TPD membranes
were reported in Table 3. The water uptake and swelling ratio of
SDPES were also displayed for comparison. As expected, an increase
in IEC and temperature led to an increase in water uptake and
swelling ratio for both SPES-x-TPD and SDPES membranes. Fig. 5
showed the water uptake as a function of IEC for the SPES-x-TPD
and SDPES membranes at 80 ꢀC, 94% RH and 80 ꢀC, 34% RH. The
swelling ratio of SPES-x-TPD and SDPES at 20 ꢀC and 80 ꢀC in water
were also measured, and the results were shown in Fig. 6. For the
membranes with low sulfonation degree (IEC < 2.2 mmol/g), the
water uptake and swelling ratio increased mildly with increasing
temperature. The membranes with high sulfonation degree
(IEC > 2.2 mmol/g) exhibited a relative rapid increase in water
sorption and swelling ratio at high temperature, attributed to the
formation of large and continuous ion network in the sulfonated
polymers [26]. It is noteworthy that the SPES-25-TPD membranes
displayed lower water uptake and swelling ratio than SDPES
membranes, although they had same IEC value. The SPES-40-TPD
membrane with the IEC of 2.09 mmoL/g still displayed excellent
dimensional stability with the swelling ratio of 9.6% at 20 ꢀC
and 15.5% at 80 ꢀC, while the traditional sulfonated poly(aryl ether
sulfone)s with similar IEC value often dissolved in water or
lost their mechanical properties due to serious swelling [27].
Moreover, the SPES-60-TPD membrane with the highest IEC value
(2.86 mmol/g) exhibited the swelling ratio of 26.7% at 20 ꢀC and
59.3% at 80 ꢀC, which was much lower than poly(aryl ether
sulfone)s containing sulfofluorenyl groups under similar IEC value
and conditions [19]. The low swelling ratio of SPES-x-TPD should be
attributed that the triptycene groups were introduced in the
copolymers. First, the introduction of large conjugated triptycene
Fig. 3. 1H (a) and 13C NMR (b) spectra of SPES-60-TPD in DMSO-d6.
a small quantity of irregular ion clusters formed. These ionic
clusters were not connected with each other but were rather
isolated. It was similar with the TEM image of SDPES (Fig. 8(e)).
However, for SPES-45-TPD membrane with the IEC of
groups could increase the
pep interaction between aromatic
polymer chains so that the water uptake and swelling ratio were
confined [28]. Second, the sulfonated poly(arylene ether)s with
sulfonic acid groups attached in side chain could show lower water
uptake and swelling ratio than main-chain type [29]. Third, struc-
tures of the polymer with triptycene groups assemble in a way to
minimize free volume and fill the concave clefts of the triptycene
groups, so the interlocking structure formed among the polymer
main chains [22]. Similarly, the interlocking structure could also
formed in the SPES-x-TPD polymer main chain which may prohibit
the over-swollen of SPES-x-TPD membranes (Fig. 6).
Fig. 4. The TGA curves of SDPES and SPES-x-TPD copolymers.

1744
F. Gong et al. / Polymer 52 (2011) 1738e1747
Table 3
Water uptake,
l value, swelling ratio and conductivity of SPES-x-TPD copolymers.
Polymer
IEC (mmol/g)
l
(94% RH)a
Water uptake (%)a
34% RH
Swelling ratio (%)
lb lc
Conductivity (S/cm)a
34% RH
94% RH
D
D
94% RH
SDPES
1.40
1.41
1.64
1.87
2.09
2.29
2.49
2.68
2.86
0.91
6.1
5.3
5.7
6.1
7.3
8.2
8.7
9.5
1.9
3.7
4.9
6.5
9.3
11.5
13.4
15.6
21.9
ed
15.4
13.6
16.9
20.8
27.1
33.7
38.9
45.9
65.3
11.6
8.1
5.2
7
13.5
5.7
9.4
12.3
15.5
18.3
23.5
27.9
59.3
e
4.5 ꢁ 10ꢂ6
1.0 ꢁ 10ꢂ5
1.5 ꢁ 10ꢂ5
3.1 ꢁ 10ꢂ5
2.3 ꢁ 10ꢂ4
9.0 ꢁ 10ꢂ4
1.9 ꢁ 10ꢂ3
2.9 ꢁ 10ꢂ3
4.5 ꢁ 10ꢂ3
3.0 ꢁ 10ꢂ3
3.6 ꢁ 10ꢂ2
2.7 ꢁ 10ꢂ2
5.7 ꢁ 10ꢂ2
8.6 ꢁ 10ꢂ2
1.5 ꢁ 10ꢂ1
1.75 ꢁ 10ꢂ1
1.9 ꢁ 10ꢂ1
2.17 ꢁ 10ꢂ1
2.5 ꢁ 10ꢂ1
1.1 ꢁ 10ꢂ1
SPES-25-TPD
SPES-30-TPD
SPES-35-TPD
SPES-40-TPD
SPES-45-TPD
SPES-50-TPD
SPES-55-TPD
SPES-60-TPD
Nafion 117
8
9.6
11.6
14.6
17.5
26.7
e
12.7
7.1
a
Measured at 80 ꢀC.
b
Measured at 20 ꢀC in water.
Measured at 80 ꢀC in water.
e Not be measured.
c
d
3.6. Proton conductivity
The proton conductivities of SDPES, SPES-x-TPD copolymers
along with that of Nafion 117 were measured at 80 ꢀC in the range of
34e94% RH. As shown in Table 3, SPES-x-TPD membranes showed
the proton conductivities in the range of 2.7 ꢁ 10ꢂ2e2.5 ꢁ10ꢂ1 S/cm
at 94% RH and 1.0 ꢁ 10ꢂ5e4.5 ꢁ 10ꢂ3 S/cm at 34% RH. As expected,
their proton conductivities increased with IEC values and
l values
(the number of water molecules per sulfonic acid unit). To clarify the
relationship between RH and proton conductivity, humidity
dependence of the proton conductivity of SPES-x-TPD, Nafion 117
and SDPES membranes were also summarized in Fig. 7. As shown in
Fig. 7, it can be seen that the conductivity showed great dependence
on humidity for low IEC membranes (IEC < 2.09). For example, the
conductivity of SPES-25-TPD (IEC ¼ 1.41 mmol/g) dropped from
2.7 ꢁ10ꢂ2 S/cm (94% RH) to 1.0 ꢁ10ꢂ5 S/cm (34% RH) at 80 ꢀC. This is
typical behavior for hydrocarbon ionomer membranes due to less
developed microphase separation than those of the perfluorinated
ionomers [30]. Increasing the IEC significantly increased the
conductivity at low humidity. The ionomers with higher IEC
(IEC > 2.5 mmol/g) showed higher or similar proton conductivity in
comparisontothatof Nafion 117 overawide humidity rangeat 80 ꢀC.
For example, the SPES-60-TPD membrane with the highest IEC of
2.86 mmol/g showed a proton conductivity of 2.5 ꢁ10ꢂ1 S/cmwhich
Fig. 5. The water uptake of SPES-x-TPD membranes and SDPES at 80 ꢀC.
Fig. 6. The swelling ratio of SPES-x-TPD and SDPES membranes at 20 ꢀC and 80 ꢀC in
water.
Fig. 7. Humidity dependence of the proton conductivity of SPES-x-TPD, Nafion 117 and
SDPES membranes at 80 ꢀC.

F. Gong et al. / Polymer 52 (2011) 1738e1747
1745
Fig. 8. TEM images of (a) SPES-55-TPD (IEC ¼ 2.68 mmol/g), (b) SPES-45-TPD (IEC ¼ 2.29 mmol/g), (c) SPES-35-TPD (IEC ¼ 1.87 mmol/g), (d) SPES-25-TPD (IEC ¼ 1.41 mmol/g) and
(e) SDPES (IEC ¼ 1.4 mmol/g).
is much higher than that of the perfluorinated Nafion membrane
(1.1 ꢁ10ꢂ1 S/cm) at 80 ꢀC and 94% RH. At 80 ꢀC and 34% RH, SPES-60-
TPD displays the conductivity of 4.5 ꢁ10ꢂ3 S/cmwhich is also higher
than that of the Nafion membrane (3.0 ꢁ10ꢂ3 S/cm). Compared with
poly(aryl ether sulfone)s containing sulfofluorenyl groups under
similar IEC value, the conductivity of SPES-x-TPD was also higher.
The excellent proton conduction of the SPES-60-TPD membrane
especially at low humidity may be explained by its excellent
connectivity of proton paths and good water-holding capability of
sulfonated triptycene groups.
3.7. Membrane stability toward oxidation
The oxidative stability of the SPES-x-TPD membranes was
evaluated in Fenton’s reagent (3 wt% H2O2 aqueous solution
containing 2 ppm FeSO4) at 80 ꢀC for 1 h as an accelerated test

1746
F. Gong et al. / Polymer 52 (2011) 1738e1747
Table 4
3.8. PEFC performance
Oxidative stability results of SPES-x-TPD membranes.
The H2/O2 fuel cell performance of the SPES-55-TPD
(IEC ¼ 2.68 mmol/g, 50 m thick) membrane and Nafion 117
(IEC ¼ 0.91 mmol/g, 226
m thick) were tested at 40 ꢀC and with
Polymer
IEC
Oxidative stabilitya:decrease in
m
Weight (%)
IEC (%)
m
SPES-25-TPD
SPES-30-TPD
SPES-35-TPD
SPES-40-TPD
SPES-45-TPD
SPES-50-TPD
SPES-55-TPD
SPES-60-TPD
SDPES
1.41
1.64
1.87
2.09
2.29
2.49
2.68
2.86
1.40
0.91
0
1
0
0
humidity of 30% RH for both electrodes. As shown in Fig. 9, the
SPES-55-TPD membrane displayed better cell performance. At a cell
voltage of 0.6 V, the current density of SPES-55-TPD membrane was
222 mA cmꢂ2 while Nafion 117 was 93 mA cmꢂ2. This should be
attributed to the high proton conductivity of SPES-55-TPD at 30%
RH. Through the practical fuel-cell operation, it also suggested that
the SPES-x-TPD membranes should be hopeful PEM materials at
low humidity.
7
0
15
53
0
0
b
þ
ec
e
e
0
þ
þ
0
Nafion 117
0
e
a
The membrane was heated in the Fenton reagent (FeSO4 (2 ppm) in 3% H2O2) at
80 ꢀC for 1 h.
b
c
4. Conclusions
þ The membrane was dissolved completely in the Fenton reagent.
e Not be measured.
A series of high molecular weight poly(aryl ether sulfone)s
containing triptycene groups PES-x-TPD (x refers to molar
percentage of TPD) were synthesized through nucleophilic
aromatic substitution polycondensation by using 2,5-triptycenediol
(TPD), bis(4-hydroxyphenyl) sulfone (BHPS) and 4,40-difluor-
odiphenyl sulfone (DFDPS). The sulfonation reaction for all copol-
ymers above was easily operated and the degree of sulfonation
could be readily and accurately controlled by adjusting the ratio of
TPD and BHPS. These ionomers generally show high thermal
stability with the degradation temperature of sulfonic acid groups
around 300 ꢀC and good mechanical strength at low humidity
regardless of high IEC value. They also displayed good water
swelling resistance. Meanwhile, due to the good water-holding
capability and well-defined phase separated structures, these novel
SPES-x-TPD membranes with high IEC value achieved high proton
conductivity in a wide range of humidity. Moreover, a H2/O2 fuel
cell using SPES-55-TPD (IEC ¼ 2.68 mmol/g) showed better
performance than that of Nafion 117 at 40 ꢀC and 30% RH. In
conclusion, the SPES-x-TPD membranes should be hopeful PEM
materials especially at low humidity.
(Table 4). As expect, the oxidative stability decreased with the
increase of IEC values. The SPES-x-TPD and SDPES membranes
maintained the shapes of the membranes in Fenton’s reagent for
low IEC ones (IEC ꢄ 2.09 mmol/g). For higher IEC membranes
(IEC ¼ 2.29 mmol/g), after 1 h in Fenton’s reagent, the membrane
broken into pieces and obvious weight loss was observed. When
the IEC value was higher than 2.29 mmol/g, the membranes were
dissolved completely in the Fenton reagent after 1 h. The similar
results also have been reported by Miyatake [19]. In comparison
with the sulfonated poly(aryl ether sulfone)s reported previously
[26], the poor oxidative stability of SPES-50-TPD, SPES-55-TPD
and SPES-60-TPD membranes was mainly due to their higher IEC
values. It has been proved that the IEC values play an important
role in oxidative stability of PEMs, when IEC value increased, the
oxidative stability decreased [24]. It should be noted that the
residual membranes after the test retained their IEC values as
confirmed by NMR. It was in same with poly(aryl ether sulfone)s
containing sulfofluorenyl groups. This suggested that the oxida-
tive degradation of these ionomers occurs on the main chain
aromatic rings. As we know, oxidative degradation of PEMs was
likely to occur at carbon atoms linking the ether bond by the
attack of hydroxyl radicals due to the high electron density at
these positions [31].
Acknowledgments
We thank the National Basic Research Program of China (No.
2009CB623401), the National Science Foundation of China (No.
50973106, 51021003 and 50825302) and the Development of
Scientific and Technoloical Project of Jilin Province (No. 20086020)
for the financial support.
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