Synthesis and proton conductivity of sulfonated, multi-phenylated poly(arylene ether)s

Article abstract of DOI:10.1002/pola.27273

A series of sterically-encumbered, sulfonated, poly(arylene ether) copolymers were synthesized and their proton conductivity examined. The series was prepared by copolymerizing a novel monomer, 2″,3″,5″, 6″-tetraphenyl-[1,1′:4',1″:4″,1″':4″', 1″″-quinquephenyl]-4,4″″-diol, with 4,4'- difluorobenzophenone and bisphenol A. Subsequent sulfonation and solution casting provided membranes possessing ion exchange capacities of 1.9 to 2.7 mmol/g and excellent mechanical properties (Young's modulus, 0.2-1.2 GPa; tensile strength, 35-70 MPa; elongation at break, 62-231%). Water uptake ranged from 34 to 98 wt% at 80 °C/100% RH. Proton conductivities ranged between 0.24 to 16 mS/cm at 80 °C/60% RH, and 3 to 167 mS/cm at 80 °C/95% RH. TEM analysis of the polymers, in the dehydrated state, revealed isolated spherical aggregates of ions, which presumably coalesce when hydrated to provide highly conductive pathways. The strategy of using highly-encumbered polymer frameworks for the design of mechanically-robust and dimensionally-stable proton conducting membranes is demonstrated. 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2579-2587 Sulfonated, multiphenylated, random copolymers of poly(arylene ether)s were synthesized for examination as proton-conducting membranes. Proton conductivities ranged from 3 to 167 mS/cm at 80 °C/95% RH. The highly-encumbered polymer framework conferred exceptional mechanical strength and dimensional stability. Copyright

Full text of DOI:10.1002/pola.27273

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Synthesis and Proton Conductivity of Sulfonated, Multi-phenylated
Poly(arylene ether)s
Hsu-Feng Lee,1 Po-Hsun Wang,1 Yi-Chiang Huang,1 Wen-Hung Su,1 Ram Gopal,1
Chun Che Lee,1,2 Steven Holdcroft,2 Wen-Yao Huang1
1Department of Photonics, National Sun Yat-Sen University, No. 70, Lienhai Rd., Kaohsiung 80424, Taiwan
2Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, BC, V5A 1S6, Canada
Correspondence to: S. Holdcroft (E-mail: holdcrof@sfu.ca) or W.-Y. Huang (E-mail: wyhuang@faculty.nsysu.edu.tw)
Received 22 February 2014; accepted 30 May 2014; published online 19 June 2014
DOI: 10.1002/pola.27273
ABSTRACT: A series of sterically-encumbered, sulfonated, poly
(arylene ether) copolymers were synthesized and their proton con-
ductivity examined. The series was prepared by copolymerizing a
80 ꢀC/60% RH, and 3 to 167 mS/cm at 80 ꢀC/95% RH. TEM analysis
of the polymers, in the dehydrated state, revealed isolated spheri-
cal aggregates of ions, which presumably coalesce when hydrated
to provide highly conductive pathways. The strategy of using
highly-encumbered polymer frameworks for the design of
novel monomer, 200,300,500,600-tetraphenyl-[1,10:4’,100:400,100’:400’,10000
-
quinquephenyl]-4,40000-diol, with 4,4’-difluorobenzophenone and
bisphenol A. Subsequent sulfonation and solution casting pro-
vided membranes possessing ion exchange capacities of 1.9 to
2.7 mmol/g and excellent mechanical properties (Young’s modu-
lus, 0.2–1.2 GPa; tensile strength, 35–70 MPa; elongation at break,
62–231%). Water uptake ranged from 34 to 98 wt% at 80 ꢀC/100%
RH. Proton conductivities ranged between 0.24 to 16 mS/cm at
mechanically-robust and dimensionally-stable proton conducting
C
V
membranes is demonstrated.
2014 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2014, 52, 2579–2587
KEYWORDS: fuel cells; ionomers; membranes; poly(arylene
ether); polycondensation; proton conductivity
proton channels.24,25 Recent research has found that localiz-
ing, or densifying, ionic rich regions on a random copolymer
architecture by way of incorporating highly sulfonated moi-
eties can be a means to achieve high proton conductivity
and enhanced oxidative/hydrolytic stability.11,12,15,20,23 For
example, Li et al. report fully aromatic triblock copolymers
based on sulfonated poly(2,6-diphenyl-1,4-phenylene oxide)s
and poly(arylene ether sulfone)s that form well-connected
hydrophilic nanochannels. The hydrophilic segments consist
of highly localized sulfonic acid groups.9 Matsumoto et al.
report novel random copolymers of poly(ether sulfone) bear-
ing pendent phenyl groups with a high density of sulfonic
acid groups to facilitate proton transport.11,12 Chen et al.
investigated the introduction of bulky, hydrophilic domains
and long hydrophobic chains to facilitate phase separation
and the formation of proton conductive channels,15 while
Seo et al. describe copolymers containing sulfonated poly(te-
traphenyl ether ketone sulfone)s as the hydrophilic domain,
demonstrating good proton conductivity and fuel cell per-
formance.21 Weiber et al. developed two distinct sulfonated
poly(arylene ether sulfone)s, and poly(arylene sulfone)s—
INTRODUCTION Polymer electrolyte membrane fuel cells
(PEMFCs) are of interest because of their high fuel conver-
sion efficiency, low noise, and the absence of toxic emis-
sions.1–5 Commercial perfluorinated sulfonic acid (PFSA)
ionomers, such as Nafion, Flemion and Aciplex have been
widely adopted because of their relative good mechanical/
chemical stability and high proton conductivity.6 However,
they suffer from a high permeability of gases, high cost, envi-
ronmental inadaptability, and limited performance at higher
temperatures.7 As a result, alternative PEM materials, partic-
ularly those based on sulfonated aromatic polymers, are
under intense investigation.8–20 A feature characteristic of
these materials is that a high degree of sulfonation is
required to achieve highly ion-conductive membranes, which
results in a high water uptake (WU),21,22 a commensurate
loss of mechanical strength and inferior fuel cell perform-
ance compared to PFSA ionomers.21,23
To realize the properties of PFSA ionomers, it appears hydro-
carbon analogues must emulate the morphology of PFSAs,
i.e., an appropriate proportion of hydrophilic and hydropho-
bic segments are required to form nanophase-separated
Additional Supporting Information may be found in the online version of this article.
C
V
2014 Wiley Periodicals, Inc.
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SCHEME 1 Synthesis of monomer 1.
varying the positions and number density of the sulfonic
acid functionality.26,27 The tetra-sulfonated copolymers pro-
vided densely located sulfonic acid groups and provided high
proton conductivity due to distinct phase separation of the
ionic domains.
ducted using standard vacuum-line techniques. In order to
remove the water produced, a slow stream of argon was
passed through the reaction vessel during the polymerization
process.
200,3000,500,600-Tetraphenyl-[1,10:40,100:400,1000:4000,10000
quinquephenyl]-4,40000-diol (1)
-
The above work provides an impetus to further examine
polymers bearing densely sulfonated/localized functionality.
The present work is further motivated by the observation
that in order to achieve a contiguous network of ion chan-
nels in a polymer membrane (without resorting to very high
ion exchange capacities) the constituent polymer must con-
tain highly localized regions sulfonic acids to facilitate self-
aggregation both of ion clusters and hydrophobic segments,
thus enhancing mechanical strength and restricting excessive
swelling. In the present work, we explore this concept by
designing bulky, rigid architectures of sulfonated poly(ary-
lene ethers) distinctly characterized by their bearing multi-
ple phenyl groups and multiple sulfonic acid functionality
per repeat unit. A novel monomer, 200,300,500,600-tetraphenyl-[1,
10:40,100:400,1000:4000,10000- quinquephenyl]-4,40000-diol (1), is herein
reported and copolymerized with 4,40-difluorobenzophenone
and bisphenol A. The inter-relationship between structure,
conductivity, and morphology of densely-sulfonated, steri-
cally encumbered, rigid poly(arylene ether)s is investigated.
The monomer was synthesized according to Scheme 1. A mix-
ture of 4,400-dibromo-20,30-50-60-tetraphenyl-[1,10;40,100]-ter-
phenyl (5 g, 7.22 mmol), 4-methoxyphenyl boronic acid (3.3 g,
21 mmol), Pd(PPh3)4 (0.05 g, 0.432 mmol), K2CO3 (4 g, 28.9
mmol), H2O (13 mL), and toluene (200 mL) was placed in a
flame-dried flask and refluxed for 48 h under hydrogen atmos-
phere. The resultant mixture was passed through a short col-
umn packed with silica gel. Evaporation of the solvent yielded a
crude product, which was purified by column chromatography
to obtain a white solid product, 4,40000-dimethoxy-200,300,500,600-
tetraphenyl-1,10:40,100:400, 1000:4000,10000-quinquephenyl (90%). A
three-necked flask equipped with a magnetic stirrer was
charged with 3 g (4.01 mmol) of the dimethoxy groups mono-
mer in dichloromethane (40 mL), to which was added to BBr3
ꢀ
(1.14 mL, 12.0 mmol) in dichloromethane (10 mL) at 278 C
under a nitrogen atmosphere. The solution was allowed to
warm at room temperature and then stirred overnight. The
solution was cooled in an ice bath and quenched with water.
The quenched reaction was extracted twice using ethyl acetate
and (1M) NaOH. The combined organic extracts were washed
with brine, dried over Na2SO4, and concentrated at a yield of
EXPERIMENTAL
1
Materials
92.1%: H NMR (500 MHz, CDCl3, d 5 ppm): 4.68 (s, 2H, OH),
4,400-Dibromo-20,30-50-60-tetraphenyl-[1,10;40,100]-terphenyl was
synthesized as described previously.28,29 Reagents and sol-
vents were purchased from Aldrich Chemical Co., Merck,
Lancaster, Fluoko, or Fisher. N,N0-Dimethylacetamide (DMAc),
dimethyl sulfoxide, toluene, CH2Cl2, methanol, ClSO3H, HCl,
NaOH, Boron tribromide, anhydrous K2CO3, and aqueous
H2O2 (30%) were obtained from commercial sources. Tolu-
ene was dried over CaH, distilled under argon atmosphere,
and deoxygenated by purging with argon for 30 min in prep-
aration for the monomer synthesis. Bisphenol A, 4,40-difluor-
obenzophenone, 4-methoxyphenyl boronic acid and
Pd(PPh3)4 were purchased from Aldrich Chemical Co. and
used without further purification. Polymerizations were con-
6.79–7.33 (m, 36H, ArH).
Polymer Synthesis
The polymers were synthesized according to Scheme 2. Con-
densation polymerization followed our previously reported
procedure.30 Polymerizations were carried out in a 50 mL
three-necked round-bottom flask equipped with a stir bar, a
Dean–Stark apparatus fitted with a condenser, and a nitrogen
inlet. The flask was charged with 1 (2 g, 2.78 mmol), 4,40-
difluorobenzophenone (1.21 g, 5.56 mmol), bisphenol A
(0.635 g, 2.78 mmol), potassium carbonate (0.84 g, 6.12
mmol), DMAc (20 mL), and toluene (15 mL). The solution
mixture was stirred at 130 to 150 ꢀC for several hours.
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SCHEME 2 Synthesis of SP1-X. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
During the reaction, a slow stream of nitrogen was passed
through the reaction vessel to remove the water produced
by the reaction and then azeotroped with toluene. After com-
plete removal of the water, the reaction solution was stirred
under reflux (160 ꢀC) for 24 h. The resultant polymer solu-
tion was diluted with 10 to 15 mL of THF and poured with
stirring into methanol (250 mL). A fibrous polymer, P1, was
collected by filtration, washed, and vacuum-dried for 24 h at
spectrometer at a resolution of 4 cm21 min21 from 4000 to
400 cm21
.
Membranes of SP1 were obtained by filtering and casting
DMAc solutions of the polymer onto a flat glass plate, drying
at 80 ꢀC for 12 h, and then under reduced pressure for
another 12 h. The ion exchange capacity (IEC) of the mem-
branes was determined by acid-base titration. Dried mem-
branes were weighed and immersed in 1.0 M HCl solution
for 24 h to protonate the sulfonic acid groups. The mem-
branes were washed thoroughly with deionized water until
pH neutral. The membranes were immersed in 1.0 M NaCl
for 24 h and the proton released titrated with 0.01 M NaOH
to a phenolphthalein end point. IEC (mmol/g) was calculated
as follows eq 1:
ꢀ
120 C.
To a solution of P1 (0.4 g) in dichloromethane (30 mL) at
room temperature, 0.748 M of chlorosulfonic acid solution in
dichloromethane (30 mL, containing 1.5 mL [22.4 mmol]
chlorosulfonic acid) was added dropwise. The reaction mix-
ture was stirred for 24 h and poured into water. The precipi-
tate formed was filtered, washed thoroughly with deionized
water until pH neutral, and dried under vacuum at room
temperature overnight to yield sulfonated polymer, SP1. P1
was sulfonated to different extents according to the above
procedure by adding 4 mL (59.9 mmol), 5 mL (74.8 mmol),
and 5.5 mL (82.3 mmol) of chlorosulfonic acid, respectively.
V
NaOH 3MNaOH
IECðmmol=gÞ5
(1)
Wdry
where VNaOH and MNaOH are the volume and concentration of
the NaOH solution, respectively, and Wdry is the weight of
the dried membrane.
Measurements
Thermogravimetric analysis (TGA) was carried out on a Per-
kin Elmer Pyris 1 over a temperature range of 100 to 600
ꢀC using a heating rate of 10 ꢀC min21 under N2. Prior to
analysis, the membranes were dried and kept in the TGA fur-
nace at 150 ꢀC under air for 20 min to remove residual
water. Thermomechanical analysis (TMA) was performed on
specimens with controlled size (length, 10 mm; width,
2 mmꢀ; thickness, 30 um) using a Seiko Pyris Diamond TMA
at 25 C.
The chemical structure of the monomer and polymers was
identified by 500 MHz 1H NMR spectroscopy (Varian UNITY
INOVA-500) using DMSO-d6 and CDCl3 as solvents. The
molecular weights of the polymer were measured by GPC
(Viscotek) with a refractive index detector (Viscotek 270).
THF was used as the eluent (flow rate, 1 mL/min), and the
molecular weights of the polymers were calibrated using
polystyrene standards. FT-IR spectroscopy of the dry mem-
brane samples was recorded by using a BRUKER Vector 70
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Oxidative stability was evaluated by immersing membranes
into Fenton0s reagent (aq. 3% H2O2, containing 2 ppm
FeSO4) at 80 ꢀC. Water uptakes were determined according
to their increased mass after soaking in deionized water at
rt for 24 h. Water uptake was calculated using eq 2:
were embedded in epoxy resin and sectioned 70 nm thick.
TEM images were taken on a JEOL, JEM-2100 (HR) TEM
using an accelerating voltage of 200 kV.
RESULTS AND DISCUSSION
W
wet 2Wdry
Synthesis and Chemical Characterization of SP1
water uptake 5
3100%
(2)
A novel bisphenol monomer, 1, possessing six possible sulfo-
nation sites, was synthesized as described, and characterized
by 1H NMR spectroscopy. A NMR spectrum of 1 is shown in
Figure 1. It consists of a singlet peak at 4.68 ppm and a mul-
tiplet of peaks at 6.8 to 7.5 ppm, assigned to the OH group
and aromatic protons, respectively, as indicated. P1 was syn-
thesized by a one-pot polymerization of monomer 1 in the
presence of 4,40-difluorobenzophenone and bisphenol A in
87% yield. The weight-average and number-average molecu-
lar weights of P1 were 879 kDa and 437 kDa, respectively,
as determined by a single peak in the GPC analysis. Polymers
were also synthesized from other molar ratios of bis(4-fluo-
rophenyl) sulfone, 4,40-(hexafluoroisopropylidene)diphenol,
and 1, but multi-modal molecular weights were produced
(as observed by GPC traces) and were not analyzed further.
Wdry
The temperature was increased from 20 ꢀC to 80 ꢀC to
investigate temperature dependence. The hydration number,
k (water molecules per sulfonic acid group), was calculated
by combining the water uptake data and the IEC value as fol-
lows eq 3:
10003ðWwet 2Wdry
IEC3Wdry 318
Þ
k5
(3)
where Wdry and Wwet are the weights of the dried and wet
samples, respectively. The dimensional swelling ratio was
calculated from the change in film dimensions as follows
eqs 4 and 5:
L
wet2Lday
The 1H NMR spectrum of the P1 is shown in Figure 2(a).
Peaks located at 7.78 ppm are assigned to the protons of 40-
difluorobenzophenone labeled i and n. The two peaks at 6.9
ppm and 7.05 ppm are assigned to the bisphenol A protons
(labeled j and k). The multiplet at 6.8 to 7.5 ppm is assigned
to aromatic protons of monomer 1, but the single peak at
4.68 ppm position (OH group of monomer 1) disappeared
upon polymerization. The 1H NMR analysis is in complete
agreement with the target structures of the copolymer com-
position (monomer 1: 4,40-(hexafluoroisopropylide-ne)diphe-
nol : bis(4-fluorophenyl) sulfone, feed ratio 0.5:0.5:1), as
shown in Figure 2(a).
DL5
3100%
(4)
(5)
Lday
T
wet2Tday
DT5
3100%
Tday
where Tdry, Twet, Ldry, and Lwet are the thickness and length
of dry and wet samples, respectively.
In-plane proton conductivity was measured by ac imped-
ance spectroscopy, using
a Solartron 1260 frequency
response analyzer (FRA) that employed a two-electrode
configuration, according to a procedure described else-
where.31 The membrane (10 3 5 mm) was placed between
the two Pt electrodes of a conductivity cell, and a 100 mV
sinusoidal ac voltage over a frequency range of 10 MHz-
100 Hz was applied. The resulting Nyquist plots were fitted
to a standard Randles-equivalent circuit to determine the
membrane resistance. Proton conductivity (r) was calcu-
lated using eq 6:
P1 was sulfonated to various extents using chlorosulfonic
acid. Generally, the introduction of sulfonic acid groups by
chlorosulfonic acid preferentially occurs on electron-rich aro-
matic rings. The structures of the SP1-X copolymers (where
X indicates IEC) was confirmed by 1H NMR spectroscopy, as
shown in Figure 2(b). The signals at 6.7–7.2 ppm become
broad after sulfonation and new signals appear at 7.6 ppm
and 7.75 ppm [labeled 1- and 7 in Fig. 2(b)], due to the
presence of the sulfonic acid group.
L
r5
(6)
RA
Baseline-corrected FTIR spectra of P1 and SP1 are shown in
Supporting Information Figure S1, off-set for clarity. The
absorption band at 1023 cm21 corresponds to Ph-O-Ph sym-
metric stretching, and the bands at 1330 cm21 and
1500 cm21 correspond to Ph-O-Ph asymmetric stretching of
the poly(arylene ether). All polymers exhibit absorption
bands at 1606 and 1651 cm21 due to C5C and C5O stretch-
ing. Characteristic bands at 1035 cm21 and 1110 cm21 are
assigned to symmetric and asymmetric stretching of the
O5S5O group. The intensity of this peak increased with
increasing degree of sulfonation (IEC), again confirming
introduction of the sulfonic acid groups. Further support of
sulfonation is provided by IEC analysis, determined by
where L (cm) is the distance between electrodes, R (X) is
the membrane resistance, and A (cm2) is the cross-sectional
area of the membrane. An ESPEC SH-241 temperature/
humidity chamber was used to measure membrane conduc-
tivity under conditions of variable temperature and humid-
ity. The membranes were equilibrated overnight in the
chamber at
humidity.
a predetermined temperature and relative
For TEM analysis, membranes were stained with sliver ions
by immersion overnight in 1 M AgNO3, rinsing with water
and drying under vacuum for 24 h. The stained membranes
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The corresponding thickness of SP1-X membranes are listed
in Table 1 and ranged from 28 to 40 mm. The water uptake
of SP1 increased with increasing IEC, as shown in Figure 3,
and were much higher than Nafion 117. The Figure 4,
shown as hydration number increased with temperature
with the exception of SP1-1.88, which remained relatively
constant. The in-plane and through-plane dimensional
length changes of SP1 membranes (listed in Table 1) were
evaluated by comparing their hydrated state and dry states.
Generally, dimensional changes increased with increasing
IEC and water content. SP1-2.22 and SP1-2.44 membranes
exhibited good dimensional stability yet high water uptake.
Their swelling ratio varied by 5 and 18% at 80 ꢀC, respec-
tively, similar to that of Nafion 117. Comparisons with
Nafion 117, indicate that the higher rigidity of the aromatic
chain of SP1 confers a distinct improvement over other
flexible chain analogues. Despite the much larger water
sorption capacities, the dimensional swelling of the SP1 is
relatively low. We attribute this to a high free volume
resulting from bulkiness of the tetra-phenylated structure,
leading to water sorption without excessive dimensional
swelling.
FIGURE 1 1H NMR spectrum of bisphenol monomer 1.
titration, which are listed in Table 1 and ranged from 1.88–
2.67 mmol/g. The degree of sulfonation of SP1-X increased
upon increasing the IEC values and ranged from 57 to 81%.
The hydrolytic stability of the membranes was examined
by immersing them in water at 140 ꢀC for 24 h (see
Table 2). Of the four types of SP1 membranes examined,
SP1-1.88, SP1-2.22, and SP1-2.44 lost 2, 9, and 10 wt %,
respectively, by dissolution, but retained their flexibility
and transparency. SP1-2.67 completely dissolved. Oxidative
stability was evaluated using Fenton0s reagent (3%
Characterization of Membranes
TGA curves of P1 and SP1 are shown in Supporting Informa-
tion Figure S2 and the data extracted are listed in Table 1.
P1 exhibits exceꢀllent thermal stability, with 5 wt % loss
occurring at 490 C (Td5%). SP1 polymers are less thermally
stable, exhibiting several distinct degradation steps, with a
major processes occurring at ꢁ250 ꢀC, attributed to lossꢀof
sulfonic acid groups. Further degradation occurs at ꢁ530 C,
presumable due to fragmentation of the SP1-X main chain.
Td5% for SP1 membranes decreased (from 291 to 258 ꢀC)
upon increasing the IEC (from 1.88 to 2.67 mmol/g), sup-
porting the assertion that degradation of the sulfonate group
is responsible for their overall degradation within this tem-
perature region.
aq.H2O2,
3 1 h. SP1-1.88
ppm FeSO4) at 80 ꢀC for
remained unaltered after 1h immersion, while SP1-2.22
and SP1-2.44 lost 2 and 4 wt %, respectively. SP1-2.67
dissolved. SP1-2.22 and SP1-2.44 membranes had good
oxidative stabilities, as compared to the findings of previ-
ous work in which a sulfonated poly(arylene ether sulfone
ketone) possessing IECs of 2.12 and 2.43 mmol/g are
reported to completely dissolve/degrade.32
FIGURE 2 1H NMR spectrum of the (a)P1, (b)SP1–2.67. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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TABLE 1 Thermal Properties, Water Uptake, k, and Dimensional Stability of SP1 Membranes
Sample Name
IECa (titr.)
Degree of Sulfonation (%)b
T
d5%(ꢀC)c
Water Uptaked
k
Dl(%)e
DT(%)e
Thickness (mm)
SP1-1.88
SP1–2.22
SP1–2.44
SP1–2.67
Nafion117
1.88
2.22
2.44
2.67
0.091
57
67
74
81
291
267
262
258
–
34
87
98
10
21
22
–
5
3
30
15
18
–
15
20
–
28
40
f
–
35
175
30
19
18
7.7
a
d
e
IEC determined by acid-base titration.
In the hydrated state (80 ꢀC and 100% RH).
Change in film length (Dl) and thickness (DT) in hydrated state (80 ꢀC
b
The degree of sulfonation was calculated by assuming six sulfonic
acid groups per monomer unit. The theoretical highest IEC values of
SP1-X is 3.3 mmol/g.(degree of sulfonation: 100%).
and 100% RH).
f
Dissolved in water.
c
5% weight loss temperature (acid form).
Proton conductivities of SP1 are observed to increase with
RH (at 80 C), as shown in Figure 5. SP1 membranes exhib-
ited lower conductivities than Nafion 117, with the highest
conductivity being 4.51 mS/cm (SP1-2.67) at 40% RH, and
16.39 mS/cm at 60% RH, compared to 10 and 20 mS/cm
for Nafion 117. However, the proton conductivity of SP1
regions represents domains of ionic clusters; bright
areas represent hydrophobic regions. The TEM images of
SP1-2.22, SP1-2.44, and SP1-2.67 reveal ionic cluster sizes
of 6 to 12, 14 to 16, and 17 to 20 nm, respectively indicating
that the ion domains increase with increasing IEC. In com-
parison, Nafion membrane exhibits ionic domains 4 to 5 nm
in diameter.32
ꢀ
membranes showed
a greater temperature dependence
(measured under 95% RH), as shown in Figure 6 and listed
in Table 2, and the conductivity increased to a greater extent
with increasing IEC. For example, the proton conductivity of
the SP1-2.67 increased from 0.05 to 0.169 S/cm between
30 ꢀC and 80 ꢀC, i.e., much greater than Nafion 117, while
the conductivities of SP1-2.44 were similar to Nafion 117.
SP1-1.88 showed very little variation in proton conductivity
with increasing temperature, similar to that observed for
water uptake.
Stress-strain curves, determined at room temperature, are
shown in Figure 8 and Table 3. SP1-1.88 exhibits a tensile
strength 70.2MPa with elongation at break of 230.7%, simi-
lar to the parent polymer, P1, but the tensile strength
decreases with IEC. SP1-X membranes exhibit tensile
strengths ranging from 35.3 to 70.2 MPa, much higher than
Nafion 117 (21.8 MPa). Elongation at break ranged from
62% to 230%, respectively, generally lower than those of the
Nafion 117 (>243%). The Young0s moduli were between
0.20 and 1.17 GPa, which is much higher than that of Nafion
117 (at 0.05 GPa). In general, the SP1 membranes tolerated
much larger deformations and larger stresses without
TEM images of SP1 under two magnifications are shown in
Figure 7. Nanoscale phase separation is observed: dark
FIGURE 4 Hydration number as a function of temperature for
SP1-X. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
FIGURE 3 Water uptake as a function of temperature for SP1-X
copolymers. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
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TABLE 2 Proton Conductivity, Hydrolytic and Oxidative Stability of SP1 Membranes
Proton
Proton
Conductivity
(mS/cm),
95%RH
Conductivity
(mS/cm), 80 ꢀC
Oxidative
Stabilitya
Hydrolytic
Stabilityb
Sample Name
IEC (mmol/g)
40%RH
60%RH
30 ꢀC
80 ꢀC
Residual Weight (%)
Residual Weight (%)
SP1-1.88
SP1–2.22
SP1–2.44
SP1–2.67
Nafion117
1.88
2.22
2.44
2.67
0.091
0.1
0.24
2.61
8.44
16.39
20
1.07
14.73
43.1
52.66
64
3
100
98
98
91
90
0.5
15
1.81
4.51
10
102
169
100
94
c
c
-
-
-
-
a
b
c
The polymer was heated in Fenton’s reagent (FeSO4 (2 ppm) in 3%
H2O2) at 80 ꢀC for 1 h.
After 24 h at 140 ꢀC.
Dissolved in water.
FIGURE 5 Proton conductivity of SP1-X membranes as a func-
tion of RH at 80 ꢀC. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
FIGURE 6 Proton conductivity of SP1-X membranes as a func-
tion of temperature at 95% RH. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 7 TEM images of stained membranes: (a) SP1-2.22 (b)
SP1-2.44, (c) SP1-2.67, membranes. Scale bar 100 nm. Inset
scale bar is 20 nm.
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ionic aggregates or absorption of water in the free volume of
the polymer—the latter being supported by the lower degree
of dimensional swelling with respect to water uptake.
This work demonstrates that the multiphenylated polymer
backbone confers high rigidity and induces free volume in
membranes that facilitate water sorption for proton conduc-
tivity, while retaining high mechanical strength. Current
research is directed to examining these materials in fuel
cells, both as the proton exchange membrane and the ion-
omer for the catalyst layer.
ACKNOWLEDGMENTS
The authors are grateful to the National Science Council of Tai-
wan (NSC 101-2923-P-110-001-MY2) and the Natural Sciences
and Engineering Research Council of Canada for financial sup-
port of this work. The authors also gratefully acknowledge Y. Y.
Chuang at Fuel Cell Center of Yuan Ze University for the TEM
measurements.
FIGURE 8 Stress–strain curves of SP1-X membranes.
breaking compared to Nafion 117, demonstrating their excel-
lent mechanical properties, again attributed to the rigidity
and free volume of the multiphenylated structure.
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