Novel polyaromatic ionomers with large hydrophilic domain and long hydrophobic chain targeting at highly proton conductive and stable membranes

Article abstract of DOI:10.1039/c1jm10950b

A novel dihydroxyl monomer bearing 18 electron rich phenyl rings were synthesized and polymerized with other monomers bearing electron deficient phenyl rings to give dense and selective sites in macromolecules for post-sulfonation, which was successfully

Full text of DOI:10.1039/c1jm10950b

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PAPER
Novel polyaromatic ionomers with large hydrophilic domain and long
hydrophobic chain targeting at highly proton conductive and stable
membranes
a
a
a
a
Dongyang Chen, Shuanjin Wang, Min Xiao, Yuezhong Meng and Allan S. Hayb
*
*
Received 4th March 2011, Accepted 19th May 2011
DOI: 10.1039/c1jm10950b
A novel dihydroxyl monomer bearing 18 electron rich phenyl rings were synthesized and polymerized
with other monomers bearing electron deficient phenyl rings to give dense and selective sites in
macromolecules for post-sulfonation, which was successfully conducted in ClSO3H/CH2Cl2 solution at
1
room temperature in a subsequential step. The chemical structures were confirmed by H NMR and
FT-IR spectra. The ionic exchange capacity (IEC) was controlled to be from 0.65 to 1.21 mequiv gꢀ1 to
afford considerable proton conductivity. Distinct phase separation was observed in the resulting
membranes from SAXS profiles. The SPAEK-5 with an IEC of 1.21 mequiv gꢀ1 gave better proton
conductivity than Nafion 117 at all tested temperatures under 100% relative humidity. The membranes
exhibited an exceeding stability when immersing in Fenton’s reagent (3 wt.% H2O2 + 2 ppm FeSO4) at
80 ꢁC. These properties make them promising candidates for electrochemical applications.
separations where the hydrophobic phase imparts mechanical
Introduction
support and the hydrophilic phase forms proton conducting
channels.6,7
Sulfonated polymeric ionomers, as one of the most distinguished
functional polymeric materials, have attracted extensive interest
for a variety of applications such as polymer electrolyte
membrane fuel cell, actuator, vanadium redox flow battery,
electrolyzer, nanofiltration, chlor-alkali industry and so on.1,2
Among all the properties proton conduction is the basic func-
tionality, whereas the mechanical strength should be satisfied for
assembly and the thermal and chemical stability should be filled
for device operation. The perfluorosulfonic acid ionomers which
integrate a hydrophobic Telfon-like backbone with hydrophilic
ionic side chain possess excellent proton conductivity at low
temperatures, which provides valuable models of efficient proton
conduction for the design of alternative candidates for different
purposes. Since these perfluorosulfonic acid ionomers possess
low ion selectivity, high electrolyte permeability, are environ-
mentally unfriendly andhave high cost, numerous works have
been done on the development of high performance proton
exchange membranes for electrochemical applications.3–5 The
high oxidative stability of perfluorosulfonic acid ionomers is
attributed to the great hydrophobicity of the fluorine atom,
which is not accepted to be environmentally benign. However,
the excellent proton conductivity is attributed to their phase
Sulfonated aromatic polymers have attracted lots of attention
owning to their high thermal and chemical stability, excellent
mechanical strength and easy manufacturing. The introduction
of the sulfonic acid group is approachable through copoly-
merization with sulfonated monomer or post-sulfonation of
parent polymers bearing sulfonation sites. However, it is
envisaged that the proton conductivities of randomly sulfonated
ionomers are too low as compared with NafionTM (Dupont) at
the same ionic exchange capacity (IEC) level. Redesign of the
polymer chemical structure is demanded for the development of
high proton conductive ionomers.8,9 Hay et al. have advanced
several linear or branched end-capped ionomers exhibiting
defined phase separation, however, the IEC of those ionomers is
limited by the balance between the amount of end group and the
molecular weight of the ionomers.10–12 Ueda et al. have devel-
oped locally and densely sulfonated ionomers possessing
improved proton conductivity due to the formation of a well-
connected proton path. They have demonstrated that the ion-
omer with an IEC of 2.38 mequiv gꢀ1 has comparable proton
conductivity with Nafion even at 30% relative humidity.13,14
Miyatake et al. have presented aromatic ionomers containing
highly sulfonated block with proton conductivity comparable to
(<40% RH) or higher than (>40% RH) that of Nafion
membranes although still with higher IEC.15 They also have
exploited the influence of acidity on the proton conductivity,
and achieved better performance than the common sulfonated
poly(fluorenyl ether sulfone)s.16
aState Key Laboratory of Optoelectronic Materials and Technologies, The
Key Laboratory of Low-carbon Chemistry & Energy Conservation of
Guangdong Province, Sun Yat-Sen University, Guangzhou, 510275,
(China). E-mail: mengyzh@mail.sysu.edu.cn; wangshj@mail.sysu.edu.cn
bDepartment of Chemistry, McGill University, 801 Sherbrooke West,
Montreal, Quebec, H3A 2K6, Canada
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ꢁ
We have previously investigated many ionomers with sulfonic
acid group on the main chain or pendent, randomly or multi-
block distributed.17–20 It is demonstrated that the aggregation
and the mobility of sulfonic acid group are of primary impor-
tance for proton conduction. Herein, we report the design of
novel ionomers bearing highly centralized and flexible sulfonic
acid groups with the aim to construct unhindered proton con-
ducting channels. The long hydrophobic chain is expected to
impact mechanical support even after the degradation of
hydrophilic segments, so as to avoid the direct contact of cathode
and anode in the corresponding cell.
150 C for 3 h to eliminate the produced water and then cooled
down to 90 ꢁC. 0.3341 g (1 mmol) decafluorobiphenyl was added
and the reaction was continued for another 2 h. Finally the
mixture was cooled down and precipitated out at water. Crude
product was collected by filtration and recrystallized from
ethanol/chloroform twice. ꢁPure crystals were obtained with 83%
1
yield (0.4502 g). m.p.: 97 C. H NMR (400 MHz, CDCl3, d):
3.83 (s, 6H, CH3), 6.88–6.94 (d, 4H, Ar H), 7.02–7.08 (d, 4H,
Ar H).
Synthesis of monomer 2. To a 25 mL three-neck round bottom
flask equipped with a N2 inlet, a Dean–Stark trap and a magnetic
stirrer, 0.5424 g (1 mmol) monomer 1 and 1.3616g (8 mmol) 4-
phenylphenol were charged with 1.3900 g (10 mmol) K2CO3,
10 mL NMP as solvent and 5 mL toluene azeotropic agent. The
mixture was heated at 145–150 ꢁC for 3 h to eliminate the
produced water then the reaction temperature was increased to
190 ꢁC and hold for 20 h. The mixture was cooled down and
poured into water. The precipitate was collected by filtration and
purified by column chromatography on a silica gel (dichloro-
methane: hexane ¼ 1 : 1). Yield: 71% (1.2382 g). m.p.: 123–
125 ꢁC. 1H NMR (500MHz, CDCl3, d): 3.66 (s, 6H, CH3), 6.42–
7.55 (m, 80H, Ar H).
Experimental
Materials
4, 40-Difluorobenzophenone and decafluorobiphenyl were
purchased from Aldrich Chemical Co. and the former was
recrystallized from ethanol. Bis(4-hydroxylphenyl)sulfone
(99.9% purity), 4-phenylphenol, 4-methoxylphenol, boron
tribromide were bought from Aladdin reagent Co., China. N,N0-
Dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), N-
methyl-2-pyrrolidone (NMP), toluene, CHCl3, CH2Cl2,
methanol, ethanol, concentrated H2SO4 (95–98%), ClSO3H,
HCl, NaOH, tert-butylamine, anhydrous K2CO3 and aqueous
H2O2 (30%) were obtained from commercial sources. DMAc and
Synthesis of monomer 3. To a solution of monomer 2 (1.7440 g,
1 mmol) in dichloromethane (20 mL) was added 3 mL BBr3
dropwise at ꢀ78 ꢁC. The mixture was stirred at ꢀ78 ꢁC for 1 h.
Then the reaction temperature was elevated to room temperature
and the reaction continued for 10 h, the mixture was diluted with
CH2Cl2, washed with water, dried over MgSO4 and concentrated
in vacuum. High purity product was obtained by column chro-
matography purification (dichloromethane) on a silica gel. Yield:
ꢀ
toluene were dried with 4 A molecule sieves prior to use.
Potassium carbonate was dried at 130 ꢁC for 10 h prior to use.
Synthesis
The synthetic processes of monomers and polymers are depicted
in Scheme 1 and Scheme 2 respectively.
ꢁ
1
75% (1.2863 g). m.p.: 140–142 C. H NMR (500MHz, CDCl3,
Synthesis of monomer 1. To a 25 mL three-neck round bottom
flask equipped with a N2 inlet, a Dean–Stark trap and a magnetic
stirrer, 0.2482 g (2 mmol) 4-methoxylphenol were charged with
0.4170 g (3 mmol) K2CO3, 10 mL DMF as solvent and 5 mL
toluene as azeotropic agent. The mixture was heated at 145–
d): 5.29 (s, 2H, OH), 6.42–7.55 (m, 80H, Ar H).
Copolymerization of poly(arylene ether ketone)s (PAEK)s
(typical procedure, PAEK-1). To a 25 mL three neck flask
equipped with a magnetic stirrer, a N2 inlet, a Dean–Stark trap
Scheme 1 Synthesis of monomers.
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Scheme 2 Copolymerization of PEAKs and sulfonation of SPEAKs and t-Bu-PAEKs.
and a condenser, monomer 3 (0.0343 g, 0.02 mmol), Bis(4-
hydroxylphenyl)sulfone (0.0245 g, 0.98 mmol), potassium
carbonate (0.0209 g, 1.5 mmol), 4, 40-Difluorobenzophenone
(0.0218 g, 1 mmol), toluene (2 mL) and DMSO ꢁ(3 mL) were
charged. The mixture was first heated at 145–150 C for 3 h to
eliminate the produced ꢁwater, then the toluene evaporated out
and brought up to 175 C, and reacted at that temperature for
another 3 h. After cooling down, the viscous mixture was poured
into ethanol to precipitate out the polymer which was collected
by filtration. The polymer was purified by redissolving it in
CHCl3 and precipitated out three times. The product was dried at
120 ꢁC in vacuum for 24 h.
in CH2Cl2 was added dropwise under vigorously stirring over
3 h. The reaction was continued for another 5 h. The resulting
pale purple product precipitated out from the solution during the
course. The precipitates were washed with hexane three times
and then with water three times. 90 wt.% of the solid was dis-
solved in 20 mL DMAc. To the solution 20 mL of 3 wt.% NaOH
aqueous solution was added to react for 5 h, then the reaction
mixture was acidified with 50 mL of 5 vol.% HCl for another 5 h.
The resulting solution was dialyzed for 3 days (Molecular Weight
Cut Off: 6000) and the solvent was removed by rotary
evaporator.
Synthesis of t-Bu-PAEKs (typical procedure). The remaining 10
wt.% polymer from the above sulfonation procedure was dis-
solved in 5 mL NMP, then an excess amount of tert-butylamine
was added and magnetically stirred for 24 h. The solvent was
removed in vacuum. The resulting solid was wash in boiling
Synthesis of sulfonated poly(arylene ether ketone)s (SPAEKs).
To a 150 mL round bottom flask, 75 mL dichloromethane and
0.4573 g (1 mmol) PAEK were introduced. After PAEK was
dissolved completely, 3.6 mL of 1 M solution of HCl (3.6 mmol)
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ethanol and boiling water both for 1 h with changiꢁng the solvent
twice. Then the solid was dried in vacuum at 100 C for 3 h.
Water uptake
The dry membranes were weighed and immersed in deionized
water at different temperature for 24 h. The water uptake was
defined as weight change to that of the dry membrane.
Membrane preparation. The synthesized ionomers were dis-
solved in DMAc and cast onto glass plates. Membranes with
thickness between 100–200 mm were obtained after being dried at
Oxidative stabilities
ꢁ
80 C for 24 h at atmosphere then 120 ꢁC for 6 h in vacuum.
Followed by release from the glass plates, membranes were
ꢁ
Tested in Fenton’s reagent (3 wt.% H2O2 + 2 ppm FeSO4) at
ꢁ
immersed in 1 M H2SO4 solution at 80 C for 2 h and boiling
80 C. The membranes with the thickness ranging from 100 mm
to 160 mm were immersed in Fenton’s reagent and conditioned at
deionized water for 3 h with changing the water at least three
times. Membranes were kept in deionized water until use.
ꢁ
80 C Water Bath Shaker.
Proton conductivity
Characterization
Measured by electrochemical impedance spectroscopy.21
Samples were cut into circles with diameter of 1 cm and soaked in
deionized water for 20 h prior to test. Proton conductivity (s) of
the membranes in the transverse direction was determined by
measuring the impedance spectrum of a cell with the given
membrane sample sandwiched between two gold electrodes.
Instrumental
Nuclear magnetic resonance (NMR) spectra were recorded at
400 MHz in a Bruker DRX NMR instrument and the chemical
shifts were listed in ppm downfield from tetramethylsilane
(TMS). Gel permeation chromatography (GPC) analysis was
performed on a Waters Breeze system equipped with a Waters
Styragel column, Waters 515 HPLC pump and Waters 2414
refractive index detector, chloroform as an elution solvent at
a flow rate of 1 mL minꢀ1 and polystyrene as standards for
calibration. HPLC analysis was performed on SHIMADZU
HPLC instrument equipped with LC-20AT pump and SPD-20A
detector. FT-IR spectra were recorded on a PerkinElmer Spec-
trum 100 Fourier transform spectrometer with membrane
samples. Inherent viscosity of ionomers was determined using an
Ubbelohde viscometer in DMAc at 20 ꢁC. Melting point was
taken on a SGW X-4 melting point apparatus. Thermal stability
was analyzed using a PerkinElmer Pyris Diamond TG/DTA
analyzer. The temperature was increased from 50 ꢁC to 600 ꢁC at
a heating rate of 10 ꢁC minꢀ1 under N2 atmosphere. The proton
conductivity was carried out on a Solartron 1255 B frequency
response analyzer coupled with a Solartron 1287 electrochemical
interface in the frequency range of 1 Hz to 1 MHz. The glass-
transition temperature (Tg) was determined on a Seiko 220 DSC
instrument at a heating rate of 10 ꢁC minꢀ1 under N2 protection.
The tensile properties were determined by SANS (Shenzhen,
China) electromechanical universal test machine (model CMT-
4014). Samples were cut into dumbbell shape and immersed in
deionized water prior to testing. Matrix-assisted laser desorption
ionization time-of-flight (MALDI TOF) mass spectra were
d
sðs=cmÞ ¼
RS
Where S and d are the face area and thickness of the membrane,
and R is derived from the low intersect of the high frequency
semicircle on a complex impedance plane with the Re(Z0) axis.
Results and discussion
Synthesis of monomers and polymers
Decafluorobiphenyl was used to synthesize linear polymers
owning to the different activity of two end fluorine atoms from
the other eight fluorine atoms.22,23 From this point, we designed
a novel difluoro monomer (monomer 3) from decafluoro-
biphenyl through three reaction steps as illustrated in Scheme 1.
It can provide 18 sulfonation sites closely connected to the
central biphenyl through 10 flexible ether bonds, which is
expected to modulate the hydrophilic phase toward larger
channels as compared to tranditional copolymers or block
architectures. The 1H NMR spectrum of monomer 3 is shown in
Fig. 1. The presence of the strong singlet at d ¼ 5.29 ppm vali-
dates the successful transformation from methoxyl to hydroxyl.
However, the peaks from 6.42 ppm to 7.55 ppm are indistin-
guishable or overlapped, in which a same phenomenon was
reported on the monomer with similar complex structure in
literature.24 Sample purity was higher than 99% according to
HPLC results. The molecular weights measured by MALDI-
TOF for monomer 2 and monomer 3 are 1742.5 and 1714.5
respectively.
recorded on
a
BRUKER DAL TONICS Ultraflex
#xX_CYR_HEX_428; with the instrument set in the positive
reflection mode. Lithium bromide and dithranol were used as
cationization reagent and matrix respectively. Small Angle
X-Ray Scattering (SAXS) spectrum was determined on Bruker
D8 Advance X-Ray Diffractometer. Samples were conditioned
at atmosphere overnight and irradiated by X-ray (Cu-Ka, l ¼
High molecular weight poly(arylene ether ketone)s (PAEKs)
were synthesized via one-pot copolymerization of monomer 3,
bis(4-hydroxylphenyl)sulfone and 4,40-difluorobenzophenone
through nucleophilic substitution reaction. As shown in Table 1,
the PAEKs containing varying amounts of monomer 3 were
obtained in high yields. The number-average molecular weights
of all samples were higher than 23 kDa, implying the high purity
of monomer 3. It is found that only the electron rich phenyl rings
of poly(aryl ether)s could be sulfonated in the presence of large
excess of ClSO3H in CH2Cl2 solution at room temperature.10 The
ꢀ
1.5406 A) at 40 kV and 30 mA.
Ion exchange capacity (IEC)
Calculated by the integral ratio of all aromatic protons and
1
protons in methyl group in H-NMR of the corresponding t-
butyl bearing polymers.
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Fig. 1 1H NMR spectrum of monomer 3.
Table 1 Basic properties of PAEKs
Mn
Mw
Sample
Monomer 3 content (%)a
Yield (%)
>95
(kDa)
(kDa)
Tg (ꢁC)
Td-5%/ꢁCb
Td-max/ꢁCc
PAEK-1
PAEK-2
PAEK-3
PAEK-4
PAEK-5
2.0
2.3
2.8
3.5
4.6
26
24
24
23
24
46
42
38
39
34
195.7
191.7
194.8
192.2
187.7
498.5
479.2
478.0
471.1
468.1
569.4
573.3
574.7
568.8
570.7
a
Molar ratio of Monomer 3 in the hydroxyl groups containing monomers; b The 5% weight loss temperature; c The maximum weight loss temperature.
introduction of sulfone and ketone groups as strong electron
withdrawing groups promises good stability of the adjacent
phenyl rings in the sulfonation process and good solubility of the
built polymer for efficient post-reaction. Selective sulfonation
was successfully carried out in CH2Cl2 using ClSO3H as sulfo-
nation agent. Sulfonated poly(arylene ether ketone)s (SPAEKs)
were obtained with high inherent viscosities as shown in Table 2.
The ionic exchange capacity (IEC) can be calculated from the
integral area ratio of all aromatic protons to protons in the
with the IEC varied from 0.65 to 1.21 mequiv gꢀ1 were obtained
(IECNafion117 ¼ 0.91 mequiv gꢀ1). The sulfonation degrees were
calculated to be larger than 80%. Good solubility in N,N0-
dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and
N-methyl-2-pyrrolidone (NMP) was found for SPAEKs, while
PAEKs only showed good solubility in CHCl3 and CH2Cl2.
The chemical structures of PAEKs were examined by 1H NMR
and FT-IR techniques, which are shown in Fig. 2 and Fig. 3
respectively. After the introduction of sulfonic acid groups,
a resonance peak of sulfonic acid group is observed as a single
and broad peak in 1H NMR spectrum (Fig. 4), which is a typical
resonance peak of active proton. In FT-IR spectrum, the
1
methyl group in the H NMR spectrum. Every t-butyl group
represents one sulfonic acid group in the polymer. The IEC
measured in this way is demonstrated to be in good accordance
with the titrated IEC value.10 The representative 1H NMR
spectrum of t-butyl bearing polymers (t-Bu-PAEK-4) is shown in
Fig. 5 (see later). An obvious resonance peak at 1.25 ppm is
observed, which is assigned to the resonance of protons in methyl
group. The resonance peaks from 6.70 ppm to 8.20 ppm are
assigned to the signal of aromatic protons. From the integral
area ratio of these two kinds of peaks, the t-butyl group content
in the polymer can be calculated and translated to the sulfonic
acid group content (the IEC value of the polymer). By control-
ling the feed ratio of monomer 3, ionomers with different IECs
can be readily synthesized. As shown in Table 2, five samples
Table 2 Basic properties of SPAEKs
IEC/
Inherent
Ionomers mequiv. gꢀ1 Solubility viscosity/dL gꢀ1 Tg/ꢁC Td-5%/ꢁC
SPAEK-1 0.65
SPAEK-2 0.82
SPAEK-3 0.98
SPAEK-4 1.07
SPAEK-5 1.21
DMF
0.41
0.44
0.48
0.53
0.44
192
188
194
201
195
355
346
345
337
336
DMAc
DMSO
NMP
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Fig. 2 1H NMR spectrum of PAEK-5.
absorption peak at 1032 cmꢀ1 of SPAEK-5 is assigned to the
asymmetric vibration of O]S]O in sulfonic acid group, which
is absent in the FT-IR spectrum of PAEK-5. Both 1H NMR and
FT-IR results indicate the successful introduction of sulfonic
acid groups.
Thermal property
One advantage of the adoption of aromatic polymers as sup-
porting materials is their good thermal properties which are
beneficial to elevate their operation temperature for potential
applications. Because of the long hydrophobic chain (accounts
for more than 95 mol%) of SPAEKs, the glass transition
temperatures (Tg) are not obviously influenced by the introduc-
tion of sulfonic acid groups and were measured to be about
Fig. 3 Normalized FT-IR spectra of PAEK-5 and SPAEK-5.
Fig. 4 1H NMR spectrum of SPAEK-5.
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Fig. 5 1H NMR spectrum of t-Bu-PAEK-4.
ꢁ
190 C as listed in Table 2. These results are similar to those of
parent PAEKs. This is quite different from the random poly-
meric ionomers and those block polymeric ionomers with rela-
tive short hydrophobic chain (Tg can not be obtained before
decomposition),18 and even the block polymeric ionomers with
certain length of hydrophobic chain (great increase in Tg after
sulfonation).25
The TG curves of PAEKs and SPAEKs are shown in Fig. 6
and Fig. 7 respectively. PAEKs exhibit good thermal stability all
ꢁ
the way up to 460 C, while SPAEKs exhibit two steps decom-
position profile (the decomposition of sulfonic acid group in the
first step and the decomposition of the backbone of the polymer
in the second step), the same as most of the reported sulfonated
poly(arylene ether)s.26 The 5% weight decomposition tempera-
tures (Td-5%) are listed in Table 2. It can be seen that all of the
ꢁ
Fig. 7 TG curves of SPAKEs.
Td-5% are higher than 330 C. With the decreasing in IEC, the
Td-5% of SPAEKs increases gradually.
Fig. 6 TG curves of PAEKs.
Fig. 8 SAXS profiles of SPAEK-2 and SPAEK-5.
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Fig. 9 Graphical illustration of the structure of SPAEKs.
Membrane morphology
The self-assembling of hydrophilic and hydrophobic phases
produces an ionomer peak in small angle X-ray scattering
(SAXS) which is attributed to the electron density difference of
the separated phases. The scattering peak is considered to be the
spacing of proton conductive channels.27 Fig. 8 shows the SAXS
profiles of SPAEK-2 and SPAEK-5 membranes. Intensive peaks
are observed, indicating periodical electron density changed in
distances of 3.8 nm (d ¼ 2p/q) and 2.6 nm for SPAEK-2 and
SPAEK-5 respectively. The spacing of proton conductive chan-
nels in SPAEK-5 is lower than that of SPAEK-2 because of its
higher IEC value to form more hydrophilic channels. From these
representative SAXS profiles it could be inferred that distinct
phase separations in SPAEKs are achieved by this novel
molecular design. The graphical illustration of the structure of
SPAEKs is shown in Fig. 9. The hydrophilic domain is composed
of sulfonated monomer3 segments, whose single molecular length
is as large as 1.74 nm (calculated using Cache software). The
hydrophobic segments account for 98ꢂ95.4 mol% of the linear
ionomers. These large hydrophilic domains and long hydro-
phobic chains should facilitate the self-assembling of each phase
during membrane preparation and account for the distinct phase
separation observed above. The highly centralized sulfonic acid
groups thus are more convenient to form inter-connected proton
conducting channels.
Fig. 10 Water uptake (a) and proton conductivity (b) of Nafion 117 and
SPAEKs as a function of temperature at 100 RH%.
Proton conductivity and water uptake
The dependence of water uptake and proton conductivity on
temperature at 100% relative humidity is shown in Fig. 10. Both
properties increase with increasing temperature for SPAEKs and
Nafion 117. Furthermore, the higher IEC of SPAEKs resulted in
the higher water uptake and proton conductivity. SPAEK-2,
which has slightly lower IEC compared to Nafion 117, exhibits
about half the proton conductivity of Nafion 117. This behavior
is not often reported in literatures. In case of only increasing the
IEC to 1.21 mequiv gꢀ1 (SPAEK-5), the proton conductivity
becomes higher than that of Nafion 117 at all the tested
temperatures. The dramatically improvement is considered to be
the result of distinct phase separation originated from the novel
structure as illustrated above. It should be mentioned that,
according to the results obtained in our measurements, the
proton conductivities of Nafion 117 are slightly lower than that
reported in literatures.
Fig. 11 The dependence of proton conductivity on relative humidity at
90 ꢁC.
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Table 3 Mechanical properties of SPAEKs
phenomenon was observed. The samples of SPAEKs did not
break into pieces in a Water Bath Shaker for more than 24 h. By
comparing with our previous similar systems under the same
testing conditions, the oxidative stability of SPAEK-5 exhibited
a dramatic improvement as shown in Table 4. During the
accelerated test, samples underwent decomposition at the
hydrophilic domains which leads to break up of the polymeric
main chain and results in loss of mechanical strength. The
SPAEKs can maintain their physical appearances for a much
longer durability with very slight changes on the surface. This is
attributed to the long hydrophobic chain which impacted the
mechanical strength after the decomposition of the hydrophilic
domains. The anti-oxidative and hydrolysis durability can endow
the membrane as a stable separator to avoid short circuit of the
electrochemical devices.
Elongation at
break (%)
Sample
Tensile strength/Mpa
SPAEK-1
SPAEK-2
SPAEK-3
SPAEK-4
SPAEK-5
22.6 ꢃ 2%
19.3 ꢃ 1%
18.1 ꢃ 2%
16.5 ꢃ 4%
15.3 ꢃ 3%
26.1 ꢃ 5%
28.8 ꢃ 2%
33.4 ꢃ 3%
38.9 ꢃ 4%
44.0 ꢃ 2%
Fig. 11 shows the dependence of proton conductivity on
ꢁ
relative humidity at 90 C. It is well accepted that the efficient
transportation of proton needs the assistance of water.1 With
increase in relative humidity, the proton conductivity shows an
obvious increase. SPAEK-5 exhibits comparable proton
conductivity with Nafion 117 under low relative humidity, which
validates the construction of well-connected proton conductive
channels. The higher proton conductivity of Nafion 117 here
should not only be ascribed to the pronounced phase separation
of hydrophilic and hydrophobic domains but also the shape of
the hydrophilic channels (cylindrical28). The hydrophobic phase
of Nafion 117 consists of crystalline domains and amorphous
domains. However, the hydrophobic phase of SPAEKs is totally
amorphous according to the DSC characterization. This intrinsic
difference leads to the different interaction between hydrophilic
and hydrophobic phases, so as to the different shape of the
hydrophilic channels. Therefore, it is of great challenge to
modulate the proton conductive channels not only from the
viewpoint of phase separation but also the interaction between
hydrophilic and hydrophobic phases. This provides a new way to
design the molecular structure of sulfonated polymeric ionomers.
Conclusions
Novel aromatic ionomers with large hydrophilic domain and
long hydrophobic chain structure were synthesized and charac-
terized for proton exchange membrane application. Selective
sulfonation was achieved through rational molecular design,
which was to control the electron density of corresponding
phenyl rings of the polymers. The resulting membranes exhibited
distinct phase separation and modulated proton conductive
channels. The ionomer with IEC as low as 0.82 mequiv gꢀ1
showed about half proton conductivity of Nafion 117 under
100% relative humidity. The SPAEK-5 with an IEC of 1.21
mequiv gꢀ1 gave better proton conductivity than Nafion 117 at
all tested temperatures under 100% relative humidity. These
specially designed ionomers exhibit highly thermal and oxidative
stability. They are promising candidates for electrochemical
applications.
Mechanical property and oxidative stability
Acknowledgements
From Table 3 it can be seen that all of the samples possess
relative high tensile strength (higher or comparable to the tensile
strength of Nafion29). The tensile strength of SPAEKs decreases
with increasing IEC value while the elongation at break gives
a reverse trend. This is because the higher IEC value should result
in higher water uptake (Fig. 10), leading to more swelling of the
membrane and lower cohesion of polymer molecules.
The authors would like to thank the China High-Tech Devel-
opment 863 Program (Grant No.: 2007AA03Z217), Guangdong
Province Universities and Colleges Pearl River Scholar Funded
Scheme (2010); Guangdong Province Natural Science Founda-
tion (Grant No.: 10151027501000096); Guangdong Education
Bureau (Key Project cxzd1004); Chinese Universities Basic
Research Founding for financial support of this work.
It is well accepted that the oxidative stability decreases with
increasing IEC value and water uptake.20 The oxidative stability
of SPAEKs is supposed to be similar to other aromatic polymeric
ionomers with the same IEC value and water uptake due to the
same degradation mechanism involved. However, an abnormal
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