Synthesis and characterization of sulfonated polyimides containing trifluoromethyl groups as proton exchange membranes

Article abstract of DOI:10.1002/cjoc.201180266

A novel sulfonated diamine, 4,4′-bis(4-amino-3- trifluoromethylphenoxy) biphenyl 3,3′-disulfonic acid (F-BAPBDS), was successfully synthesized by nucleophilic aromatic substitution of 4,4′-dihydroxybiphenyl with 2-chloro-5-nitrobenzotrifluoride, followed by reduction and sulfonation. A series of sulfonated polyimides of high molecular weight (SPI-x, x represents the molar percentage of the sulfonated monomer) were prepared by copolymerization of 1,4,5,8-naphathlenetetracarboxylic dianhydride (NTDA) with F-BAPBDS and nonsulfonated diamine. Flexible and tough membranes of high mechanical strength were obtained by solution casting and the electrolyte properties of the polymers were intensively investigated. The copolymer membranes exhibited excellent oxidative stability due to the introducing of the CF₃ groups. The SPI membranes displayed desirable proton conductivity (0.52 × 10^-1-0.97 × 10^-1 S·cm ^-1) and low methanol permeability (less than 2.8 × 10 ^-7 cm²·s^-1). The highest proton conductivity (1.89 × 10^-1 S·cm^-1) was obtained for the SPI-90 membrane at 80°C, with an IEC of 2.12 mequiv/g. This value is higher than that of Nafion 117 (1.7 × 10^-1 S·cm^-1). Furthermore, the hydrolytic stability of the obtained SPIs is better than the BDSA and ODADS based SPIs due to the hydrophobic CF₃ groups which protect the imide ring from being attacked by water molecules, in spite of its strong electron-withdrawing behaviors. Copyright

Full text of DOI:10.1002/cjoc.201180266

FULL PAPER
Synthesis and Characterization of Sulfonated Polyimides
Containing Trifluoromethyl Groups as Proton Exchange
Membranes
Zhang, Qingshuang(张清爽)
Li, Qiaoling*(李巧玲)
Department of Chemistry, School of Science, North University of China, Taiyuan, Shanxi 030051, China
A
novel sulfonated diamine, 4,4'-bis(4-amino-3-trifluoromethylphenoxy) biphenyl 3,3'-disulfonic acid
(
2
F-BAPBDS), was successfully synthesized by nucleophilic aromatic substitution of 4,4'-dihydroxybiphenyl with
-chloro-5-nitrobenzotrifluoride, followed by reduction and sulfonation. A series of sulfonated polyimides of high
molecular weight (SPI-x, x represents the molar percentage of the sulfonated monomer) were prepared by copoly-
merization of 1,4,5,8-naphathlenetetracarboxylic dianhydride (NTDA) with F-BAPBDS and nonsulfonated diamine.
Flexible and tough membranes of high mechanical strength were obtained by solution casting and the electrolyte
properties of the polymers were intensively investigated. The copolymer membranes exhibited excellent oxidative
stability due to the introducing of the CF groups. The SPI membranes displayed desirable proton conductivity
3
－
1
－
1
－
1
－
7
2
－
1
(
0.52×10 —0.97×10 S•cm ) and low methanol permeability (less than 2.8×10 cm •s ). The highest pro-
－
1
－
1
ton conductivity (1.89×10 S•cm ) was obtained for the SPI-90 membrane at 80 ℃, with an IEC of 2.12 me-
－
1
－
1
quiv/g. This value is higher than that of Nafion 117 (1.7×10 S•cm ). Furthermore, the hydrolytic stability of the
obtained SPIs is better than the BDSA and ODADS based SPIs due to the hydrophobic CF groups which protect
3
the imide ring from being attacked by water molecules, in spite of its strong electron-withdrawing behaviors.
Keywords sulfonated copolyimide, proton exchange membrane, proton conductivity, fuel cell
Introduction
matic polyimides derived from 1,4,5,8-naphthalene-
tetracarboxylic dianhydride (NTDA) have superior
chemical resistance, excellent thermal and mechanical
stability, and good film forming ability, which are just
required properties for PEMs used in fuel cell systems.
However, their stability towards water at high humidity
atmosphere and temperature was not enough to satisfy
the practical requirements because of the hydrolysis of
Polymer electrolyte membrane fuel cells (PEMFCs)
including direct methanol fuel cells (DMFCs) have been
attracting more and more attention in the ubiquitous
energy-dependent world because of the world-wide en-
1
-4
ergy crisis and their high power efficiency. Polymer
electrolyte membrane (PEM) plays a key role in the
PEMFC system in terms of fuel cell performance. The
function of a PEM in a fuel cell is to separate effectively
the anode and cathode gases and to conduct protons un-
der a variety of operating conditions and over the entire
life time of the fuel cell. Sulfonated perfluoropolymers
such as Nafion (Dupont) can satisfy many of these
PEMs requirements and have found practical use in
PEMFCs. Nafion membranes, however, have some spe-
cific limitations including their very elevated cost, high
fuel permeability, and loss of the preferable properties
at temperature above 80 ℃. One of the important chal-
lenges in the current fuel cell research is to develop a
highly conductive and durable membrane that can serve
as an alternative to the perfluorinated ionomers. Non-
fluorinated aromatic hydrocarbon polymers have at-
9
imide rings. A variety of polyimides (PIs) have been
molecularly designed on the basis of knowledge ac-
quired on the structure-property relationships to meet
with the specific requirements for optional performance
in PEMFC applications. Some of SPIs sulfonated dia-
7
,15-18
mines displayed better water stabilities.
1
9
Okamoto et al. reported that sulfonated copolyim-
ides with more flexible sulfonated diamines showed
much better water stability than other sulfonated aro-
matic polyimides. Watanabe et al. showed that polyim-
ides bearing aliphatic sulfonic acid groups enhanced
water stability. The improvement in water stability of
polyimides was attributed to the higher basicity of the
1
3,15
17
sulfonated diamine.
ported on the successful synthesis of sulfonated poly-
imides bearing a CF groups. The synthesized material
Zhang and co-workers re-
1
-6
tracted great interest in recent years.
7
-18
Some earlier reports
have demonstrated that aro-
3
*
E-mail: zqs_sbk@yahoo.cn
Received December 9, 2010; revised March 11, 2011; accepted March 22, 2011.
Project supported by the Natural Science Foundation of Shanxi Province (No. 2009011099) and the Shanxi Province Abroad Study Foundation (No.
008064).
2
1460
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2011, 29, 1460— 1466

Synthesis and Characterization of Sulfonated Polyimides Containing Trifluoromethyl Groups
exhibited high proton conductivity, excellent oxidative
stability and an improved water stability, thus showing
potential as a polymer for proton exchange membranes.
They attributed the improved oxidative and hydrolytic
hydrate (30 mL) was slowly added at the reflux tem-
perature and the mixture was stirred at this temperature
for about 24 h. The reaction solution was filtered when
hot to remove Pd/C, and the filtrate was dried by rotary
evaporation. The crude product was recrystallized from
3
stabilities to the CF groups which protect the polymer
chains from being attacked by water or radicals mole-
cules.
ethanol to afford 16.5 g (84%) of yellow powders. m.p.
1
76.0—76.5 ℃. H NMR (DMSO-d
6
) δ: 5.48 (br, 4H,
It is spectulated that the incorporation of CF
3
groups
NH
2
), 6.82 (dd, J＝1.3, 2.7 Hz, 2H, ArH), 6.87 (d, J＝
and the flexible ether groups into SPIs can lead to sub-
stantial improvements in polymer properties including
good proton conductivity, resistance to oxidation as well
as excellent mechanical properties. Therefore, a series
of sulfonated polyimides were synthesized by copoly-
merization of a novel sulfonated diamine [4,4'-bis(4-
amino-3-trifluoromethylphenoxy) biphenyl 3,3'-disul-
fonic acid, F-BAPBDS] with NTDA and 4,4'-diami-
nodiphenyl ether. The thermal and chemical stability,
mechanical strength, water absorption and proton con-
ductivity of SPIs have been investigated.
3.0 Hz, 4H, ArH), 6.94 (d, J＝6.7 Hz, 4H, ArH), 7.54 (d,
J＝9.0 Hz, 4H, ArH). Anal. calcd for C26
18 6 2 2
H F N O C
61.91, H 3.60, N 5.55; found C 61.87, H 3.62, N 5.59.
Synthesis of 4,4'-bis(4-amino-3-trifluoromethyl-
phenoxy)biphenyl 3,3'-disulfonic acid (F-BAPBDS)
100 mL three-necked flask equipped with a mechanical
stirring device was charged 5.0 g (10 mmol) of F-BAPB.
The flask was cooled in an ice-bath, after which 8 mL of
concentrated sulfonic acid was slowly added under stir-
ring. After the sulfuric acid had been completely added,
the mixture was stirred at 0 ℃ for 0.5 h and then
slightly heated until the BANPB was completely dis-
solved. The mixture was again cooled in an ice-bath,
Experimental
3
and 2.0 mL of fuming sulfonic acid (60% SO ) was
Materials
slowly added. Subsequently, the stirring of the mixture
was continued at 0 ℃ for 0.5 h after which it was
slowly heated to 50 ℃ and kept at this temperature for
4
,4'-Dihydroxybiphenyl, 2-chloro-5-nitrobenzotri-
fluoride and 4,4'-diamino-diphenyl ether (ODA) were
purchased from Aldrich and used as received. 1,4,5,8-
Naphathlene-tetracarboxylic dianhydride (NTDA) (Al-
drich) was purified by vacuum sublimation before use.
Sulfuric acid (95%) and fuming sulfuric acid (60%)
were used as received. All other solvents were used
without further purification. Ultra-pure water was ob-
tained from a Millipore Milli-Q purification system.
6
h. After cooling to room temperature, the mixture was
poured into 120 g of crushed ice. The solid was filtered
off, and then dissolved in ethanol. The resulting solution
3
was basified with Et N and the yellow precipitate was
filtered off, washed with ethanol, and dried in vacuum
1
at 90 ℃. 5.6 g of product was obtained (yield 85%). H
NMR (in the Et
3
N salt form, DMSO-d
6
) δ: 7.91 (d, J＝
6
6
.0 Hz, 2H, ArH), 7.36 (dd, J＝6.0, 12.3 Hz, 2H, ArH),
Monomer synthesis
.81 (s, 2H, ArH), 6.68 (s, 4H, ArH), 6.57 (d, J＝15.4
Synthesis of 4,4'-bis(4-nitro-2-trifluoromethyl-
phenoxy) biphenyl (F-BNPB) To a 250 mL round-
bottomed flask were charged with 4,4'-dihydroxybi-
phenyl (7.4 g, 0.04 mol), 2-chloro-5-nitrobenzotrifluo-
ride (20.3 g, 0.09 mol) and DMSO (100 mL). Potassium
carbonate (6.9 g, 0.05 mol) was added. After 30 min of
stirring at room temperature, the mixture was heated at
Hz, 2H, ArH), 5.53 (s, 4H, NH
2
), 3.08 (t, J＝210 Hz,
12H), 1.07 (t, J＝120 Hz, 18H).
Polymer synthesis
Synthesis of F-BAPBDS-based copolyimides
SPIs were synthesized according to the method previ-
1
0
ously reported as follows (SPI-80): To a 100 mL com-
pletely dried three-necked flask were added 0.532 g (0.8
mmol) of F-BAPBDS, 8 mL of m-cresol, and 0.68 mL
1
00 ℃ for 12 h. Then, the reaction mixture was poured
into 400 mL of methanol/water (V∶V＝10∶1), and the
precipitated yellow solid was collected by filtration. The
yield of product was 21.44 g (95.0%). The crude prod-
uct was recrystallized from DMF/methanol to give fine,
3
of Et N successively under nitrogen flow. After the
F-BAPBDS was completely dissolved, 0.268 g (1 mmol)
of NTDA, 0.040 g (0.2 mmol) of nonsulfonated diamine
1
(ODA) and 0.244 g of benzoic acid were added. The
yellow crystals. m.p. 140.0 — 140.5 ℃ ; H NMR
mixture was stirred at room temperature for a few min-
(
DMSO-d
6
) δ: 7.10 (d, J＝6.0 Hz, 2H, ArH), 7.22 (d,
utes and then heated at 80 ℃ for 4 h and 180 ℃ for
J＝9.0 Hz, 4H, ArH), 7.78 (d, J＝9.0 Hz, 4H, ArH), 8.4
2
0 h. After cooling to 80 ℃, additional 5 mL of
(
dd, J＝1.5, 3.4 Hz, 2H, ArH), 8.46 (d, J＝6.0 Hz, 2H,
m-cresol was added to dilute the highly viscous solution,
and then the solution was poured into acetone. The fi-
ber-like precipitate was filtered off, washed with ace-
tone thoroughly, and dried in vacuum oven for 12 h at
14 6 2 6
ArH). Anal. calcd for C26H F N O : C 55.33, H 2.50,
N 4.96; found C 55.38, H 2.57, N 5.02.
Synthesis of 4,4'-bis(4-amino-2-trifluoromethyl-
phenoxy)biphenyl (F-BAPB) To a 100 mL three-
necked flask equipped with a magnetic stirring device
and nitrogen inlet were charged F-BNPB (22.6 g, 0.04
mol), 10% Pd/C (0.2 g) and ethanol (200 mL). Subse-
quently, under a nitrogen atmosphere, hydrazine mono-
1
50 ℃ to give product with 97% yield. FT-IR: 1710
－
1
(
ν
sym C＝O), 1666 (νasym C＝O) and 1348 (νC—N imide) cm .
Copolymers SPI-60, SPI-70 and SPI-90 were pre-
pared by following the same procedure as above, except
Chin. J. Chem. 2011, 29, 1460— 1466
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1461

Zhang & Li
FULL PAPER
that the feed ratio of F-BAPBDS to ODA was 6∶4, 7∶
(IEC) of SPI-x was calculated from the molar ratio of
sulfonated diamine to nonsulfonated diamine in feed,
3
and 9∶1, respectively.
Membrane preparation The polymers were dis-
and also was determined through titration. The mem-
＋
solved in m-cresol to form a 8—10 wt% solution at 80
branes in the H form were immersed in a 1 mol/L
＋
NaCl solution for 24 h to liberate the H ions (the H
＋
＋
℃
. Then the solution was filtered and cast on a glass
＋
sheet. The solvent was evaporated at 120 ℃ for 12 h.
The as-cast membranes were soaked in ethanol for 12 h
to remove the residual solvent, and then treated with 1.0
mol/L sulfonic acid at room temperature for 48 h for
proton exchange. The proton-exchange membranes
were thoroughly washed with water and then dried in
vacuum at 150 ℃ for 10 h.
ions in the membrane were replaced by Na ). The H
ions in solution were then titrated with 0.01 mol/L
NaOH.
Proton conductivity and methanol permeability
The proton conductivity of SPI and Nafion® mem-
branes was measured by a four-electrode ac impedance
method from 0.1 Hz to 100 kHz, 10 mV ac perturbation
and 0.0 V dc rest voltage. Impedance spectra were re-
corded using a Princeton Applied Research Model 273A
Potentiostat (Model 5210 frequency response detector,
EG&G PARC, Princeton, NJ). The membranes were
fixed in a measuring cell made of two outer gold wires
to feed current to the sample and two inner gold wires to
Polymer characterization
1
Measaurements H NMR were measured at 300
MHz on a Bruker AV300 spectrometer (Germany).
FT-IR spectra of copolyimides were obtained with a
Bio-Rad digilab Division FTS-80 FT-IR spectrometer
2
0
(Cambridge, MA). The inherent viscosities were deter-
measure the voltage drops. Conductivity measure-
ments under fully hydrated conditions were carried out
with the cell immersed in liquid water. The proton con-
ductivity (σ) of the membranes was calculated using the
following equation:
mined on 0.5 g/dL concentration of polymer in m-cresol
with an Ubbelohde capillary viscometer at (30±0.1) ℃.
The thermogravimetric analyses (TGA) were obtained
in nitrogen with a Perkin-Elemer TGA-2 thermogra-
vimetric analyzer (Inspiratech 2000 Ltd, UK) at a heat-
ing rate of 10 ℃/min. Tensile measurements were per-
σ＝d/L
s s
•W •R
formed with
a
mechanical tester Instron-1211
where the d is the distance between reference electrodes,
and L and W are the thickness and width of the mem-
instrument (Instron Co., U.S.A.) at a speed of 1 mm/min
at ambient humidity (ca. 30% relative humidity).
Water uptake and dimensional change The
membrane (50—70 mg per sheet) was dried at 100 ℃
under vacuum for 12 h until constant weight as dry ma-
terial was obtained. It was immersed into deionized wa-
ter at room temperature for 12 h and then quickly taken
out, wiped with tissue paper, and quickly weighted on a
microbalance. The water uptake of the membranes was
calculated according to Eq. (1).
s
s
brane respectively.
The methanol permeability was determined at room
temperature (20 ℃) by using a cell basically consisting
of two-half-cells separated by the membrane, which was
－
1
fixed between two rubber rings. Methanol (2 mol•L )
was placed on one side of the diffusion cell, and water
was placed on the other side. Magnetic stirrers were
used on each compartment to ensure uniformity. The
concentration of the methanol was measured by using a
SHIMADZU GC-1020A series gas chromatograph.
Peak areas were converted into methanol concentration
with a calibration curve. The methanol permeability was
calculated by Eq. (4).
water uptake＝(Wwet－Wdry)/Wdry×100%
(1)
where W and W are the weight of the dry and the
dry
wet
corresponding water-swollen membranes, respectively.
The dimension changes of the membranes were
measured in thickness and in the plane direction, which
were characterized by Eqs. (2) and (3).
A DK
C (t)＝
C (t－t )
B
A
0
VB
L
∆
T
L
c
＝(Twet－Tdry)/Tdry
(2)
(3)
where C
A
and C
B
are the methanol concentration of feed
side and permeated through the membrane, respectively.
are the effective area, the thickness of
membrane and the volume of permeated compartment,
∆
c
＝(Lwet－Ldry)/Ldry
A, L and V
B
where T and L are the thickness and diameter of the
wet
wet
membrane equilibrated at 70% relative humidity (RH)
for 24 h, respectively; Tdry and Ldry refer to those of the
membrane equilibrated in liquid water for 12 h.
respectively. DK is defined as the methanol permeabil-
0
ity. t is the time lag.
Oxidative stability A small piece of membrane
Results and discussion
sample was soaked in Fenton’s reagent (30% H
taining 30 mg/L FeSO ) at room temperature (about 20
). The stability was evaluated by recording the time
2 2
O con-
Synthesis and characterization of monomer and
polymers
4
℃
when membranes began to dissolve and dissolved com-
pletely.
The synthesis of the new sulfonated diamine mono-
mer F-BAPBDS was achieved in three steps as shown
in Scheme 1. The diphenolate anion, which was formed
Ion exchange capacity Ion exchange capacity
1
462
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Chin. J. Chem. 2011, 29, 1460— 1466

Synthesis and Characterization of Sulfonated Polyimides Containing Trifluoromethyl Groups
by reacting 4,4'-dihydroxybiphenyl with potassium car-
bonate, underwent aromatic nucleophilic displacement
reaction with 5-chloro-2-nitrobenzotrifluoride to yield
dinitro compounds, 1,4-bis(4-nitro-3-trifluoromethyl-
phenoxy)-benzene (F-BNPB). The reduction of F-
BNPB was conducted with 10% Pd/C and hydrazine
monohydrate at 80 ℃ for 24 h to give the compound
F-BAPB in high yield (84%). Then, the F-BAPBDS was
prepared by sulfonation with fuming sulfonic acid (10%
m-cresol that was widely applied for polynaphthalimide,
using benzoic acid as the catalyst as shown in Scheme 2.
The degree of sulfonation (DS) of the copolymer was
readily controlled through the monomer feed ratios of
F-BAPBDS to ODA so that a series of copolymers with
different IEC values could be obtained. The copolymers
were denoted as SPI-x, where x was the molar fraction
of F-BAPBDS in the feed. The SPBIBI polymers were
obtained in almost quantitative yields (above 96%) and
displayed reduced viscosity values ranging from 1.09 to
1.26 dL/g (Table 1) indicating high molecular weight of
the copolymers. All copolyimides in salt form are solu-
ble in m-cresol, DMSO, DMAc and DMF, and form a
flexible and tough film by casting from the solution.
Figure 2 displays the FT-IR spectra of SPI-x. The strong
absorption bands around 1710 (νsym C＝O), 1666 (νasym
SO
3
) at 50 ℃ for 2 h and precipitated as the inner salt.
The sulfonation only occurred in the 3,3'-positions of
the central biphenyl rings in the case of this reaction
temperature since these two positions were more reac-
tive than the other ones. The two phenyl rings to which
the amino and CF
due to the strong electron-withdrawing effect of the
protonated amino and CF groups. Its structure was
3
groups attached were less reactive
－
1
3
C＝O) and 1348 (νC—N imide) cm were assigned to the
1
confirmed by H NMR analysis (Figure 1). The assign-
ments of each proton are also given in the Figure 1, and
this spectrum agrees well with the proposed structure.
The novel F-BAPBDS-based SPIs were prepared by the
traditional high temperature polymerization method in
naphthalenic imide rings. The bands around 1197 and
－
1
1034 cm were assigned to asymmetric and symmetric
S＝O stretching vibration of sulfonic acid groups and
the intensity of this peak increased with the increasing
sulfonated monomer ratio.
Scheme 1 Synthesis of the sulfonated diamine
Scheme 2 Synthesis of the sulfonated copolyimides
Table 1 Various properties of the SPI membranes
b
－
1
IEC /(mequiv•g )
a
－
1
Sample
η /(dL•g )
Tensile strength/MPa
Young’s modulus/GPa Elongation at break/%
Calculated
1.69
Measured
SPI-60
SPI-70
SPI-80
1.26
1.16
1.21
1.09
1.64
1.81
1.94
2.10
98.4
97.5
92.3
89.0
1.09
1.04
0.97
0.98
29.8
28.9
32.8
28.5
1.85
1.99
SPI-90
2.12
a
－
1
b
0
.5 g•dL in m-cresol at 30 ℃. Calculated, IEC calculated from DS; measured, IEC measured with titration.
Chin. J. Chem. 2011, 29, 1460— 1466
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
1463

Zhang & Li
FULL PAPER
1
Figure 1 H NMR of sulfonated diamine.
erties of the polymers are listed in Table 1. The mem-
branes demonstrated tensile stresses of 89.0—98.4 MPa,
Young’s moduli of 0.97—1.09 GPa, and elongation at
break values of 28.5%—32.8%. These results indicated
that the SPI membranes were, at high degrees of sul-
fonation, strong and tough enough for fuel cell applica-
tions.
Water uptake and dimensional change
It is well-known that proton transport through a sul-
fonated polymer membrane is strongly dependent on the
membrane hydration state. However, excessively high
levels of water uptake would result in membrane fragil-
ity and dimensional changes, which in turn would give
rise to a loss of mechanical properties. The water uptake
data are reported in Table 2 along with the water swell-
ing ratio at 20 ℃.
Figure 2 FT-IR spectra of SPI membranes in proton form.
Thermal and mechanical properties
Figure 3 shows the TG curves of the F-BAPBDS-
based SPIs in proton form. The first step of weight loss
up to 150 ℃ was due to the evaporation of absorbed
water. The second step of weight loss was attributed to
the decomposition of sulfonic acid groups. The desul-
fonation temperature was 250 ℃, which was similar to
The water uptake (WU) of SPI-x membranes with
IEC from 1.69 to 2.12 mequiv•g was in the range of
－
1
3
5%—63%. The number of water molecules absorbed
per sulfonic acid group (λ) was in the range of 11.5—
16.5 for the copolymers, which was lower than that
2
1
19
(
20—25) for the other SPIs. For example, the SPI-80
that for ODADS or BAPBDS -based SPIs. The last
weight loss, which started around 560 ℃, was attrib-
uted to the decomposition of the polymer backbone.
membrane (IEC＝1.99) showed lower water uptake
(54%) and λ value (14.0) than those of SPI-x based on
NTDA and 4,4'-diaminodiphenyl ether-2,2'-disulfonic
2
1
acid (WU＝87%, λ＝24.8, IEC＝1.95), and SPI-x
from NTDA and 4,4'-bis(4-aminophenoxy)biphenyl-
19
3
,3'-disulfonic acid (WU＝63%, λ＝18.5, IEC＝1.89).
The relatively lower WU of SPI-x could be interpreted
3
by the hydrophobic CF groups which decrease the hy-
drophilicity of polymer chains.
The membranes dimensional change along the
in-plane and through-plane directions of the sample due
to water absorption under fully hydrated conditions is
summarized in Table 2. As can be seen, a higher water
uptake tends to result in a more considerable dimen-
sional change. The F-BAPBDS-based SPI membranes
exhibited isotropic membrane swelling in water with
similar swelling ratio in thickness direction than in
plane direction. This is similar with the NTDA-
Figure 3 TGA curves of SPI membranes in proton form.
2
2
BAPBDS, but different from the cases of Nafion and
2
3
NTDA-BDSA/ODA, where the change in thickness
was much larger than that in plane direction.
Good mechanical properties of the PEMs are re-
quired for fuel cell applications. The mechanical prop-
1
464
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Chin. J. Chem. 2011, 29, 1460— 1466

Synthesis and Characterization of Sulfonated Polyimides Containing Trifluoromethyl Groups
Table 2 Water uptake, swelling ratio of the SPI membranes and Nafion 117
Size change/%
T ∆L
－
1
Sample
IEC/(mequiv•g )
Water uptake/% (m/m)
λ
∆
SPI-60
SPI-70
SPI-80
SPI-90
1.69
1.85
1.99
2.12
1.98
1.95
1.89
0.90
35
43
11.5
12.9
14.0
16.5
22.2
24.8
18.5
12.1
10
10
12
14
—
—
—
—
8
9
50
10
13
—
—
—
—
63
2
1
NTDA-BDSA/ODA
79
2
1
NTDA-ODADS/ODA
87
2
4
NTDA-BAPBDS/BAPB
Nafion 117
63
19.6
strongly dependent upon the water uptake since the
methanol permeated through the membranes as complex
Proton conductivity and methanol permeability
In PEM fuel cells, the proton conductivity of the
membrane is particularly important since it plays a sig-
nificant role in fuel cell performance. The proton con-
ductivities of obtained SPI membranes are reported in
Table 3. For comparison, Nafion 117 was measured
under the same conditions and resulted in a value of
＋
＋
forms such as CH
3
OH
2
and H
3
O . As expected, the
methanol permeability of all the SPI membranes was
observed to be lower than that of Nafion 117 mem-
branes (Table 3). For example, the methanol permeabil-
－7
2
－1
ity of the SPI-80 membrane was 1.9×10 cm •s ,
－6
2
－1
－
1
－1
while the value of Nafion 117 was 2.4×10 cm •s .
0
.9×10 S•cm at 20 ℃, which is similar to that
2
4
reported by Okamoto et al. The proton conductivities
of SPIs were close to that of Nafion 117 and ranged
Table 3 Proton conductivity and methanol permeability of SPI
and Nafion 117 membranes
－
1
－1
－1
from 0.52×10 to 0.97×10 S•cm at 20 ℃, which
are related to IEC and water uptake. The temperature
dependence of the proton conductivities of SPI mem-
branes and Nafion 117 measured in liquid water are de-
picted in Figure 4. The proton conductivity of SPIs
showed a linear temperature-dependence. As expected,
an increase in temperature results in an increase in pro-
ton conductivity based on the simplified diffusion
mechanism and thermal motion of protons in channels
within membranes. SPI-90 with 63% water uptake gave
Methanol
Proton conductivity/
－
1
Sample
SPI-60
permeability/
φ
(S•cm )
2
－
1
(
cm •s )
20 ℃
80 ℃
0.093
0.143
0.164
0.189
0.170
－
－
－
－
7
－
－
－
－
－
5
5
5
5
4
0.9×10
1.1×10
1.9×10
2.8×10
5.8×10
6.6×10
4.4×10
3.5×10
3.7×10
0.052
0.073
0.085
0.097
0.090
7
7
7
SPI-70
SPI-80
SPI-90
－
7
Nafion 117
24×10
－
1
－1
a conductivity of 0.97×10
S•cm
at 20 ℃ and
－
1
－1
1
.89×10 S•cm at 80 ℃, which was higher than
In order to understand the performance tradeoff be-
tween the permeability and conductivity, an investiga-
tion was carried out using the selectivity as a represen-
tation of the transport characteristics of both the proton
and methanol (σ/P) of the SPIs and Nafion 117 mem-
branes. The results are also shown in Table 3. The SPI
membrane was approximately 10 times more selective
than that of Nafion 117.
－
1
－1
－1
those of Nafion 117 (0.90×10 S•cm and 1.7×10
－
1
S•cm , respectively) measured at the same conditions.
For a fully hydrated membrane, the methanol trans-
port across a proton exchange membrane should be
Membrane stability to water and oxidation
The stability of sulfonated polyimide membranes in
water has been the subject of many researches because
it is one of the important factors affecting membrane
performance. The hydrolysis stability was measured by
immerging the samples in distilled water at 80 ℃ and
evaluated by the loss of mechanical strength (broken
when lightly bent). Table 4 lists the water stability of
the SPI-x membranes. Obviously, the SPI-x membranes
displayed better water stability than those main-chain-
type SPIs. For example, the SPI-80 is a little brittle for
4
6 h and did not break into pieces for more than 120 h
Figure 4 Temperature dependence of proton conductivity of
SPI membranes in water.
after being soaked in water at 80 ℃, but the SPIs based
Chin. J. Chem. 2011, 29, 1460— 1466
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
1465

Zhang & Li
FULL PAPER
2
1
on the NTDA-BDSA/ODA (1/1) and NTDA-ODADS/
drophilic HO• radicals and water molecular. At 80 ℃,
2
1
ODA (1/1) are brittle for 5.5 and 24 h, respectively.
The improved water stability results from the lower wa-
the SPI-90 membrane gave an IEC value of 2.12
－
1
－1
mequiv•g and the proton conductivity of 1.89×10
－
1
ter uptake and the hydrophobic CF
3
groups which could
S•cm . This value was higher than that of Nafion 117
－
1
－1
protect the polymer main chains from being attacked by
water molecules. However, the water stability of the
SPI-x membranes was far poor than that of SPIs based
(1.7×10 S•cm ). In addition, the SPI membranes
displayed much lower methanol permeability than that
of Nafion 117, and thus a higher selectivity.
19
BPDADS (＞1000 h, at 80 ℃ in water) due to the
lower basicity of sulfonated diamine. Although the hy-
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b
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－
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a
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a
b
Measured at 80 ℃; 30 ℃ in 30% H
2
O
2
containing 30 mg/L
2
c
FeSO ; The time when the membrane broke into pieces after
being shaken drastically; The time when the membrane dis-
4
d
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4 2 2
at 25 ℃ and the results were also summarized in Table
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1
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HOO•) occurring mainly in or in the vicinity of the wa-
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2
－
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2
1
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A novel sulfonated diamine of 4,4'-bis(4-amino-3-
trifluoromethylphenoxy) biphenyl 3,3'-disulfonic acid
22
3
(F-BAPBDS) was synthesized successfully and used for
2
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2
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groups which
protect the polymer chains from being attacked by hy-
(E1012092 Li, L.; Lu, Z.)
1
466
www.cjc.wiley-vch.de
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2011, 29, 1460— 1466