Sulfonated poly(arylene-co-naphthalimide)s synthesized by copolymerization of primarily sulfonated monomer and fluorinated naphthalimide bichlorides as novel polymers for proton exchange membranes

Article abstract of DOI:10.1021/ma060275k

A novel sulfonated aromatic dichloride monomer was successfully prepared by the reaction of 2, 5-dichlorobenzophenone with fuming sulfuric acid. Copolymerization of this monomer in the form of sodium salt (1) with N-(4-chloro-2-trifluoromethylphenyl)-5-ch

Full text of DOI:10.1021/ma060275k

Macromolecules 2006, 39, 6425-6432
6425
Sulfonated Poly(arylene-co-naphthalimide)s Synthesized by
Copolymerization of Primarily Sulfonated Monomer and Fluorinated
Naphthalimide Dichlorides as Novel Polymers for Proton Exchange
Membranes
Zhiming Qiu,†,‡ Shuqing Wu,†,‡ Zhiying Li,‡,§ Suobo Zhang,*,† Wei Xing,§ and
Changpeng Liu§
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China, Graduate School of Chinese Academy of
Sciences, Changchun, China, and State Key Laboratory of Electroanalytical Chemistry,
Changchun, China
ReceiVed February 6, 2006; ReVised Manuscript ReceiVed July 14, 2006
ABSTRACT: A novel sulfonated aromatic dichloride monomer was successfully prepared by the reaction of 2,
5-dichlorobenzophenone with fuming sulfuric acid. Copolymerization of this monomer in the form of sodium
salt (1) with N-(4-chloro-2-trifluoromethylphenyl)-5-chloro-1,8-naphthalimide (2) or bis(N-(4-chloro-2-trifluo-
romethylphenyl)-1,4,5,8-naphthalimide (3) generated two series of novel poly(arylene-co-naphthalimide)s I-x and
II-x where x represents the content of the sulfonated monomer. The synthesized copolymers with the -SO3H
group in the side chains possessed high molecular weights revealed by their high viscosity and the formation of
tough and flexible membranes. The copolymers exhibited excellent stability toward water and oxidation due to
the introduction of the hydrophobic CF3 groups. The sulfonated copolyimides that incorporated with 1,8-
naphthalimide (I-x) exhibited better hydrolytic and oxidative stabilities than those with 1,4,5,8-naphthalimide.
Copolymer I-50 membrane endured for more than 83 h in Fenton’s reagent at room temperature. The mechanical
properties of I-50 membrane kept almost unchanged after immersing membrane in boiling water for 196 h. The
proton conductivities of copolymer films increased with increasing IEC and temperature, reaching values above
6.8 × 10-1 S/cm at 80 °C. Consequently, these materials proved to be promising as proton exchange membranes.
Introduction
-CF2-SO3H in Nafion. Addition of more sulfonic acid conduc-
tors may increase conductivity, but it may lead to undesirable
large swelling and thus result in a dramatic loss of mechanical
properties. Aromatic polyimides are known to have superior
solvent-resistance, excellent mechanical strength and good film
forming ability because of the strong charge-transfer interaction
between the polymer chains.12 Okamoto and co-workers have
reported that sulfonated copolyimides from naphthalene-1,4,5,8-
tetracarboxylic dianhydride (NTDA) and more flexible sul-
fonated and nonsulfonated diamines show much better water
stability than other sulfonated aromatic polymers.13 Watanabe
et al. had shown that NTDA-based sulfonated polyimides with
bulky fluorenyl groups having unique water uptake behavior
and produced higher conductivities than Nafion.14 They also
reported that polyimides bearing aliphatic sulfonic acid groups
had enhanced water stability. The improvement in water stability
of polyimides was attributed to the higher basicity of the
sulfonated diamine.15,16
Polymer electrolyte membrane fuel cells (PEMFCs) have been
identified as promising power sources for transport, stationary,
and portable applications due to their low emissions and high
conversion efficiency.1-3 Improvement in performance of the
polymer electrolyte membrane (PEM) has been the focus in the
development of PEMFCs because the PEM is one of the key
components for successful PEMFC fabrications.4 Nafion
(Dupont), a perfluorosulfonic acid polymer, is a currently used
PEM in commercial systems. Nafion is chemically robust in
oxidizing environments and has good proton conductivity when
hydrated. However, some specific limitations exist for Nafion
membranes including very high cost, high gas permeability, and
loss of the preferable properties at high temperature.5 These
limitations have stimulated many efforts in the development of
new PEM materials, aiming to reduce costs and to overcome
technical drawbacks of Nafion.6 Sulfonated aromatic polymers,
such as poly(ether ether ketone),7 poly(ether sulfone)s,8 poly-
(arylene ether)s,9 polyimide,10 and poly(p-phenylene)s11 are
widely investigated as candidate PEM materials. Although each
of such sulfonated aromatic polymers has its own advantages,
most of them have failed to meet the requirement of high con-
ductivity and durability under fuel cell operating conditions.
Their lower conductivity can be attributed to chain rigidity, lack
of ion channels, and lower acidity of Ar-SO3H than that of
For PEMFC applications, the long-term durability of the
polymer exchange membrane is an important issue. Therefore,
the materials with chemical stability are highly required. Among
such sulfonated aromatic polymers, poly(p-phenylene)s (PPP)
have shown excellent thermal-oxidative stability and mechanical
property. However, they are characterized by poor film forming
ability.11,17
The objective of our work is to design and synthesize sul-
fonated polymers having both high proton conductivity and good
thermal oxidation stability. In this paper, we report the synthesis
of poly(arylene-co-naphthalimide)s by copolymerization of such
aromatic dichloride monomers (Scheme 1). The resulting
copolymers possessed the CF3 protected naphthalimide structural
* To whom correspondence should be addressed. E-mail: sbzhang@
ciac.jl.cn.
† State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences.
‡ Graduate School of Chinese Academy of Sciences.
§ State Key Laboratory of Electroanalytical Chemistry.
10.1021/ma060275k CCC: $33.50 © 2006 American Chemical Society
Published on Web 08/16/2006

6426 Qiu et al.
Macromolecules, Vol. 39, No. 19, 2006
Scheme 1. Syntheses of Monomers
into water at room temperature for a given time, the changes of
thickness and diameter were calculated from
characteristics. The activated sulfonic acid groups are separated
from naphthalimide moiety and localized on the meta- position
of electron withdrawing carbonyl groups in the side chain of
copolymers. The copolymers display good proton conductivity,
resistance to hydrolysis and oxidation, along with excellent
mechanical properties. A sulfonated monomer, sodium 3-(2,5-
dichlorobenzoyl)benzenesulfonate (1) and hydrophobic fluori-
nated naphthalimide dichloride comonomers (2) and (3) were
also synthesized for this study.
∆Tc ) (T - Ts)/Ts
∆Lc ) (L - Ls)/Ls
(2)
where Ts and Ls are the thickness and diameter of the membrane
equilibrated at 70% RH, respectively; T and L refer to those of the
membrane immersed in liquid water for 5 h.
Oxidative Stability. A small piece of membrane sample with a
thickness of about 40 µm was soaked in Fenton’s reagent (30 ppm
FeSO4 in 30% H2O2) at room temperature. The stability was
evaluated by recording the time when membranes began to dissolve
and dissolved completely.
Experimental Section
Materials. N,N-Dimethylacetamide (DMAc) was dried over
CaH2, distilled under reduced pressure, and stored over 4 Å
molecular sieves. Isoquinoline (99%) was used as received from
Shanghai Kuilin Chemical Co., Ltd. Reagent grade anhydrous NiBr2
was dried at 220 °C under vacuum. Triphenylphosphine (PPh3) was
recrystallized from hexane. Zinc dust was stirred with acetic acid,
filtrated, washed thoroughly with ethyl ether, and dried under
vacuum. 5-chloro-1,8-naphthalic anhydride and 1,4,5,8-naphtha-
lenetetracarboxylic dianhydride were used as received from Beijing
Multi. Technology Co., Ltd. 4-Chloro-2-(trifluoromethy)aniline
received from JS (Tianjin) Chemical & Metallurgical Co., Ltd was
used after distillation. 2,5-Dichlorobenzophenone was prepared by
an AlCl3 catalytic Friedel-Crafts acylation of 1, 4-dichlorobenzene
with benzoyl chloride according to a previously described proce-
dure.18
Ion Exchange Capacity. Ion exchange capacity (IEC) was
determined through titration. The membranes in the H+ form were
immersed in a 1 N NaCl solution for 24 h to liberate the H+ ions
(the H+ ions in the membrane were replaced by Na+ ions). The
H+ ions in solution were then titrated with 0.01 N NaOH.
Proton Conductivity. The proton conductivities of the copoly-
mer membranes were evaluated using electrochemical impedance
spectra in the temperature range of 20-80 °C. The impedance
measurements were carried out on a Solartron 1255B Frequency
Response analyzer and a Solartron 1470 Battery Test Unit (Solartron
Inc., U.K.) coupled with a computer. The membranes with 1 cm2
in area were sandwiched between two stainless steel blocking
electrodes. The samples were allowed to equilibrate at the desired
temperature for 0.5 h. The impedance spectra were recorded with
the help of ZPlot/ZView software (Scribner Associates Inc.) under
an ac perturbation signal of 10 mV over the frequency range of 1
MHz to 1 Hz.
Monomer Synthesis. Synthesis of Sodium 3-(2,5-Dichloroben-
zoyl)benzenesulfonate (1). To a 50 mL three-neck flask equipped
with a magnetic stirred bar was charged 10 mmol of 2,5-
dichlorobenzophenone. The flask was cooled in an ice bath.
Concentrated sulfuric acid (95%, 1.5 mL) was then added with
stirring. After 2,5-dichlorobenzophenone was dissolved, 3 mL of
fuming sulfuric acid (SO3 60%) was slowly added to the flask.
The reaction mixture was stirred at room temperature for 2 h and
then at 80 °C for additional 2 h. After cooling to room temperature,
the slurry was carefully poured into 100 mL of cooled 30% NaOH
aqueous solution. The resulting white precipitate was filtered and
then redissolved in 30 mL of water. After the solution was adjusted
to pH ) 7 with 0.5 M HCl, NaCl solid (8 g) was added to the
solution, resulting in a white precipitate. The precipitate was filtered,
washed with saturated NaCl aqueous solution, and dried at 80 °C
in a vacuum. Yield was 97%. 1H NMR (ppm, D2O): 8.09 (s, 1H),
7.9-8.0 (d, 1H), 7.7-7.8 (d, 1H), 7.57-7.59 (t, 1H), 7.45-7.50
(m, 3H). 13C NMR (ppm, DMSO-d6): 194.5 (s), 143.3 (s), 137.4
(s), 135.2 (s), 132.4 (s), 132.1 (s), 131.5 (s), 131.0 (s), 130.8 (s),
Measurements. 1H and 13C NMR spectra were measured at 300
MHz on an AV600 spectrometer. FT-IR spectra were obtained with
a Bio-Rad digilab Division FTS-80 FT-IR spectrometer. The
inherent viscosities were determined on 0.5 g/dL concentration of
polymer in NMP with an Ubbelohde capillary viscometer at 30 (
0.1 °C. The thermogravimetric analyses (TGA) were obtained in
nitrogen with a Perkin-Elemer TGA-2 thermogravimetric analyzer
at a heating rate of 10 °C/min. Tensile measurement were performed
with a mechanical tester Instron-1211 instrument at a speed of 1
mm/min. The relative humidity was 50% RH.
Water Uptake. The membrane (30-40 mg per sheet) was dried
at 80 °C under vacuum for 6 h until constant weight as dry material
was obtained. It was immersed into deionized water at room
temperature for 4 h. Then the membranes were taken out, wiped
with tissue paper, and quickly weighed on a microbalance. Water
uptake of the membranes was calculated from
WU ) (Ws -Wd)/Wd
(1)
where Ws and Wd are the weight of dry and corresponding water-
swollen membranes, respectively.
Dimensional Change. Dimensions change of the copolymer
membranes was investigated by immersing the round shape samples

Macromolecules, Vol. 39, No. 19, 2006
Sulfonated Poly(arylene-co-naphthalimide)s 6427
Figure 1. 1H NMR and 13C NMR spectra of monomer 1.
129.2 (s), 128.8 (s), 128.6 (s), 125.9 (s). FT-IR: 1675 cm-1 (CdO
stretching), 1233 and 1044 cm-1 (asymmetric and symmetric
stretching vibration of sodium sulfonate groups).
solution was filtered and cast on a glass sheet. The solvent was
evaporated by heating from 60 to 70 °C until the membranes were
dry and then dried in a vacuum oven at 100 °C for 48 h. The
thickness of the resulting membranes was in the range 25-40 µm.
Synthesis of N-(4-Chloro-2-trifluoromethylphenyl)-5-chloro-
1,8-naphthalimide (2). In a 500 mL round-bottomed flask, 41.02
g (0.10 mol) of 5-chloro-1, 8-naphthalic anhydride, 19.56 g (0.10
mol) of 4-chloro-2-(trifluoromethyl)aniline, 7.2 mmol of isoquino-
line, and 300 mL of acetic acid were added. The solution was
refluxed for 24 h. The reaction mixture was then poured into 500
mL of ethanol. The precipitated was filtered, washed with etha-
nol, and dried in a vacuum. The product was obtained by
sublimation under vacuum at 200 °C in the yield of 76.8% (31.52
g). Mp: 278-279 °C.1H NMR (DMSO-d6): 8.76-8.73 (1H, d),
8.66-8.64 (1H, d), 8.51-8.49 (1H, d), 8.13-8.00 (4H, m), 7.81-
7.78 (1H, d): IR (KBr): 1716 and 1672 cm-1 (CdO of imide),
1353 cm-1 (C-N stretching), 781 cm-1 (CdO bending). Anal.
Calcd for C19H8Cl2F3NO2 (410.17): C, 55.64; H, 1.97; N, 3.41.
Found: C, 57.12; H, 2.03; N, 3.89.
Results and Discussion
Monomer Syntheses. The synthetic route of monomers is
outlined in Scheme 1. Monomer (1), sodium 3-(2, 5-dichlo-
robenzoyl)benzenesulfonate, was synthesized with 97% yield
by the reaction of 2,5-dichlorobenzophenone with fuming
sulfuric acid at 80 °C for 2 h and subsequent neutralization with
NaOH. The molecular structure and composition of the sodium
sulfonate salt compound (1) was confirmed by FT-IR, 1H NMR,
and 13C NMR spectroscopies. The FT-IR spectrum (KBr sub-
strate) showed a intense absorption band at 1675 cm-1 (CdO
stretching) and bands at 1100 and 1044 cm-1 (asymmetric and
1
symmetric stretch vibration of sodium sulfonate groups). H
NMR and13C NMR spectra of (1) confirmed that the sulfonic
group was bonded to the meta-position of carbonyl groups
(Figure 1). Naphthalimide dichloride monomer (2) was synthe-
sized by the reaction of 5-chloro-1,8-naphthalic anhydride and
4-chloro-2-(trifluoromethyl)aniline in acetic acid, and puri-
fied by subliming under vacuum at 200 °C. It was used as a
hydrophobic comonomer for copolymerization with the mono-
mer (1). Incorporation of hydrophobic trifluoromethyl groups
ortho to the imide nitrogen could protect the polymer main
chains from being attacked by water molecules containing highly
oxidizing radical species. For comparison study, naphthalimide
dichloride (3) was also synthesized from naphthalene-1,4,5,8-
tetracarboxylic dianhydride (NTDA) and 4-chloro-2-trifluoro-
methylaniline. The chemical structures of 2 and 3 were
Synthesis of Bis(N-(4-chloro-2-trifluoromethylphenyl)-1,4,5,8-
naphthalimide) (3). This compound was prepared from 1,4,5,8-
naphathlenetetracarbonxylic dianhydride and 4-chloro-2-(trifluo-
romethyl)aniline using the same procedure as described above, then
sublimed under vacuum at 250 °C. The yield was 87%. Mp > 300
°C. 1H NMR spectrum in DMSO-d6: 8.79 (4H, s), 8.11-8.08 (4H,
m), 7.93-7.90 (2H, d). FT-IR: 1714 and 1684 cm-1 (CdO of
imide), 1349 cm-1 (C-N stretching), 771 cm-1 (CdO bending).
Anal. Calcd for C28H10Cl2F6N2O4 (623.29): C, 53.96; H, 1.62; N,
4.49. Found: C, 55.05.12; H, 1.73; N, 4.20.
General Procedure for Syntheses of Sulfonate Copolyimides.
NiBr2 (0.32 g, 1.42 mmol), PPh3 (2.60 g, 9.98 mmol), and zinc
dust (5.20 g, 80.00 mmol) were placed in a 250 mL three-necked
round-bottomed flask. The flask was evacuated and filled with
nitrogen three times. Dry DMAc (20 mL) was added via syringe
and the mixture became red brown in 20 min. Then, the monomer
1 (10 mmol) and comonomer (2 or 3) (10 mmol) were added and
stirred at 90 °C for 4 h. The resulting viscous mixture was diluted
with 50 mL of DMAc, filtered, and then poured into 200 mL of 10
wt % HCl/acetone. The polymer was collected by filtration, washed
with acetone, and dried in a vacuum at 200 °C for 24 h.
1
confirmed by FT-IR and H NMR (Figure 2).
Polymer Syntheses. As shown in Scheme 2, two series of
sulfonated copolymers were synthesized by Ni(0)-catalyzed
coupling polymerization of monomer 1 with 2 or 3, respectively.
The polymerizations were carried out under the reaction
conditions reported in the literature.18 The degree of sulfonation
(DS) of the copolymer was readily controlled through the
monomer feed ratios of 1 to 2 or 3. The copolymers were
Membrane Preparation. The sulfonated copolymers were
dissolved in N-methylpyrrolidinone (NMP) to form a 2-3%
solution at 80 °C under argon atmosphere for 8 h. Then the NMP

6428 Qiu et al.
Macromolecules, Vol. 39, No. 19, 2006
Figure 2. 1H NMR spectrum of 2 in DMSO-d6.
Scheme 2. Synthesis of Copolymer I and II
denoted as I-x and II-x, where x is the mole fraction of the
monomer 1 in the feed. The FT-IR spectra of copolymers are
shown in Figure 3. The strong absorption bands around 1716
cm-1 (VsymCdO), 1658 cm-1 (VasymCdO) and 1348 cm-1 (VCN
imide) are assigned to the naphthalenic imido rings. The band
s around 1100 and 1031 cm-1 were assigned to the stretch
vibration of sulfonic acid groups. The intensity of this peak
to 4:1, as expected for the composition of II-50. The peak at δ
) 148.5 could be assigned to the carbon attached -SO3H group.
The sulfonation degree values calculated from the ratio of the
signal intensity of C1 to that of C3 in II-50 was about 50%,
which are in very close agreement with the monomer feed ratio
during copolymerization. All copolymers are soluble in polar
organic solvents (DMAc, NMP) and form a flexible and tough
film by casting from the solution. Their solubility behavior is
shown in Table 1. Intrinsic viscosity values of all copolymers
were higher than 1.52dL/g in NMP at 30 °C, which indicated
successful copolymerization in producing high molecular weight
copolymers. Figure 5 shows the thermal stability of the
copolymer I-70 and II-70 investigated by TGA. It can be seen
that the polymers thin films exhibited a typical three-step
degradation pattern. The first weight loss up to ca. 200 °C is
ascribed to the loss of water molecules, absorbed by the highly
hygroscopic -SO3H groups. The second weight loss of around
300 °C is due to the decomposition of sulfonic acid groups by
the desulfonation. The third stage weight loss around 500 °C is
assigned to the decomposition of polymer main chain.
increased with an increasing sulfonated monomer ratio. The 13
C
NMR spectrum of II-50 is shown in Figure 4. The signal at δ
) 163.0 and δ ) 196.8 ppm was assigned to the carbonyl carbon
atom of imide ring (C1) and benzophenone units (C2),
respectively. The integration ratio of C1 to that of C2 is close
Water Uptake, Water Stability, and Mechanical Proper-
ties. The water uptake of sulfonated polymers is known to have
a profound effect on membrane conductivity and mechanical
properties.19 Water molecules dissociate acid functionality and
facilitate proton transport. However, excessively high levels of
water uptake can result in membrane fragility and dimensional
change, which lead to the loss of mechanical properties. The
water uptake and swelling ratio of copolymer membranes were
determined by measuring the changes in the mass and size,
respectively, before and after hydration. As shown in Table 2,
the room-temperature water uptake of II-50 and II-70 films with
Figure 3. FT-IR spectra of sulfonated polyimides.

Macromolecules, Vol. 39, No. 19, 2006
Sulfonated Poly(arylene-co-naphthalimide)s 6429
Figure 4. 13C NMR spectrum of II-50 in DMSO-d6.
Table 1. Solubilities of Sulfonated Copolyimides in Acidified Forms
solvents
decreased interchain interaction. In addition, the copolymer I-50
synthesized from less symmetrical monomer 2 contains regio-
irregular naphthalimide structure, which is produced by three
possible link types of monomer 2, such as head to head, head
to tail, and tail to tail. The breaking regular structures loosened
the film packing and markedly increased the interchain space
in which water molecules could be confined.
polymer ηa (dL/g) NMP DMSO DMAc MeOH DMF acetone water
I-40
I-50
I-70
I-90
II-50
II-70
2.57
2.35
2.37
1.92
1.52
1.57
++b
++
++
++
++
++
- -
+-
+-
+-
+-
+-
++
++
++
++
++
++
- -
- -
+-
+-
+-
+-
+-
+-
+-
- -
- -
- -
- -
- -
- -
- -
- -
- -
+-
- -
- -
+-
- -
- -
The stability of sulfonated polyimide membranes in water
has been the subject of much research because it is one of the
important factors affecting membrane performance.10a,c,e,13 In
general, polyimides display relatively low hydrolytic stability
due to the inherent hydrolytic feasibility of imido ring. Introduc-
ing the diamine moieties with high basicity proved to be highly
effective for improving the water stability of polyimides be-
cause the high basicity of the sulfonated diamine moieties
depresses the hydrolysis of imido ring.15,16 In the present study,
copolyimides I-x and II-x all contain the CF3 group, which is
unfavorable to depress the hydrolysis of imido ring due to the
electron withdrawing nature of the CF3 group. However,
incorporation of the hydrophobic trifluoromethyl group on the
o-anilide position could have a positive effect on the stability
since it could protect the polymer main chains from being
attacted by water molecules containing highly oxidative radical
species. The another common structural feature between I-x and
II-x is that the sulfonic acid groups are attached to deactivated
phenyl ring in the side chain of copolymers. It has been reported
that the attachment of the sulfonic acid group on the pendent
chain is favorable to enhance the proton conductivity and
improve water stability of polymer membranes.20 Although
having such similarities in the features of their structures,
copolymers I-50 and II-70 still showed large differences in water
stability as shown in Table 3. The films I-50 and II-70 displayed
similar initial mechanical properties with a tensile strength,
elongation at break, and Young’s modulus in the ranges
60-65 MPa, 7-9%, and 1.28-1.80 GPa, respectively. After
soaking in the boiling water for 23 h, the II-70 film became
somewhat brittle and was broken after being lightly bent at 25
°C in water. The results indicated the poor stability of II-70
toward water. However, polyimide I-50 displayed excellent
water stability as the initial tensile strength only decreased to
56 MPa after soaking in water for 196 h. The enhanced stability
of I-50 toward water could be resulted from its unique
naphthalimide structure. Copolymer I-x had two carbonyl groups
in the naphthalimide moieties whereas copolymer II-x had four
carbonyl groups. Therefore, copolymer I-x should possess
decreased positive charge density in carbonyl groups compared
with that of copolymer II-x because of the electron withdrawing
characteristics of carbonyl groups. In addition, the 1,8-naph-
thalimide moiety is smaller and less rigid than the 1,4,5,8-
naphthalimide moiety, which may also contribute to the excellent
a ηinh was measured at a concentration of 0.5 g/dL in NMP at 30 °C.
b Key: (++) soluble at room temperature; (+-) soluble by heating; (- -)
insoluble.
Figure 5. Thermogravimetric analysis thermograms of I-70 and
II-70.
IEC of 1.11 and 1.89 is 20 and 30% corresponded to absorption
of, respectively, 9 and 10 water molecules per -SO3H group.
This is in the same range as Nafion, which absorbs 11 water
molecules per sulfonic acid group at room temperature.3 The
water uptakes observed for copolymer I-x varied from 48 to
259% for IEC values varying from 1.28 to 3.36 mequiv g-1
,
which are much higher than that of II-x with a comparable IEC
values. For example, copolymer I-50 (IEC ) 1.65 mequiv/g)
shows almost 2 times water uptake (61%) than that of II-70
(31%) (IEC ) 1.89 mequiv/g). The content of hydrophobic CF3
groups in copolymer I-50 (11.5 wt %) is almost same as that in
copolymer II-70 (11.9 wt %). Therefore, the difference of water
uptake between copolymer I-50 and II-70 can be attributed to
their difference in molecular structure. Comonomer 3 is
structurally symmetrical, the copolymer II-70 synthesized from
3 possessed a linear rigid-rod chain structure (linkage carbon
(A) fall in a line) (Figure 6). However, comonomer 2 is not
symmetrical, and the resulting copolymer I-50 has a bent nature
of chain structure, which leads to enhanced free volume and

6430 Qiu et al.
Macromolecules, Vol. 39, No. 19, 2006
Table 2. Water Uptake, Size Chang, IEC, and Proton Conductivity of Copolymer Membranes
IECa (mequiv/g)
water uptake (% W/W)
size change
membrane
thickness (µm)
calculated
measured
in water
70% RH
σ (S/cm) at 80 °C
∆t
∆l
I-40
I-50
I-70
I-90
II-50
II-70
30
34
41
35
25
35
1.30
1.67
2.47
3.36
1.13
1.92
1.28
1.65
2.41
3.27
1.11
1.89
48
61
118
259
20
28
35
53
108
10
18
0.075
0.260
0.687
-
0.025
0.075
0.15
0.19
0.45
0.80
0.12
0.15
0.02
0.03
0.07
0.19
0.01
0.02
31
a Calculated, IEC calculated from DS; measured, IEC measured with titration.
Figure 6. Chain structure of copolymers I-50 and II-70.
Table 3. Mechanical Properties Soaking in Water at 100 °C for
Copolymer
soaking maximum elongation Young’s
IEC
time
(h)
stress
(MPa)
at break
(%)
modulus
(GPa)
membrane (mmol‚g-1
)
I-50
1.67
0
48
96
196
0
60
57
57
56
65
-
9
7
6
7
7
-
1.28
1.27
1.29
1.27
1.80
-
II-50
1.13
23
water stability of I-50 because the flexible chain structure is
favorable to improve the stability of polymers toward water.13
Oxidative Stability. Membrane stability toward oxidation
was examined by observing the dissolving behavior in Fenton’s
reagent (30 ppm FeSO4 in 30% H2O2) at 25 °C, which is one
of the standard tests for oxidative stability. Copolymer II-70
becomes brittle in the solution after 50 h. The membrane had
completely dissolved after 93 h. Copolymer I-50 showed better
oxidation stability and it took 263 h until the membrane
complete disappearance in the solution. These results suggested
that the new copolyimide membranes displayed excellent
stability toward oxidation and much better than that of the
polyimide reported in the literature.21 For example, under
identical test conditions, polyimide containing trifluoromethyl
groups dissolved in the solution within 9 h.22 Again, the
hydrophobic trifluoromethyl groups that are localized on the
ortho-position of the imido bond in copolymers may play an
important role in the observed oxidation stability enhancement.
Coplymer I-50 showed much better stability than that of
copolymer II-70, which is probably due to their difference in
structure of naphthalimide moieties and membrane morphology.
Figure 7. Temperature dependence of the proton conductivities of
Nafion 117 and SPI.
room temperature. Proton conductivity can be different with
the difference of experimental approaches and instruments. For
comparison, Nafion 117 was measured under the same condi-
tions and resulted in a value of 0.09 S/cm at 25 °C, which is
similar to that reported by Okamoto et al.13b Copolymer I-40,
-50, and -70 with IEC 1.30-2.47 showed proton conductivities
in the range 4.3 × 10-2 to 6.2 × 10-2 S/cm at 25 °C. Their
proton conductivities increased both with IEC and with tem-
perature. The temperature dependence of the proton conductivi-
ties of copolymer membranes I-x, II-x, and Nafion 117
measured in liquid water are summarized in Figure 7. When
considering both proton conductivity and water stability,
copolymer I-50 with IEC 1.67 has the best combination of
properties for application in PEM for FC. Its proton conductivity
reaches 2.6 × 10-1 S/cm, at 80 °C), which is higher than that
Proton Conductivity. The proton conductivities of copoly-
mers membrane were measured after the membranes were
initially hydrated by immersion in deionized water for 24 h at

Macromolecules, Vol. 39, No. 19, 2006
Sulfonated Poly(arylene-co-naphthalimide)s 6431
Figure 8. SEM micrographs of polymer membranes: (a) I-50; (b) I-70; (c) I-90; (d) II-70.
of Nafion 117 (1.5 × 10-1 S/cm, at 80 °C). Copolymer I-50
and I-70 membrane also showed sharper increase in the proton
conductivity with the temperature than that of Nafion 117.
Among the two types of copolymer I and II, copolymer I
showed higher conductivities with comparably similar IEC
values. For example, the proton conductivity of I-50 with IEC
1.65 is about 3.5 times higher than that of II-70 with IEC 1.89
at 80 °C. These proton conducting properties seem to reflect
their water uptake behavior.
SEM. Figure 8 shows the morphology of sulfonated copoly-
mer. The lighter regions represent localized ionic domains; the
dark regions represent hydrophobic domains. The enhancement
in the size of pores with the degree of sulfonation of the
membranes can be observed clearly. The micrographs provide
direct evidence of biphasic morphology for the polymers. The
ionic aggregates are visibly connected to yield a continuous ionic
network. The density of the ionic pathways increases with ionic
content as evidenced from Figure 8a-d and explains why the
ionic conductivity rises to relatively high values.
1,4,5,8-naphthalimide moiety with a similar IEC values. When
considering both proton conductivity and water stability,
copolymer I-50 with IEC 1.67 has the best combination of
properties for application in PEM for FC. Its proton conductivity
reaches 2.6 × 10-1 S/cm, at 80 °C), which is higher than that
of Nafion 117 (1.5 × 10-1 S/cm, at 80 °C). Consequently, these
materials proved to be promising as proton exchange mem-
branes.
Acknowledgment. We thank the National Basic Research
Program of China (No. 2003CB615704) and the National
Science Foundation of China (No. 20474061) for the financial
support.
References and Notes
(1) Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells 2001, 1, 5.
(2) Costamagna, P.; Srinivasan, S. J. Power Sources 2001, 102, 253.
(3) Rikukawa, K.; Sanui, K. Prog. Polym. Sci. 2000, 25, 1463.
(4) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345.
(5) (a) Inzelt, G.; Pineri, M.; Schultze, J. W.; Vorofyntsev, M. A.
Electrochim. Acta 2000, 45, 240. (b) Savadogo, O. J. New Mater.
Electron. 1998, 1, 47.
Conclusion
(6) (a) Hickner, M. A.; Ghassem, H.; Kim, Y. S.; Einsla, B. R.; McGrath,
J. E. Chem. ReV. 2004, 104, 4587. (b) Li, Q. F.; He, R. H.; Jensen, J.
O.; Bjerrum, N. J. Chem. Mater. 2003, 15, 4896. (c) Kerres, J. A. J.
Membr. Sci. 2001, 185, 3.
(7) Jones, D. J.; Rozie`re, J. J. Membr. Sci. 2001, 185, 41. (b) Xing, P.;
Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Wang, K.;
Kaliaguine, S. J. Membr. Sci. 2004, 229, 95. (c) Gil, M.; Ji, X.; Li,
X.; Na, H.; Hampsey, J. E.; Lu, Y. J. Membr. Sci. 2004, 234, 75. (d)
Xing, P.; Robertson, G. D.; Guiver, M. D.; Mikhailenko, S. D.;
Kaliaguine, S. Macromolecules 2004, 37, 7960. (e) Vetter, S.;
Ruffmann, B.; Buder, I.; Nunes, S. P. J. Membr. Sci. 2004, 260,
181.
(8) (a) Nolte, R.; Ledjeff, K.; Bauer, M. Malhaupt, R. J. Membr. Sci.
1993, 83, 211.(b) Kerres, J.; Cui, W.; Reichle, S. J. Polym. Sci., Polym.
Chem. 1996, 34, 2421. (c) Wang, F.; Hickner, M.; Kim, Y. S.;
Zawodzinski, T. A.; McGrath, J. E. J. Membr. Sci. 2002, 197, 231.
(d) Karlsson, L. E.; Jannasch, P. J. Membr. Sci. 2004, 230, 61. (e)
Willes, K. B.; Wang, F.; McGrath, J. E. J. Polym. Sci., Polym. Chem.
2005, 43, 2964.
The primarily sulfonated aromatic dichloride monomer and
hydrophobic dichloride monomers containing trifluoromethyl-
substituted 1,8-naphthalimide or 1,4,5,8-naphthalimide moieties
were synthesized and used as comonomers to generate oxidative
and water stable copolymers for proton exchange membranes.
The synthesized copolymers with the -SO3H group in the side
chain of polymers possessed high molecular weights, revealed
by their high viscosity and the formation of tough and flexible
membranes. The copolymer membranes exhibited excellent
water and oxidative stabilities due to the introduction of the
hydrophobic CF3 groups on the ortho-position of imido groups,
which could protect the polymer main chains from being
attacked by water molecules containing highly oxidizing radical
species. The sulfonated copolyimides containing 1,8-naphthal-
imide moiety (I-x) exhibited better hydrolytic and oxidative
stabilies, and higher proton conductivity than those containing
(9) (a) Cabbaso, I.; Yuan, Y.; Mittelsteadt, C. US Patent 5,989,742, 1999.
(b) Miyatake, K.; Oyaizu, K.; Tsuchida, E.; Hay, A. S. Macromolecules

6432 Qiu et al.
2001, 34, 2065. (c) Miyatake, K.; Chikashige, Y.; Watanabe, M.
Macromolecules, Vol. 39, No. 19, 2006
(15) Asano, N.; Miyatake, K.; Watanabe, M. Chem. Mater. 2004, 16, 2841.
Macromolecules 2003, 36, 9691.
(16) Asano, N.; Aoki, M.; Suzuki, S.; Miyatake, K.; Uchida, H.; Watanabe,
(10) (a) Genies, C.; Mercier, R.; Sillion, B.; Cornet, N.; Gebel, G.; Pineri,
M. Polymer 2001, 42, 359. (b) Zhang, Y.; Litt, M. H.; Savinell, R.
F.; Wainright, J. S. Polym. Prepr. 2000, 41, 1561. (c) Guo, X.; Fang,
J.; Watari, T.; Tanaka, K.; Kita, H.; Okamoto, K. Macromolecules
2002, 35, 6707. (d) Miyatake, K.; Zhou, H.; Uchida, H.; Watanabe,
M. Chem. Commun. 2003, 368. (e) Yin, Y.; Fang, J.; Watari, T.;
Tanaka, K.; Kita, H.; Okamoto, K. J. Mater. Chem. 2004, 14, 1062.
(f) Einsla, B. R.; Kim, Y. S.; Hickner, M. A.; Hong, Y. T.; Hill, M.
L.; Pivovar, B. S.; McGrath, J. E. J. Membr. Sci. 2005, 255, 141. (g)
Yasuda, T.; Miyatake, K.; Hirai, M.; Nanasawa, M.; Watanabe, M. J.
Polym. Sci., Part A: Polym. Chem. 2005, 43, 4439.
(11) (a) Kobayashi, T.; Rikukawa, M.; Sanui, K.; Ogata, N. Solid State
Ionics 1998, 106, 219. (b) Ghassemi, H.; Ndip, G.; McGrath, J. E.
Polymer 2004, 45, 5855.
(12) Ghosh, M. K., Mittal, K. L., Eds. Polyimides fundamental and
applications; Marcel Dekker: New York, 1996; p 7.
M. J. Am. Chem. Soc. 2006, in press.
(17) (a) Marrocco, M.; Gagne, R. R. US Patent 5,227,457, 1993. (b) Wang,
Y.; Quirk, R. M. Macromolecules 1995, 28, 3495. (c) Wang, J.;
Sheares, V. V. Macromolecules 1998, 31, 6769.
(18) Wu, S.; Li, W.; Zhang, S. Polymer 2005, 46, 8396.
(19) Kopitzke, R. W.; Linkous, C. A.; Anderson, H. R.; Nelson, G. L. J.
Eletrochem. Soc. 2000, 147, 1677.
(20) (a) Wang, L.; Meng, Y. Z.; Wang, S. J.; Hay, A. S. J. Polym. Sci.,
Part A: Polym. Chem. 2004, 42, 1779. (b) Miyatake, K.; Hay, A. S.
J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 3211. (c) Gao, Y.;
Robertson, G. P.; Guiver, M. D.; Mikhailenko, S. D.; Li, X, Kaliaguine,
S. Macromolecules 2004, 37, 6748. (d) Yin, Y.; Yamada, O.; Suto,
Y.; Mishima, T.; Tanaka, K.; Kita, H.; Okamoto, K. J. Polym. Sci.,
Part A: Polym. Chem. 2005, 43, 1545. (e) Lafitte, B.; Puchner, M.;
Jannasch, P. Macromol. Rapid Commun. 2005, 26, 1464.
(13) (a) Fang, J.; Guo, X.; Harada, S.; Watari, T.; Tanaka, K.; Kita, H.;
Okamoto, K. I. Macromolecules 2002, 35, 9022. (b) Watari, T.; Fang,
J.; Tanak, K.; Kita, H.; Okamoto, K. I. J. Membr. Sci. 2004, 230,
111.
(14) Miyatake, K.; Zhou, H.; Watanabe, M. Macromolecules 2004, 37,
4956.
(21) Fang, J.; Guo, X.; Harada, S.; Watari, T.; Tanaka, K.; Kita, H.;
Okamoto, K. Macromolecules 2002, 35, 9022.
(22) Miyatake, K.; Zhou, H.; Matsuo, T.; Uchida, H.; Watanabe, M.
Macromolecules 2004, 37, 4961.
MA060275K