Synthesis and characterization of ionic conducting sulfonated polybenzimidazoles

Article abstract of DOI:10.1002/pola.23938

By copolymerization of the disodium-2, 2′-disulfonate-4,4′- oxydibenzoic acid (SODBA), the 4,4′-oxybis(benzoic acid) (OBBA) with the bis 3, 4-(diaminophenyl)sulfone (BDAPS), a series of high molecular weight sulfonated polybenzimidazoles (sPBI) were prepa

Full text of DOI:10.1002/pola.23938

Synthesis and Characterization of Ionic Conducting Sulfonated
Polybenzimidazoles
2
1
JULIEN JOUANNEAU,1 LAURENT GONON,2 GERARD GEBEL, VINCENT MARTIN,1 REGIS MERCIER
´
´
1Laboratoire des Mate´riaux Organiques a` Proprie´te´s Spe´cifiques, UMR 5041 CNRS/Universite´ de Savoie,
chemin du canal 69360 Solaize, France
2INAC/Laboratoire des Structures et Proprie´te´s d’Architectures Mole´culaires, UMR 5819 CEA-CNRS-UJF, 17 rue des Martyrs,
38054 Grenoble cedex, France
Received 17 September 2009; accepted 15 January 2010
DOI: 10.1002/pola.23938
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: By copolymerization of the disodium-2,20-disulfo-
nate-4,40-oxydibenzoic acid (SODBA), the 4,40-oxybis(benzoic
acid) (OBBA) with the bis 3,4-(diaminophenyl)sulfone (BDAPS),
a series of high molecular weight sulfonated polybenzimida-
zoles (sPBI) were prepared by varying the ratio of monomers
SODBA/OBBA. Polymers with ion exchange capacity (IEC) rang-
ing from 0 to 3.2 meqHþ/g were obtained. The chemical struc-
ture of the sPBI was confirmed by NMR, Fourier-transform
infra-red (FTIR). Although the sPBI display a very poor solubil-
ity in organic solvents, they are, in the ammonium salt form,
soluble in polar aprotic solvents such as DMF, NMP, or DMSO.
Tough and ductile membranes from solution casting method
were prepared. The water uptake and the ionic conductivity
ꢀ
C
V
were determined at 30 and 90 C. 2010 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 48: 1732–1742, 2010
KEYWORDS: fuel cell; membrane; polycondensation; polyelec-
trolyte; sulfonated polybenzimidazole
INTRODUCTION Over the last decade, considerable interests
have been focused on the new clean energy sources. Because
of their high energy efficiency, the hydrogen fuel cell is one
of the most attractive technologies. However, before to be
integrated as power source in the future electric vehicles,
the performances and the durability at high temperatures
form part of the chemical structure of at least one of the
monomers involved in the polymerization reaction. Also,
depending on the type of polymers synthesized, this route
allows obtaining block copolymers presenting both hydro-
philic and hydrophobic sequences.18 Concerning the synthe-
sis of sPBI these different approaches were considered. The
sulfonation was carried on either by a thermal treatment of
sulfuric acid doped PBI,19–22 or by N alkylation of the imid-
azole ring.23–28 sPBI preparation by the polymerization of
sulfonated monomers was first reported by Uno et al.29 and
later by Arnold and coworkers,30,31 but without connection
with fuel cell membranes. More recently, other research
groups have used this approach for the synthesis of sPBI for
ꢀ
(>100 C) of one major component of this fuel cell, namely,
the electrode membrane assembly (MEA) still needs to be
improved. In other words, the design of ionic conducting
polymers with improved properties is of a major importance.
Indeed, if the sulfonated perfluoro polymer membranes1
display very good electrochemical properties, its stabili-
ty appears rather limited at high temperatures. On the
other hand, a number of aromatic polymers, such as poly-
imides,2,3 polysulfones,4,5 polybenzoxazoles,6 poly(ether ether
ketone)s,7,8 were intensively studied as an alternative solu-
tion. The design of ionic conducting membranes from these
polymers deals mainly with sulfonic acid groups containing
macromolecular structures, which can be obtained either by
postsulfonation9–12 or by polymerization of sulfonated mono-
mers.2,13–17 Although the first method is very convenient
because many polymers potentially interesting are commer-
cially available, however, it is rather difficult to control pre-
cisely the amount of grafted sulfonic acid groups and their
position along the macromolecular backbone. Conversely, for
the second approach, the sulfonation extent of polymers is,
in general, easier to control because the sulfonic acid groups
fuel cell membrane application.32–39 In
a similar way,
McGrath and coworkers40 have synthesized sulfonated poly
(arylene ether benzimidazole) copolymers via a nucleophilic
aromatic polycondensation. Finally, an original route to
obtain protonic conducting PBI membranes consists in dop-
ing PBI with acid compounds such as phosphoric acid14,41–45
giving ionic conducting membranes with good performances
at high temperatures.
As a part of our continuing research on the synthesis of ionic
conducting aromatic heterocyclic polymers, the synthesis of
sPBI from SODBA and BDAPS monomers was addressed in
this work. In varying the ratio SODBA/ODBA monomers, dif-
ferent sPBI were prepared as random copolymers or
Correspondence to: R. Mercier (E-mail: mercier@lmops.cnrs.fr)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1732–1742 (2010) 2010 Wiley Periodicals, Inc.
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ARTICLE
sequenced copolymers with an average sulfonated block
length of 5 or 10 repeat units. Polymer blends involving the
unsulfonated PBI and the 100% sPBI were also investigated.
SODBA Monomer Synthesis
In a 500 mL three-necked round-bottom flask equipped with
a magnetic stirrer, a condenser, a nitrogen inlet/outlet and
containing 150 mL of 30% fuming sulfuric acid, and 75 g of
ODBA (0.29 mol) were added. The resulting reaction mixture
was then heated 2 h at 110 ꢀC. The final brown homogene-
ous solution was cooled down to room temperature before
to be slowly poured in NaCl saturated aqueous solution. The
resulting precipitate was filtered off and recrystallized twice
in water. After drying under vacuum for 20 h at 120 ꢀC,
102 g of expected product as white needle-like crystals were
obtained. Yield: 76%. IR (KBr, cmꢁ1): 3474, 2941, 1719
(CO2H), 1251, 1084, 1031 (SO3Na), 1155 (PhAOAPh), 936,
EXPERIMENTAL
Materials
4,40-Oxydibenzoic acid (ODBA) from Atochem was washed
with hot tetrahydrofuran (THF) before use. The 30% fuming
sulfuric acid and the sodium chloride were purchased from
Aldrich. The bis-3,4-diaminophenylsulfone (BDAPS) was
kindly provided by Konishi Chemical Ind. Co. and recrystal-
lized in acetonitrile before use. Benzoic acid and 1,2-benze-
nediamine were purchased from Aldrich and purified by sub-
limation. Dimethylsulfoxide (DMSO) was purchased from
VWR Chemical Co. Eaton reagent was prepared from phos-
phorous pentoxyde and methanesulfonic acid (Aldrich).
1
898, 767, 704 (1,2,4-trisubstituted benzene), 619 (CAS). H-
NMR (DMSO-d6, 20 ꢀC, ppm): 8.44 (d, J ¼ 2.1 Hz, 2H), 7.96
(dd, J ¼ 8.4, 2.1 Hz, 2H), 6.89 (d, J ¼ 8.7 Hz, 2H). NMR 13C
(DMSO-d6, 20 ꢀC, ppm): 166.36, 156.93, 137.50, 132.31,
129.75, 125.45, 120.46.
Methods
Eaton Reagent Preparation
NMR spectra were recorded on a Bruker AC250, with fre-
quencies of 250 MHz and 62.9 MHz for H and 13C analysis,
1
A total of 3 g of phosphorous pentoxide (P2O5) and 20 mL
of methanesulfonic acid were placed in a 50 mL flask with a
magnetic stirrer. The mixture was stirred at 40 ꢀC till com-
plete dissolution of P2O5. This mixture was used as a solvent
for both polymerization and synthesis of benzimidazole
model compounds, and prepared just before use.
respectively. The solvent used was DMSO-d6 with tetrame-
thylsilane (TMS) as internal reference. Fourier transformed
infra-red (FTIR) spectra were performed on a NICOLET 20SX
spectrophotometer. Samples were prepared as KBr pressed
pellets. Thermogravimetric analyses (TGA) were achieved
with a TA instrument 2950 TGA. The analyses were done
Model Compounds Synthesis
ꢀ
ꢀ
with a heating rate of 10 C/min, from 20 to 600 C. The in-
herent viscosity was determined at 30 ꢀC from a 0.5 g/dL
polymer solution in H2SO4 using an Ubbelhode viscometer.
Size exclusion chromatography (SEC) was performed on a
Waters apparatus equipped with 2 Plgel Mixed-D columns
and refractive index detector. DMF containing 0.05 mol/L of
LiBr was used as the solvent at a flow rate of 1.0 mL/min at
70 ꢀC. Polymer solution was filtered through a 0.45-lm
PTFE. Calibration was made with polystyrene standards. The
titration of polymer acid functions was done as follows: A
piece of 30 mg of polymer membrane were immersed during
24 h in 20 mL of a 2 M NaCl aqueous solution. Then titra-
tion was performed with a 0.05 M NaOH aqueous solution,
using the Radiometer Analytical Titramaster 880 ABU52
instrument. Water uptake of membranes was measured by
immersing for 24 h three samples (around 20 mg each) of
membrane in a water bath. Afterward, each sample was
taken out, quickly wiped with tissue paper and weighed on a
microbalance. The water uptake can then be calculated as
the ratio (in weight) of adsorbed water on the dry sample
weight. The water uptake values reported correspond to the
average value of the three samples. For the conductivity
measurement, films were previously hydrated for 24 h at
ambient temperature in liquid water, sandwiched between
two platinum electrodes and then placed in a sealed cell
under controlled atmosphere. The complex impedance
method was used to measure the proton conductivity of
membranes. Frequency dependence of the cell impedance
was measured at 5 mV over the frequency range from
100 Hz to 10 MHz with a Schlumberger SI 1255 frequency
response analyser.
2,20-Disulfo-4,40-bis(2-benzimidazole)-diphenylether (1)
1.297 g of SODBA (2.806 mmol), 0.607 g of 1,2-benzenedi-
amine (5.612 mmol), and 10 g of Eaton reagent were placed
into a 50 mL three-necked round-bottom flask equipped
with a reflux condenser, a nitrogen inlet/outlet, and a mag-
netic stirrer. The nitrogen flow was very weak to avoid the
sublimation of 1,2-benzenediamine. The reaction mixture
was heated to 130 ꢀC and kept at this temperature for 2 h.
The reaction solution was then poured in 100 mL of water,
resulting in a very thin precipitate. Because of difficulties
encountered for the filtration of the very thin formed pow-
der, centrifugation was preferred to isolate the compound,
which was then washed carefully with water and centrifuged
again to give 1.04 g of a clear pink powder that was dried
under vacuum at 120 ꢀC for 15 h. Yield: 66%. IR (KBr,
cmꢁ1): 3430 (NH), 1633, 1599, 1286 (benzimidazole), 1253,
1085, 1029 (SO3), 616 (CAS), 1155 (PhAOAPh). 1H-NMR
ꢀ
(DMSO-d6, 20 C, ppm): 8.72 (d, J ¼ 2.2 Hz, 2H), 8.13 (dd, J
¼ 8.5, 2.2 Hz, 2H), 7.63 (q, J ¼ 3.0 Hz, 4H), 7.25 (q, J ¼ 3.2
13
ꢀ
Hz, 4H), 7.13 (d, J ¼ 8.5 Hz, 2H). C-NMR (DMSO-d6, 20 C,
ppm): 156.96, 150.63, 139.52, 138.72, 129.03, 127.03,
124.20, 123.14, 122.56, 115.06.
4,40-Bis(2-benzimidazole)-diphenylether (2)
1.789 g of ODBA (6.928 mmol), 1.498 g of 1,2-benzenedi-
amine (13.856 mmol), and 13 g of Eaton reagent were
placed into
a 50 mL three-necked round-bottom flask
equipped with a reflux condenser, a nitrogen inlet/outlet,
and a magnetic stirrer. The nitrogen flow was very weak to
avoid the sublimation of 1,2-benzenediamine. The tempera-
ture was slowly increased to 130 ꢀC and the reaction
SYNTHESIS AND CHARACTERIZATION OF sPBI, JOUANNEAU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
mixture was kept for 1 h. The clear reaction mixture
obtained was then poured in 100 mL of water. The precipi-
tate obtained was filtered off and washed twice with water,
and once in ethyl acetate. After drying under vacuum 20 h at
120 ꢀC, 2.50 g of expected product as clear brown powder
was obtained. Yield: 90%. IR (KBr, cmꢁ1): 3417, 3144 (NH),
1626, 1605, 1539, 1292 (benzimidazole), 1145, 1068
(PhAOAPh). 1H-NMR (DMSO-d6, 20 ꢀC, ppm): 8.30 (d, J ¼
8.4 Hz, 4H), 7.67 (q, J ¼ 3.0 Hz, 4H), 7.35 (d, J ¼ 8.4 Hz,
4H), 7.28 (q, J ¼ 3.0 Hz, 4H). 13C-NMR (DMSO-d6, 20 ꢀC,
ppm): 157.88, 150.86, 139.20, 128.82, 125.67, 122.47,
119.50, 115.18.
ers endcapped with diamine functions. (2) Corresponds to
chain extension by allowing the reaction of the sulfonated
oligomers with the nonsulfonated monomers. As general pro-
cedure, the synthesis of the 50% sulfonated copolymer with
a targeted average length sequence of five sulfonated repeat
units is reported hereafter.
In a 100 mL three-necked round-bottom flask equipped with
a nitrogen inlet/outlet and a mechanical stirrer, 1.098 g of
SODBA (2.375 mmol), 0.793 g (2.850 mmol) of BDAPS, and
18 g of Eaton reagent were first introduced. After complete
dissolution of the monomers at 80 ꢀC, the temperature was
increased to 130 ꢀC and maintained for 14 h. The reaction
mixture was then cooled to 80 ꢀC and 0.736 g of ODBA
(2.850 mmol), 0.661 g of BDAPS (2.375 mmol), and 12 g of
Eaton reagent were added. After complete solubilization of
the added monomers, the solution was heated again at
130 ꢀC and kept at this temperature for 14 h. The work up
to isolate the polymers was the same as for the random
copolymers.
2,20-Diphenyl-5,50-sulfonyldibenzimidazole (3)
2.064 g of BDAPS (7.416 mmol), 1.811 g of benzoic acid
(14.832 mmol), and 15 g of Eaton reagent were placed into
a 50 mL three-necked round-bottom flask equipped with a
reflux condenser, a nitrogen inlet/outlet, and a magnetic stir-
ꢀ
rer. The temperature was slowly increased to 120 C and the
reaction mixture was kept for 1 h. The clear reaction mixture
obtained was then poured in 300 mL of water. The precipi-
tate obtained was filtered off and washed twice with boiling
water. After drying under vacuum 20 h at 120 ꢀC, 3.12 g of
expected product as white powder was obtained Yield: 93%.
IR (KBr, cmꢁ1): 3427 (NH), 1631, 1600, 1287 (benzimida-
zole), 1253, 1183 (SO2), 616 (CAS). 1H-NMR (DMSO-d6,
30 ꢀC, ppm): 8.35 (s, 2H), 8.29 (d, 4H), 7.91 (m, 4H), 7.67
(m, 6H). 13C-NMR (DMSO-d6, 30 ꢀC, ppm): 154.54, 141.04,
138.99, 136.04, 131.32, 129.38, 128.60, 127.26, 121.83,
115.67, 115.61.
Membrane Preparation
Polymer (1.0 g) was dissolved at 150 ꢀC in 15 mL of DMSO
containing triethylamine. The solution was stirred overnight,
filtered through a Whatman 30 lm filter, and then casted
onto a clean glass substrate. The solvent was evaporated at
ꢀ
60 C overnight under nitrogen flux, and then at 200 ꢀC for
1 h (the temperature was gradually increased from 60 to
ꢀ
200 C). After cooling back to room temperature, membrane
was lifted off the glass plates by immersion in water. To
remove the solvent trace, the membrane was dried under
vacuum for 5 h at 120 ꢀC, and then immersed in water and
methanol. The membrane was then acidified in a 2 M HCl so-
lution for overnight and rinsed thrice with water. A series of
thin flexible membranes having a thickness ranging from 20
to 60 l were obtained following this procedure.
Polymer Synthesis
Homo Polymers and Random Copolymers
The typical procedure used to obtain the 50% sulfonated
random copolymer is as follows. First the water content of
the monomers was determined to avoid the unbalance ratio
of the monomers. In a 100 mL three-necked round-bottom
flask equipped with a nitrogen inlet/outlet and a mechanical
stirrer, 0.960 g of SODBA (2.076 mmol), 0.536 g of ODBA
(2.076 mmol), 1.156 g of BDAPS (4.152 mmol), and 25 g of
Eaton reagent were introduced. After complete dissolution of
Polymer Blend Membranes
A series of DMSO solution containing both sulfonated homo-
polymer sPBI100 in the ammonium form and the nonsulfo-
nated polymer nsPBI were prepared. By varying the ratio of
the two polymers, blends having different relative IEC were
obtained. To prepare membranes from these blend solutions
similar aforementioned conditions were used. Transparent
membranes were obtained.
ꢀ
the monomers at 80 C, the temperature of the reaction mix-
ture was gradually increased to 130 ꢀC and maintained for
14 h. Addition of 20 g of solvent was necessary during the
reaction to keep the viscosity of solution not too high. Then
the reaction mixture was poured in 300 mL of water giving
a fiber-like precipitate. The polymer was then crushed. The
resulting polymer particles were filtered off and washed sev-
eral times in boiling water (typically three times) until to get
neutral pH of the washing water. The white powder obtained
was dried overnight at 120 ꢀC under vacuum to give 2.21 g
of polymer (98% yield). The nonsulfonated homopolymer
was synthesized following the similar operating conditions.
Only the polymerization temperature was changed (110 ꢀC
RESULTS AND DISCUSSION
Monomer Synthesis
SODBA has been synthesized (Scheme 1) as previously
reported by Einsla et al.6 Only the final work up was slightly
different. So the precipitation of the reaction medium in sat-
urated NaCl solution was found a convenient method to iso-
late the expected product. After two crystallizations in water,
polymer grade SODBA monomer was obtained with a quite
good yield. The chemical structure of SODBA was confirmed
by FTIR and NMR analysis. In comparison with the FTIR
spectrum of ODBA, those of SODBA clearly display three new
absorption bands at 1251, 1084, 1031 cmꢁ1 assigned to the
O¼¼S¼¼O stretching vibration of the SO3Na group. As
ꢀ
instead of 130 C).
Sequenced Copolymer
The synthesis of sequenced copolymers involves a two-step
reaction: (1) Concerns the preparation of sulfonated oligom-
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ARTICLE
SCHEME 1 Synthesis of SODBA.
mentioned in the Einsla’s publication, the 1H-NMR showed
three peaks in the aromatic region at 8.4, 7.9, and 6.8 ppm.
Now taking in consideration the observed coupling effects
between the different protons, we can affirm that the sulfo-
nation occurred on the two aromatic rings in the ortho posi-
tion of the ether bond. The 13C-NMR spectrum was also in
good accordance with the expected chemical structure.
compounds,ꢀa melting temperature was shown by DSC. Only
a Tg at 181 C for model compound 3 was observed.
Polymer Synthesis
Originally, PBI synthesis was carried out by melt polymerization
of aromatic bis diamines with aromatic dicarboxylic acids or
derivatives.46 More recently, Inoue et al.47 used polyphosphoric
acid (PPA), which is known to be a very good solvent for differ-
ent condensation reactions. However Tomlin et al.48 reported
some problems regarding the synthesis of high molecular
weight PBI in PPA. Before the condensation, polymerization is
very far advanced, they pointed out the crystallization of low
molecular weight oligomers out of PPA. Moreover, PPA is a
highly viscous solvent setting a real problem of handling.
Model Synthesis
The synthesis of model compounds from the three mono-
mers used for the preparation of sPBI and shown Scheme 2
was performed to define the better experimental conditions
to obtain high molecular weight fully cyclized PBI. Thereby,
it was shown from 13C-NMR study of reaction products, that
benzimidazole ring formation in the Eaton’s reaꢀgent occurs
quantitatively at temperature not much that 130 C. For that,
the peak appearance at 156 ppm characteristic of the
ANHAC¼¼NA (benzimidazole ring) was followed as well as
the total disappearance of peaks around 170 ppm relative to
carbonyl of both amide and carboxylic functions. These
model compounds were also of particular interest to get
For these reasons, we found it was more convenient to use
the Eaton reagent49 for the synthesis of our sPBI as
described by Kim et al.50 In contrast with PPA, Eaton reagent
is a low viscous solvent. Furthermore, the polymer can be
separated more easily simply by pouring the reaction solu-
tion in H2O, with no need of repeated washing with a basic
solution to remove completely the remaining solvent, as it is
necessary when polyphosphoric acid is used. The polymers
obtained through this process have their sulfonic groups on
the acid form. A series of sPBI, listed in Table 1, were pre-
pared from SODBA, ODBA, and BDAPS. So sPBI with an IEC
ranging from 0 to 3.20 mequiv./g were obtained. Random
copolymers (Scheme 3) were considered as well as
1
data regarding the chemical shifts of H and 13C-NMR, which
were helpful for the analysis of polymer spectra. On the
other hand, we have determined their molecular weights by
SEC to estimate the difference between the measured and
the true value and to use it for the determination of a cor-
rected number average molecular weight of both sulfonated
oligomers and sulfonated polymers. For any of these model
SCHEME 2 Synthesis of model compounds.
SYNTHESIS AND CHARACTERIZATION OF sPBI, JOUANNEAU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Composition of sPBI Synthesized
IEC (mequiv./g)
Measured by
Acronym
Structure
% SODBA
% ODBA
Calculated
NMR
Titration
nsPBI
Homopolymer
Random
Sequenced, 5a
Sequenced, 10a
Random
Sequenced, 5a
Sequenced, 10a
Random
0
100
50
50
50
25
25
25
10
0
0
0
0
sPBI50rand
sPBI50seq5
sPBI50seq10
sPBI75rand
sPBI75seq5
sPBI75seq10
sPBI90rand
sPBI100
50
50
50
75
75
75
90
100
1.84
1.84
1.84
2.57
2.57
2.57
2.96
3.20
1.84
1.85
1.89
2.57
2.55
2.55
2.91
3.20
1.80
2.06
1.98
2.46
2.62
2.47
2.86
3.12
Homopolymer
a
Average number of repeat units of sulfonated sequence of the polymer.
sequenced ones with sulfonated block length of 5 or 10
repeat units (Scheme 4). Afterward, the acronyms s-oligo5
and s-oligo10 were chosen to identify the oligomers having
respectively, an average of 5 or 10 sulfonated repeat units.
To confirm the structure of the sequenced copolymers, SEC
analyses were performed on the sulfonated oligomers pre-
pared in the first step of the synthesis. Considering that the
molecular weight values obtained from polystyrene calibra-
tion curves were too much different from those expected, we
have decided to determine the molecular weight of the
model 1 by the same method and in the same conditions to
calculate the ratio Mn /Mn which was found to be 2.6 and
sulfuric acid was used as solvent. The values listed Table 3
are consistent with high molecular weight polymers. An
investigation by SEC was also possible using sPBI in lithium
sulfonate form because soluble in DMF. The number-average
molecular weights (Mn ) obtained, varied from 77,000 to
exp
227,000. As for the sulfonated oligomers, such high values
cannot be considered as representative values of molecular
weights of these polymers. On the other hand, the corrected
Mn values calculated from the relation used for the oligomers
are more credible (see Table 3).
NMR Structural Characterization
exp
theo
From NMR data of the model compounds, we were able to
clearly analyze the 13C spectra of polymers and confirm their
structure. As an example, the spectrum of the sulfonated
homopolymer sPBI100 is reported Figure 1. As aforemen-
tioned, the peak appearing at 154 ppm is characteristic of
use it to determine an Mn ¼ Mn /2.6 of the two sulfo-
corr
exp
nated oligomers (see Table 2).
The inherent viscosity (ginh) measurement of the sPBI was
also carried out. To prevent ionic interactions concentrated
SCHEME 3 Synthesis of sPBI by copolymerization of SODBA, ODBA with BDAPS.
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ARTICLE
SCHEME 4 One pot two-step synthesis route of sequenced copolymer.
one carbon of the imidazole ring, whereas this appearing at
139 ppm corresponds to the carbon bonded to sulfonic acid
group. It is of particular interest to notice that the benzimid-
azole ring on this sPBI possesses two isomers, depending
on the position of the NAH. This group can be either in
para position or in meta position of the sulfone bridging
group. Because of the presence in the chain of both isomers,
most of the carbon peaks appear in the spectrum as large
peaks.
units to the total number of repeat units, also expressed as
the sulfonation degree (SD) is given by the equation:
ðArea 7:15Þ
SD ¼
ꢂ 100 ðexpressed in %Þ
ðArea 7:15Þ þ ðArea 7:3Þ2
The calculated SD of the copolymer sPBI75, for example, was
found to be 75%, in good accordance with the target value.
Information regarding the chemical structure of the sPBI can
1
also be obtained from H-NMR analysis (Fig. 2). Although the
TABLE 2 Molecular Weight Determination of Sulfonated
0
0
0
aromatic protons H1,2,3 and H1 ,2 ,3 of the nonsulfonated and
sulfonated units have similar chemical shifts at 8.2 and 7.8
Oligomers
a
theo
b
exp
0
0
0
ppm, those corresponding to the protons H4 ,5 ,6 of the sulfo-
nated phenyl ring are well distinguished from H4,5 of the
nonsulfonated phenyl ring. So, the chemical shift of H5 was
Acronym
Mn
Mn
Mn
corr
Model 1
s-oligo5
s-oligo10
a
562
1,480
7,170
562
2,760
4,760
3,400
6,520
0
found to be at 7.3 ppm, whereas this corresponding to H5
12,380
appears at 7.15 ppm. These specific resonance frequencies
allowed us to determine the chemical composition of the sul-
fonated copolymers. The proportion of sulfonated repeat
Calculated from the theoretical molecular structure (g/mol).
Determined from SEC analysis (in polystyrene equivalent g/mol).
b
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 3 Molecular Weight Determination of SPBI
a
exp
a
Acronym
ginh (dL/g)
Mn
Mn
Mw
Mw/Mn
exp
corr
nsPBI
1.78
1.07
1.84
1.80
1.27
2.11
1.30
1.11
1.41
117,000
77,000
45,000
29,600
63,500
68,100
43,800
87,300
42,700
49,600
58,800
229,000
177,000
370,000
367,000
260,000
417,000
230,000
238,000
307,000
1.95
2.30
2.00
2.10
2.30
1.80
2.10
1.90
2.00
sPBI50rand
sPBI50seq5
sPBI50seq10
sPBI75rand
sPBI75seq5
sPBI75seq10
sPBI90rand
sPBI100
165,000
177,000
114,000
227,000
111,000
129,000
153,000
a
Expressed in polystyrene equivalent g/mol.
The SD determination of each copolymer is reported Table 1
as equivalent ionic exchange capacity (IEC) value. All these
values founded are very close to the theoretical IEC values.
These results show the interest to use functionalized mono-
mers to well control the extent of functional groups intro-
duced in the polymer chain.
by the interactions SO3H-benzimidazole in the membrane
that make the Hþ/Naþ exchange difficult.
Thermal Properties
The thermal stability of the sPBI was studied by TGA. As it
can be seen Figure 3, the nonsulfonated PBI do not exhibit
any significant weight loss before the degradation of the
polymer backbone up to 450 ꢀC, whereas for all the sulfo-
nated PBI, a first weight loss between 350 and 470 ꢀC was
observed. This first weight loss can be related to the sulfonic
acid group loss of the sPBI. This phenomenon corresponds
to a desulfonation reaction. Such analysis can be considered
as a simple method to determine the extent of sulfonation of
a polymer (see Table 4). However, it is less precise than the
proton NMR analysis because of the possible overlapping of
the end of temperature domain corresponding to the desul-
fonation and this due to the polymer backbone degradation.
Titration
The IEC of the polymers synthesized can also be determined
by titration. Acidified membranes were cut into small pieces
and stirred overnight in a 2 M NaCl solution. The mixture
was then titrated with a 0.05 M NaOH solution. The protons
of the membrane are very difficult to exchange with the Naþ
ions, so it was very important to wait for the stabilization of
the pH-meter before each addition of NaOH. One-inflection
titration curves are obtained, and it was possible to deter-
mine an experimental value of the IEC from the equivalent
volume. These values reported Table 1 are quite in good
agreement with the values expected from the monomer feed
ratio. However, this experimental determination seems less
precise than the NMR characterization. This can be explained
Based on the TGA analysis in dynamic mode, all the sPBI
synthesized exhibit good thermal stability, but this cannot be
directly linked with the behavior of the material during a
FIGURE 1 13C-NMR spectra of the sPBI100 homopolymer.
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ARTICLE
FIGURE 2 1H-NMR spectra of the homopolymers nsPBI (a), the copolymer sPBI75rand (b), and sPBI100 (c).
long term use at high temperature. To complete these data,
an isothermal mode TGA analysis was performed on the
sPBI100 at 200 C under air. The thermogram presented Fig-
nated polymers until to reach the thermal degradation
domain
ꢀ
Water Uptake
ure 4 does not display any significant weight loss even after
50 h. This result can be considered as a good indication of
the sPBI thermal stability, but of course, do not reflect
exactly its thermal behavior in fuel cell operating conditions
at this temperature Glass transition temperature (Tg) of the
sPBI (random copolymers and homopolymers) was studied
by TMA. Unfortunately, no Tg was observed for the sulfo-
It is well-known that the proton transport through sulfonated
polymer membranes is strongly dependent on their hydra-
tion state. As a consequence, the ability of the membranes
TABLE 4 Thermal Properties of the SPBI
Desulfonation
(wt %)b
Sulfonation T5%
Tg
Acronym
Rate (%)
(ꢀC)a,b Calculated Measured (ꢀC)
nsPBI
0
482
418
424
421
407
411
412
399
401
–
–
336
sPBI50rand
50
15
15
15
21
21
21
24
26
20
17
18
24
24
24
27
28
sPBI50seq5 50
sPBI50seq10 50
sPBI75rand
75
sPBI75seq5 75
sPBI75seq10 75
sPBI90rand
90
sPBI100
100
a
FIGURE 3 TGA analysis of nsPBI (a), sPBI50rand (b), sPBI75r-
Temperature at which the 5% weight loss was recorded by TGA.
Sulfonic acid form (SO3H) of the sPBI.
b
and (c), sPBI90rand (d), and sPBI100 (e).
SYNTHESIS AND CHARACTERIZATION OF sPBI, JOUANNEAU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 4 Isothermal TGA analysis of sPBI100 at 200 ꢀC under air.
to absorb water is a critical key point. The data obtained
from measurements done at 30 ꢀC and 90 ꢀC, are reported
Table 5, along with the corresponding values of membrane
ionic conductivity.
FIGURE 5 Water uptake of sPBI in liquid water at 30 ꢀC,
expressed in wt % (black dots) or in number of water mole-
cules per sulfonic acid group (clear dots).
The water uptake of sPBI membranes are also represented
as function of the degree of sulfonation Figure 5. The hydro-
philic effect of SO3H is clearly visible as the water uptake
increases with the degree of sulfonation up to 46% for the
sulfonated homopolymer sPBI100. However, for the number
of H2O molecules per sulfonic acid group (k), no significant
change was observed with the degree of sulfonation of poly-
mers. All values ranged from 6.7 to 8.7 H2O/SO3H, which is
quite low in comparison with the value generally obtained
with the Nafion (k ¼ 12).
In Figure 5, the values are reported obtained for different
polymer architectures but having the same degree of sulfo-
nation. Based on these results, one can consider that the
blockness of polymers does not change the water uptake
properties. On the other hand, after soaking in water at
90 ꢀC, all the membranes absorb less water than at lower
temperature. This very particular behavior has already been
observed by Qing et al.32–35 and seems typical of sPBI pre-
pared from sulfonated diacid monomers.
The water uptake property of sPBI100 was also studied at
higher temperature in a high pressure reactor and repre-
sented Figure 6. The membrane exhibits outstanding
capacity to maintain good mechanical stability and moderate
swelling up to 180 ꢀC, the limit temperature of the reactor.
Up to 140 ꢀC the water uptake remains lower than 50% in
weight. Above this temperature, the dramatic increase of
water uptake is probably the consequence of weaker ionic
interactions between the polymer chains and hence, a better
affinity for water. Even at 180 ꢀC, the k value is only about
13.5, which is comparable with Nafion one at ambient
temperature.
Ionic Conductivity
Conductivity is the major parameter used to estimate the
propensity of the membrane to proton transport. All meas-
urements were carried out in hydrated state (100% relative
humidity), and reported Table 5. As it is currently observed
TABLE 5 Water Uptake and Conductivity of sPBI Membranes
Water Uptake at 30 ꢀC
Water Uptake at 90 ꢀC
Conductivity (S/cm)
Acronym
wt %
k (H2O/SO3H)
wt %
k (H2O/SO3H)
30 ꢀC
90 ꢀC
nsPBI
7.0
27.5
28.8
26.6
24.9
30.8
32.0
33.1
32.6
37.1
36.2
45.9
8.1
22.1
23.5
23.1
21.7
26.3
31.6
32.3
31.6
33.5
31.5
36.5
1.9E-06
–
–
sPBI50rand
sPBI50seq5
sPBI50seq10
sPBI50blend
sPBI75rand
sPBI75seq5
sPBI75seq10
sPBI75blend
sPBI90rand
sPBI90blend
sPBI100
8.3
8.7
8.0
7.5
6.7
6.9
7.1
7.0
7.0
6.8
8.0
6.7
7.1
7.0
6.6
5.7
6.8
7.0
6.8
6.3
5.9
6.3
3.8E-05
3.3E-05
1.2E-05
2.9E-05
1.8E-04
7.1E-05
1.2E-04
1.2E-04
9.3E-04
5.0E-04
1.3E-03
1.4E-05
–
1.7E-05
1.2E-04
4.7E-05
7.3E-05
1.2E-04
1.8E-04
–
2.0E-04
–, too low to be correctly measured.
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ARTICLE
ꢀ
30 and 90 C were found to be limited considering the very
high IEC of the sPBI. The ionic conductivity of such high IEC
sPBI is quite low in comparison with aromatic sulfonated poly-
mers, even at high temperature and high relative humidity.
Because a part of sulfonic acid functions interacts with benz-
imidazole units, the proton transport through membranes is
seriously affected. However, the 100% sulfonated PBI
(sPBI100) which offers a good balance between oxidative sta-
bility, moderate water uptake at high temperature, and con-
ductivity can be considered as a good potentially candidate for
high temperature fuel cell membrane application.
The authors thank the CNRS (Centre National pour la Recher-
che Scientifique), the CEA (Commissariat a` l’Energie Atomique),
and the GDR PACTE for financial support. They also thank
S. Jacob and G. Larramona (IMRA Europe) for conductivity
measurements.
FIGURE 6 Water uptake of sPBI100 in liquid water at high
temperature.
for ionomers, the conductivity is dependent on the IEC of
the polymers. But surprisingly, conductivities of sPBI at 30
ꢀC are very low compared with other kinds of sulfonated
polymers displaying similar IEC. Even the most sulfonated
PBI do not exhibit conductivity higher that 0.2 mS/cm. Few
other research groups25,27,28 working on sulfonated PBI al-
ready observed such low conduction properties. The interac-
tions sulfonic acid–benzimidazole ring previously described
are likely responsible of this phenomenon. The mechanism
generally admitted for proton transport in hydrated ionom-
ers is a jump mechanism from one sulfonic acid group to
another, assisted by water. In the case of sPBI, the SO3H are
subject to interactions with the benzimidazole units of the
polymer backbone and hence, the proton is shared between
both groups, yielding to an intermediate form between sul-
fonic acid/benzimidazole and sulfonate/benzimidazolium. So
a fraction of acid functions can be supposed to be not really
available for proton transport. Despite the lower water
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