Intrinsically proton-conducting benzimidazole units tethered to polysiloxanes

Article abstract of DOI:10.1021/ma047482+

Polysiloxanes with pendant benzimidazole units have been prepared by free radical thiolene coupling reactions of 2-(2-benzimidazolyl)ethanethiol and vinyl-functional polysiloxanes. The latter polymers were prepared by anionic ring opening copolymerization of 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclosiloxane and 1,3,5-hexamethylcyclosiloxane. Copolymers with different degrees of benzimidazole functionality were conveniently obtained by varying the monomer ratios. The coupling reaction was found to be very efficient, and the vinyl groups were completely consumed, as confirmed by NMR and FTIR spectroscopy. The glass transition temperature (T_g) of the benzimidazole functional copolymers increased dramatically with the benzimidazole content at low contents to reach a plateau value just above 50°C at a content of approximately 33 mol % benzimidazole functional siloxane residues in the copolymer. Conductivity measurements carried out at 60 and 140°C indicated that the level of polymer segmental mobility, and thus the T_g, was the most decisive parameter for the proton conductivity at low temperatures (60°C), while the benzimidazole concentration was more important at elevated temperatures (140°C). A conductivity of 7 × 10^-6 S/cm was reached at 140°C by a polygiloxane carrying 57 mol % benzimidazolegrafted siloxane residues under completely anhydrous conditions.

Full text of DOI:10.1021/ma047482+

Macromolecules 2005, 38, 3283-3289
3283
Intrinsically Proton-Conducting Benzimidazole Units Tethered to
Polysiloxanes
J. Christian Persson and Patric Jannasch*
Department of Polymer Science & Engineering, Lund University, P.O. Box 124,
SE-221 00 Lund, Sweden
Received December 7, 2004; Revised Manuscript Received January 31, 2005
ABSTRACT: Polysiloxanes with pendant benzimidazole units have been prepared by free radical thiol-
ene coupling reactions of 2-(2-benzimidazolyl)ethanethiol and vinyl-functional polysiloxanes. The latter
polymers were prepared by anionic ring opening copolymerization of 1,3,5,7-tetramethyl-1,3,5,7-
tetravinylcyclosiloxane and 1,3,5-hexamethylcyclosiloxane. Copolymers with different degrees of benz-
imidazole functionality were conveniently obtained by varying the monomer ratios. The coupling reaction
was found to be very efficient, and the vinyl groups were completely consumed, as confirmed by NMR
and FTIR spectroscopy. The glass transition temperature (T
increased dramatically with the benzimidazole content at low contents to reach a plateau value just above
0 °C at a content of approximately 33 mol % benzimidazole functional siloxane residues in the copolymer.
Conductivity measurements carried out at 60 and 140 °C indicated that the level of polymer segmental
mobility, and thus the T , was the most decisive parameter for the proton conductivity at low temperatures
g
) of the benzimidazole functional copolymers
5
g
(
60 °C), while the benzimidazole concentration was more important at elevated temperatures (140 °C).
-6
A conductivity of 7 × 10 S/cm was reached at 140 °C by a polysiloxane carrying 57 mol % benzimidazole-
grafted siloxane residues under completely anhydrous conditions.
Introduction
imidazole, benzimidazole, and pyrazole have in many
respects properties similar to those of water if compared
Proton-conducting polymers have a key function in
many important technical devices, such as polymer
9
relative to their respective melting points. Especially
the amphoteric behavior and the ability to form inter-
molecular hydrogen bonds are important characteristics
which are related to the proton conductivity of both
water and imidazoles. However, in contrast to water,
the latter compounds may be covalently tethered to a
suitable polymer. This allows for the preparation of fully
polymeric materials in which the proton-conducting
species are prevented from being leached out and lost.
A crucial point when molecularly designing these poly-
electrolyte fuel cells (PEMFCs), electrochemical sensors,
and electrochromic devices.1,2 Especially PEMFC tech-
nology has attracted considerable interest due to its high
energy efficiency and environmentally friendly quali-
3
,4
ties. The polymer membranes in PEMFCs must be
able to facilitate fast proton conduction and at the same
time be thermally and mechanically stable. It is con-
siderably more favorable to operate PEMFCs at high
temperatures, e.g., above 100 °C, due to the many
3
-5
mers is to allow for a high local mobility of the
advantages reached at the system level.
The high
heterocycles after the immobilization.1
0,11
Thus, a suf-
proton conductivities reached by the membranes of
today are mainly facilitated by water or other volatile
low molecular weight compounds present in the polymer
membrane. Consequently, a loss of these species will
immediately result in a parallel loss of conductivity. For
example, Nafion, the most widely used polymer mem-
brane, quickly loses conductivity at temperatures above
ficiently low glass transition temperature (Tg) seems to
be essential to reach highly conductive systems. Im-
mobilization of the heterocycles further implies that no
proton transport can take place via a so-called vehicle
mechanism, where the protons diffuse as protonated
species through the material. Instead, the transport
must rely entirely on structure diffusion where the
protons are transferred between the charge-carrying
heterocycles in a complex manner. This is similar to the
1
00 °C due to the evaporation of water. The replacement
of water by less volatile compounds is one possible way
to improve the membranes to enable operation above
1
00 °C. For example, strong oxo acids, e.g., phosphoric
“Grotthuss” mechanism of proton transfer, which is
well-known for water.1
2-14
Recently, Schuster et al.
acid and sulfuric acid, have been complexed with basic
6
,7
polymers such as polybenzimidazole. However, since
the potential risk of depleting the conducting phase still
exists, many problems remain unsolved. There is con-
sequently a need to explore new concepts and new
polymeric materials that are capable of fast proton
conduction in the absence of any volatile compounds,
and that are durable at elevated temperatures.8
showed, using a series of model compounds with imi-
dazole tethered to the chain ends of oligo(ethylene
oxide)s, that it is possible to reach conductivities of 20-
8
0 µS/cm at 120 °C in systems which rely only on
10,11
structure diffusion.
In addition, similar oligomers
based on benzimidazole tethered to both ends of oligo-
(
ethylene oxide)s have recently been studied in our
Kreuer has pointed out the possibility to develop fully
laboratory and were found to reach conductivities
polymeric proton-conducting membranes based on ni-
between 0.2 and 70 µS/cm at 110 °C, depending on the
length of the oligo(ethylene oxide) spacer. Further-
2
trogen-containing heterocycles. Heterocycles such as
15
more, two fully polymeric proton conductors consisting
of imidazole tethered to a polystyrene and benzimida-
zole tethered to a branched siloxane network have
*
To whom correspondence should be addressed. E-mail:
patric.jannasch@polymer.lth.se.
1
0.1021/ma047482+ CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/16/2005

3284 Persson and Jannasch
Macromolecules, Vol. 38, No. 8, 2005
Scheme 1. Synthesis of
-(2-Benzimidazolyl)ethanethiol (BET)
Scheme 2. Preparation of PVMSx by Anionic Ring
2
4
Opening Copolymerizations of the Monomers V and
D
3
recently been investigated by Hertz et al.16 Conductivi-
ties as high as 700 µS/cm at 200 °C were obtained for
the former system.
was stored above molecular sieves prior to use. Before initia-
tion using a 2.5 M solution of n-butyllithium (Acros), the
reaction solution was cooled to -40 °C, carefully degassed, and
left under a blanket of argon. All reactions were run for 40 h
under an argon atmosphere, and the polymerizations were
In the present work we have focused on linear
polysiloxanes tethered with benzimidazole. Polysilox-
anes have a high degree of segmental mobility and
consequently low Tg values, and the stability issue of
benzimidazole is less critical than that of imidazole.
Several different procedures to tether various functional
groups to polysiloxanes have been reported in the
literature. For example, Hooper et al. have tethered
oligo(ethylene oxide) chains onto a polysiloxane back-
finally terminated by addition of a small excess of Me
3
SiCl
(
Aldrich, 98%). The polymer products were precipitated twice
in MeOH to remove unreacted species, and then dried under
vacuum to remove the remaining solvent.
The copolymer samples were designated PVMSx, where x
denotes the mole percent of vinylsiloxane residues in the
1
7
1
bone by hydrosilylation. In other studies functional
polymer, as determined by H NMR spectroscopy. In the study,
a commercial PVMS copolymer (United Chemical Technolo-
gies, Inc.) containing 5 mol % vinylsiloxane residues was also
included. The polymers were characterized by NMR spectros-
copy and chromatography, as described below. Table 1 sum-
marizes some general structural data of the PVMS copolymers.
Tethering Benzimidazole Functionalities onto the
PVMS Copolymers. A 100 mL two-necked, round-bottomed
glass reactor was fitted with a heater and reflux condenser
and was further equipped with a magnetic stirrer. The
reactants were dissolved in THF and then transferred to the
reactor. The temperature was slowly increased to the boiling
point of THF (66 °C), and the reactor was carefully degassed
groups have been tethered to polymers via thiol-ene
1
8-20
free radical coupling reactions.
We have in the
present work combined the excellent segmental mobility
of the polysiloxanes with the intrinsic proton-conducting
properties of benzimidazole to establish structure-
property relationships for these new materials. For this
reason, a modulated method of synthesis was chosen
to conveniently obtain relevant model polymers with
different molecular structures. A series of polymeric
proton-conducting compounds were thus synthesized by
tethering benzimidazole units onto polysiloxane back-
bones via short aliphatic thioether spacers. The polysi-
loxane precursor backbones were prepared by anionic
ring opening polymerization, and the coupling reaction
was subsequently performed via a thiol-ene free radical
reaction. Polymers having different concentrations of
benzimidazole were prepared by varying the composi-
tion of the precursor copolymers. The present paper
focuses on the synthesis and structural characterization
of these novel polymers. In addition, the solubility and
the thermal properties, as well as proton conductivity
data, will be discussed. In a future paper we will report
and discuss the conductivity behavior and the effect of
doping the benzimidazole-tethered polymers with se-
lected oligomeric and polymeric acids.
with N
performed under reflux in THF using an excess of BET, and
,2′-azobis(2-methylpropionitrile) (AIBN) (Acros, 98%) as the
2
before the initiation of the reaction. The reactions were
2
radical source. The relative amounts of the reactants cor-
responded to the molar ratio [thiol]:[vinyl]:[AIBN] equal to 10:
5
2
:3. A slight overpressure of N gas was kept in the reactor
during the reaction to avoid air. All reactions were carried out
for 40 h to convert all the vinyl groups into thioether links. In
accordance with the designation of the PVMS copolymers, the
benzimidazole-functionalized polymers were designated as
bimiPVMSx, where x denotes the molar percentage of benz-
imidazole-grafted siloxane residues in the polymer. After the
reaction, bimiPVMS5 and bimiPVMS16 were precipitated in
methanol (PROLABO, >98.5%) while bimiPVMS33 and bimi-
PVMS57 were precipitated in ethyl acetate (KEBO, >99.5%)
because of the higher benzimidazole content of the latter.
Finally, the samples were carefully dried under vacuum and
stored in an argon-filled glovebox until analysis.
Experimental Section
1
13
Synthesis of 2-(2-Benzimidazolyl)ethanethiol. 2-(2-
Benzimidazolyl)ethanethiol (BET) was readily prepared by
reacting a slight excess of 3-mercaptopropionic acid (Aldrich,
Characterization. H and C NMR spectra were recorded
with a Bruker 400 MHz spectrometer using chloroform-d or
dimethyl sulfoxide-d as solvent depending on the benzimida-
6
99+%) with 1,2-diaminobenzene (Aldrich, 98%) in 4 M aqueous
zole content of the samples (bimiPVMS5 and bimiPVMS16,
chloroform; bimiPVMS33 and bimiPVMS57, DMSO). The
compositions of the copolymers and the results of the coupling
reactions were determined by comparing the integrated NMR
HCl at 100 °C during 40 h of reflux as shown in Scheme 1. In
a typical synthesis procedure, 7.83 g (73.8 mmol) of 3-mer-
captopropionic acid was reacted with 6.57 g (60.8 mmol) of 1,2-
diaminobenzene in 30 mL of a 4 M HCl solution. After the
reaction, distilled water was added to the mixture to obtain a
total volume of 500 mL. After neutralization using the required
volume of a 4 M aqueous NaOH solution, the BET precipitated
as a white powder. Finally, the product was carefully rinsed
with water to remove excess 3-mercaptopropionic acid and salt.
Preparation of the Copolymers. The precursor copoly-
mers were synthesized by copolymerizations of cyclosiloxanes.
The monomers 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclosi-
3
signals from the vinyl protons and the SiCH protons. The
PVMS copolymer compositions are shown in Table 1. To
determine the origin of the different signals, various correla-
1
1
tion experiments were also performed, including H- H cor-
1
1
1
13
relation ( H- H COSY) and H- C heteronuclear multiple
quantum coherence (HMQC). Infrared spectra were obtained
using a Bruker IFS 66 FTIR spectrometer with the samples
in the form of pressed tablets of ground mixtures of polymer
and KBr salt, or directly applied films of viscous liquids on
KBr tablets. The samples were subsequently analyzed in
loxane (V
98%) were purchased from Fluka and Aldrich, respectively,
and were used without further purification. Anionic ring
opening copolymerizations of V and D using different mono-
mer ratios were carried out in anhydrous THF (Labscan,
9.8%) at room temperature according to Scheme 2. The THF
4 3
) (g97%) and 1,3,5-hexamethylcyclosiloxane (D )
-
1
(
transmission mode at a resolution of 4 cm .
Differential scanning calorimetry (DSC) measurements were
performed using a TA Q1000 instrument. The samples were
carefully dried in a vacuum oven and then quickly transferred
to aluminum sample pans which were hermetically sealed. The
4
3
9

Macromolecules, Vol. 38, No. 8, 2005
Benzimidazole Tethered to Polysiloxanes 3285
Table 1. Copolymerization and PVMSx Data
copolymer compositiona
monomer feed
ratio [V4]/[D3]
copolymerization
time (h)
(mol % vinylsiloxane
copolymer
residues, x)
Mnb
Mw/Mn
PVMS5c
PVMS16
PVMS33
PVMS57
5
16
33
57
13300
15800
10600
10800
2.2
1.4
1.4
1.3
0.5
1.125
3
40
40
40
a
Determined by H NMR. Determined by size exclusion chromatography (SEC) using polystyrene standards. c Commercial sample
1
b
obtained from United Chemical Technologies, Inc.
samples were analyzed at a scan rate of 10 °C/min from -90
to +250 °C. To investigate the thermal stability of the
copolymers, thermogravimetric analysis (TGA) was performed
on a TA Q500 analyzer under nitrogen purge from room
temperature to 600 °C at a heating rate of 10 °C/min.
The neat PVMS copolymer samples were analyzed by size
exclusion chromatography (SEC). Three Shodex columns in
series (KF-805, -804, and -802.5) and a refractive index
detector were employed in the setup. All chromatograms were
recorded at room temperature and at a THF elution rate of 1
mL/min. The calibration was based on polystyrene standards
purchased from Polymer Laboratories, Ltd.
The solubility of the different bimiPVMSx samples was
investigated by placing pieces of dry samples in a range of
selected solvents contained in test tubes. The samples were
then left for 24 h at 22 °C. If the samples were then found to
be completely dissolved after visual inspection, they were
considered as soluble; if not they were considered as non-
soluble.
To measure the proton conductivity of the bimiPVMS
samples, they were sandwiched between two gold-plated
electrodes, either as a liquid or as a pressed tablet. Carefully
dried samples were prepared in an argon-filled glovebox to
avoid water contamination. The sample dimensions were
defined by circular Teflon spacers having thicknesses ranging
from 100 to 250 µm, and diameters between 10 and 12 mm.
Conductivity data were collected at 60 and 140 °C using a
Novocontrol high-resolution dielectric analyzer in the fre-
-
1
7
quency range 10 to 10 Hz at a voltage amplitude of 1.0 V.
The dc conductivities were then determined at the frequency
that corresponded to the minimum imaginary response in
2
1
complex conductivity plots.
Results and Discussion
Copolymer Synthesis. Polysiloxanes tethered with
benzimidazole units via short aliphatic thioether spacers
were prepared and characterized as intrinsic proton
conductors. The syntheses were performed in three
steps involving first preparation of BET, then copoly-
merizations of the monomers V4 and D3 to form PVMS
copolymers, and finally thiol-ene coupling reactions
between the PVMS copolymers and the BET.
1
Figure 1. H NMR spectra of BET (A), PVMS16 (B), and
bimiPVMS16 (C). Please note the absence of the shifts from
the vinyl protons in spectrum C, indicating the complete
conversion of the vinyl groups in the coupling reaction.
Three PVMS copolymer samples with different com-
positions were prepared by anionic ring opening copo-
lymerization (Scheme 2). Also, a commercial PVMS
sample was included in the investigation (PVMS5). The
compositions of the copolymers were determined by
BET was synthesized by reacting 3-mercaptopropionic
acid with 1,2-diaminobenzene under acidic conditions,
as shown in Scheme 1. BET was subsequently charac-
1
terized by H NMR spectroscopy, and characteristic
1
comparing the integrated H NMR signals from the
shifts from the protons of the benzimidazole unit and
the carbon chain were observed (Figure 1). The integrals
of the signals corresponded well to the relative number
of the various protons in BET. It was also noted that
the signal from the thiol proton was absent. Moreover,
no signals from residual reactants were detected, which
indicated a high purity of the product. Analysis of the
reaction mixture by FTIR spectroscopy showed that the
protons of the vinyl groups between 5.7 and 6.1 ppm
with the integrated signal of the SiCH3 protons at
approximately 0.1 ppm. It should be pointed out that
the vinylsiloxane residues were arranged in groups of
four due to the structure of the monomer V4. We were
not able to determine the sequence distribution of the
vinylsiloxane residues in the present samples. However,
it was observed in our experiments that the ratio of V4
residues in the copolymers was below the monomer feed
ratio of the V4 monomer charged to the reactor at the
start of the copolymerizations. This suggested that the
reactivity of V4 was lower than the reactivity of D3 under
-
1
strong absorption of the carbonyl group at 1700 cm
gradually disappeared as the reaction proceeded (not
shown). Simultaneously, absorption bands from the
-1
imidazole ring appeared at around 1540 cm (N-H)
and at 1623 cm- (CdN).
1

3286 Persson and Jannasch
Macromolecules, Vol. 38, No. 8, 2005
Scheme 3. Preparation of Benzimidazole-Grafted
Polydimethylsiloxane via the Free Radical Thiol-Ene
Coupling Reaction
Scheme 4. Proposed Reaction Steps in the Thiol-Ene
2
3,24
Coupling Reaction
these conditions. This is in agreement with the finding
of Herczynska et al., who also observed a lower reactiv-
ity of V4 than that of D3 during anionic copolymerization
in toluene, which caused concentration gradients of
vinylsiloxane units along the copolymer chains.22 As
seen in Table 1, the molecular weights of the PVMS
copolymers prepared in this study ranged from 10 600
to 15 800, and the polydispersity index (PDI) was in the
range from 1.3 to 1.4 when calibrated with polystyrene
standards. The lower molecular weight of the vinyl-rich
copolymers was probably a direct consequence of the
lower reactivity of the monomer V4 as compared to D3.
Figure 2. FTIR spectra of PVMS57 (A) and bimiPVMS57 (B).
The dashed lines indicate the positions of the absorption peaks
2
arising from -CHdCH (b), imidazole N-H deformation (9),
and imidazole CdN ([).
Copolymer Solubility. The solubility of the bimi-
PVMSx copolymers was investigated in a wide range of
solvents. Expectedly, the solubility was highly depend-
ent on the degree of benzimidazole functionalization (x)
of the copolymers. The solubilities of the various samples
in a number of selected solvents, arranged according to
their hydrogen-bonding capacity and solubility param-
eter (δ), are shown in Table 2. As anticipated, due to
their hydrophobicity, the PVMS copolymers were readily
soluble in toluene, but not in hydrogen-bonding solvents
such as methanol. The solubility properties then changed
quite dramatically as increasing amounts of benzimi-
dazole were incorporated and the polymers became
increasingly hydrophilic and hydrogen bonded. Sample
bimiPVMS57 was soluble in methanol but not in
toluene, which was the reverse situation compared to
that found for the PVMS copolymers. Because of its
amphoteric and hydrogen-bonding character, the pres-
ence of benzimidazole promoted the solubility of the
copolymers in polar and hydrogen-bonding solvents. It
was noted that the bimiPVMSx copolymers were all
soluble in the moderately hydrogen-bonding THF and
in the strongly hydrogen-bonding 1-butanol, while no
copolymers were soluble in acetonitrile or water. Also,
it was found that the solubility of the copolymers in
methanol was the reverse of that in diethyl ether and
chloroform. It was finally observed that all the solutions
foamed when shaken, which indicated that the copoly-
mers had an amphiphilic character.
As shown in Scheme 3, the coupling reaction between
PVMS and BET was achieved via a free radical thiol-
ene coupling reaction. The coupling reaction has been
reported to involve the propagation of a thiyl radical
through the vinyl groups of the PVMS copolymers.2
A new thiyl radical is then regenerated by chain transfer
from the resulting carbon radical to a thiol, as outlined
in Scheme 4. The modifications of the PVMS copolymers
were monitored by observing the disappearance of the
3,24
1
H NMR signal arising from the protons of the vinyl
groups (Figure 1). Simultaneously, signals from the
protons of the thioether chain and the benzimidazole
unit emerged as the reaction proceeded. After 40 h of
reaction, all vinyl groups had been efficiently converted
into thioether links. It was also possible to monitor the
reaction by following the disappearance of the absorp-
tion from the CH2 wag vibration of the vinyl group at
-
1
9
60 cm , using FTIR (Figure 2). Furthermore, the IR
absorption bands from the N-H deformation and CdN
in the imidazole rings of the product were observed at
-
1
1
540 and 1623 cm , respectively. In addition, the
1
3
complete conversion was confirmed by C NMR by
observing the disappearance of the shifts arising from
the vinyl carbons (not shown). All the bimiPVMSx
samples were slightly yellow after the purification. It
was noted that bimiPVMS5 was a highly viscous liquid,
bimiPVMS16 was a soft solid, and the other samples
were quite brittle solid materials at room temperature.
Thermal Properties. All the bimiPVMSx copoly-
mers were fully amorphous materials and showed single
glass transition temperatures (Tg). As shown in Figure
3, the Tg values of the bimiPVMSx copolymers increased
dramatically in comparison to those of the PVMS
copolymers. The latter copolymers all showed a Tg close

Macromolecules, Vol. 38, No. 8, 2005
Table 2. Solubility of the bimiPVMSx Samples at 22 °C
solventa
THF,
Benzimidazole Tethered to Polysiloxanes 3287
polymer
sample
water,
methanol, 1-butanol,
DMSO,
diethyl ether, acetonitrile, chloroform,
toluene,
δ ) 18 (p)
δ ) 48 (s) δ ) 30 (s)
δ ) 23 (s) δ ) 25 (m) δ ) 19 (m)
δ ) 15 (m)
δ ) 24 (p)
δ ) 19 (p)
PVMS5
-
-
-
-
-
-
-
-
+
+
-
+
+
+
+
-
-
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
+
+
+
-
-
+
+
+
-
-
bimiPVMS5
bimiPVMS16
bimiPVMS33
bimiPVMS57
a
The symbols “+” and “-” indicate solubility and nonsolubility, respectively. Solubility parameters (δ, MPa1/2) were obtained from the
Polymer Handbook,2 and the letters s, m, and p denote strongly, moderately, and poorly hydrogen-bond-forming solvents, respectively.
5
Figure 4. Thermal stability as evaluated by TGA under
nitrogen.
Figure 3. Influence of the benzimidazole content of the
bimiPVMSx copolymers on the T , as evaluated by DSC.
g
to -125 °C, while bimiPVMS57 had a Tg at 57 °C, as
measured by DSC. The major reason for the large
increase of the Tg was most likely the result of the very
strong hydrogen-bonding character of the benzimidazole
units, causing both intra- and intermolecular hydrogen-
bonding interactions. The hydrogen bonds between the
benzimidazole units thus suppressed the mobility of the
polymer chain segments to a large extent. However,
after approximately 33 mol % benzimidazole-grafted
siloxane residues was reached, the effect of additional
benzimidazole on Tg seemed to stabilize, and the Tg
values leveled out to reach a plateau at around 53 °C.
This implies that it might be possible to incorporate
additional benzimidazole units without any further
significant increase of Tg, which may favor high con-
ductivities at high temperatures.
The DSC analysis also revealed a weak exothermic
peak at approximately 165 °C for both the PVMS and
the bimiPVMS copolymers. These exotherms were, after
DSC heating-cooling-heating cycles, found to be ir-
reversible and might arise from limited degradation
reactions. A similar exothermic peak was found at 250
Figure 5. Weight loss of the first degradation step recorded
by TGA as a function of the BET content of the bimiPVMS
copolymers. The dashed line indicates the diagonal of the plot
(
x ) y).
°
C for a neat poly(dimethylsiloxane) sample, indicating
weight loss was nicely found to be equivalent to the BET
content of the bimiPVMS copolymers, as shown in
Figure 5. This indicated the relatively weak nature of
the C-S-C bonds. The second weight loss occurred at
about 360 °C and most probably involved a drastic
degradation of the polysiloxane backbones. In fuel cells,
where both oxidative and reducing conditions are present
in combination with high temperatures, the material
must be durable enough to resist the aggressive envi-
ronment. Here it should again be emphasized that the
materials prepared and studied in the present investi-
the limited thermal stability of polysiloxanes. In this
context it is important to note that the conductivity
measurements discussed below were carried out below
1
50 °C.
Figure 4 shows the traces from the thermogravimetri-
cal analysis of the bimiPVMSx samples carried out
under a nitrogen atmosphere. The first weight loss
occurred at temperatures above 200 °C, and was found
to increase with the content of benzimidazole-grafted
siloxane residues (x). Indeed, the magnitude of this

3288 Persson and Jannasch
Macromolecules, Vol. 38, No. 8, 2005
contents, and the difference in Tg between bimiPVMS33
and bimiPVMS57 was only 4 °C. A further increase of
the benzimidazole content will thus increase the number
of proton carriers without suppressing the segmental
mobility significantly. If the material is to be used at
high temperatures, e.g., 140 °C, it may be considered
advantageous for the conductivity to incorporate a
maximum amount of proton-carrying species. However,
as seen in Figure 6, the increase of the conductivity with
the concentration of benzimidazole seems to level off at
high concentrations.
Hertz et al. have previously reported conductivities
of approximately 30 µS/cm at 140 °C and 0.2 µS/cm at
7
0 °C for a fully polymeric material based on benzimi-
dazole tethered to an inorganic siloxane network poly-
mer via flexible spacers.16 This value may be compared
with that of the sample with the highest conductivity
in our study, bimiPVMS57, reaching conductivities of
7
µS/cm at 140 °C and 0.6 nS/cm at 70 °C. The higher
conductivity in the former system might be due to the
longer spacer units, which facilitated the local mobility
of the benzimidazole units. However, it is difficult to
make a detailed analysis because no data, e.g., on the
Tg of the benzimidazole-tethered polysiloxane networks,
were reported. We have previously reported a conduc-
tivity of 70 µS/cm at 110 °C for benzimidazole tethered
to oligo(ethylene oxide) chain ends.15 It should be noted,
however, that the Tg of this oligomer was -30 °C, which
was considerably lower than for bimiPVMS57. As
demonstrated by Schuster et al., there are possibilities
to further improve the conductivities of imidazole-based
systems by doping the materials through addition of
suitable acidic species which introduce excess protons
into the hydrogen-bonded networks.1
Figure 6. Influence of the benzimidazole content of the
bimiPVMSx copolymers on the proton conductivity at 60 and
140 °C.
gation were model polymers to establish structure-
property relationships which may serve as guidelines
for the molecular design of materials intended for use
in fuel cells. Having said this, it might be possible to
improve the stability of the C-S-C bonds in the present
case by converting the weak thioether links to sulfoxide
or sulfone links through carefully controlled oxida-
2
6,27
tions.
The latter links are known to have a much
higher chemical stability than the thioether link.
Proton Conductivity. The proton conductivity was
measured by impedance spectroscopy with the bimi-
PVMSx samples sandwiched between two gold-plated
electrodes. The bulk conductivity (σ) of a sample can be
described as the sum of the contributions of all the
charge-carrying species according to
0,11
Conclusions
Benzimidazole units have been tethered to polysilox-
anes via short aliphatic thioether spacers to obtain
samples to study structure-property relationships for
this kind of intrinsically proton-conducting polymer. The
conversion of the vinyl groups of the PVMS copolymers
into thioether links proceeded readily via thiol-ene
coupling, and was quantitative in all experiments. The
functionalizations resulted in a dramatic increase of the
Tg, which can be explained by the strong intra- and
intermolecular interactions existing between the teth-
ered benzimidazole units. All samples were shown to
be stable up to 165 °C under nitrogen purge according
to DSC data. Obviously, in comparison to the present
samples, the thermal stability needs to be greatly
improved to withstand the aggressive chemical and
physical conditions in a fuel cell. The proton conductivity
was found to be promoted by high benzimidazole
contents and low Tg values. These parameters are
however conflicting in the present materials. Generally,
low benzimidazole contents will result in low Tg values,
but also in low proton carrier concentrations. On the
other hand, high benzimidazole contents will provide
more efficient paths for the proton transport due to the
higher carrier concentrations, but will be accompanied
by unfavorably high Tg values. A maximum conductivity
of 7 µS/cm at 140 °C was reached by the studied
copolymers under completely dry conditions. However,
doping with suitable polymeric and oligomeric acids is
expected to increase the conductivity, and this will be
the next step in our investigation of these new materi-
als. Also, the effects of spacer length and the nature of
the spacer still remain to be explored.
σ ) n µ q
i i i
(1)
∑i
where n is the number of charge carriers, µ is their
mobility, and q is the electric charge of the carriers.
Figure 6 shows the influence of the benzimidazole
content (x) on the proton conductivity at 60 and 140 °C.
At 60 °C, bimiPVMS5 displayed a superior conductivity
because of its higher segmental mobility (lower Tg) and
the conductivity gradually decreased when the benz-
imidazole content was increased. However, at 140 °C
the situation was reversed. At this temperature the
conductivity of bimiPVMS57 was the highest because
of its higher benzimidazole content. The more pro-
nounced temperature dependence of the copolymers
with high benzimidazole content indicated higher acti-
vation energies for the proton transport in these sys-
tems. This shows that the proton carrier density and
the segmental mobility are closely interconnected in this
kind of system. An increase in the benzimidazole
content caused a suppression of the segmental mobility
in the polymers, and vice versa, which was consistent
with the dramatic increase of the Tg, as seen in the DSC
study. It is therefore necessary to reach a suitable
balance between the charge carrier density and the
segmental mobility if these kinds of materials are to be
used in a broad temperature interval.
As mentioned above, the influence of the benzimida-
zole content on the Tg seemed to stabilize at high

Macromolecules, Vol. 38, No. 8, 2005
Benzimidazole Tethered to Polysiloxanes 3289
Acknowledgment. We thank the Swedish Research
Council for financial support.
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