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Author's personal copy
Journal of Membrane Science 304 (2007) 173–180
Ionic crosslinking of ionomer polymer electrolyte
membranes using barium cations
Jeffrey V. Gasa a , R.A. Weiss a,b,∗ , Montgomery T. Shaw a,b
a
b
Polymer Science Program, University of Connecticut, Storrs, CT 06269-3136, United States
Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269-3136, United States
Received 27 May 2007; received in revised form 19 July 2007; accepted 20 July 2007
Available online 26 July 2007
Abstract
Acidic polymer electrolyte membranes (PEM) based on sulfonated poly(ether ketone ketone) (SPEKK) with relatively moderate to high levels of
sulfonation (ion-exchange capacity (IEC) > 1.7 meq/g) have excellent proton conductivities (∼0.1 S/cm), but they absorb excessive amounts of water
at elevated temperatures (ca. 60–90 ◦ C). To reduce the water sorption of the membranes and improve their mechanical properties and dimensional
stability, the protons in acidic SPEKK membranes were partially exchanged with divalent barium cations to create ionic crosslinks between the
sulfonate groups attached to the aromatic rings of the PEKK. The degree of crosslinking was varied by changing the degree of neutralization of
the ionomer. The proton conductivity, water uptake, and methanol permeability at various levels of sulfonation and degree of crosslinking were
measured. The conductivity was reduced by crosslinking, but the thermal stability, swelling, and barrier properties were improved. Crosslinking
reduced the water swelling at room temperature by nearly a factor of two and prevented membrane dissolution at temperatures up to 80 ◦ C. In a
water/methanol mixture (72/28, v/v), swelling was reduced by a factor of four. The balance of transport and mechanical properties could be varied
to produce a viable PEM for a direct methanol fuel cell by adjusting the crosslink density.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Ionomers; Proton exchange membranes; Selectivity; Conductivity; Neutralization
1. Introduction
Perfluorosulfonic acid membranes, such as Nafion® , are
presently considered the standard material for PEM fuel cell
(PEMFC) technology because of their high proton conductivities and excellent electrochemical and mechanical stability in
the fuel cell environment. However, these membranes possess
several properties that are unfavorable when methanol is used
as fuel, which most researchers consider to be the main hindrance to the technological and commercial success of direct
methanol fuel cells (DMFC). A disadvantage of Nafion® membranes for the DMFC application is the crossover of methanol
through the membrane, which adversely affects the performance
(cell voltage) and fuel utilization efficiency of the cell. Although
the proton conductivity and oxidative resistance of Nafion®
∗ Corresponding author at: Department of Chemical Engineering, University
of Connecticut, Storrs, CT 06269-3136, United States. Tel.: +1 860 486 4698;
fax: +1 860 486 6048/4.
E-mail address: [email protected] (R.A. Weiss).
0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.memsci.2007.07.031
membranes are both excellent, the durability and methanol
crossover of Nafion® become problematic especially when thinner membranes and high methanol feed concentrations are used
to increase power density. As a result, considerable research
has been focused recently on developing PEMs with a better balance of transport, mechanical, and aging properties. A
key challenge is to improve the resistance of the membrane
to methanol permeation without compromising other essential
properties, specifically proton conductivity.
In our previous work [1], we found that SPEKK
with relatively moderate to high levels of sulfonation (ca.
IEC > 1.7 meq/g) had proton conductivities comparable to
that of Nafion® (ca. 0.1 S/cm at 98% relative humidity and
20 ◦ C). However, these membranes exhibited poor dimensional,
mechanical, and barrier properties in an aqueous environment at
elevated temperatures (ca. 60–90 ◦ C), which are typical operating conditions in a DMFC. The poor properties were attributed
to the excessive water sorption of the membranes at high water
activities (ca. > 50 wt% of water at 98% relative humidity). In
the work reported in this paper, the objective is to reduce the
water and methanol sorption by crosslinking the membrane.
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The idea of crosslinking of PEM is not new, but most of the
crosslinking studies done previously pertain to chemical (covalent) crosslinking [2–5]. The crosslinking procedure discussed
in this paper is a relatively less studied technique, which is ionic
crosslinking. The approach is to exchange with divalent cations
some of the protons in acidic membranes. The salt produced by
the exchange reaction should have very low water solubility and
form a stable crosslink. To the best of the authors knowledge,
the application of the abovementioned crosslinking procedure
to PEMs is a novel idea.
In general, crosslinking alters several important properties of
polymers. The more common effects of crosslinking are reduction in the sorption in swelling solvents, increase in transition
temperatures, and increase in plateau modulus [2–10]. In some
cases, crosslinking can also enhance the tensile strength and
toughness of the polymer [11,12]. It can also alter the permeability and permselectivity of the polymer [13,14].
In PEM applications, previous studies have indicated that
chemically crosslinked membranes can be superior to Nafion®
membranes in terms of properties pertinent to DMFC applications such as methanol permeability, swelling properties, and
proton conductivity [2–5]. In the different crosslinked systems
that they studied, the common conclusion was that the proton
conductivities of their membranes were comparable to Nafion®
but the methanol permeabilities were about one order of magnitude lower [2–5]. They also reported that the swelling of their
membranes were lower than Nafion® membranes.
In this study, it was postulated that the ionic crosslinking of
acidic SPEKK would give similar desirable effects such as low
swelling and low permeability of SPEKK membranes in polar
solvents such as methanol and water. The approach was to use
very acidic SPEKK (IEC > 1.5 meq/g) and crosslink some of the
sulfonate groups with divalent cations. Barium ions are known
to form salts that have extremely low solubility in water, and
thus were chosen to be the crosslinking species. Very acidic
membranes were used to compensate for the loss of protons due
to the ion-exchange process, and therefore maintain high conductivity. The specific objective of this work was to determine
the effect of barium crosslinking on the swelling, thermal stability, and transport properties (proton conductivity and methanol
permeability) of PEMs.
2. Experimental
2.1. Ionically crosslinked SPEKK membranes
SPEKK membranes with IECs of 1.7 and 2.06 meq/g were
prepared by sulfonating a 5% (w/v) solution of PEKK with a
53/47 (v/v) mixture of concentrated sulfuric acid and fuming sulfuric acid at room temperature. To crosslink the membranes, the
protons in acidic SPEKK membranes were partially exchanged
with barium cations from dilute barium acetate solutions. These
cations, which are divalent, can link together two sulfonate
groups to attain electro-neutrality (Fig. 1). When many sulfonate groups coming from different SPEKK chains are linked
together by these divalent cations, a network of SPEKK chains
can be formed. The fraction of protons exchanged with barium,
denoted here as the exchange fraction, is directly related to the
degree of crosslinking. Although the magnitude of the mechanical and transport properties changed with IEC, the effects of
Ba-crosslinking of the two different membranes was the same.
There are two possible crosslinking mechanisms that can take
place in these materials. Ionic crosslinks as shown be Fig. 1
occur as a necessity of achieving electrical neutrality in the
material; the effect of these crosslinks is likely to be relatively
independent of temperature. Also possible, however, are physical crosslinks due to ion–dipole associations of the Ba-sulfonate
groups, which can produce ionic aggregation, often termed
ion-clusters in the ionomer literature. The properties of the
physical crosslinks are expected to be temperature-dependent.
Ion-clusters are distinguished from the ionic crosslinks in that
they provide multifunctional crosslinks produced by nano-phase
separation of ion-rich domains of the order of 1–5 nm in size in
typical highly neutralized ionomers. In the discussion of the
effects of Ba-crosslinking in this paper, the crosslinks are simply referred to as ionic, because formation of ionic crosslinks
must occur due to the need for electronic neutrality, as shown
in Fig. 1. Whether or not physical crosslinks, i.e., ion-clusters,
also occurred requires structural analysis of the polymer using
techniques such as small angle X-ray scattering, which was not
done in this study.
To fine-tune the degree of crosslinking, the acidic SPEKK
membranes were immersed in dilute solutions (0.1 wt%) of barium acetate at various immersion times at room temperature.
After soaking in Ba(OAc)2 solution, the barium-crosslinked
membranes were thoroughly washed with copious amounts of
water to extract residual barium acetate solution absorbed by
the membrane and to remove acetic acid, which is a byproduct
of the exchange reaction (Fig. 1). The membranes were preswollen in water prior to immersion in barium acetate solution
to remove residual solvent (NMP) from the casting procedure
and also to promote the migration of barium ions into the membrane and minimize localization of the crosslinking reaction on
the membrane surface.
The degree of barium exchange was calculated as the number fraction of sulfonic acid groups substituted with barium.
Fig. 1. Schematic of the barium-crosslinking reaction of acidic SPEKK membranes.
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175
The amount of sulfonic acid groups was measured by acid–base
titration following the previously established procedure for
measuring the IEC of SPEKK [1]. The working equation for
fractional substitution xE was:
xE =
IEC0 − IECE
IEC0
(1)
where IEC0 is the IEC of the original sample and IECE is the
IEC of the sample after substitution, but based on its original
dry weight.
2.2. Characterization of the barium-crosslinked SPEKK
membranes
2.2.1. Proton conductivity
Impedance spectroscopy has been widely used to measure the
proton conductivity of PEMs [15–25]. The conductivity of the
membranes was measured using a Hewlett-Packard Agilent®
4284A LCR meter covering a frequency range of 20–106 Hz.
The applied voltage was 50 mV. Membrane conductivities were
measured using a custom-made cell based on the design of
Zawodzinski et al. [15]. Cells of this design measure conductivity along the plane of the membrane.
2.2.2. Water sorption
The water sorption of the membranes was measured by placing films in a controlled humidity environment and weighing
them every 24 h until the mass of the membranes reached equilibrium, as judged by the point where the changes in mass
were merely random weighing errors. The membranes were then
dried in vacuum at 100 ◦ C for 24 h and re-weighed. Water sorption is defined here as the mass of the absorbed water divided
by the mass of the wet membrane, i.e., the mass fraction of
water.
2.2.3. Thermal stability
The thermal stability of the polymers was measured with a
TA Instruments Hi-Res TGA 2950 thermogravimetric analyzer
using a nitrogen atmosphere and a heating rate of 20 ◦ C/min.
The samples were first dried for 10 min at 120 ◦ C in the TGA
furnace prior to the scan to remove any residual water absorbed
by the relatively hydrophilic sulfonated polymers during sample
transfer.
2.2.4. Methanol permeability
The methanol permeabilities of the Ba-crosslinked SPEKK
membranes were measured using the cell shown in Fig. 2. The
design of this cell is a variation of that used by Walker et al. to
measure methanol permeation through membranes based on sulfonated aromatic hydrocarbons and also perfluorosulfonic acid
membranes (Nafion 117® ) [26].
The lower part of the cell, which was a 20-mL glass vial,
was filled with 10 mL methanol. Methanol vapor in equilibrium
with the liquid diffused along the concentration gradient through
the membrane, which was clamped between the mouth of the
vial (2 cm in diameter) and the cap. The cap had a 1-cm hole
so that the methanol that diffused through the membrane could
Fig. 2. Schematic of the permeation cell.
escape. The cell was placed inside a drying oven, which provided
temperature control and an air draft that maintained the methanol
concentration and humidity in the environment above the surface
of the membrane at a minimum. The mass of the methanol inside
the cell was measured as a function of time. For the results
reported here, the methanol permeability (P) was calculated by
applying Fick’s first law:
J = −D
dCm
Cb
Cb
= −DK
= −P
dx
L
L
(2)
where J is the molar flux of methanol, D the methanol diffusivity, K the partition coefficient or the solubility of methanol
in the membrane, Cm the methanol concentration in the membrane, Cb the methanol concentration in the gas phase, and L is
the thickness of the membrane. In this device, it is difficult to
quantify accurately the value of Cb . Although Cb at the inner
surface of the membrane could be approximated using the vapor
pressure of MeOH, it is difficult to estimate Cb at the outer surface of the membrane. It is often assumed that Cb = 0, but a finite
methanol concentration near the cap can introduce appreciable
error in the calculation of J. To circumvent this problem, a reference sample with known methanol permeability (i.e., Nafion® )
was tested and the methanol permeabilities of the samples were
reported relative to the methanol permeability of the reference
sample. In this way, the driving force term (Cb ) in Eq. (2)
cancels out because all the samples were measured in one batch,
and therefore were exposed to the same conditions (same driving
force). The resulting relationship for the relative permeability is
then
Ju Lu
Pu
=
Jr L r
Pr
(3)
where u and r refer to the unknown and reference samples,
respectively. The reference sample was a Nafion 115® membrane (Dupont de Nemours, Wilmington, DE) with a thickness
of 127 ␮m as reported by DuPont.
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Fig. 3. Time evolution of the barium-exchange reaction in SPEKK with an IEC
of 1.7 meq/g using 0.1 wt% barium acetate solution.
3. Results and discussion
3.1. Ion-exchange reaction
Fig. 3 shows the time evolution of the exchange reaction.
It can be seen that a broad range of exchange fractions can be
obtained and can almost reach completion (100% exchange).
The analysis of the kinetics of exchange reaction is quite complicated because there are two competing rates that have to be
deconvoluted, the rate of barium exchange and the diffusion rate
of barium ions. The data obtained in this study was not enough
to get an accurate model of the kinetics of such ion-exchange
process.
Fig. 4. Influence of the degree of barium crosslinking or the exchange fraction
on the water sorption of the SPEKK membrane with an IEC of 1.7 meq/g.
3.3. Water sorption vs. temperature
3.2. Water sorption
Fig. 5 shows the water sorption of the SPEKK
(IEC = 1.7 meq/g) and barium-exchanged membranes as a function of temperature and exchange fraction. The water sorption of
the neat SPEKK membrane with an IEC of 1.7 meq/g exhibited
a strong temperature dependence. At temperatures below 45 ◦ C,
the temperature dependence of the water sorption was weak,
but above 45 ◦ C, there was a sharp upturn in the water sorption. That was probably due to the glass-to-rubber transition
of the water-swollen SPEKK. With increasing exchange frac-
The purpose of modifying SPEKK membranes through
crosslinking is to reduce the water sorption of these membranes
in aqueous environments especially for those membranes with
relatively high sulfonation levels. Fig. 4 shows the equilibrium
water sorption of barium-exchanged SPEKK membranes (IEC
of the original SPEKK was 1.7 meq/g) as a function of the
exchange fraction or the degree of crosslinking. These water
sorption data were taken at 23.8 ◦ C and 98% relative humidity.
The equilibrium water sorption monotonically decreased with
increasing exchange fraction. As with other crosslinkable systems [6–10], this result is an expected one because the barium
ionic crosslinks reduce the chain mobility, thereby reducing the
ability of these membranes to expand and absorb water.
If the sorption data is extrapolated to 100% exchange fraction, the water sorption is not zero; it is about 17 wt%. However,
this does not imply that the barium crosslinks are dissociated.
The membranes were pre-swollen in water prior to immersion in
barium acetate solution for crosslinking; thus the energetically
most favorable state in equilibrium with water is a somewhat
swollen condition even at 100% exchange fraction. This is analogous to the formation of highly swellable hydrogel networks
from monomer solutions as opposed to neat monomer [26].
Fig. 5. Influence of temperature and barium-exchange fraction on the water
sorption at 25 ◦ C of barium-exchanged SPEKK membranes. The numbers in
parentheses represent the mol% of H+ neutralized by Ba2+ . The IEC of the
untreated SPEKK was 1.7 meq/g.
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tion, the position of the upturn shifted to higher temperatures
and the magnitude of the sorption values decreased. Above 64%
barium exchange, the upturn disappeared and the water sorption became almost temperature-independent. This shows that
the barium crosslinking not only reduced the magnitude of the
water sorption but also improved the thermal stability of the
swelling of these membranes in an aqueous environment. The
excessive swelling exhibited by the neat SPEKK membrane at
temperatures above 45 ◦ C is undesirable because it compromises
the mechanical integrity, dimensional stability, and the barrier
properties of the membrane.
3.4. Swelling in water–methanol mixtures
In direct methanol fuel cells, the usual practice is to use dilute
solutions of methanol in water as fuel to minimize methanol
crossover and swelling of the membrane. The limitation on the
concentration of methanol in contact with the anode unfavorably
limits the performance of the cell. It is desirable that the cell be
operated at higher methanol concentrations to increase power
density and reduce the fuel volume, which are especially essential for portable applications. Higher methanol concentrations
are also desirable for easier water management. Therefore, it is
important to gain an understanding of the swelling properties of
PEM in methanol–water mixtures.
The swelling of the barium-exchanged SPEKK membranes
in water–methanol mixtures is shown in Fig. 6. The activity
of methanol in water–methanol mixtures varies in a relatively
linear fashion with methanol mole fraction over the range
shown in Fig. 6 (activity coefficient drops slightly from about
1.6 to around 1.3) [27]. The sorption of neat SPEKK membrane with an IEC of 1.7 meq/g exhibited a strong dependence
on methanol concentration, meaning that this polymer prefers
methanol over water. The slope of the SPEKK membrane data
in Fig. 6 increases monotonically with increasing methanol concentration. As the degree of crosslinking increased, the swelling
Fig. 6. Sorption of water at 25 ◦ C by barium-exchanged SPEKK membranes
as a function of methanol concentration and exchange fraction. The numbers
in parentheses represent the mol% of H+ neutralized by Ba2+ . The IEC of the
untreated SPEKK was 1.7 meq/g.
177
became less dependent on methanol concentration or methanol
activity. Above 64% barium exchange, the fluid uptake became
virtually independent of methanol activity, which suggests that
the barium crosslinking of SPEKK membranes eliminated the
favorable interaction with methanol, relative to water. Swelling
is reduced to a convenient value of around 20% regardless of the
fluid composition.
3.5. Ionic conductivity
In the barium-exchange reaction, the mobile protons are
replaced by the much less mobile barium ions. The mobility
of barium ions is much lower because these ions are bulkier and
have far less ionic character. Therefore, the ionic conductivity
of the membranes is expected to decrease upon ion-exchange
with barium, and that is confirmed in Fig. 7. The conductivity
decreased dramatically with increasing degree of crosslinking. If
the conductivity of the barium-exchanged SPEKK membranes
is plotted against the concentration of the remaining protons
(Fig. 8), expressed here in terms of the number of sulfonic
acid groups per mole of SPEKK repeat unit, the conductivity
of the barium-exchanged membranes overlaps with that of the
untreated or the neat SPEKK membranes. It appears that the barium disulfonate groups have very little contribution to the ionic
conductivity. This implies that the barium ions indeed have very
low mobility and the conductivity is controlled solely by the
protons.
3.6. Methanol permeability and permselectivity
In direct methanol fuel cells, methanol crossover is an important problem, especially for portable applications where the
current densities are relatively low. At low current densities, the
rate of methanol oxidation is low and unreacted methanol concentrates at the anode, promoting diffusion of methanol through
Fig. 7. Ionic conductivity of barium-exchanged SPEKK membranes as a function of exchange fraction.
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Fig. 8. Ionic conductivity of barium-exchanged SPEKK membranes and
untreated SPEKK membranes as a function of the number of remaining sulfonic
acid groups per repeat unit.
the membrane. To reduce crossover, the intrinsic property of the
membrane that has to be minimized is the methanol permeability. In this study, the ionic crosslinking of SPEKK using divalent
cations such as barium was expected to reduce the methanol permeability by reducing the swelling of the membrane, which will
reduce both the solubility and diffusivity contributions to the
permeability (see Eq. (2)). The crosslinks themselves may also
decrease methanol diffusivity because of attraction between the
penetrant and the ionic clusters.
Fig. 9 shows the methanol permeability of the bariumexchanged SPEKK membranes relative to that of Nafion® .
The methanol permeability of the original SPEKK membrane
(IEC = 2.06 meq/g) was only about 40% of that of Nafion® ,
and the permeability decreased with increasing degree of ionic
crosslinking. For a barium exchange of only 20%, the methanol
Fig. 9. Methanol permeability of the barium-exchanged SPEKK (2.06 meq/g)
membranes as a function of the exchange fraction.
Fig. 10. Influence of barium-exchange fraction on the proton conductivity of
the barium-exchanged SPEKK membranes normalized with respect to the conductivity of Nafion® .
permeability of the SPEKK membrane was about one-fifth that
of Nafion® . The proton conductivities of the barium-crosslinked
SPEKKs (initial IEC = 2.06 meq/g) relative to the conductivity
of Nafion® are shown in Fig. 10. The conductivity of the unneutralized SPEKK was about 40% higher than that of Nafion® . For
a barium exchange of 10%, the conductivity of the crosslinked
SPEKK was comparable to that of Nafion® ; however, as shown
in Fig. 9, the methanol permeability was about 80% lower than
that of Nafion® .
The ratio of the proton conductivity and the methanol
permeability, referred to as permselectivity or selectivity of
the membranes is plotted for the SPEKK membranes (initial
IEC = 2.06 meq/g) as a function of barium exchange in Fig. 11.
There seems to be a maximum in the selectivity at around
14% exchange, but there are not enough data points to make
a definitive conclusion about the statistical significance of this
maximum. The selectivity at 14% exchange was about five times
greater than that of Nafion® . Above 20% exchange, the selectivity dropped to about the same selectivity as for Nafion® .
Fig. 11. Influence of barium-exchange fraction on the permselectivity of the
barium-exchanged SPEKK membranes normalized with respect to the permselectivity of Nafion® .
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Fig. 12. Derivative TGA curves of the barium-exchanged SPEKK (1.7 meq/g)
membranes for various exchange fractions.
3.7. Thermal stability
TGA curves of the barium-exchanged SPEKK membranes
(initial IEC = 1.7 meq/g) as a function of the degree of barium
exchange are shown in Fig. 12. The ordinate in this plot is the
derivative of the sample mass with respect to temperature. A
mass loss step in the integral TGA curve (mass vs. temperature)
will be manifested as a peak in the derivative plot. The derivative
plot simply makes it easier to resolve small mass loss steps and to
pin-point the temperature associated with each step. In the TGA
derivative curve for the neat SPEKK membrane, the large peak
centered around 350 ◦ C is due to desulfonation, while the broad
peak around 500 ◦ C is due to degradation of the PEKK backbone.
For the barium-exchanged SPEKK membranes, the desulfonation peaks are systematically shifted to higher temperatures, and
result in lower mass loss. There is in addition a new peak centered
around 750 ◦ C, which is not present in the TGA curve of the neat
SPEKK membrane; this may be due to desulfonation of the neutralized sulfonic acid groups. One possible cause of the increase
in the first desulfonation temperature is a decrease in diffusion
due to the higher polymer viscosity; another is that an increase in
pH suppresses the desulfonation reaction. Gaining further information concerning the nature of the degradation reactions would
require analysis of the products, which is beyond the scope of this
work.
4. Conclusions
Crosslinking SPEKK membranes using barium cations was
achieved and the degree of crosslinking could be controlled by
using dilute aqueous solutions of barium acetate. Although the
crosslinking reduced the proton conductivity of SPEKK membranes, the methanol permeability and methanol sorption were
reduced significantly. There seemed to be an optimum degree
of crosslinking, about 14% barium exchange, where the ratio of
the proton conductivity to the methanol permeability, the selectivity, was highest. But, more samples with various degrees of
crosslinking need to be tested to confirm the statistical significance of this optimum crosslink density. Statistical analysis of
179
the data obtained in this study showed that there is a decreasing
relationship between selectivity and the degree of crosslinking. However, when a highly sulfonated PEKK was used (e.g.,
IEC = 2.06 meq/g), a barium exchange of 10%, still yielded a
membrane with a proton conductivity comparable to that of
Nafion® , but with a methanol permeability 80% lower than
that of Nafion® . Crosslinking with barium ions also greatly
improved the thermal stability of the SPEKK membranes under
dry conditions, and crosslinking mitigated the problem of excessive water sorption of SPEKK membranes at elevated tempe
ratures.
It appears from the results presented in this paper that the
preparation of viable and robust PEM membranes may be possible by using a highly sulfonated polymer, even one that is
close to water-soluble, that is partially neutralized to form
a partial ionomer salt. With a divalent cation (e.g., barium),
intermolecular crosslinks that form from the ion-bridge due
to the metal sulfonate, renders the polymer less swellable by
water and also may improve the membranes mechanical properties. However, investigation is needed into the question of
the durability of the crosslinks in a working fuel cell. While
preliminary experiments indicated that Ba++ can be displaced
by H+ in an acidic solutions, the situation in a fuel cell is
somewhat different in that all the anions are immobile; thus,
the only escape for Ba++ is to move to the cathode. Its presence there could impede the normal cathode reactions, but it
is not likely to be washed out. In the materials area, additional work is needed to determine whether other cations also
may work to improve membrane selectivity and mechanical
properties. In this case, dipole–dipole aggregation of the metal
sulfonate groups, similar to what occurs in metal salt ionomers of
much lower ion-exchange capacity, may provide the crosslinks.
Similarly, other divalent cations or higher valency ions such
as Al(III) or Ti(IV) may be of interest. The higher valency
cations may provide stronger crosslinking and better water resistance and mechanical properties using less exchange of the
protons.
Acknowledgment
This work was partially supported by a grant from the Dept.
of Energy, 10761-001-05.
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