Journal of Membrane Science 320 (2008) 215–223
Contents lists available at ScienceDirect
Journal of Membrane Science
journal homepage: www.elsevier.com/locate/memsci
Structured polymer electrolyte blends based on sulfonated
polyetherketoneketone (SPEKK) and a poly(ether imide) (PEI)
Jeffrey V. Gasa a , R.A. Weiss a,b , Montgomery T. Shaw a,b,∗
a
b
Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, United States
Department of Chemical Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, United States
a r t i c l e
i n f o
Article history:
Received 21 November 2007
Received in revised form 25 March 2008
Accepted 30 March 2008
Available online 26 April 2008
Keywords:
Proton exchange membrane
Polymer blend
Electric field alignment
a b s t r a c t
The performance and economics of proton-exchange membrane (PEM) fuel cells are highly dependent on
the membranes used to separate the fuel and oxidant. While maintaining reasonable cost, the membrane
must feature a number of desirable properties including high proton conductivity. Blends of polymers
are one approach to tailoring PEM properties; however, blending to achieve mechanically and chemically
robust membranes has generally resulted in reduced conductivity. The objective of this work was to
demonstrate the use of field alignment of the proton-conducting domains to increase the conductivity in
a polymer blend PEM. A blend of sulfonated poly(etherketoneketone) (SPEKK) and a polyether imide (PEI)
was used to illustrate this method. Blends of SPEKK/PEI with a 3:7 mass ratio were aligned using electric
field strengths varying from 0 to 30 V/mm and frequencies varying from 0 to 10 kHz. In general, the degree
of alignment agreed with theoretical predictions for the alignment of drops or particles suspended in a
fluid with a different dielectric constant, e.g., when the frequency of the applied ac field was increased,
the threshold field for phase alignment increased and the diameter of the oriented columnar structures
decreased. Alignment resulted in up to three orders of magnitude increase in conductivity at low humidity.
By careful selection of temperature and residual solvent content, alignment was shown to be possible in
the melt state, which is essential for an economic process for producing alignment-enhanced membranes.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Perfluorosulfonic acid membranes, such as Nafion® , are
presently considered the benchmark electrolyte membrane for fuel
cells, because of their high proton conductivities and excellent
electrochemical and mechanical stability in the fuel cell environment [1]. However, these membranes possess several properties
that are unfavorable for DMFC application, which most researchers
consider to be the main hindrance to the technological and commercial success of this type of fuel cell. The 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. These protonexchange membranes (PEMs), which are mostly polymers that
contain sulfonic acid groups, tend to have high swelling in methanol
and high methanol permeability.
∗ Corresponding author at: Polymer Program, Institute of Materials Science, and
Chemical, Materials and Biomolecular Engineering Department, University of Connecticut, Storrs, CT 06269-3136, United States.
E-mail address: [email protected] (M.T. Shaw).
0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.memsci.2008.03.075
A possible route for reducing the swelling of PEMs is to blend the
acidic polymer with neutral polymers or slightly acidic polymers
that are more resistant to swelling in methanol. Obviously, incorporation of a non-conductive material in a conductor may cause a
reduction in the proton conductivity of the membrane. It has been
reported recently that the proton conductivities of the two-phase
SPEKK–PEI blends are relatively low at low SPEKK volume fractions
(<0.4) because the protons are confined to discrete SPEKK droplets
[2]. The insulating matrix (PEI) surrounding these droplets impedes
the transport of protons from one droplet to another. However, the
hydrophobic (PEI) polymer matrix provides dimensional stability,
resistance to methanol permeation, and reduced swelling of the
blend membranes by water and methanol. To create a continuous
path for proton transport and thereby enhance the proton conductivity of these blends, it is desirable to align the conductive SPEKK
droplets along the direction of current flow.
It has been demonstrated by several researchers that the morphology of multi-phase systems can be significantly influenced by
the application of an electric field because of interfacial polarization
that distorts the applied field [3–7]. For phase-separated systems
exposed to external electric fields, interfacial polarization occurs if
the dielectric constant or the conductivity of the dispersed phase
differs from that of the matrix. The “positive” ends of the dispersed
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particles attract the “negative” ends of the neighboring dispersed
particles. If there is sufficient mobility within the matrix phase, the
dispersed particles are drawn close to each other, forming strings
of particles (also called pearl-chain formation) aligned along the
direction of the applied field. This phenomenon has been exploited
particularly in the area of electrorheological fluids [8].
About 20 years ago, Moriya and co-workers demonstrated pearlchain formation of poly(ethylene oxide) (PEO) in a polystyrene (PS)
matrix in the melt state [4]. More recently, Venugopal et al. did
similar, but more extensive work on various blends systems (e.g.,
PEO–PS, poly(methyl methacrylate)–PS and poly(vinyl acetate)–PS)
[5]. In cases where the interfacial energy is low, strings of droplets
can coalesce and form columnar structures that are aligned along
the direction of the field. In blends where the dispersed phase is
conductive, such a structure can have much higher conductivity
than the isotropic counterpart, which would be highly desirable in
the design of a PEM. Oren et al. [9] and later Brijmohan et al. [10]
demonstrated this effect for mixtures of sulfonated cross-linked
PS particles (SXLPS) dispersed in a poly(dimethylsiloxane) (PDMS)
matrix. Similarly, SXLPS has been dispersed and aligned in a thermoplastic SPEKK matrix to achieve very high proton conductivities
[11]. The goal of the research described herein was to apply the
same principle to polymer electrolyte blend membranes based on
SPEKK. The scope of the study was limited to a demonstration of
feasibility of alignment using a variety of blend compositions (IEC
range of the SPEKK, ratio of blend components) and the conductivity benefits that result from alignment. A detailed parametric study
of material and process variables was beyond the scope of the study.
tions were cast onto clean glass plates at 70 ◦ C and air dried in a
hood until most of the solvent had evaporated. Final drying was
done in vacuum at 110–120 ◦ C for 48 h. The dried membranes were
then soaked in de-ionized water at 20 ◦ C for 48 h to remove residual
NMP. For in-plane alignment, the membranes were about 100 ␮m
thick × 3 mm × 30 mm, with the latter two dimensions being determined by the size of the gap between the electrodes of cell A (Fig. 1).
For through-plane alignment in the melt state, the membrane sample was a circular disc with a thickness of ∼0.5 mm and a diameter
of 25 mm.
2.3. Preparation of anisotropic membranes by solvent casting
under an electric field
The polymer solutions were cast under ac electric fields using
the casting cell A illustrated in Fig. 1. The design of cell A was
adopted from an earlier study conducted by Krause and coworkers [5–7]. In this cell, the field is applied along the plane of the
membrane (in-plane). The geometry of cell A makes morphological studies more convenient because the sample can be viewed
perpendicular to the field and directly under a microscope. During
casting, these cells were connected to a high-voltage ac power supply (a TENMA Jupitor 2010 power supply and a Trek Model 609D-6
high voltage amplifier). The morphology of the blend membranes
was very adequately characterized with a Nikon Labophot optical
microscope using transmission mode. Good contrast was achieved
without phase-contrast optics or stains.
2.4. Preparation of anisotropic membranes by melt-state
electric-field alignment
2. Experimental
2.1. Materials
The PEKK used was OXPEKK-SP (Oxford Performance Materials, New Britain, CT) with a terephthaloyl (T) to isophthaloyl (I)
ratio of 6:4. Sulfonation of PEKK followed procedures similar to
those described previously [12]. The PEI component was a commercial polyetherimide (General Electric Ultem® 1000). The chemical
structures of the two polymers are shown in Scheme 1.
2.2. Solution blending and preparation of isotropic membranes
Various ratios of SPEKK and PEI were dissolved in N-methyl
pyrrolidone (NMP) to form 7.5 wt% polymer solutions. These solu-
Electric-field alignment of the SPEKK-rich domains in the thickness direction of a membrane during solvent casting is, in practice,
more difficult than aligning them in the longitudinal direction,
because it is difficult to apply a uniform field to the sample without
impeding the evaporation of solvent. We approached this problem
by using a very fine supported mesh as an upper electrode to allow
solvent evaporation. However, the high fields needed to align the
SPEKK dispersed phase could not be maintained because the electrical resistance of the sample was very low due to the short-path
length and large area for current flow, combined with the inherently
high conductivity of ionomers.
To remedy this, melt alignment was used. To accomplish this,
solutions containing 10 wt% of a 3/7 mass ratio of SPEKK to PEI
Scheme 1. Chemical structure of polymers used.
J.V. Gasa et al. / Journal of Membrane Science 320 (2008) 215–223
217
Fig. 1. Apparatus for casting membranes under applied electric field.
were cast at 70 ◦ C on an aluminum plate. The resulting membranes were dried at 110 ◦ C in vacuum to remove most, but not
all of the solvent. In this condition, the Tg of the solvent-plasticized
PEI matrix was about 150 ◦ C. The membranes were subsequently
heated to 200 ◦ C inside cell B shown in Fig. 1. The temperature
was held at 200 ◦ C for 100 min and then reduced to room temperature. The field was maintained during the entire thermal history.
The morphology of these membranes was observed with scanning electron microscopy, which is superior to light microscopy
for imaging the cross-sections formed by cryogenic fracture of the
membrane.
2.5. Conductivity measurements
The conductivity of the membranes was measured in the thickness direction and along the plane of the membranes. The in-plane
proton conductivity was measured using the impedance technique,
as described in previous publications [11,13,14]. Nyquist plots were
used to calculate the conductivity, using the higher Z -intercept of
the semi-circle at lower frequency. The conductivity was determined at 98% relative humidity and room temperature (23 ◦ C),
using a Hewlett–Packard Agilent® 4284A LCR meter covering a
frequency range of 1 to 106 Hz. The applied voltage was 50 mV.
The humidity was controlled using a 0.1% LiCl solution in a glass
desiccator. An equilibrium RH of 98% was achieved in 4 h and the
membranes were equilibrated for 8 h before the measuring the conductivity. The conductivities were re-measured after another 8 h
in the humidity chamber to check if the membranes had reached
their equilibrium hydration. The equilibrium hydration of all of the
specimens was ∼9 to 10 water molecules per sulfonate group [11].
The through-plane conductivity of the membranes was measured on dry films using the casting apparatus (cell B in Fig. 1) as
the conductivity cell. The principal reason for this procedure was
that the membrane adhered strongly to the discs and could not be
removed unharmed. The conductivity values were recorded immediately after cooling the sample from the melt alignment process
to ensure that moisture absorption from ambient environment was
minimal. This assumption was validated by performing two consecutive frequency sweeps. Invariably the differences were found to be
relatively insignificant (<5%), which is reasonable because only the
edge of the sample was exposed to environment.
3. Results and discussion
3.1. Estimation of the threshold field for alignment
By equating the dielectrophoretic (ordering) force to the Brownian randomizing force, Schwan and Sher derived an equation for the
threshold field magnitude (Ep ) required for the alignment process
to occur [15,16]. The expression for Ep is
εd + 2εm ε −ε
Ep = m
d
kT
2ε0 εm r 3
1/2
(1)
where εm and εd are the dielectric constants, or relative permittivities, of the matrix and the dispersed phase, respectively, r is
the droplet radius, ε0 is the dielectric permittivity of free space, k
is the Boltzmann’s constant, and T is the temperature. Note that
Fig. 2. Frequency dependence of the room temperature (T ∼ 23 ◦ C) relative permittivities of SPEKK (IEC = 2.01 mequiv./g) and PEI measured by dielectric spectroscopy.
Both samples were in the dry state.
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3.2. Morphology of the structured membranes
3.2.1. In-plane alignment
An optical micrograph of a SPEKK–PEI blend that was solvent
cast in the presence of an electric field (50 V/mm, 20 Hz) is shown
in Fig. 3b. For comparison, the unaligned counterpart is also shown
in Fig. 3a. The mass fraction and IEC of SPEKK in this blend were 0.2
and 1.26 mequiv./g, respectively. The blend that was solvent cast in
the field formed columnar structures that were oriented along the
direction of the applied electric field. These structures appeared to
be similar to those found by Venugopal et al. [5] for different blend
systems that did not contain an ionomer component. For the result
shown in Fig. 3b, the applied field was 50 V/mm, which is lower
than the estimated Ep values discussed earlier. The possible reason
for this is that during the alignment process there is trace water
Fig. 3. Optical micrograph of a 20% SPEKK–PEI blend solvent cast from NMP at 110 ◦ C
without electric field (a) and with an electric field (b). The field amplitude was
50 V/mm with a frequency of 20 Hz.
a significant dielectric contrast |εm − εd | is required for the alignment process to occur; otherwise, the required field will exceed the
dielectric breakdown strength of the polymer (Eq. (1)).
The frequency dependences of the relative permittivities of
the SPEKK and PEI in the dry state, as measured by dielectric spectroscopy, are shown in Fig. 2. The dielectric constant of
SPEKK with an IEC of 2.0 mequiv./g exhibited strong frequency
dependence, whereas the PEI dielectric constant was almost frequency independent. Therefore, the dielectric contrast between
the two components was a strong function of frequency, and
so was Ep . The dielectric contrast decreased with increasing frequency. Because Ep is inversely proportional to the dielectric
contrast, it was expected that Ep would increase with increasing
frequency. This was confirmed experimentally and the results are
discussed in a latter section. The strong frequency dependence
of the SPEKK dielectric constant is most likely due to the presence of highly polar sulfonic acid groups, which are not present
in PEI.
Eq. (1) was used to estimate Ep for the SPEKK–PEI blend system,
using the values for the dielectric contrast taken from Fig. 2 and the
actual size of the droplets measured from the optical micrographs,
e.g., Fig. 3a. For a frequency of 20 Hz, the dielectric contrast εd − εm
was about 3.9. By inspection of the optical micrographs of these
blends, the droplet radii ranged between 1 and 4 ␮m. From these
values, the calculated Ep ranged from 0.1 to 1.0 kV/mm (note that
Ep is a strong function of r, i.e., Ep ∼ r−3/2 ).
Fig. 4. SEM micrograph of a cryo-fractured section of an SPEKK–PEI blend melt cast
at 200 ◦ C: (a) without electric field and (b) under ac electric field applied along the
thickness of the membrane. The field amplitude was 1 kV/mm with a frequency of
20 Hz. The mass fraction of SPEKK (IEC = 1.92 mequiv./g) in the membrane was 0.3.
J.V. Gasa et al. / Journal of Membrane Science 320 (2008) 215–223
219
present in the blend that may increase the dielectric contrast, as
it would tend to collect in the ionomer-rich phase. Alternatively,
the NMP solvent itself, with a dielectric constant of ε = 32.6, could
have increased the dielectric constant of the SPEKK-rich phase more
than that of the PEI-rich phase. We may speculate that the NMP
concentration in the SPEKK-rich phase was higher, and the solventimparted mobility increased the ability of the sulfonic acid groups
to align at 20 Hz, thus leading to enhanced polarization.
3.2.2. Through-plane alignment
Fig. 4(b) is an SEM image of a cryo-fractured section of an
SPEKK–PEI blend that was aligned in the thickness direction in the
melt state. The mass ratio of SPEKK to PEI in this blend was 3/7.
A significant amount of residual NMP (about 10–15 wt% according
to TGA data) from the solvent casting was present in the blend.
That amount of solvent was retained purposefully to plasticize
the blend and aid in the melt-casting process. It would be difficult to melt process the blend without using plasticizers because
the Tg ’s of both SPEKK and PEI are above 200 ◦ C and desulfonation
begins at around 250 ◦ C. Fig. 4(b) shows that alignment of the dispersed phase, though apparent, was not very distinct, which was
probably because the alignment process was very slow and the
exposure time to the applied field was too short (100 min). Because
it would be impractical to extend the exposure time or increase the
amount of solvent in the blend, a matrix with a lower Tg (styreneacrylonitrile copolymer (SAN)) was used to facilitate the alignment
process and better demonstrate the concept of alignment in the
conductivity direction of a membrane. Fig. 5(b) is an SEM image of
a cryo-fractured section of an SAN–SPEKK blend that was aligned
in the melt. The blend composition and casting conditions were the
same as those used for the sample in Fig. 4, except that SAN was
used instead of PEI for the continuous phase. Note that the aligned
domains (columnar structures) are more distinct in this blend than
those in PEI–SPEKK blends. As the SAN–SPEKK blend is not suitable
for PEM application, no further characterization of this material was
attempted.
3.2.3. Influence of blend composition
Blends of SPEKK and PEI with different compositions were solvent cast under an electric field and the micrographs of the resulting
membranes are shown in Fig. 6. Similar structures were obtained
for mass fractions of SPEKK less than 0.5 (Fig. 6a–c). When the
SPEKK-rich phase was continuous (Fig. 6d), there no observable
alignment of the dispersed PEI-rich phase. In theory, when the
dielectric constant of the dispersed phase is less than that of the
matrix, the dispersed phase should still align along the direction
of the applied electric field [5,7]. But such was not observed in this
system, probably because the PEI-rich droplets were much smaller
than the SPEKK-rich droplets and, therefore, required much higher
field magnitudes for alignment. Higher fields could not be applied
without causing dielectric breakdown. Note again that Ep is a strong
function of r (Eq. (1)).
3.3. Proton conductivity
3.3.1. In-plane alignment
The proton conductivities of the aligned SPEKK–PEI blends
are plotted in Fig. 7, which also includes the conductivities of
the unaligned counterparts for comparison. The conductivity was
enhanced by between one and two orders of magnitude upon alignment. There is a suggestion of a mild maximum in the enhancement
factor ( aligned / unaligned ) around a SPEKK mass fraction of 0.3 (a
student t-test analysis yields a p value of 0.031, which indicates that
the hypothesis of no maximum can be safely discarded with only a
3.1% chance of being in error). This is consistent with the observa-
Fig. 5. SEM micrograph of a cryo-fractured section of an SPEKK–SAN blend melt cast
at 200 ◦ C: (a) without electric field and (b) under ac electric field applied along the
thickness of the membrane. The field amplitude was 1 kV/mm with a frequency of
20 Hz. The mass fraction of SPEKK (IEC = 1.92 mequiv./g) in the membrane was 0.3.
tion that at SPEKK weight fractions <0.3, some of the columns were
short and did not form a continuous path for proton transport. Thus,
the enhancement factor was relatively low. On the other hand, for
SPEKK mass fractions >0.3, the enhancement effect was also relatively low because at high concentrations of the conductive phase,
some of the droplets in the unaligned membranes formed continuous pathways for protons through natural (not field-induced)
percolation.
3.3.2. Through-plane alignment
The complex resistivity of the structured PEI–SPEKK blend
as a function of frequency is shown in Fig. 8. These data were
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Fig. 6. Optical micrographs of SPEKK–PEI blends solvent cast at 110 ◦ C under ac electric field applied along the plane of the membrane. The field amplitude was 0.5 kV/cm
with a frequency of 20 Hz. The mass fractions of SPEKK (IEC = 1.26 mequiv./g) in the micrographs shown above are the following: (a) 0.1, (b) 0.2, (c) 0.3 and (d) 0.9.
gathered using dry membranes (see Section 2), so the resistivity is correspondingly high. The much higher conductivity
(lower resistivity) observed for the structured membrane is in
accord with the structure shown in Fig. 4. The resistivity was
about three orders of magnitude lower for the structured membrane.
3.4. Influence of field magnitude and frequency
Fig. 7. Proton conductivity of aligned vs. unaligned blends of SPEKK and PEI. The
IEC of SPEKK component was 2.2 mequiv./g. The electrical field conditions for the
in-plane alignment were 0.5 kV/cm at 20 Hz. The conductivity of the neat SPEKK
(IEC = 2.2 mequiv./g) was about 0.01 S/cm [17].
Fig. 8. Complex resistivity magnitude of aligned and the unaligned SPEKK–PEI
membranes shown in Fig. 4. The electrical field conditions for through-plane alignment were 0.5 kV/cm at 20 Hz. For these measurements, the membranes were dry
(as cast, with and without field).
Fig. 9 shows the morphology of SPEKK–PEI blends cast at various field magnitudes and at a constant frequency of 10 kHz.
J.V. Gasa et al. / Journal of Membrane Science 320 (2008) 215–223
221
Fig. 9. Optical micrographs of SPEKK–PEI blends solvent cast at 85 ◦ C under ac field with a frequency 10 kHz and at various field amplitudes, in V/mm: (A) 5, (B) 10, (C) 15,
(D) 20, (E) 25 and (F) 30. The arrow indicates the direction of the applied field. The weight fraction of SPEKK in the membrane was 0.2 with an IEC of 2.67 mequiv./g. The scale
bar in (A) applies to all micrographs.
The results clearly show that there was a threshold field magnitude (Ep ) of ∼25 V/mm required to produce structures that
were preferentially aligned along the direction of the applied
field. Fig. 10 suggests that Ep increases with increasing frequency.
The Ep values for this blend system are much lower than those
used by Venugopal et al. (1 kV/mm) to align blends of non-
sulfonated polymers [5]. This is probably due to the increase in
either the dielectric constant or the conductivity of PEKK upon
sulfonation, which in effect increases the dielectric or conductivity contrast between the two phases. According to Eq. (1),
Ep decreases with increasing dielectric or conductivity contrast
[16].
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J.V. Gasa et al. / Journal of Membrane Science 320 (2008) 215–223
Fig. 10. Influence of frequency on the threshold field magnitude required to produce
structured (anisotropic) membranes. The SPEKK ionomer used for these data had an
IEC of 2.67 mequiv./g and the mass fraction of SPEKK in the blend was 0.2.
At field magnitudes below Ep , the size of the dispersed phase
increased with increasing field magnitude (Fig. 10), possibly
because the mutual attractive force between the droplets due to
dielectrophoresis was higher and, therefore, the rate of droplet
coalescence was higher. Above Ep , the diameter of the columnar
structures appeared to decrease when the frequency was increased,
as shown in Fig. 11. We can fairly safely hypothesize that the diam-
eter of the columns decreased with increasing frequency, because
at higher frequencies the polarization amplitude of the droplets
due to the applied ac field is lower, and therefore the probability of
droplet coalescence is less.
Application of a dc field led to formation of structures resembling electrical trees [18], as illustrated in Fig. 11A. The use of ac
fields for alignment minimizes ionic polarization and, therefore,
reduces the chances of electrolytic degradation leading to failure.
While important, designing a practical continuous process for
making aligned blend films was beyond the scope of this work,
which was limited to a demonstration of feasibility. However, one
can imagine that the casting and drying steps would use conventional equipment, while the alignment step would require multiple
passes through sets of electrically charged rolls held at the alignment temperature. Membrane thickness would be limited by the
phase size, while the width would depend mostly upon the casting
step. Supporting the membrane on a metallic web might be needed
to prevent breakage during the processing.
4. Conclusions
It has been demonstrated in this study that structured membranes for proton transport can be achieved by the application of
electric fields during casting of these blend membranes. The size of
the phase domains formed under electric field was found to depend
Fig. 11. Optical micrographs of SPEKK–PEI blends solvent cast at 85 ◦ C under 50 V/mm ac electric field at various frequencies: (A) 0 Hz, (B) 0.1 Hz, (C) 10 Hz and (D) 10 kHz.
The IEC of SPEKK was 2.67 mequiv./g. The scale bar in (A) applies to all micrographs.
J.V. Gasa et al. / Journal of Membrane Science 320 (2008) 215–223
on the magnitude and frequency of the applied field. Therefore, the
size of the columnar structures, which can be conceived as channels for proton transport in a fuel cell, can be tailored by proper
selection of the field magnitude and frequency, in addition to the
nature of the two polymers comprising the blend. The structuring
of SPEKK–PEI and SPEKK–SAN blends in the melt state has also been
demonstrated, along with a significant increase in the conductivity
due to the structuring. With many other variables available (component structure and molecular weight, temperature, composition,
etc.) it is reasonable to imagine that membranes with a wide range
of properties can be made using practical processing techniques.
Acknowledgments
This work was partially supported by grant from the Department
of Energy, 10761-001-05, and Civil, Mechanical and Manufacturing Innovation Program of the National Science Foundation, CMMI
0727545.
References
[1] G. Hoogers, Fuel Cell Technology Handbook, CRC Press, Boca Raton, FL, 2003.
[2] J.V. Gasa, R.A. Weiss, M.T. Shaw, Influence of blend miscibility on the proton
conductivity and methanol permeability of polymer electrolyte blends, J. Polym.
Sci., Polym. Phys. Ed. 44 (2006) 2253–2266.
[3] C.G. Garton, Z. Krasucki, Bubbles in insulating liquids—stability in electric field,
Proc. R. Soc. London A 280 (1964) 211–226.
[4] S. Moriya, K. Adachi, T. Kotaka, Effect of electric field on the morphology of pole(ethylene oxide) polystyrene blend, Polym. Commun. 26 (1985)
235–237.
223
[5] G. Venugopal, S. Krause, G.E. Wnek, Morphological variations in polymer blends
made in electric fields, Chem. Mater. 4 (1992) 1334–1343.
[6] K. Xi, S. Krause, Droplet deformation and structure formation in two-phase polymer/polymer/toluene mixture in an electric field, Macromolecules 31 (1998)
3974–3984.
[7] G. Venugopal, S. Krause, Development of phase morphologies of poly(methyl
methacrylate) polystyrene toluene mixtures in an electric field, Macromolecules 25 (1992) 4626–4634.
[8] T.C. Jordan, M.T. Shaw, Electrorheology, MRS Bull. 76 (1991) 38–43.
[9] Y. Oren, V. Freger, C. Linder, Highly conductive ordered heterogeneous ion
exchange membranes, J. Membr. Sci. 239 (2004) 17–26.
[10] S.B. Brijmohan, S. Swier, R.A. Weiss, M.T. Shaw, Proton exchange membranes
based on sulfonated crosslinked polystyrene micro particles dispersed in
poly(dimethyl siloxane), Polymer 47 (2006) 2856–2864.
[11] J.V. Gasa, S. Boob, R.A. Weiss, M.T. Shaw, Proton exchange membranes
composed of slightly sulfonated poly(ether ketone ketone) and highly sulfonated crosslinked polystyrene particles, J. Membr. Sci. 269 (2006) 177–
186.
[12] S. Swier, J. Gasa, M.T. Shaw, R.A. Weiss, Sulfonation reaction kinetics of
poly(ether ketone ketone) using a mixture of concentrated and fuming sulfuric
acid, Ind. Eng. Chem. Res. 43 (2004) 6948–6954.
[13] T.C. Chilcott, M. Chan, L. Gaedt, T. Nantawisarakul, A.G. Fane, H.G.L. Coster, Electrical impedance spectroscopy characterization of conducting membranes. I.
Theory, J. Membr. Sci. 195 (2002) 153–167.
[14] T.A. Zawodzinski, M. Neeman Jr., L.O. Sillerud, S. Gottesfeld, Determination
of water diffusion coefficients in perfluorosulfonate ionomeric membranes, J.
Phys. Chem. 95 (1991) 6040.
[15] H.P. Schwan, L.D. Sher, Alternating-current field-induced forces and their biological implications, J. Electrochem. Soc. 116 (1969) 170–174.
[16] H.A. Pohl, Dielectrophoresis, Cambridge University Press, Cambridge, UK,
1978.
[17] S. Swier, Y.S. Chun, J. Gasa, M.T. Shaw, R.A. Weiss, Sulfonated poly(ether ketone
ketone) ionomers as proton exchange membranes, Polym. Eng. Sci. 45 (2005)
1081–1091.
[18] C.C. Ku, R. Liepins, Electrical Properties of Polymers, Chemical Principles,
Hanser, Munich, 1987, pp. 106–128.