Influence of Blend Miscibility on the Proton Conductivity
and Methanol Permeability of Polymer Electrolyte Blends
JEFFREY V. GASA,1 R. A. WEISS,1,2 MONTGOMERY T. SHAW1,2
1
Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136
2
Department of Chemical Engineering, Institute of Materials Science, University of Connecticut, Storrs,
Connecticut 06269-3136
Received 30 November 2005; revised 4 April 2006; accepted 14 April 2006
DOI: 10.1002/polb.20865
Published online in Wiley InterScience (www.interscience.wiley.com).
The influence of miscibility on the transport properties of polymer electrolyte
blends composed of a proton conductor and an insulator was investigated. The protonconductive component in the blends was sulfonated poly(ether ketone ketone) (SPEKK),
while the nonconductive component was either poly(ether imide) (PEI) or poly(ether sulfone) (PES). The phase behavior of PEI-SPEKK blends was strongly influenced by the
sulfonation level of the SPEKK. At low sulfonation levels (ion-exchange capacity (IEC)
¼ 0.8 meq/g), the blends were miscible, while at a slightly higher level (IEC ¼ 1.1 meq/g),
they were only partially miscible and for IEC 1.4 meq/g they were effectively immiscible over the entire composition range. The PES-SPEKK blends were miscible over the
entire range of SPEKK IEC considered in this study (0.8–2.2 meq/g). At high IEC
(2.2 meq/g) and at low mass fractions of SPEKK (<0.5), the miscible blends (PES-SPEKK)
had higher proton conductivities and methanol permeabilities than the immiscible ones
(PEI-SPEKK). The opposite relationship was observed for high mass fractions of SPEKK
(>0.5). This behavior was explained by the differences in morphology between these two
blend systems. At low IEC of SPEKK (0.8 meq/g), where both PEI-SPEKK and PESSPEKK blend systems exhibited miscibility, the transport properties were not significantly
C 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2253–2266, 2006
different. V
Keywords: miscible blend; PEI; PEKK; PES; proton-exchange membrane
ABSTRACT:
INTRODUCTION
Recent advances in direct methanol fuel cell
technology are quite dramatic and have placed
this technology on the brink of commercialization. While there are several bottlenecks in the
current art, many regard the membrane, which
must separate the methanol fuel and oxidant yet
transport hydrogen ions, as the key polymer
challenge because of the need for greater chemi-
Correspondence to: M. T. Shaw (E-mail: montgomery.shaw@
uconn.edu)
Journal of Polymer Science: Part B: Polymer Physics, Vol. 44, 2253–2266 (2006)
C 2006 Wiley Periodicals, Inc.
V
cal resistance for operation with methanol as the
fuel.1 These proton-exchange membranes (PEMs),
which are mostly polymers that contain sulfonic
acid groups, tend to have high methanol permeability, which lead to high crossover of methanol
fuel from the anode to the cathode of the fuel cell.
Methanol crossover is undesirable because it
reduces fuel utilization efficiency and adversely
affects the performance of the cell because of a
mixed-potential effect.1 Methanol crossover must
be low, especially for portable applications wherein
the current densities are relatively low. At low
current densities, the rate of methanol oxidation
is low and therefore unreacted methanol concentrates at the anode, promoting diffusion of methanol through the membrane.
2253
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GASA, WEISS, AND SHAW
Perfluorosulfonic acid (PFSA) membranes, such
as Nafion1, have been historically the standard material for PEM in DMFC. However, the limitations
of PFSA membranes in DMFC applications, such as
high methanol permeability and poor mechanical
stability under swollen conditions, and the high cost
of its manufacture have motivated researchers to
develop PEMs based on aromatic hydrocarbons.2,3
These types of polymers are known to have excellent mechanical and barrier properties, and therefore are usually called engineering thermoplastics.
Several studies have shown that PEMs based on
sulfonated aromatic hydrocarbons exhibit proton
conductivities comparable to Nafion1.2,3 However,
high swelling in methanol remains a problem for
both types of PEMs.
Blending of a hydrophobic (nonsulfonated) polymer with an acidic polymer has become a widely
used approach for the design of PEMs with improved resistance to methanol. A variety of polymer pairs have been considered, in particular,
blends of Nafion1 with fluoropolymers4–7 and hydrocarbons.8,9 More relevant to this work are binary blends of sulfonated poly(ether ether ketone)
(SPEEK) with poly(ether imide) (PEI),10–16 poly
(aryl ether sulfone) (PES),17–19 and poly(2,6-dimethyl phenylene oxide).10
Obviously, incorporation of a nonconductive
material in a conductor may cause a reduction in
the proton conductivity of the membrane. However, Mikhailenko et al. reported that it is possible
to improve the conductivity of SPEEK by blending
it with relatively small amounts of PEI, a nonconductive engineering thermoplastic.12 They found
that blending with PEI first results in an increase
and then a decrease in proton conductivity at PEI
concentrations greater than 5%. PEEK and PEI
are known to be miscible,20,21 but sulfonated
PEEK and PEI are immiscible, according to SEM
studies done by Mikhailenko et al.12 However, sev-
eral others have reported that sulfonated PEEK
and PEI are miscible, based on thermal analysis.14–16,22
The objective of our study was to investigate
the influence of blend miscibility on the transport
properties of polymer electrolyte membranes composed of an acidic polymer and a nonsulfonated
polymer. The acid polymer used in this study was
sulfonated poly(ether ketone ketone) (SPEKK).
SPEKK is an ionomer similar to SPEEK, but with
higher transition temperatures and thermo-oxidative stability.11,16,23 The preparation of SPEKK
and its potential use as a PEM in fuel cells were
described previously.23–26 Poly(ether ketone ketone) (PEKK) itself is a relatively new engineering thermoplastic that has high temperature stability, excellent chemical and solvent resistance,
and excellent mechanical properties. PEKK is
actually a family of copolymers with different
ratios of terephthaloyl (T) and isophthaloyl (I)
moieties (Fig. 1). In this study, the polymer of
principal focus was the acid form of SPEKK with
a T/I ratio of 6/4. PEKK with this T/I ratio is miscible with PEI.20 However, SPEKK with relatively
high levels of sulfonation is immiscible with
PEI.25 Therefore, we expect different levels of
miscibility when different sulfonation levels of
SPEKK are blended with PEI. Another blend system that we have investigated in this work comprised SPEKK and PES, which is also an engineering thermoplastic. Swier et al. reported that
SPEKK/PES blends are miscible.26
The purpose of this work was to compare the
transport properties (proton conductivity and
methanol permeability) of immiscible blends
(SPEKK/PEI) with that of miscible ones (SPEKK/
PES). Knowledge of the conductivity–miscibility
relationships should be useful for designing ionomer blend systems for direct methanol fuel cell
applications.
Figure 1. Chemical structure of PEKK.
Journal of Polymer Science: Part B: Polymer Physics
DOI 10.1002/polb
INFLUENCE OF BLEND MISCIBILITY AND CONDUCTIVITY
EXPERIMENTAL
Similarly,
Sulfonation of PEKK
Weq ¼
The PEKK used was OXPEKK-SP (Oxford Performance Materials, New Britain, CT), which has
a terephthaloyl (T) to isophthaloyl (I) ratio of 6:4
(Fig. 1). The effect of increasing the isophthaloyl
content is to reduce crystallinity and lower the Tg.
Sulfonation was achieved by dissolving the polymer (5% w/v) in a mixture of 53/47 (v/v) concentrated sulfuric acid (96.3%) and fuming sulfuric
acid (30% free SO3).23,24 The sulfonation time and
temperature were varied to achieve different levels of sulfonation.24 The resulting sulfonated polymer (SPEKK) was precipitated by drop-wise addition of the solution (total volume of solution was
about 200 mL) into 1.5 L of rapidly stirred de-ionized water. The SPEKK was filtered and washed
repeatedly with de-ionized water to remove excess
acid. After washing, it was soaked in copious
amounts (5 g of polymer to 2 L of water) of de-ionized water for 24 h to remove residual acid. The
washing process was followed by measuring the
pH of the soaking solution; washing continued
until the reading was neutral. The washed SPEKK
was air dried in a fume hood overnight at room
temperature and then under vacuum at 120 8C for
3 days.
The sulfonation level was determined from the
carbon/sulfur ratio obtained from elemental analysis and/or by titration of the sulfonic acid groups.
Previous results showed good agreement (discrepancies were no greater than 0.04 meq/g) between
the two methods.24 For the titration method,
SPEKK was ion-exchanged with excess saturated
aqueous sodium chloride solution overnight. The
HCl product was then titrated with a normalized
sodium hydroxide solution using phenolphthalein
as an acid–base indicator. The sulfonation level
may be expressed as: (i) a degree of sulfonation
(Xs) defined as the average number of sulfonate
groups per repeat unit; (ii) equivalent weight
(Weq), which is the mass of polymer per sulfonate
group; or (iii) as a concentration of sulfonate
groups, expressed as ion-exchange capacity (IEC,
equivalents per mass). For SPEKK, the relationship between degree of sulfonation (Xs) and IEC
in terms of milliequivalents of SO3H per gram of
SPEKK (meq/g) is:
300 ðIECÞ
Xs ¼
1000 81 ðIECÞ
2255
ð1Þ
Journal of Polymer Science: Part B: Polymer Physics
DOI 10.1002/polb
1000 300 81Xs
¼
IEC
Xs
ð2Þ
The numbers 300, 81, and 1000 refer to the molecular weight of the PEKK repeat unit, molecular
weight of sulfonic acid, and conversion factor,
respectively. The units of Weq are grams of polymer per equivalent of sulfonate groups. The maximum IEC of SPEKK is 4.35 meq/g (i.e., two SO3H
groups per repeat unit). Either Weq or IEC are
most useful for comparing blends that have complex structures.
Blend Preparation
Blends of SPEKK with either PEI or poly(ether
suflone) (PES) were prepared by codissolving the
two polymers (at various mass ratios) in N-methyl
pyrrolidone (NMP) to form 5 wt % polymer solutions. These solutions were cast onto clean glass
plates at 60 8C and air dried in a fume hood until
most of the solvent had evaporated. After drying,
the membranes were soaked in de-ionized water
at 20 8C for 48 h to remove residual solvent.
Blends of unmodified PEKK with PEI and PES
proved difficult to make because of the crystallinity of PEKK reduced its solubility in reasonable
solvents at low temperatures.
Impedance Spectroscopy
Impedance spectroscopy has been widely used to
measure the proton conductivity of PEMs.11,27–35
The conductivity of the membranes was measured
using a Hewlett-Packard Agilent1 4284A LCR
meter covering a frequency range of 20–106 Hz.
The applied voltage was 50 mV. Membrane conductivities were measured using a cell described
by Zawodzinski et al.27 This cell measures the
conductivity along the plane of the membrane,
which generally is easier than through-the-plane
measurements because the membrane resistance
can be made arbitrarily high by spacing the electrodes far apart. If the membrane is isotropic, as
ours appeared to be, the results should be independent of the current direction. Measurements
were made at 98% relative humidity and room
temperature (23 8C).
Water Sorption
The water sorption of the membranes was measured by placing films in a controlled humidity
2256
GASA, WEISS, AND SHAW
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 8C 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.
Microscopy
The morphology of the composite membranes was
studied using a Nikon Labophot optical microscope in transmission mode and an environmental
scanning electron microscope (ESEM; Philips
ESEM 2020). Refractive index contrast was very
high, eliminating the need for staining or interference optics. For ESEM, the membranes were
cryo-fractured in liquid nitrogen and images were
made of the section thickness.
Methanol Permeability
The methanol permeabilities of the membranes
were measured using the cell shown in Figure 2.
The design of this cell is a variation of the device
used by Walker et al. to measure methanol permeation through membranes based on sulfonated
aromatic hydrocarbons and also PFSA membranes, such as Nafion1 117.36
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
Figure 2. Sectional view of the permeation-measuring
cell. [Color figure can be viewed in the online issue,
which is available at www.interscience.wiley.com.]
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 escape. The
cell was placed inside a drying oven, which provided temperature control (30 8C) and an air draft
that maintained the methanol concentration
above the surface of the membrane at a minimum. The humidity was not controlled, but was
measured and found to be relatively constant at
(14 6 2)%. The mass of the methanol inside the
cell was measured as a function of time to find the
molar flux. For the results reported here, the
methanol permeability (P) was calculated by applying Fick’s first law:
J ¼ D
dCm
Cb
Cb
¼ DK
¼ P
dx
L
L
ð3Þ
where J is the molar flux of methanol, D is the
methanol diffusivity, K is the partition coefficient
or the solubility of methanol in the membrane,
Cm is the molar concentration of methanol in the
membrane, Cb is the methanol concentration in
the gas phase, and L is the thickness of the membrane. Use of the driving force based on concentrations or partial pressures in the gas phase lead to a
host of units for permeability. It is often convenient
to refer to the driving force in terms of a molar concentration, that is, mol/m3, which yields units of
m2/s for permeability. Assuming equilibrium and
ideal-gas mixtures, the molar concentration of
methanol in the vapor phase will not vary with the
total pressure, but will be given by the simple
expression aiP0i /RT, where ai is the activity in the
condensed phase and P0i is the pure-component
vapor pressure at temperature T. For condensable
vapors, there is the obvious issue of liquid versus
vapor molar concentration; with pure methanol,
the liquid Cb is about 0.025 mol/cm3 (25 kg mol/
m3), while for the equilibrium vapor at room temperature Cb is about 6.7 106 mol/cm3 (0.0067 kg
mol/m3).
In this simple device (Fig. 2), which superficially resembles the setup in ASTM E 96-95, it is
difficult to quantify accurately the value of DCb.
Although Cb adjacent to the inner surface of the
membrane should be close to the equilibrium concentration (e.g., mol/m3) of MeOH, it was difficult
to estimate Cb adjacent to the outer surface of the
membrane because of the unknown factors controlling the mass transfer away from the film. To
circumvent this problem, a reference sample with
known methanol permeability (here, Nafion 1121)
was also tested and the methanol permeabilities of
Journal of Polymer Science: Part B: Polymer Physics
DOI 10.1002/polb
INFLUENCE OF BLEND MISCIBILITY AND CONDUCTIVITY
2257
the samples were reported relative to the methanol permeability of the reference sample. In this
way, the driving force term (DC) in eq 3 cancels out
because all the samples were measured in one
batch, and therefore were exposed to the same conditions (same driving force). Thus
Jsample Lsample
Psample
¼
Jreference Lreference Preference
ð4Þ
At high SPEKK, the PEI-SPEKK membranes
swelled and distorted excessively; thus only membranes with SPEKK concentrations below 40%
were included in the comparison of the two blend
systems. Fortunately, it was in this range that the
major differences due to miscibility were found.
Figure 4. Optical micrographs of PEI-SPEKK blends
wherein the IEC of SPEKK is 0.8 meq/g for both
blends and the SPEKK weight fractions are (a) 0.7 and
(b) 0.5. The images were taken at room conditions
(23 8C, 48% relative humidity).
RESULTS AND DISCUSSION
Phase Behavior of PEI-SPEKK Blends
Figure 3. Optical micrographs of the neat SPEKK
membrane with an IEC of 0.8 meq/g (a). Under a cross
polarizer, a micrograph of the same membrane is
shown in (b). The images were taken at room conditions (23 8C, 48% relative humidity).
Journal of Polymer Science: Part B: Polymer Physics
DOI 10.1002/polb
It has been reported in the literature that PEKK
and PEI are miscible.20 However, we found that
upon sulfonation of PEKK, these two polymers
become immiscible at a certain level of sulfonation. The miscibility behavior of PEI-SPEKK
blends was studied by microscopic techniques
but the phase behavior could not be supported by
thermal analysis because the glass transitions of
the two components were too close to each other
(Tg of PEI ¼ 215 8C, Tg of SPEKK ¼ 180–200 8C
for IEC ¼ 0.8–2.2 meq/g, respectively).23,25
An optical micrograph of the neat SPEKK
membrane with an IEC of 0.8 meq/g is shown in
Figure 3(a). The pure SPEKK membrane exhibited birefringence under cross-polarized light as
2258
GASA, WEISS, AND SHAW
Figure 5. Optical micrographs of PEI-SPEKK blends wherein the IEC of SPEKK is
1.1 meq/g for all the blends and the SPEKK weight fractions are (a) 0.3, (b) 0.5, (c) 0.7,
and (d) 0.9. The images were taken at room conditions (23 8C, 48% relative humidity).
shown in Figure 3(b), which is an indication of
crystallinity. It was previously reported that
wide-angle X-ray scattering of SPEKK at this
sulfonation level (0.8 meq/g) showed diffraction
peaks due to crystallinity.23 Almost certainly, the
rough texture in the micrograph of the pure
SPEKK membrane was also due to crystallinity.
The blends (Fig. 4) showed no evidence of phase
separation and the morphology was more homogeneous (smoother and more transparent) than
the pure SPEKK. These are indications of miscibility. No phase separation was observed over the
entire composition range at an SPEEK sulfonation level of 0.8 meq/g. At a slightly higher IEC
for SPEKK (1.1 meq/g), evidence of a miscibility
gap appeared (Fig. 5). At SPEKK mass fractions
less than 0.3, the blends appeared to be miscible
(no droplets formed). At a 0.3 mass fraction of
SPEKK, small droplets of SPEKK appeared,
which was confirmed by sulfur microanalysis of
the dispersed phase using energy dispersive X-ray
spectroscopy (EDS). Between 30 and 70% SPEKK,
the blends were phase-separated, with an indication of a spinodal decomposition process at
0.5 mass fraction. At and above 70% SPEKK, the
blends were miscible again. At an IEC of SPEKK
1.5 meq/g, the blends were phase-separated
(see Fig. 6) over the entire composition range
considered in this study (SPEKK mass fractions
from 0.1 to 0.9). ESEM of the cryo-fractured sections of the blends (Fig. 7) confirmed that the
droplets were indeed solid polymeric phase and
not artifacts such as water droplets or air bubbles. The findings from the optical microscopy of
the PEI-SPEKK blends at room temperature
(23 8C) are summarized in an isothermal phase
diagram shown in Figure 8. There seems to be a
lower critical IEC value.
Journal of Polymer Science: Part B: Polymer Physics
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INFLUENCE OF BLEND MISCIBILITY AND CONDUCTIVITY
2259
et al. examined the PES blend, which because of
its miscibility might be most sensitive to water,
using AFM and TEM, and could find no evidence
of nano-phase separation.26 One hypothesis is that
the water-insensitive PES retards the kinetics of
phase separation substantially. In this case, rapid
phase separation might occur at higher temperatures.
Figure 6. Optical micrographs of PEI-SPEKK blends
wherein the IEC of SPEKK is 2.2 meq/g for both
blends and the SPEKK weight fractions are (a) 0.4 and
(b) 0.5. The images were taken at room conditions
(23 8C, 48% relative humidity).
Phase Behavior of PES-SPEKK Blends
In contrast to the phase behavior of the PEISPEKK blends, the PES-SPEKK blends were miscible over the entire range of IEC values and mass
fractions of the SPEKK component considered in
this work (0.8–2.2 meq/g). Figure 9(a) shows an
optical micrograph of a 50/50 PES-SPEKK blend
wherein the IEC of SPEKK was 0.8 meq/g and
one wherein the IEC of SPEKK was 2.2 meq/g
[Fig. 9(b)]. Neither blend showed any evidence of
phase separation. Other compositions and IEC
values between 0.8 and 2.2 meq/g exhibited similar miscibility behavior.
For any of these blends, as well as other
blends of this nature, a concern is the influence
of water absorption on the phase behavior. Swier
Journal of Polymer Science: Part B: Polymer Physics
DOI 10.1002/polb
Figure 7. Electron micrographs (ESEM) of a neat
SPEKK membrane with an IEC of 2.0 meq/g (a) and a
blend with 20% PEI (b). The images were taken at
room conditions (23 8C, 48% relative humidity).
2260
GASA, WEISS, AND SHAW
Figure 8. Isothermal phase diagram of PEI-SPEKK
showing miscible and immiscible blends at various IEC
versus concentration. The temperature was 20 8C.
in either the dispersed phase or the matrix.
Because the discontinuity in the conductivity of
the blends was observed at SPEKK weight fractions between 0.4 and 0.5, the percolation threshold is expected to be close to these values, which
was quite evident from the morphology of these
blends.
To explore more quantitatively the effect of
mixing on the conductivity of these blends, a theoretical model and a scaling equation were compared with the data. The theoretical model used
was based on an improved effective-medium
theory (EMT) developed by Nakamura.37 The limitation of the simple EMT is that it can only be
applied to composites with small volume fractions
of the dispersed phase (0.1 or less), where local
Proton Conductivity
The proton conductivity of the PEI-SPEKK
blends as a function of the concentration of the
ionic conductor (SPEKK) is shown in Figure 10.
At 15% SPEKK, the conductivity was about four
orders of magnitude lower than for pure SPEKK.
As the SPEKK content increased, the conductivity increased monotonically with decreasing slope
in a semilog scale [Fig. 10(a)]. A step increase in
conductivity was observed between SPEKK mass
fractions of 0.4–0.5, which can be explained by
comparing the morphology of these two blends. At
a SPEKK mass fraction of 0.4, the SPEKK droplets in this two-phase blend are separated from
each other [Fig. 6(a)]. Surrounding these ion-conducting droplets is a relatively insulating matrix
(PEI) that hinders the migration of protons from
one droplet (conductor) to the other (the solubility
of SPEKK at this IEC is small, but finite, so the
conductivity is not expected to be 0). At a SPEKK
mass fraction of 0.5, interconnectivity of some
SPEKK droplets was observed [Fig. 6(b)], which
is a reasonable explanation for its higher conductivity relative to the blend containing 40 wt %
SPEKK. At SPEKK mass fractions greater than
0.5, the conductivity increased monotonically
with increasing SPEKK concentration. It is in
this concentration range also that the morphology
changed from an interconnected but bi-continuous structure to one featuring fully dispersed
droplets of PEI in a continuous SPEKK phase.
Sulfur microanalysis using EDS showed that the
blends were highly phase-separated over the entire composition range; only one component (either pure SPEKK or pure PEI) could be detected
Figure 9. Optical micrographs of 50/50 PES-SPEKK
blends wherein the IEC of SPEKK are (a) 0.8 meq/g
and (b) 2.2 meq/g. The images were taken at room conditions (23 8C, 48% relative humidity).
Journal of Polymer Science: Part B: Polymer Physics
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INFLUENCE OF BLEND MISCIBILITY AND CONDUCTIVITY
2261
when compared with the other component, which
is true for the SPEKK/PEI system. In such a case,
the model simplifies to
r ¼ r1
Figure 10. Proton conductivity (23 8C and 98% relative humidity) of PEI-SPEKK blends wherein the IEC
of SPEKK is 2.2 meq/g. (a) The solid line is based on
the improved EMT developed by Nakamura,37 eq 5. (b)
Solid line comprises two scaling laws connected by an
exponential sigmoid. [Color figure can be viewed in the
online issue, which is available at www.interscience.
wiley.com.]
field effects are unimportant.38 In this work, local
field effects could not be ignored because the
blend compositions covered almost the entire
range (0.15–0.9). Therefore, the improved EMT
by Nakamura was more suitable to this work because this theory extends the validity of the EMT
to the entire composition range (0–1). According
to Nakamura’s theory, a two-phase composite can
be considered as a random mixture of both series
and parallel connectivity patterns of the two
phases. The equation he developed is quite cumbersome but it becomes relatively simple if the
conductivity of one of the phases is negligible
Journal of Polymer Science: Part B: Polymer Physics
DOI 10.1002/polb
VðV Vc Þ
1 Vc
V > Vc
ð5Þ
where V is the volume fraction, r1 is the conductivity, and Vc is the percolation volume fraction
threshold of the conductive phase (SPEKK).
The density of SPEKK was assumed to be close
to that of PEI such that the SPEKK mass fraction
on the abscissa in Figure 10(a) can be considered
equivalent to the volume fraction of SPEKK. The
specific gravity of PEI (Ultem 1000TM) is about
1.27, whereas PEKK is about 1.31. It was reported by Zaidi et al. that sulfonation reduced
the density of PEEK because of reduction in crystallinity.13 A similar reduction in density of PEKK
upon sulfonation was expected; thus, the near
equivalence of the densities is probably a very
reasonable assumption. We tried to estimate the
specific gravity of SPEKK (IEC ¼ 2.2 meq/g) by
measuring the dimensions of the membrane using
a micrometer. The mass of the membrane was
measured by using an analytical balance. The approximate density was 1.24 6 0.4 g/cm3.
On the basis of the modeling results, the percolation volume fraction threshold (Vc) and the
conductivity of the SPEKK-rich phase (r1) were
about 0.33 6 0.12 and 0.16 6 0.02 S/cm, respectively. The regressed Vc (0.33) is higher than the
theoretical value of 0.16 for an irregular array of
the conductive particles.39 According to a description provided by Lux, the viscosity of the polymer
blend has an inhibiting influence on the equilibrium phase separation process such that Vc can
be higher than the ‘‘true’’ equilibrium value.39
This is especially true for the SPEKK-PEI blend
where phase separation in the ternary (NMPSPEKK-PEI) solution only occurred when the total
polymer concentration reached about 20 wt %
(starting from 5 wt %) during solvent casting. At
20 wt % polymer concentration, the ternary solution was already quite viscous.
Equation 5 does not apply at low SPEKK weight
fractions (<0.4). Theoretically, the conductivity of
the composite should be 0 when the volume fraction of the conductive phase is lower than the percolation threshold. However, thermodynamics dictates that the miscibility of SPEKK in the nonconductive matrix (PEI) will be finite, which could
cause leakage of ions from one conductive droplet
to another. If the conductivity data are extrapolated to 100% PEI content, the assumption that
2262
GASA, WEISS, AND SHAW
the conductivity of one phase (PEI) is negligible
when compared with the other phase (SPEKK) is
still valid. The missing impedance data at V ¼ 0,
0.05, and 0.1 were obtained, but the conductivities
could not be calculated because the impedance
plots for the three composites exhibited capacitive
behavior (phase angle ¼ 908; no ionic conduction) over the entire frequency range of 1 Hz to
1MHz.
Another possible reason for the finite conductivity at low SPEKK levels is the seemingly bimodal size distribution of the droplets [Fig. 7(b)].
Swier et al. reported that in SPEKK-PEI blends,
the droplet size increased with increasing weight
fraction of SPEKK.25 At low weight fractions of
SPEKK, the small droplets might have percolated at around 0.17 volume fraction. As the volume fraction of SPEKK increases, larger droplets
appear, but the smaller droplets are still present
[Fig. 7(b)]. At a total SPEKK volume fraction of
around 0.4 (sum of smaller and larger droplets),
the larger droplets conceivably reached their percolation threshold, which caused the sudden increase in conductivity.
A two-stage scaling equation is considered in
Figure 10(b). This log–log plot shows very clearly
the transition between two conductivity regions:
one below and one above the percolation threshold of a SPEKK-rich phase. Above the percolation
limit of about x ¼ 0.45, the scaling exponent is
about 3, while below the percolation limit, the
slope is significantly higher (P < 0.001) at about
4. In the latter region for the blend, which has
low concentrations of SPEKK, the proton concentration is increasing in a linear fashion in the single phase, while at concentrations above the solubility limit of SPEKK, highly conductive but discrete regions are increasing in number and sized
within a fixed-conductivity matrix. This combination of changes could be responsible for the apparently higher power found at low concentrations.
It should be noted that the Nakamura equation
(eq 5) has no explicit scaling exponent, but tends
to be quadratic at high concentrations. It is developed assuming a uniform droplet size.
Figure 11(a) compares the proton conductivities of PES-SPEKK blends (miscible) with those
of PEI-SPEKK blends (immiscible) wherein the
IEC of SPEKK was 2.2 meq/g for both blend systems. There is an apparent crossover of the two
conductivity curves. At low weight fractions of
SPEKK (<0.5), the conductivities of the miscible
blends were higher than those of the immiscible
ones. However, this relationship reverses at higher
Figure 11. (a) Proton conductivity (23 8C and 98%
relative humidity) of PEI-SPEKK and PES-SPEKK
blends wherein the IEC of SPEKK is 2.2 meq/g; and
(b) log-log plot of PES-SPEKK conductivity results.
[Color figure can be viewed in the online issue, which
is available at www.interscience.wiley.com.]
weight fractions of SPEKK; the immiscible blends
became more conductive than the miscible ones.
(Chance probability of two runs comprising four
positive and six negative numbers is 1%). This
crossover in conductivity curves can be explained
by the morphology of these blends. At low weight
fractions of SPEKK, the sulfonic acid groups in
the immiscible blends (droplet-wise morphology)
are contained within the droplets and percolation
of the acid groups is hindered by the insulating
matrix (PEI). In the miscible blend, the sulfonic
acid groups are dispersed homogeneously throughout the membrane and percolation can still be achieved, although the presence of the nonconductJournal of Polymer Science: Part B: Polymer Physics
DOI 10.1002/polb
INFLUENCE OF BLEND MISCIBILITY AND CONDUCTIVITY
ing component (PES) dilutes the concentration of
protons and reduces the conductivity. In contrast,
for the immiscible blend at high mass fractions of
SPEKK (>0.5), the conductive SPEKK matrix can
swell and conduct protons almost independently
from the nonconductive PEI droplets because the
concentration of dissolved PEI in the SPEKK is relatively low. The clustering of sulfonic acid groups
within the conductive phase is somewhat similar
to the pure conductive component (SPEKK). The
effect of the insulating droplets is to reduce the
overall concentration of charge carriers in the blend
and increase the tortuosity of the proton pathways, and thereby reduce the conductivity. In the
miscible blends, the PES component can affect the
clustering of the sulfonic acid groups in SPEKK
because PES chains are present locally in between
SPEKK chains and can hinder both intrachain
and interchain clustering of sulfonic acid groups.
Figure 11(b) depicts the conductivity in the
same fashion for PES-SPEKK blends as is done in
Figure 10(b) for the PEI-SPEKK mixtures. Assuming a single scaling law for the entire composition
range for this miscible blend gives a power of 4.2 6
0.6, which is indistinguishable from that (3.9 6
0.3) for the low-concentration region of the PEISPEKK blends, suggesting similar effects. The
sinuous behavior of the data in Figure 10(b) is
statistically significant according to the test for
runs of residuals; e.g., the probability of finding
three runs with 10 points is only about 4%. However, there is no obvious reason why the conductivity should behave in the fashion shown if the
system is indeed miscible. This is discussed in
more detail later.
The crossover in the conductivity curves can
also be associated with the water absorption of
these blends [Fig. 12(a)]. At high SPEKK mass
fractions (>0.5), the water uptakes of the immiscible blends were higher than those of the miscible ones, but this relation flips at low SPEKK
mass fractions (<0.5). Figure 12(b) shows the
hydration number (k) or the number of water per
sulfonic acid groups in the blends. At high
SPEKK mass fractions (>0.5), the hydration
number is almost constant for both blend systems and the immiscible blends have higher hydration number than the miscible ones. From
SPEKK mass fraction of 0.5–0.4, the hydration
number of the immiscible blends drops from
about 9 to 7.5 water molecules per sulfonic acid,
whereas the miscible blends remained almost
constant at about 8. The abrupt increase in hydration number at low SPEKK concentrations is
Journal of Polymer Science: Part B: Polymer Physics
DOI 10.1002/polb
2263
Figure 12. Equilibrium water sorption (a) and hydration number (b) of PEI-SPEKK and PES-SPEKK blends
wherein the IEC of SPEKK is 2.2 meq/g. Membranes
were equilibrated at 98% relative humidity and room
temperature (23 8C). [Color figure can be viewed in the
online issue, which is available at www.interscience.
wiley.com.]
trivially due to the contribution of the water
uptake of the matrix that becomes prominent at
low SPEKK weight fractions.
As discussed earlier, both blend systems (PEISPEKK and PES-SPEKK) were miscible when the
IEC of SPEKK was low enough, i.e., 0.8 meq/g.
Figure 13 shows the proton conductivity of the
2264
GASA, WEISS, AND SHAW
Figure 13. Proton conductivity (23 8C and 98% relative humidity) of PEI-SPEKK and PES-SPEKK blends
wherein the IEC of SPEKK is 0.8 meq/g.
two blend systems wherein the IEC of SPEKK
was 0.8 meq/g. There are no apparent differences
in the conductivities between the two blend systems, which mean that the influence of PES and
PEI on the conductivity of these miscible blends
was similar. This also supports the claim that the
crossover in the conductivity curves of the two
blend systems at high IEC of SPEKK was due to
morphology differences and not due to inherent
differences between the two secondary components
(PEI and PES).
Attempts to reach a more global description of
the conductivity in terms of the Equivalent Box
Model,40 or similar parallel-series models, were
frustrated by the large number of parameters in
these model, even when simplified, relative to
the number of conductivity observations.
Methanol Permeability
Figure 14 is a plot of the mass of methanol that
diffused through the blend membranes as a function of time. The molar flux (J) is obtained by
dividing the slope of this plot converted to g/s by
the molecular weight of methanol (32 g/mol) and
Figure 14. Steady-state mass flow of methanol through
(a) PEI-SPEKK and (b) PES-SPEKK blends. Measurements were taken at 30 8C. [Color figure can be viewed
in the online issue, which is available at www.interscience.
wiley.com.]
Journal of Polymer Science: Part B: Polymer Physics
DOI 10.1002/polb
INFLUENCE OF BLEND MISCIBILITY AND CONDUCTIVITY
the area normal to the flux (cm2). The diameter
of the hole in the cap was used to calculate the
area. The thickness-normalized molar flux (J multiplied by L) for Nafion 1151 was 1.08 108 mol/
cm s, which is close to what Walker et al. reported
(1.39 108 mol/cm s) using pure methanol on
one side of the membrane while sweeping the
other side with nitrogen gas.36 A possible reason
for their higher value is that they did the measurements at a higher temperature (40 8C). In
their paper, they did not report the calculated
value for the methanol permeability. By turning
the vial upside down, we measured with Nafion
1151 a permeability of 4 106 cm2/s based on
the molar concentration in the pure liquid, which
compares with 1.7 106 measured by Einsla
et al. using Nafion 1171.41
Figure 15 shows the methanol permeability of
the blend membranes relative to that of Nafion1.
As expected, the methanol permeability increased
with increasing concentration of SPEKK for both
blend systems. The IEC of SPEKK in these blends
was 2.2 meq/g. At low mass fractions of SPEKK
(<0.5), the immiscible PEI-SPEKK blends exhibited lower methanol permeabilities than the miscible PES-SPEKK blends. This behavior is again
Figure 15. Methanol permeability of PEI-SPEKK
and PES-SPEKK blends relative to Nafion 1151. Measurements were taken at 30 8C. [Color figure can be
viewed in the online issue, which is available at www.
interscience.wiley.com.]
Journal of Polymer Science: Part B: Polymer Physics
DOI 10.1002/polb
2265
due to the respective morphologies of these blends
and the explanation is analogous to that of the
conductivity behavior. In the immiscible blends,
the PEI matrix impedes the permeation of methanol from one SPEKK droplet to another, whereas
in the miscible blends, wherein the SPEKK chains
are dispersed throughout the blend, there is a continuous and uniform path for methanol transport.
The methanol permeabilities of the insulating
components (PEI and PES) were both on the low
end and are almost the same in magnitude. Therefore, the observed difference between the permeabilities of the immiscible blends and those of the
miscible blends were not due to an intrinsic difference between the permeabilities of the two nonconducting components.
CONCLUSIONS
The influence of blend miscibility on the transport properties of polymer electrolyte blends was
investigated. Miscible blends (PES-SPEKK) can
have remarkably different proton conductivities
and methanol permeabilities when compared with
immiscible blends that have the same overall
IEC. These differences in the transport properties
can be explained by the differences in the morphology between these two blend systems.
Blends of PEI and SPEKK exhibited different
miscibility behavior depending on the sulfonation
level of the SPEKK component. Miscible blends
were obtained at IEC values of SPEKK of up to
0.8 meq/g. Above 0.8 meq/g, PEI-SPEKK blends
were immiscible and formed dispersed droplet
morphologies. On the other hand, blends of
SPEKK and PES were miscible over the entire
investigated range of blend compositions and of
SPEKK IEC values of 0.8–2.2 meq/g. At high
IEC of the SPEKK component (2.2 meq/g), the
proton conductivities and methanol permeabilities of the miscible blends (PES-SPEKK) were
significantly higher than those of the immiscible
blends (PEI-SPEKK) at low SPEKK mass fractions. At high mass fractions, the relation was reversed. At low IEC of SPEKK (0.8 meq/g), where
both PEI-SPEKK and PES-SPEKK blend systems
exhibited miscibility, the transport properties were
not significantly different. This indicates that the
contributions of PES and PEI to the transport
properties of the blends are the same. This also
supports the claim that the crossover in the conductivity versus blend composition curves of the
two blend systems at high IEC of SPEKK is due to
2266
GASA, WEISS, AND SHAW
morphology differences and not due to inherent
differences in the transport properties between the
two nonconductive components (PEI and PES).
The authors gratefully acknowledge financial support
from the Army through the Connecticut Global Fuel
Cell Center and the Department of Energy (Grant
10761-001-05). We also thank S. Boggs for use of the
impedance spectroscopy equipment.
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Journal of Polymer Science: Part B: Polymer Physics
DOI 10.1002/polb