A block
copolymer of poly(vinylbenzyl chloride)-b-polystyrene
(PVBC-b-PS) was synthesized through
nitroxide-mediated polymerization, then blended with poly(2,6-dimethyl-1,4-phenylene
oxide) (PPO) at different compositions, and solution cast to prepare
a series of blend films. Differential scanning calorimetry analysis
of the PVBC-b-PS and PVBC-b-PS blended
with the PPO showed a single glass transition temperature for each
of the compositions examined, suggesting that all components in the
blend membranes are compatible. The benzyl chloride groups in the
blend films were converted to quaternary ammonium groups by reaction
with trimethylamine, and the functionalization reached high conversion
as characterized by ion exchange capacity (IEC) measurements. The
PPO blend anion exchange membranes (AEMs) show improved mechanical
properties compared to the styrenic copolymer, particularly in a hydrated
condition. The membranes were subjected to thermal and THF/water annealing
procedures to study the effect on membrane properties. Small-angle
X-ray scattering (SAXS) experiments indicated the formation of a phase-separated
morphology in the membrane after annealing with solvent vapor in the
presence of water. The ionic conductivities of the blended membranes
show an expected increase with increasing IEC and corresponding water
uptake. Ionic conductivity and water uptake were found to increase
significantly after either annealing in the presence of water and
THF or high-temperature annealing in the presence of water. The highest
hydroxide conductivity reached 43 mS/cm measured at 60 °C in
water.
Herein, we report
the anion and water transport properties of an
anion-exchange membrane (AEM) comprising a block copolymer of polyethylene
and poly(vinylbenzyl trimethylammonium) (PE-b-PVBTMA)
with an ion-exchange capacity (IEC) of 1.08 mequiv/g. The conductivity
varied little among the anions CO3
2–,
HCO3
–, and F–, with
a value of E
a ≈ 20 kJ/mol and a
maximum fluoride conductivity of 34 mS/cm at 90 °C and 95% relative
humidity. The Br– conductivity showed a transition
at 60 °C. Pulsed gradient stimulated spin echo nuclear magnetic
resonance (PGSE NMR) experiments showed that water diffusion in this
AEM is heterogeneous and is affected by the anion present, being fastest
in the presence of F–. We determined the methanol
self-diffusion in this membrane and observed that it is lower than
that in Nafion 117, because of the lower water uptake. This article
reports the first measurements of 13C-labeled bicarbonate
self-diffusion in an AEM using PGSE NMR spectrometry, which was found
to be significantly slower than F– self-diffusion.
Back-calculation of the bicarbonate conductivity using the Nernst–Einstein
equation gave a value that was significantly lower than the measured
value, implying that bicarbonate transport involves OH– in the transport mechanism. Fourier transform infrared spectroscopy,
PGSE NMR spectrometry, and small-angle X-ray scattering (SAXS) indicated
the presence of different types of waters present in the membrane
at different length scales. The SAXS data indicated that there is
a water-rich region within the hydrophilic domains of the polymer
that has a temperature dependence in intensity at 95% relative humidity
(RH).
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