Redox-responsive
anion exchange membranes were developed using photoinitiated free-radical
polymerization and reversible oxidation and reduction of viologen.
The membranes were formulated using poly(ethylene glycol diacrylate)
and diurethane dimethacrylate oligomers, dipentaerythritol penta-/hexa-acrylate
cross-linker, photoinitiators, and 4-vinylbenzyl chloride as precursors
for functionalization. In the membrane, 4,4′-bipyridine reacted
with the 4-vinylbenzyl chloride residues, and subsequently, unreacted
amines were methylated with iodomethane to obtain viologen as both
the ion carrier and redox-responsive group. Upon oxidation, viologen
supports two cations, where the reduced form only contains one cation.
Thus, the redox responsiveness changed the membrane ionicity by a
factor of 2. The area-specific resistance of the membranes in the
oxidized, +2, state was lower than in the reduced, +1, state. The
resistance increased between 40.6 ± 0.1 and 111.6 ± 0.1%,
depending on membrane thickness, with the most significant increment
being a resistance change from 4.88 × 10–4 Ω
m2 in the oxidized state to 1.03 × 10–3 Ω m2 in the reduced state. Membrane permselectivity
in the reduced, +1, state was between 15.9 ± 0.1 and 26.5 ±
0.01% lower than in the oxidized, +2, state, with no change in water
uptake, spanning an average of 0.87 ± 0.02 in the oxidized state
to an average of 0.7 ± 0.01 in the reduced state. Upon reduction,
membrane ion-exchange capacity decreases, increasing ionic resistance
and decreasing membrane permselectivity due to a reduction in fixed
charge concentration without a measurable change in water uptake.
This trend is not generally observed for ion-exchange membranes and
explains that the changes in transport properties result from changes
in ionicity, not water uptake or domain size. The reversibility and
stability of the stimuli responsiveness were confirmed by the absence
of transport property changes after redox cycling.