Anion Exchange Membranes with Dynamic Redox-Responsive Properties

Abstract

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.

Nonporous Cation Exchange Membranes with pH-Responsive Properties as Chemical Valves

“…Moreover, it was observed that the water uptake of the samples increased as the cycle number increased, which further supported this hypothesis. Along with our previously reported redox-responsive membranes, this article explores the mechanisms behind nonporous, stimuli-responsive ion exchange membranes. Understanding different strategies to manipulate the transport properties of dense ion exchange membranes will allow these membranes to explore capabilities such as integrated sensors, actuators, and drug delivery systems.…”

“…Although less studied, nonporous ion exchange membranes are also possible platforms to present reversible or “switchable” transport properties in the presence of an external stimulus . Our group has previously developed redox-responsive anion exchange membranes by incorporating viologen as the anion carrier and stimuli-responsive group . Viologen undergoes a reversible conversion between two oxidation states, in which the charge of the ionic moiety changes from +2 in the oxidized state to +1 in the reduced state.…”

“…However, most research has been directed toward porous ion exchange membranes. − In these types of membranes, acrylic acid chains are tethered to the pores of a porous membrane substrate, which can then adopt either an extended or a compacted conformation dependent on environmental pH, changing the effective pore size and ultimately, the transport properties. We previously stated the importance of investigating stimuli-responsive properties on dense ion exchange membranes, generally used in electrochemical separation processes in which a potential or a concentration gradient acts as the driving force, e.g., fuel cells and electrodialysis . In this study, we sought to develop nonporous, pH-responsive membranes as chemical gates using acrylic acid monomer as the stimuli-responsive ion carrier.…”

Mechanically robust hydrophobized double network hydrogels and their fundamental salt transport properties

“…Such mechanical support structures increase the composite membrane thickness, thereby reducing flux and limiting efficiency. In response to this challenge, freestanding semi‐IPNs have been prepared using a highly charged first network, often with an arylene backbone, and a rigid, second network, usually crosslinked or coordinated to the first 5–7,28–34 . While these materials have provided relatively low water content (<50 wt%) and high ion exchange capacity (IEC >1 mmol/g), they often exhibit low strains at break, and high Young's moduli.…”

“…In response to this challenge, freestanding semi-IPNs have been prepared using a highly charged first network, often with an arylene backbone, and a rigid, second network, usually crosslinked or coordinated to the first. [5][6][7][28][29][30][31][32][33][34] While these materials have provided relatively low water content (<50 wt%) and high ion exchange capacity (IEC >1 mmol/g), they often exhibit low strains at break, and high Young's moduli. Alternatively, DNHs with reduced water content (~44 wt% water 35,36 ) have been reported, yet they lack strongly disassociating fixed charge groups, while DNHs with the requisite ion exchange groups have only been reported down to ~75-80 wt% water.…”