Mechanically Tough and Chemically Stable Anion Exchange Membranes from Rigid-Flexible Semi-Interpenetrating Networks

A series of tough and chemically stable semi-interpenetrating network anion exchange membranes (SIPN AEMs) composed of a rigid and ion-conductive component, quaternized poly(2,6-dimethyl phenylene oxide) (QAPPO), and a hydrophilic, cross-linked, flexible poly(ethylene glycol)-co-poly(allyl glycidyl ether) (xPEG-PAGE) component were synthesized. The SIPN AEMs containing both rigid and flexible polymer constituents exhibited outstanding mechanical strength and flexibility and were much tougher than conventional QAPPO membranes. The introduction of the hydrophilic network ensured SIPN AEMs with high hydration numbers, which contributed to the high ion conductivity of these materials. The physical properties of the SIPN AEMs could be varied by the mass fractions of the QAPPO and xPEG-PAGE components, and a trade-off was observed between the samples' conductive and swelling properties. Among the compositions studied, SIPN-60-2 (Mass_QAPPO-60/Mass_PEG-PAGE = 2:1) with an IEC of 1.43 mmol/g was found to have balanced ionic conductivity (67.7 mS/cm at 80°C) and swelling ratio (26% at 80°C). The alkaline stabilities of the SIPN AEMs were evaluated in 1 M NaOH at 80°C for 30 days. Good mechanical (72% and 74% retention in tensile strength and elongation at break, respectively) and dimensional (11% increase in water uptake) stability was retained by the SIPN AEM due to the presence of the alkali-resistant xPEG-PAGE network. The quaternary ammonium groups in SIPN-60-2 were found to be relatively stable (24% and 26% decrease in IEC and OH^- conductivity in 1 M NaOH at 80°C for 30 days, respectively), and the low initial IEC and the high dimensional stability of the membrane protected the cation from severe degradation during the extended aging test.