Alkali Resistant Anion Exchange Membranes Based on Saturated Heterocyclic Quaternary Ammonium Cations Functionalized Poly(2,6-dimethyl-1,4-phenylene oxide)s

Quaternization of poly(2,6-dimethyl-1,4-phenylene oxide)s (PPOs) with a series of saturated heterocyclic compounds was carried out via the Mannich reaction to fabricate anion exchange membranes (AEMs) with high alkaline chemical stability and ion conductivity. The heterocyclic quaternary ammonium cations include 1-methylpyrrolidinium (MPy), 1-ethylpyrrolidinium (EPy), 1-butylpyrrolidinium (BPy), 1-methylpiperidinium (MPrD), 1-ethylpiperidinium (EPrD) and N-methylmorpholinium (NMM). For comparison, trimethylammonium (TMA) and 1-methylimidazolium (MeIm) functionalized PPO membranes were fabricated as well. The physicochemical properties of the prepared membranes were investigated in terms of water uptake, swelling, ion exchange capacity, mechanical properties, conductivity and tolerance to alkalies. The AEMs having saturated heterocyclic quaternary ammonium groups exhibited better resistance to nucleopholic attack of hydroxide ions than PPO-TMA and PPO-MeIm membranes, especially the PPO-MPy membrane, which retained its initial conductivity of 87.5% after immersed in 1M KOH at 80°C for 508 h.