Mechanically stable poly(arylene ether) anion exchange membranes prepared from commercially available polymers for alkaline electrochemical devices

The alkaline stability of poly(arylene ether) backbones in anion exchange polymer electrolyte membranes (AEMs) derivatized with quaternary benzyl N, N-dimethylhexylammmonium (DMH⁺) and trimethylammonium (TMA⁺) cation groups were investigated in poly(2,6-dimethyl 1,4-phenylene) oxide (PPO) and Udel polysulfone (PSF) polymers. Previous studies have demonstrated that quaternary ammonium and phosphonium groups trigger backbone degradation in commercially available poly(arylene ether)-based AEMs, despite the base polymers being resilient to alkaline solutions. Herein, we demonstrate that the electron withdrawing or donating character in the poly(arylene ether) backbone ultimately decides whether or not the prepared AEMs will become brittle in alkaline media due to cation triggered backbone degradation. Mitigation of cation triggered backbone degradation was only achieved when electron withdrawing substituents (not including the cation), such as sulfone or bromine groups, were eliminated from the polymer backbone (or, alternately, when electron donating groups were present). Hence, PPO AEMs prepared through chloromethylation, as opposed to free radical bromination, were resistant to backbone hydrolysis in alkaline media because each cation-functionalized repeat unit had two electron donating methyl groups rather than a single methyl group. In summary, this paper presents some design rules for preparing mechanically stable poly(arylene ether) AEMs from low cost, commercially available polymers for alkaline electrochemical devices.