Mechanically Stable Poly(aryl ether) Anion Exchange Membranes for Alkaline Applications

The alkaline stability of poly(aryl ether) backbones in anion exchange polymer electrolyte membranes (AEMs) featuring quaternary benzyl N, N- dimethylhexylammmonium (DMH⁺) and trimethylammonium (TMA⁺) groups were investigated in poly(2,6-dimethyl 1,4-phenylene) oxide (PPO) and Udel^® polysulfone (PSF) polymers. Previous works have demonstrated that quaternary ammonium and phosphonium groups trigger polymer backbone degradation in commercially available poly(aryl ethers) despite the non-cation functionalized polymers being resilient in alkaline media. Recently, DMH⁺ cations in PPO was shown to eliminate cation triggered backbone degradation, but it was unknown whether or not implementation of this cation would also eliminate cation triggered backbone degradation in PSF. Herein, we demonstrate that the electron withdrawing or donating character in the poly(aryl ether) backbone, ultimately decides whether or not the prepared AEMs will become brittle in alkaline media. For instance, DMH⁺ at the benzyl position in Udel^® PSF does not mitigate cation triggered backbone degradation. PPO backbones functionalized with bromine at the aryl position in addition to having DMH⁺ cations did not prevent cation triggered backbone degradation either. Mitigation of cation triggered backbone degradation was only achieved when electron withdrawing substituents (not including the cation), such as sulfone or bromine, were not incorporated in the polymer backbone. PPO AEMs prepared through chloromethylation, rather than 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 closing, some design rules will be presented for preparing mechanically stable poly(aryl ether) AEMs from low cost, commercially available polymers for alkaline applications.