Hydroxide Ion Conducting Membranes Dually Functionalized with Cationic and Zwitterionic Groups

A series of ether-free poly(p-terphenyl piperidinium)s carrying both cationic and zwitterionic groups were prepared and evaluated as anion-exchange membranes (AEMs). First, a zwitterionic monomer (zwPip) was synthesized by sulfoalkylation of N-methyl-4-piperidone (Pip) using 1,4-butane sultone. Pip and zwPip were then employed in superacid-mediated polyhydroxyalkylations with p-terphenyl to prepare precursor copolymers containing 20, 30, and 40 mol % of zwPip units. Next, the Pip units were fully quaternized in a Menshutkin reaction with iodomethane. AEMs cast from these copolymers with ion-exchange capacities between 1.8 and 2.2 m equiv g–1 displayed excellent hydroxide ion conductivities, reaching up to 171 mS cm–1 at 80 °C. At lower temperatures, 20 and 40 °C, the water uptake decreased with increasing zwitterionic content, most probably because of the increased ionic cross-linking through piperidinium–sulfonate complexes. AEMs containing 30 and 40% zwPip units showed only a minor ionic loss (