Simultaneous, Synergistic Control of Ion Exchange Capacity and Cross-Linking of Sterically-Protected Poly(benzimidazolium)s

Abstract

The application of highly charged anion exchange membranes is often limited by strong water sorption leading to excessive swelling and eventual dissolution, especially at elevated temperatures. The cross-linking of polymers has been shown to be an excellent mitigation strategy but this often restricts membrane and ionomer processing methods. Here, we explore the reaction, stability, and utility of a cross-linking agent for the recently discovered class of cationic polymers: methylated, C2-protected poly(benzimidazolium)s. In situ reaction and formation of p-xylyl cross-linking groups is found to provide novel, highly functionalized, hydroxide-stable membranes and films with enhanced anion conductivities and superior mechanical properties compared to un-cross-linked polymers. The versatility of this strategy will be important in the design of polymers for the preparation of stable, hydroxide-conducting films, membranes, and ionomers.