Hyperbranched poly(ether sulfone)s: preparation and application to ion-exchange membranes

Abstract

Hyperbranched polymers possess unique characteristics such as low viscosity, high solubility in organic solvents and a high degree of functionality at the terminal position. However, hyperbranched polymers also possess weak mechanical properties. In this study, hyperbranched poly(ether sulfone)s (HBPES) possessing sulfonic acid moieties at the terminal position were prepared by reacting electrophilic sulfonium ions with electron-rich benzene rings from two kinds of AB2 monomers. To address the weakness of HBPES, hybrid materials containing linear and hyperbranched PES were investigated. Linear and hyperbranched multiblock copolymers were successfully prepared by reacting oligomeric linear PES and AB2 monomers. From the multiblock copolymers, linear and hyperbranched PES blends were prepared and applied as ion-exchange membranes for fuel cells. Films containing various ratios of linear and hyperbranched PES terminated by sulfonyl chloride groups were prepared via casting from a solution of DMAc. The results indicated that tethers between linear and hyperbranched polymers were important for controlling the size of the phases and for preventing the dissolution of HBPES in water; thus, linear and hyperbranched polymers were crosslinked by simply heating blends of both films. Finally, hybrid films containing sulfonic acid groups were obtained by hydrolyzing the sulfonyl chloride moieties. The resultant films showed ion-exchange capacities that were comparable to that of a Nafion 117 membrane (Dupont, Tokyo, Japan). Hyperbranched poly(ether sulfone)s (HBPES) possessing sulfonic acid at the terminal point were prepared by the electrophilic reaction of sulfonium ions to electron-rich benzene rings starting two kinds of AB2 monomers. First, linear and hyperbranched multiblock copolymers were successfully prepared by the reaction of oligomeric linear PES and AB2 monomer. Next, from these multiblock copolymers, a linear and hyperbranched PES blend was prepared and applied as ion-exchange membranes for fuel cells. The blend films showed comparable ion-exchange capacity with Nafion 117 membrane.