Synthesis and characterization of benzoxazine-containing, crosslinkable, and sulfonated polymer through Diels–Alder reaction for direct methanol fuel cells

Introduction of cross-linked structure to sulfonated polymers is an effective approach to reduce the methanol permeability of the polymers for application in direct methanol fuel cells. In this work, a crosslinkable sulfonated polymer (sPBz) has been prepared through Diels–Alder reaction with a sulfonated bismaleimide and a benzoxazine-containing bisfuran as monomers. The polymer possesses both sulfonic acid groups and crosslinkable benzoxazine groups. The sulfonic acid group reduces the temperature of the thermal polymerization of benzoxazine groups, and the polymerization of benzoxazine group results in cross-linked sPBz polymer (CR-sPBz). CR-sPBz has showed good film formability, high mechanical strength, flexibility, and solvent resistance. The methanol permeability and proton conductivity of CR-sPBz is 2.52*10−7 cm2 s−1 and 48 mS cm−1 (60 °C), respectively, corresponding to a selectivity (proton conductivity/methanol permeability) of 1.86*105 S s cm−3. The selectivity of CR-sPBz is about 3.4 times of the value of the commercially-available Nafion 117 membrane. The high selectivity of CR-sPBz warrants its performance and suitability for use as a proton exchange membrane for direct methanol fuel cells.