Terminally-crosslinked sulfonated poly(fluorenyl ether sulfone) as a highly conductive and stable proton exchange membrane

Sulfonated poly(fluorenyl ether sulfone) (PFES) with a terminally-crosslinked network structure was prepared by heat-induced crosslinking of the allyl-terminated telechelic sulfone polymers using a bisazide. The crosslinked polymer membrane with a sulfofluorenyl moiety of 70% (PFES-70) showed excellent hydrolytic, dimensional, and mechanical stability. The crosslinked PFES-70 membrane revealed a proton conductivity of 0.04 S/cm at 20 °C, which increased significantly with increasing temperature. The conductivity of 0.38 S/cm at 100 °C for PFES-70 was higher than that of both non-crosslinked (0.33 S/cm) and Nafion® (0.17 S/cm). In addition, the crosslinked PFES-70 showed a methanol permeability of only 1.4% (3.9×10 −8 cm 2 /s) compared to Nafion®.