Synthesis and characterization of a novel poly(arylene ether sulfone) containing pendent imidazole groups for high temperature proton exchange membranes

A series of poly(arylene ether sulfone) containing pendent imidazole groups (PSf-Im-x) have been successfully synthesized based on a novel monomer 2,2′-bis-(2-methyl-imidazol-1-yl-methyl)-biphenyl-4,4′-diol (MIPO). The pendent imidazole groups along the polymer chain were expected to provide functional sites for the acid–base interaction with the doping phosphoric acid (PA) when they are used as polymer electrolyte membranes for high temperature fuel cell applications. The PA content of the linear PSf-Im-x membranes is about 172.3–235.8% in 85 wt% H3PO4 at room temperature. The volume swelling of these membranes is 114.4–194.0%, lower than that of polybenzimidazole (PBI) with similar PA content. The proton conductivities of the membranes are 0.021–0.053 S cm−1 at 140 °C under absolutely dehydrated state. The low volume swelling and good proton conductivity may be attributed to the "side-chain-type" structures of pendent imidazole groups, which facilitate ion transport. To obtain higher acid doping while maintaining mechanical properties, cross-linked membranes were prepared by the reaction of the imidazole group of the polymer and p-xylene dichloride. The PA content of the membranes with 20% cross-linking is 313.2% in 85 wt% H3PO4 at 80 °C. The stress at breaking and the proton conductivity of the membrane is 3.2 MPa at room temperature and 0.063 S cm−1 at 140 °C in an absolutely dehydrated state.