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 degrees 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 degrees 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 degrees C in an absolutely dehydrated state.