Synthesis and properties of soluble sulfonated polybenzimidazoles from 3,3′-disulfonate-4,4′-dicarboxylbiphenyl as proton exchange membranes

3,3′-Disulfonate-4,4′-dicarboxylbiphenyl (SCBP) was synthesized via sulfonation followed by oxidation. Sulfonated polybenzimidazoles (sPBI) with various sulfonation degrees were prepared by polycondensation of 3,3′-diaminobenzidine with SCBP and 4,4′-dicarboxylbiphenyl or 2,2-bis(4-carboxyphenyl) hexafluoropropane in PPMA. sPBI derived from 4,4′-dicarboxylbiphenyl is insoluble in common solvents, while sPBI from 2,2-bis(4-carboxyphenyl)hexafluoropropane show good solubility in polar solvents such as DMAC, DMSO, and NMP owing to the flexible hexafluoropropylidene moieties in the backbone. This sPBI ionomer could form the transparent, tough, and flexible membrane by solution casting and exhibit high thermal stability. To our surprise, sPBI membranes show an anomalous water uptake and swelling behavior, which exhibits a decreasing tendency with temperature. This is considered to be due to the secondary relaxation of sPBI at about 60 °C. In addition, sPBI membranes indicate excellent oxidative stability and low proton conductivity. The TEM investigation demonstrates that the hydrophilic ionic channels of sPBI become bigger and bigger as the sulfonation degree rises, leading to the result that the water uptake and proton conductivity enhance with increasing sulfonation degree. The sPBI membranes are a candidate material for proton exchange membranes.