Preparation and properties of polybenzimidazoles with sulfophenylsulfonyl pendant groups for proton exchange membranes

Abstract

4′-Sulfonate-2,5-dicarboxyphenyl sulfone (SCPS) was synthesized by nucleophilic substitution reaction followed by oxidation reaction. A series of polybenzimidazoles (PBIs) with controlled contents of sulfophenylsulfonyl pendant groups were prepared by polycondensation of SCPS and 2,5-dicarboxyphenyl sulfone with 3,3′-diaminobenzidine. Different from the earlier reports, these soluble PBI with sulfonic acid groups were achieved by attachment of sulfophenylsulfonyl pendant groups. They showed excellent solubility in common organic solvents and thus could be cast into the membrane for fuel cell applications. Moreover, they exhibited high thermal properties as well as oxidative and hydrolytic stability. The sulfoarylated PBI membranes showed low water uptake and swelling, but they exhibited low proton conductivity due to the intermolecular acid–base interaction like sulfonated polybenzimidazoles reported previously. However, they are still interesting matrix materials for proton exchange membranes since their proton conductivity could be improved by the blending or doping method. In addition, the sulfoarylated PBI membranes showed an increasing hydrophilic/hydrophobic nanophase separation morphology with sulfonation degree, leading to the results that their water uptake, swelling, and proton conductivity increased but their oxidative stability decreased with increasing sulfonation degree.