Synthesis and properties of soluble sulfonated polybenzimidazoles derived from asymmetric dicarboxylic acid monomers with sulfonate group as proton exchange membrane

Asymmetric dicarboxylic acid monomers, namely, 6-sulfonate-1,4-naphthalene dicarboxylic acid (SNAA) and 6,8-disulfonate-1,4-naphthalene dicarboxylic acid (DSNAA), were designed and synthesized. Subsequently, mono-sulfonated polybenzimidazoles (sPBI-N100) were synthesized by polycondensation of SNAA and 3,3′-diaminobenzidine. Other four series of sulfonated polybenzimidazoles (sPBI) were also prepared using DSNAA as sulfonated monomer. The incorporation of SNAA or DSNAA moieties increases the asymmetry of polymer chains and disrupts their regular packing. Soluble sulfonated polybenzimidazoles were thus achieved by enhancing the asymmetry of polymer chains for the first time. They show high thermal and oxidative stability. Moreover, sPBI membranes exhibit appropriate water uptake, low swelling, and excellent mechanical properties. However, they display low proton conductivity like other sPBI because of intermolecular acid–base interactions, but their proton conductivity could be improved by the doping or blend method. In addition, sPBI-N100 with the lowest ion exchange capacity shows the highest proton conductivity among all the sPBI, due to its particular morphology. These sPBI are a promising material for proton exchange membranes.