Synthesis and properties of sulfonated polybenzothiazoles with benzimidazole moieties as proton exchange membranes

Sulfonated polybenzothiazoles with benzimidazole moieties were successfully prepared by polycondensation. For comparison purpose, sulfonated polybenzothiazoles were also synthesized. Rigid-rod sulfonated polybenzothiazoles generally show poor solubility in organic solvents, whereas these sulfonated polybenzothiazoles with high sulfonation degree are soluble, but the membranes cast from them swell excessively in water. Thus they could not be used as proton exchange membranes (PEMs). The comparison study demonstrates that the incorporation of benzimidazole moieties to sulfonated polybenzothiazoles greatly enhances the solubility of products. The resulting copolymers thus could be cast into the membranes as PEMs. The benzimidazole moieties in the resulting polymers improve thermal properties, dimensional stability, tensile strength, Young's modulus, and oxidative stability, but they decrease proton conductivity and the elongation at break, due to the ionic cross-linking and hydrogen bonds. Even so, sulfonated polybenzothiazoles with appropriate content of benzimidazole moieties (10 mol%) could still display a high proton conductivity of 0.094 S cm−1 at 80 °C and an elongation at break of 26.1%. These ionomers exhibited excellent overall properties such as excellent mechanical properties, high thermal and oxidative stability, low swelling, and high proton conductivity, thus they might be a candidate for proton exchange membranes.