Proton exchange membranes derived from sulfonated polybenzothiazoles containing naphthalene units

Abstract Sulfonated polybenzothiazoles were synthesized by polycondensation of 6-sulfonate-1,4-naphthalene dicarboxylic acid, 2,5-diamino-1,4-benzenedithiol dihydrochloride and either 1,4-naphthalene dicarboxylic acid or 2,2-bis(4-carboxyphenyl) hexafluoropropane. The two series are denoted sPBT-NA and sPBT-6F, respectively. While the sPBT-NA series are insoluble in common solvents, the sPBT-6F series are soluble due to the incorporation of hexafluoroisopropylidene moieties, which impart more chain flexibility rather than disrupting the regular packing of the polymer main chain. The sPBT-6F series exhibited high proton conductivity, high thermal and oxidative stability, good mechanical properties and appropriate water uptake and dimensional swelling behavior. For example, the proton conductivity of sPBT-6F90 was much higher than that of Nafion 117, and the proton conductivity of sPBT-6F80, sPBT-6F85 and sPBT-6F95 is also as high as that of Nafion 117. The combined properties of the present sulfonated polybenzothiazoles are commendable for proton exchange membranes.