Soluble sulfonated polybenzothiazoles derived from 3,3′-disulfonate-4,4′-dicarboxylbiphenyl for proton exchange membranes

Two series of sulfonated polybenzothiazoles were synthesized by polycondensation of 2,5-diamino-1,4-benzenedithiol dihydrochloride and 3,3′-disulfonate-4,4′-dicarboxylbiphenyl with 4,4′-dicarboxylbiphenyl or 2,2-bis(4-carboxyphenyl) hexafluoropropane, which were termed as sPBT-DP and sPBT-BP, respectively. The first series is insoluble in common polar solvents because of its rigid structure. In contrast, the sPBT-BP series are soluble in DMSO and NMP, due to the flexible hexafluoroisopropylidene moieties in the backbone. Thus they could be cast into homogeneous membrane and evaluated as proton exchange membranes. The studies illustrated that they showed high thermal and oxidative stability as well as excellent mechanical properties. Moreover, they exhibited high proton conductivity and outstanding dimensional stability. For example, sPBT-BP57.5 displayed a proton conductivity of 0.094 S/cm and an in-plane swelling of 9.7% as well as a through-plane swelling of 35% at 80 °C. The sPBT-BP ionomers are a promising material for proton exchange membranes.