Novel sulfonated polybenzothiazoles with outstanding dimensional stability for proton exchange membranes

Soluble sulfonated polybenzothiazoles (sPBTs) were achieved by incorporation of flexible ether groups to the backbone for the first time. The corresponding hexafluoroisopropylidene-containing sPBTs were also prepared by direct polycondensation for comparison. Both series of sPBTs could be cast into homogeneous membranes and showed high thermal and oxidative stability as well as excellent mechanical properties. Furthermore, they also exhibited high proton conductivity and outstanding dimensional stability. For example, they displayed a proton conductivity of 0.11–0.13 S/cm at 80 °C, but only exhibited an in-plane swelling of 14–15% and a through-plane swelling of 7.1–30% even at 98 °C. Interestingly, the swelling in the plane and thickness direction presented no obvious change with increasing temperature. This is because sPBTs possess strong intermolecular interactions, which was verified by dynamic mechanical analysis. Both series of sPBTs are a promising material for proton exchange membranes.