Sulfonated Polybenzothiazoles: A Novel Candidate for Proton Exchange Membranes

Sulfonated polybenzothiazoles (sPBT) with high molecular weight as well as excellent solubility were synthesized for the first time, which was achieved by attaching the phenylsulfonyl pendant groups or incorporating the hexafluoroisopropylidene moieties to the polymer backbone. Such sulfonated polybenzothiazoles thus could be cast into homogeneous membranes and could be further evaluated as proton exchange membranes. These sPBTs showed high thermal stability and high proton conductivity as well as low swelling. For instance, the hexafluoroisopropylidene-containing sPBT with a disulfonation degree of 65% exhibited a Td5 of 380 °C, a proton conductivity of 0.11 S/cm, and a swelling of 15.5% at 80 °C. In addition, the sPBT membranes showed excellent oxidative and hydrolytic stability. In comparison with the phenylsulfonyl pendant-group-containing sPBT membranes, the hexafluoroisopropylidene-containing sPBT membranes with an equivalent ion exchange capacity showed much narrower ionic channels because of the hydrophobic hexafluoroisopropylidene moieties. This made the latter display better dimensional stability, oxidative stability, and hydrolytic stability than the former. This investigation illustrated that sulfonated polybenzothiazoles are a novel candidate for proton exchange membranes.