Synthesis of soluble sulfonated polybenzimidazoles derived from 2-sulfonate terephthalic acid

Abstract

2-Sulfonate p-terephthalic acid (sTA) was synthesized, and sulfonated polybenzimidazoles (sPBI) were prepared by direct polycondensation of 3,3'-diaminobenzidine (DAB) and sTA with 4,4'-oxybis(benzoic acid) (OBBA) or 2,2'-bis(4-carboxylphenyl)-hexafluoropropane (BCHFP). Thus the ether groups and hexafluoroisopropylidene units were incorporated into the backbone of the sPBI in order to improve the flexibility and solubility. The sPBI based on OBBA showed poor solubility, while the sPBI containing hexafluoroisopropylidene units were soluble in common solvents. sPBI-20 ∼ sPBI-60 could form tough and transparent membranes by solution casting. sPBI indicated high thermal stability, the 5% weight loss temperature (T5%) which was higher than 520 °C, decreased with the increase of degree of sulfonation. sPBI-20 ∼ sPBI-60 showed no glass transition at temperatures lower than 380 °C. In addition, sPBI showed low water uptake, low swelling ratio as well as excellent resistance to oxidation. But sPBI exhibited low proton conductivity because of the strong acid-base interactions between basic benzimidazole and sulfonic acid groups.