Synthesis of novel proton‐conducting highly sulfonated polybenzimidazoles for PEMFC and the effect of the type of bisphenyl bridge on polymer and membrane properties

Six series of novel highly sulfonated polybenzimidazoles (sPBIs) with high molecular weight were prepared by direct polycondensation between 3,3′‐diaminobenzidine and original multisulfonated dinuclear dicarboxylic acids containing bridging ether, sulfone, and hexafluoroisopropylidene moieties. All reactions were carried out in polyphosphoric acid, which acts as both solvent and catalyst. The degree of sulfonation was modulated in the final products by varying the proportion of sulfonated to nonsulfonated dicarboxylic acids used in the synthesis. The high purity of the disulfonated and tetrasulfonated monomers allows wholly sulfonated homopolymers to be obtained. Confirmation of the chemical structure and the degree of sulfonation were derived from 1 H nuclear magnetic resonance spectroscopy. Inherent viscosity was estimated as between 0.70 and 5.33 dL g −1 for sPBIs with ion exchange capacity in the range 0.87–4.68 mequiv g −1 . Dynamic thermogravimetric analysis in air showed no weight loss below 350 °C (heating rate 5 °C min −1 ). The nature of the bisphenyl bridge has clear influence on the water uptake and proton conduction properties of the resulting sPBI membranes, with hexafluoroisopropylidene links providing materials of highest conductivity as well as favoring film‐forming characteristics.