Dependence of some properties of phosphoric acid doped PBIs on their chemical structure

For three polybenzimidazoles (PBIs) with different chemical structure interaction with o-phosphoric acid (PA), basicity, leaching of PA from the membranes, mechanical properties and proton conductivity in doped state were investigated. Basicity of the PBIs was discussed with respect to chemical structure of the polymers. The experimentally obtained pKa values demonstrated that introduction of lengthy fragments between benzimidazole cycles in the repeating unit of the PBI facilitated increase of basicity of the polymer. It was shown that increase of the concentration of basic fragments in the repeating unit of PBI does not necessarily lead to increase of the amount of PA absorbed at doping. On the contrary, increasing of the polymer's basicity led to more intense absorption of PA. Acid absorption at doping was found to adhere to 3-stage isotherm. Leaching of PA from the membranes was virtually unaffected by the polymer's basicity. The study of mechanical properties of the doped PBIs demonstrated that the best mechanical strength was achieved for PBI, which had the highest inherent viscosity. Proton conductivity of the obtained membranes ranged from 2.5 × 10−2 to 5.3 × 10−2 S cm−1 at 180 °С.