Synthesis and characterization of rigid-rod sulfonated polyimides bearing sulfobenzoyl side groups as proton exchange membranes

A novel wholly aromatic diamine, 2,2′-bis(3-sulfobenzoyl)benzidine (2,2′-BSBB), was successfully prepared by the reaction of 2,2′-dibenzoyl-benzidine (2,2′-DBB) with fuming sulfuric acid. Copolymerization of 1,4,5,8-naphathlenetetracarboxylic dianhydride with 2,2′-BSBB and 2,2′-DBB generated a series of rigid-rod sulfonated polyimides. The synthesized copolymers with the –SO3H group on the side chain of polymers possessed high molecular weights revealed by their high viscosity and the formation of tough and flexible membranes. The copolymer membranes exhibited excellent oxidative stability and mechanical properties due to their fully aromatic structure extending through the backbone and pendent groups. They displayed clear anisotropic membrane swelling in water with negligibly small dimensional changes in the plane direction of the membrane. The proton conductivities of copolymer membranes increased with increasing IEC and temperature, reaching value above 1.25 × 10−1 S/cm at 20 °C, which is higher than that of Nafion® 117 at the same measurement condition. They displayed reasonably high proton conductivity due to the higher acidity of benzoyl sulfonic acid group, the larger interchain spacing, which is available for water to occupy, taking their lower water uptake (WU) into account. Consequently, these materials proved to be promising as proton exchange membrane.