Polycondensation of structurally divergent tetraamine monomers with dicarboxylic acids to synthesize polybenzimidazole copolymers for polymer electrolyte membranes

Abstract

Polybenzimidazole copolymers were synthesized by varying compositions of tetraamines monomers in polymerization feed with a single dicarboxylic acid instead of conventional method where two acids were used. In our effort to promote 2,6-bis(3,4-diaminophenyl)-4-phenylpyridine (Py-TAB) as an alternative tetraamine monomer to conventionally used 3,3',4,4'-tetraaminobiphenyl (TAB) for synthesizing readily processable pyridine bridged polybenzimidazoles (Py-PBIs), two series of random copolymers (PBI-co-Py-PBI) were synthesized by polymerizing Py-TAB and TAB with isophthalic acid or terephthalic acid to produce meta (mPBI-co-mPy-PBI) and para (pPBI-co-pPy-PBI) connected copolymers, respectively. For the first time in the PBI literature, copolymers were synthesized by varying the relative compositions of tetraamines (TAB and Py-TAB) in the polymerization feed with a single dicarboxylic acid (DCA) instead of the traditionally used method where two DCAs with variable compositions were polymerized with a single tetraamine. The solubility and hence the processability of the copolymers were improved significantly upon introduction of Py-PBI in the copolymer. The detailed characterizations of both meta and para series copolymers compellingly established that thermal, chemical and mechanical stabilities can be easily modulated according to need by altering the relative compositions of PBI and Py-PBI. The phosphoric acid (PA) loading of the copolymers increased gradually with increasing Py-PBI content since the bulky pyridine moiety facilitated the absorption of PA. The presence of pyridine functionality and a larger PA loading caused a higher proton conductivity of PA doped copolymer membranes.