Soluble Polybenzimidazoles for PEM: Synthesized from Efficient, Inexpensive, Readily Accessible Alternative Tetraamine Monomer

Tetraamine 2,6-bis(3′,4′-diaminophenyl)-4-phenylpyridine (Py-TAB) was found to be an efficient, readily accessible, inexpensive monomer for synthesizing pyridine bridge polybenzimidazoles (Py-PBIs). The Py-TAB monomer replaced the frequently and conventionally used 3,3′,4,4′-tetraaminobiphenyl (TAB) monomer for synthesizing polybenzimidazoles (PBIs) structure. Py-TAB monomer showed wider scope for synthesis of PBIs with various dicarboxylic acids. Py-PBIs displayed superior solubility in low boiling solvent and hence eliminated the processability issues of PBIs which often restricted the large scale use. Comparable and in some instances superior thermal, mechanical and oxidative stability of Py-PBI proved that the Py-TAB is the most attractive alternative tetraamine monomer to TAB for synthesis of PBIs. The membranes obtained from Py-PBIs displayed significantly lower water uptake and swelling than the conventional PBIs due to the presence of bulky Py-TAB monomer. The presence of pyridine moiety in Py-PBIs helped to load larger amount of phosphoric acid (PA) compare to conventional PBIs. The proton conductivity of PA doped Py-PBI membranes were higher than the conventional PBI membranes owing to the higher PA loading and the complex bulky structure. Photophysical studies demonstrated the complexity of Py-PBIs structure. In summary, 3-fold objectives, namely, (I) alternative tetraamine monomer, (II) soluble PBI, and (III) higher proton conducting PBI based PEM, were achieved in this study.