Investigation of sequence isomer effects in AB-polybenzimidazole polymers

In the present work, a unique series of random polybenzimidazole (PBI) copolymers consisting of the recently reported novel isomeric AB-PBI (i-AB-PBI) and the well known AB-PBI were synthesized. The i-AB-PBI incorporates additional linkages (2,2 and 5,5) in the benzimidazole sequence when compared with AB-PBI. Random copolymers, varying in composition at 10 mol % increments, were synthesized to evaluate the effects of sequence isomerism in the polymer main chain without altering the fundamental chemical composition or functionality of a polymer chain consisting of 2,5-benzimidazole units. Polymer solutions were prepared in polyphosphoric acid (PPA) and cast into membranes using the sol–gel PPA process. The resulting polymers were found to have high inherent viscosities (>2.0 dL/g) and showed elevated membrane proton conductivities (∼0.2 S/cm) under anhydrous conditions at 180 °C. Fuel cell performance evaluations were conducted, and an average output voltage ranging from 0.5 to 0.60 V at 0.2 A/cm² was observed for hydrogen/air at an operational temperature of 180 °C without applied backpressure or humidification. Herein, we report for the first time glass transition (T_g) temperatures for AB-PBI, i-AB-PBI, and an anomalous T_g effect for the series of randomized PBIs. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 619–628