N-alkyl polybenzimidazole: Effect of alkyl chain length

Despite the presence of bulk literature on polybenzimidazole (PBI), unavailability of readily soluble and processable PBI remains as the tallest challenge for the end-use. N-alkyl PBIs (N-PBIs) were synthesized by grafting the alkyl chain in the imidazole backbone to resolve this key constrain. The chain length of substituted alkyl groups was varied to evaluate the influence of chain size on the structures and properties of N-PBIs. Significant enhancement of solubility of N-PBIs compared to parent PBI in formic acid offered the opportunity to fabricate the homogeneous mechanically tough membranes with minimal efforts. The substituted long alkyl chains pushes apart the PBI chains and hence increases the face-to-face packing distance by breaking the self-association between the chains; resulted into less rigid highly soluble N-PBIs. Alkyl chain length dependent weight loss at ∼300 °C, presence of two glass transition temperatures and peculiar deep rubbery modulus in storage modulus vs. temperature plots resembled the copolymer molecular structure of N-PBIs. Hydrophobic character of alkyl chains and loosely packed structure of N-PBIs facilitated decrease in water uptake and swelling of the N-PBI membranes compared to parent PBI membrane. The temperature dependent proton conductivity of PA loaded N-PBIs membranes were found to be satisfactory.