N-substitution of polybenzimidazoles: Synthesis and evaluation of physical properties

Abstract Series of N -substituted polybenzimidazoles (PBI) were synthesized using selective alkyl groups with varying bulk and flexibility, viz., methyl, n -butyl, methylene trimethylsilane and 4- tert -butylbenzyl. PBI-I based on 3,3′-diaminobenzidine (DAB) and isophthalic acid and PBI-BuI based on DAB and 5- tert -butyl isophthalic acid were chosen for N -substitution. Structural characterizations of substituted polymers by FT-IR and 1 H NMR revealed elimination of hydrogen bonding. Evaluation of their physical properties revealed that N -substitution rendered better solvent solubility in common organic solvents, more open polymer matrix, but reduced thermal properties in comparison to their respective parent PBI. 4- tert -butylbenzyl, methylene trimethylsilane or n -butyl group substituted polymers were soluble even in chlorinated solvents (CHCl 3 and TCE). Substantial variations in gas permeability of inert gases, He and Ar and attractive P He / P Ar selectivity, especially after methyl group substitution depicted potential of these materials for gas separation.