Variation in acid moiety of polybenzimidazoles: Investigation of physico-chemical properties towards their applicability as proton exchange and gas separation membrane materials

A series of polybenzimidazoles (PBIs) were prepared from 3,3′-diaminobenzidine (DAB) and substituted aromatic dicarboxylic acids. Effects of added polarity, bulk and isomerism in the dicarboxylic acid moiety on the properties of formed aromatic polybenzimidazoles were investigated. Solution polycondensation procedure was optimized for individual case of PBI synthesis in order to obtain inherent viscosity of ≥1 dL/g. Analysis of physical properties, water uptake, acid doping (H₃PO₄ and H₂SO₄) and gas permeability was performed. All these PBIs exhibited high thermal stability, good solvent solubility and amorphous nature. The uptake of H₃PO₄ varied from 9 to 20.1 moles per repeat unit (mol/RU), H₂SO₄ uptake varied from 3.39 to 3.81 mol/RU, while water uptake varied from 1.8 to 3.6 mol/RU of PBI. The dibromoterephthalic acid and tert-butylisophthalic acid based PBI showed the highest H₃PO₄ uptake in the series, while tert-butylisophthalic acid based PBI exhibited the highest water uptake. Acid uptake was correlated with swelling of the PBI matrix, while density estimation of H₃PO₄-doped PBI by He gas expansion method could be correlated to the physical state of PBI. 5-tert-Butylisophthalic acid and 4,4′-(hexafluoroisopropylidene)bis(benzoic acid) based PBI exhibited higher H₂ and O₂ permeability than other PBIs. The ideal gas selectivity for O₂/H₂ was considerably higher for most of the PBIs than conventional gas separation membrane materials. These analyses suggested that some of these PBIs have a potential to be used as a PEM or gas separation membrane material.