Structure and dynamic properties of Polybenzimidazolium and Polybenzimidazolide in a mixture of salt and water using all atom molecular dynamics simulations

Abstract

Polybenzimidazoles (PBIs) are Ionenes with the charged benzimidazole present in the polymeric backbone. PBIs are classified as cationic or anionic ionones based on their charge, and they exhibit outstanding thermomechanical properties. PBIs have been extensively employed as membranes for a range of complex gas and liquid separations, and so on. In this paper, we employ all-atom molecular dynamics simulations to explore the behavior of two different forms of polybenzimidazole polymers in salt-water mixtures. The two polymers, namely polybenzimidazolides (PBI) (anionic form) and polybenzimidazoliums (PBIm) (cationic form), were considered in two different water-mixed salts such as (a) Sodium chloride (NaCl) and (b) Calcium chloride (CaCl₂). We report on the intramolecular and intermolecular structural and dynamic properties of PBI and PBIm in saltwater. Our findings indicate that the radius of gyration (R_g) of both PBI and PBIm decreases as the concentrations of NaCl and CaCl₂ increase. Analysis of the dihedral angle distribution reveals that the gauche conformation predominates for both polymers in the water-salt mixtures. The radial distribution function analysis between the nitrogen and carbon atoms of the imidazole group on PBI and PBIm and the salt ions shows notable structural peaks. In contrast, PBI and PBIm exhibit relatively weak interactions with water molecules. The hydrogen bonding interactions between PBI and water are particularly significant and contribute to various structural changes observed. In summary, this paper offers a comprehensive atomic-level description of the structural transitions responsible for the conformational behaviors of both PBI and PBIm.