N‑Substituted Fluorinated Polybenzimidazoles: An Easy Post-Modification Strategy for Improved CO₂ Separation

This study details synthesizing and characterizing a series of N-substituted aromatic polybenzimidazoles (N-PBIs) obtained via post-polymerization modifications of fluorinated polybenzimidazole (PBI-6F). The modifications involved the introduction at imidazole nitrogen of aliphatic chains bearing tertiary amine groups, affording N,N-dimethylethylamine and N,N-dimethylpropylamine N-substituted PBIs, PBI-DMEA, and PBI-DMPA, respectively. Their properties were compared with the performance of N-substituted PBI bearing a pure propyl chain, without an amine group in the end (PBI-Pr). All N-PBIs demonstrated enhanced processability, exhibiting solubility not only in dipolar aprotic solvents but also in such ordinary solvents as chloroform and THF. Their performance as gas separation membranes was tested and showed that the membranes with amine groups in the substituent had enhanced permselectivity for the CO₂/CH₄ gas pair, which was rather explained by an increase in the solubility parameter of CO₂. However, the presence of the aliphatic amines also resulted in an unusually low decomposition temperature for these polymers. The mechanism of thermal decomposition and solubility and diffusivity parameters were confirmed by corresponding calculations.