From Low to High Density: The Effect of Ionic Modifications on Permselectivity in Biimidazole-Based Polyimides

Abstract

In this work, a family of fluorinated polyimides featuring trifluoromethyl-substituted biimidazole rings per repeating unit was synthesized and systematically modified to evaluate the impact of ionic functionalization and ionic liquid (IL) incorporation on membrane structure and gas transport behavior. The base polyimide, PI-(CF₃-Im)-6FDA, was quaternized with methyl iodide to yield the ionic variant PI-(CF₃-Im-Me)-6FDA, which was subsequently blended with [C₄mim][Tf2N] IL to form a composite membrane (PI-(CF₃-Im-Me)-6FDA+IL). Structural and thermal characterizations confirmed successful synthesis and revealed progressive densification of the polymer matrix upon ionic modification and IL addition, as evidenced by reduced d-spacing, fractional free volume, and mechanical stiffness. Gas permeability measurements showed a consistent decrease across the series (neutral > ionic > ionic + IL), however, this biimidazole-based materials exhibited significantly enhanced permeability compared to previously reported mono-imidazole analogues. Pulsed field gradient (PFG) NMR study of CO2 and CH4 self-diffusion revealed that electrostatic interactions play a prominent role in CO2 diffusion inside the ionic membranes, especially when the IL is added to the membrane. The activation energy of self-diffusion was observed to increase across the series (neutral < ionic < ionic + IL) for CO2, while no difference, within uncertainty, was observed for CH4. An increased level of local intramembrane mobility observed for CO2 as a result of the IL addition indicates a preference for CO2 molecules to diffuse in the IL-rich membrane environments. No such local mobility increase was observed for CH4.