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 (PIs) 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 PI, was quaternized to yield the corresponding ionic form and subsequently blended with IL to form a composite membrane. Gas permeability measurements showed a consistent decrease across the series (neutral > ionic > ionic + IL), however, these biimidazole-based materials exhibited significantly enhanced permeability compared to previously reported mono-imidazole analogues. Pulsed field gradient (PFG) NMR studies of CO₂ and CH₄ self-diffusion revealed that electrostatic interactions play a prominent role in CO₂ diffusion inside the ionic membranes, especially when IL is present. 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.