Impact of Ionic Modifications on Polyimide Properties for Gas Separation Applications

In this study, we report the synthesis and investigate the structural and gas transport characteristics of polyimides containing an imidazole ring in the main chain. First, a neutral polyimide (PI-Im-6FDA) was synthesized through polycondensation, followed by quaternation of the imidazole rings, producing an ionic-polyimide (PI-Im(Me)-6FDA). Interestingly, although the neutral form of the polyimide was able to form free-standing films, the ionic-polyimide was not able to form a self-standing film by itself. However, when free ionic liquid (IL) was added, good quality self-standing films were readily achieved (PI-Im(Me)-6FDA + IL). These three materials exhibited significant thermal stability, with a 10% weight loss occurring only when T > 400 °C. The solubility profiles of the polymers in various solvents demonstrated that the ionic modification enhances solubility in common organic solvents like acetone. The molecular weights were determined to be 91 kDa for PI-Im-6FDA and 84 kDa for PI-Im(Me)-6FDA, indicating substantial polymeric chain lengths. Additionally, computational simulations indicate an increase in density and a decrease in free volume fraction with ionic content, suggesting a more compact polymer structure, as evidenced by reduced d-spacing values. Further, macroscopic gas transport measurements show that PI-Im-6FDA exhibited superior overall permselectivity performance, while PI-Im(Me)-6FDA + IL enhances the permeability of larger gases like CH4 and N2, making it a promising material for specific gas separation applications. Microscopic self-diffusion measurements performed by pulsed field gradient nuclear magnetic resonance (PFG NMR) techniques reveal a significant increase in the gas self-diffusivity upon IL addition. This result is in qualitative agreement with the transport diffusivity data from the macroscopic transport measurements. PFG NMR data also show that all materials have uniform transport properties on the length scales exceeding the smallest values of the root mean square displacements (1−2 μm) used in the measurements.