Impact of Ionic\nModifications on Polyimide Properties\nfor Gas Separation Applications

In this study, we report the synthesis and investigate\nthe structural\nand gas transport characteristics of polyimides containing an imidazole\nring in the main chain. First, a neutral polyimide (<b>PI-Im-6FDA)</b> was synthesized through polycondensation, followed by quaternation\nof the imidazole rings, producing an ionic-polyimide (<b>PI-Im(Me)-6FDA)</b>. Interestingly, although the neutral form of the polyimide was able\nto form free-standing films, the ionic-polyimide was not able to form\na self-standing film by itself. However, when free ionic liquid (IL)\nwas added, good quality self-standing films were readily achieved\n(<b>PI-Im(Me)-6FDA</b> + IL). These three materials exhibited\nsignificant thermal stability, with a 10% weight loss occurring only\nwhen <i>T</i> > 400 °C. The solubility profiles\nof\nthe polymers in various solvents demonstrated that the ionic modification\nenhances solubility in common organic solvents like acetone. The molecular\nweights were determined to be 91 kDa for <b>PI-Im-6FDA</b> and\n84 kDa for <b>PI-Im(Me)-6FDA</b>, indicating substantial polymeric\nchain lengths. Additionally, computational simulations indicate an\nincrease in density and a decrease in free volume fraction with ionic\ncontent, suggesting a more compact polymer structure, as evidenced\nby reduced <i>d</i>-spacing values. Further, macroscopic\ngas transport measurements show that <b>PI-Im-6FDA</b> exhibited\nsuperior overall permselectivity performance, while <b>PI-Im(Me)-6FDA</b> + <b>IL</b> enhances the permeability of larger gases like\nCH<sub>4</sub> and N<sub>2</sub>, making it a promising material for\nspecific gas separation applications. Microscopic self-diffusion measurements\nperformed by pulsed field gradient nuclear magnetic resonance (PFG\nNMR) techniques reveal a significant increase in the gas self-diffusivity\nupon IL addition. This result is in qualitative agreement with the\ntransport diffusivity data from the macroscopic transport measurements.\nPFG NMR data also show that all materials have uniform transport properties\non the length scales exceeding the smallest values of the root mean\nsquare displacements (1−2 μm) used in the measurements.