Experimental and computational studies on the effects of C(2) methylation on the properties and gas separation performance of polyimide-ionene membranes

Abstract

As methylation of the imidazolium C(2) position is known to affect the intermolecular properties of ionic liquids (ILs), polyimide (PI)-ionenes were designed to determine what effects the presence of a C(2)–Me group might have on the properties and membrane performance characteristics relative to analogous PI-ionenes with a C(2)–H group. A commonly used IL (1-butyl-3-methylimidazolium bistriflimide, [C₄mim][Tf₂N]) was added to the synthesized polymers which formed homogeneous PI-ionene + IL composites that were amenable to the formation of flexible films suitable for membrane testing. The gas permeation behaviors of the resultant membranes were measured for H₂, CO₂, N₂, and CH₄. The 6FDA-containing PI-ionene exhibited greater gas permeabilities than the PMDA-containing material, while both materials exhibited comparable selectivity for CO₂ relative to other gases. The permeability–selectivity relationships were visualized on Robeson plots, and these PI-ionene membranes were comparable to other ionene and poly(IL) materials. Molecular dynamics (MD) simulations and quantum mechanics (QM) calculations were performed on the C(2)–H and C(2)–Me monomeric units to determine their conformations and other structural properties. The results obtained are useful in the further development of ionene polymers for gas separation membranes.