Ionic Polyimides: Hybrid Polymer Architectures and Composites with Ionic Liquids for Advanced Gas Separation Membranes

Polyimides and ionic liquids (ILs) are two classes of materials that have been widely studied as gas separation membranes, each demonstrating respective advantages and limitations. Both polyimides and ILs are amenable to modification/functionalization based on selection of the requisite precursors. However, there have been but a handful of reports considering how polyimides and ILs could be integrated to obtain fundamentally new materials with synergistic properties. In this manuscript, we demonstrate a new and versatile way to synthesize polyimides with imidazolium cations directly located within the polymer backbone to form polyimideionene hybrids, or ionic polyimides. Our strategy for synthesizing ionic polyimides does not require the use of amino-functionalized ILs. Instead, the imidization reaction occurs prior to polymerization in the formation of an imidazole-functionalized diimide monomer. This monomer is then reacted via step-growth (condensation) polymerization with p-dichloroxylene via Menshutkin reactions, simultaneously linking the monomers and creating the ionic components. The resultant ionic polyimide is amenable to thermal processing (e.g., extrusion, melt-pressing) and capable of forming thin films. Upon soaking thin films of the ionic polyimide in a widely used IL, 1-butyl-3-methylimidazolium bistriflimide ([C4mim][Tf2N]), a stoichiometric absorption of the IL into the ionic polyimide was observed, forming an ionic polyimide + IL composite. The gas separation performances of ionic polyimide and ionic polyimide + IL composite membranes were studied with respect to CO2, N-2, CH4, and H-2. The neat ionic polyimide exhibits low permeability to CO2 and H2 (similar to 0.9 and similar to 1.6 barrers, respectively) and very low permeability to N2 and CH4 (similar to 0.03 barrers for both). For the ionic polyimide + IL composite, the permeabilities of CO2, N-2, and CH4 increase by 18002700%, while H-2 permeability only increased by similar to 200%. The large increases in permeability for CO2, N-2, and CH4 are due to greatly increased gas diffusivity through the material, with gas solubility essentially unchanged with the IL present. The ionic polyimide and ionic polyimide + IL composite were characterized using a number of techniques. Most interestingly, X-ray diffractometry (XRD) of the films reveals that the ionic polyimide + IL composite displays a sharp peak, indicating that the ionic polyimide may experience supramolecular assembly around the IL. Although the performances of these first ionic polyimide and ionic polyimide + IL composite membranes fall short of Robesons Upper Bounds, this work provides a strong foundation on which ionic polyimide materials with more sophisticated structural elements can be developed to understand the structureproperty relationships underlying the ionic polyimide platform and ultimately produce high-performance gas separation membranes.