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

Abstract

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 polyimide−ionene 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, 1butyl-3-methylimidazolium bistriflimide ([C 4 mim][Tf 2 N]), 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 CO 2 , N 2 , CH 4 , and H 2 . The neat ionic polyimide exhibits low permeability to CO 2 and H 2 (∼0.9 and ∼1.6 barrers, respectively) and very low permeability to N 2 and CH 4 (∼0.03 barrers for both). For the ionic polyimide + IL composite, the permeabilities of CO 2 , N 2 , and CH 4 increase by 1800−2700%, while H 2 permeability only increased by ∼200%. The large increases in permeability for CO 2 , N 2 , and CH 4 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 Robeson's Upper Bounds, this work provides a strong foundation on which ionic polyimide materials with more sophisticated structural elements can be developed to understand the structure−property relationships underlying the ionic polyimide platform and ultimately produce high-performance gas separation membranes.

Understanding the effects of backbone chemistry and anion type on the structure and thermal behaviors of imidazolium polyimide‐ionenes

“…The solids were filtered and dried in a vacuum oven at 120 °C, yielding the product as a light tan powder (19.39 g, 65%). 1 H NMR data were consistent with published values …”

“…PMDA‐API was synthesized following a procedure previously published in the literature . PMDA (15.000 g, 68.7 mmol) and API (17.647 g, 141 mmol) were added along with 150 mL of NMP to a 250 mL round‐bottom heavy‐walled pressure vessel (Ace Glass) sealed with a threaded polytetrafluoroethylene (PTFE) cap and a DuPont Kalrez® O‐ring.…”

“…The large majority of ionene polymers known in the literature have been synthesized via Menshutkin reactions between 3° diamines and dihaloalkane or similar species . Imidazole, a ubiquitous N ‐heterocycle with reactivity similar to 3° amines, is also highly suitable for ionene synthesis when starting from bis(imidazole) species such as 1,1′‐(1,6‐hexanediyl)bis(imidazole) …”

Ionic Liquids‐Based Membranes for Carbon Dioxide Separation

“…If the trans‐membrane pressure difference exceeds the capillary forces keeping the ILs, the ILs will be protruded from the pores and the SILMs may fail in practical use. Ionic liquid based composite membranes (ILCMs), which combine the preeminent film forming ability of polymer and the high selectivity of ILs, may address the above problems . Abdollahi et al .…”

“…Tuning the PILs’ backbones (i.e, substituent and anion variation) of PILs can affect the gas permeation properties . Shaligram et al .…”