Ionic polyimide membranes containing Tröger's base: Synthesis, microstructure and potential application in CO2 separation

In an attempt to obtain polymeric gas separation membranes simultaneously with high permeability and high selectivity, herein, series of novel ionic polyimides containing Tröger's base (TB) unit have been synthesized following an efficient synthetic methodology including the ionization of imidazole moiety and ion-exchange reaction. The effects of polyimide backbone structure, bonding mode of the ion pair and ion species on microporous structure, molecular packing behavior, mechanical property and gas separation property of the prepared ionic polyimide membranes were investigated. Our results reveal that the incorporated TB unit with rigid and in-built amine structure and an external attractive interaction for CO2 of particular ions have a synergistic effect on increasing the permeability and selectivity of polyimide membranes by retarding chain packing and increasing CO2 affinity. Typically, the prepared ionic membranes modified by the lateral substituted bis(trifluoromethanesulphonyl)imide ([TFSI]-) show a maximal CO2 permeability of 90.1 Barrer with the CO2/CH4 and CO2/N2 selectivities of 37.5 and 21.9, respectively, which is around 120% higher than that of its parent polyimide membrane in CO2 permeability. Meanwhile, the covalent integration of ions in polyimide backbone creates a sufficient stability of the membranes and endows them with tensile strengths of over 50 MPa, more superior than those of most reported supported ionic liquid membranes (SILMs) for gas separation applications. This study extends a new method for constructing high permeable polyimide materials for CO2 gas separation.