Effects of ionic liquid doping on gas transport properties of thermally rearranged poly(hydroxyimide)s

Abstract

In this study we present a novel and simple approach to improve the gas separation performance of the thermally rearranged membranes, which involves doping the polyimide precursors (HPI) with ionic liquid (IL), and carrying out its degradation along with the conversion process to polybenzoxazole (PBO) in order to facilitate the formation of larger and /or more numerous free volumes. A series of aromatic (co)poly(hydroxyimide)s based on 6FDA and HAB/4MPD diamines in different molar ratios, as well as BPADA-HAB poly(hydroxyimide) were synthesized as the precursors to be doped with IL and thermally rearranged to PBO. The structural modifications of the precursor backbone were applied to study the impact of IL on the physical properties, thermal conversion process, as well as gas transport properties of the doped polymers with different chain rigidities. The pure and IL doped (co)polyimides and their thermally rearranged counterparts were characterized by WAXD, DSC, TGA, tensile tests, and PALS, and examined in pure gas permeation experiments. TR conversion temperature was considerably reduced by IL doping (e.g. by 126 °C). This effect depended on the precursor chemical structure and the IL content. After thermal rearrangement of the IL doped HPIs, the membrane permeability to gases increased significantly compared to the un-doped analogues (e.g. 2 fold increase for O2 permeability). The permeability increase was larger for the higher IL content and the precursor chain rigidity. This was accompanied by a relatively small loss in selectivity leading to the performance shift towards the 2008 upper bound. The differences in permeability among the samples correlated with the free volume size from PALS. In particular, a very good correlation was obtained (r2 = 0.958), when the data fitted to the Cohen-Turnbull model concerned only PBO samples with low cohesive energy density. Further studies on HPIs doped with IL with lower degradation temperatures are suggested to mitigate polymer degradation and explore this new method for the design of improved gas separation membrane materials.