Grafting thermally labile molecules on cross-linkable polyimide to design membrane materials for natural gas purification and CO₂capture

Abstract

A novel strategy to design molecularly the cavity size and free volume of flexible polyimide materials via thermal treatment of rigid and cross-linkable polyimides grafted with thermal liable side beta-cyclodextrin (CD) molecules is demonstrated in this study. The spaces occupied by the labile groups may become microvoids after low-temperature thermal degradation while the rigid polyimide backbone prevails. The thermal induced cross-linking reaction among polyimide chains may create ultra-fine micro-pores that integrally connect with microvoids. As a result, the thermally cured membranes fabricated from dense polyimide precursors show gas separation performance surpassing the trade-off lines, with tough and flexible mechanical properties. Thermal annealing at 425 °C produces polyimide membranes with the best CO2 permeability of 4016 Barrer with reasonable gas pair selectivity.