Polyimide Membranes with Internal Free Volume for Gas Separation

Abstract

These days, polymer membranes have served as a key element in many useful scientific and technological applications such as sensors, fuel cells, water purification and desalination and gas and vapor separation. With the advent of a new era, demanding energy-saving and environment-friendly innovative processes, however, conventional polymer membranes for gas separation are confronted with some drawbacks ? poor chemical and thermal stability with relatively low mass transport rates and low separation efficiency. To compete effectively with other materials, it is necessary for polymer membranes to achieve both high permeability and selectivity as well as to impart strong environmental adaptability. Here we show new polyimide membranes having high internal free volume for gas separation. High internal free volume in polyimide membranes was achieved by incorporating triptycene-based diamine into polyimide structure. Triptycene is the simplest member of iptycenes, and in triptycene the three bulky phenyl groups are joined together by a single hinge as the bridgehead carbons. An unusual combination of properties of triptycene makes it useful in polymer design with high internal free volume, expecting high diffusion coefficients of gases and increased sorption capability. As such, triptycene-based polyimide membranes show relatively higher gas permeability and selectivity as compared to other polyimide membranes with rigid or semi-rigid structures. In addition, they exhibit good solubility to common organic solvents, which means they could be processed as useful membrane forms such as thin-film composites or asymmetric membranes by solution-casting or phase inversion methods.