Leveraging molecular scale free volume generation to improve gas separation performance of carbon molecular sieve membranes

Carbon molecular sieve (CMS) membranes have proven to be promising candidates for next-generation gas separations. Modification of polymeric precursors is a critical tool that permits fine-tuning of CMS structure and performance for a wide variety of gas mixtures. Here, we make a targeted alteration to a polyimide CMS precursor through substitution of free-volume-generating trifluoromethyl groups for aliphatic methyl groups on the polymer backbone. Gas separation performance shows a vast improvement, demonstrated by up to an eightfold increase in gas permeability as well as higher mixed gas separation factors in some cases. We investigate these properties, and their dependence on pyrolysis temperature, with detailed measurements of gas sorption and permeation in CMS dense film membranes with additional analysis through classical materials characterization methods. Our observations indicate that addition of free-volume-generating groups into polymeric precursors is a powerful tool for developing state-of-the-art CMS membranes, especially in cases when high permeability is an important design parameter.