Gas transport and physical aging of a highly microporous polymer as probed by positron annihilation lifetime spectroscopy

An interest in lowering the energy and capital costs for light gas separations and hydrogen production from reforming fossil fuels is a goal of many researchers. The reforming step of fossil fuels produces a mixture of light gases and syngas (CO/H2) that is purified by pressure swing adsorption or cryogenic technologies. These technologies are exception but an interest in the potential of polymeric membranes as an alternative to pressure swing adsorption and cryogenics is of interest. However, Robeson noted an upper bound relationship between permeability and selectivity for several gas pairs for polymers. Generally, high Tg amorphous polymers offered the best permselectivity for industrially relevant gas separations. There have been a number of investigations to mimic the molecular sieving characteristics of zeolites and carbon molecular sieves by modulating polymer chain packing and mobility. Schmidhauser and Koros have shown that incorporating a spirobisindane moiety into the polymer backbone can lead to an increase in permselectivity for certain gas pairs. Recently, McKeown et al built upon this finding by producing a series of ladder polymers based on the spirobisindane moiety. We have prepared one of these ladder polymers and evaluated its permselectivity, free volume, and initial aging characteristics.