(725d) Estimating CO2 Permeability From Atomistic Models of Pims for Screening of Gas Separation Performance

There is tremendous demand for a porous material that has the ability to selectively separate the individual components from a gaseous mixture. Several materials are being pursued, where polymeric membranes offer several distinct advantages as potential adsorbent candidates. Polymers of intrinsic microporosity (PIMs) are a class of glassy polymers that create large free volumes as a result of bulky, rigid, and contorted backbones. However, a fundamental trade-off in membrane performance exists, where there is a balance between high permeability and high selectivity for any given gas pair and both are necessary for industrial viability. It has been shown, both experimentally and through molecular simulations, that modest modifications in the functionality of adsorbent materials may result in profound changes in the gas adsorption properties. However, to date, a systematic computational analysis to develop the design principles necessary to understand and exploit the adsorbing potential of amorphous polymers has been elusive due to the difficulty in generating a physically accurate molecular model and computationally expensive calculations required for performance evaluation.

Through predictive molecular simulations, several structural functionalities on the PIM backbone are examined in this work, and it is found that the pore volume can be substantially increased with certain functional groups. More specifically, large bulky groups present on the spirocenter site of contortion were most effective at prohibiting efficient packing of the polymer chain, thus increasing surface area. Typically an increase in total pore volume of adsorbent improves permeability at the expense of gas pair selectivity. However, it is also shown, by means of an estimated permeability calculation using free volume theory in combination with grand canonical Monte Carlo adsorption simulations, that the permeability of the hypothetical functionalized PIMs may be increased without subsequent decreasing of mixed-gas selective performance for CO₂/N₂ and CO₂/CH₄.