A molecular modeling study of entropic and energetic selectivities in air separation with glassy polymers

Air separation is challenging due to the similarities in the sizes and ener getics of oxygen and nitrogen. Although polymer membrane-based technology h as achieved some success in replacing conventional air separation methods, the effectiveness of polymers has been shown to fall short of the economica lly attractive region occupied by inorganic microporous materials. Koros an d co-workers have recently proposed that this lack of performance is a mani festation of the low entropic selectivity in polymers, possibly due to chai n mobility or free volume effects. In this work, we address the effects of chain mobility on selectivity using molecular models and transition-state t heory. We employ the methodology recently developed;by Greenfield and Theod orou (Macromolecules 1998, 31, 7068) in which the polymer degrees of freedo m can be explicitly included in the hopping rate calculations. About 100 ox ygen and nitrogen jump events are studied in three different glassy polypro pylene configurations. To examine the effects of polymer rigidity, two sepa rate cases are considered for each jump; in the first case, the polymer mod el is held completely rigid during the event, while in the second the polym er torsional degrees of freedom are allowed to participate. The results sho w that the effects of polymer flexibility are reflected most significantly in the energy barriers, with the entropy barriers only marginally affected. Whereas the energetic selectivity can be reduced by 4 orders of magnitude in going from the rigid model to the flexible one, the entropic selectivity generally shows little change. The results are discussed in the context of current experimental and theoretical understanding of these systems.