(308d) Understanding Gas Separations Using Polymeric Membranes

Polymeric membranes, which present an efficient solution for emergent technologies, such as CO₂ capture and natural gas purification, are effective at the selective and efficient transport of gases. Here we critically delineate the underpinnings of these separation technologies using a suite of molecular simulation methods. Most of the current understanding of the separation efficiency of glassy polymer membranes is based on an ill-defined quantity, the free-volume. Our new finding, based on computer simulations on coarse grained simulations, is that the local, vibrational dynamics of a dense glassy polymer, as characterized by the long-time plateau value of the mean-squared displacement of the frozen matrix, defines the appropriate dynamic "free-volume" metric. In a related vein, it has been long thought that there is an â??upperâ? bound when permeability (related to the flux) vs. selectivity (product purity) is plotted for a host of glassy membranes and gas solute pairs. This notion has become more tenuous with the knowledge that the upper bound is not fixed, but that it â??movesâ? to improved permeability and better selectivity values with newer classes of polymers being synthesized. We shall use computer simulations to address this issue, specifically the existence of an upper bound and how it is affected by the architecture of the polymer chains in question.