(740a) Gas Separation in Cross-Linked Liquid Crystal Polymer Membranes

Authors: 
Martin, S. M., Virginia Tech
Rabie, F., Virginia Polytechnic Institute and State University
Marand, E., Virginia Tech

We have synthesized stable, cross-linked, liquid crystalline polymer (LCP) films for membrane separation applications using a combination of acrylate mesogenic and non-mesogenic monomers and a dimethacrylate cross-linker via an in situ free radical polymerization technique.  The phase behavior of the LCP membranes was characterized using DSC and X-ray scattering, and indicated the formation of a nematic liquid crystalline (LC) phase above the glass transition temperature.  Single gas transport of CO2, H2, CH4, propane and propylene in the cross-linked LCP membranes was investigated over a range of temperatures in the LC mesophase and the isotropic phase.  The solubility of the gases was dependent not only on the condensability in the LC mesophase, but also on favorable molecular interactions of certain penetrant gas molecules with the ordered polar mesogenic side chains of the LCP.  Selectivities for various gas pairs generally decreased with increasing temperature and were discontinuous across the nematic/isotropic transition. Higher cross-link densities in the membrane generally led to decreased selectivity at low temperatures when the main chain motion was limited by the lack of mesogen mobility in the ordered nematic phase. However, at higher temperatures increasing the cross-link density increased selectivity as the cross-links acted to limit chain mobility.