Influence of nanostructure on light gas separations in cross-linked lyotropic liquid crystal membranes

Novel, nanostructured lyotropic liquid crystal polymer membranes were fabricated, characterized and tested for their performance in separations involving CO2, N2, O2, CH4 and H2. Freestanding, cross-linked films with and without an ordered hexagonal nanostructure, but with the same chemical composition, exhibit different gas solubility, diffusion and permeability properties. The presence of an ordered nanostructure was observed to slow diffusion of all gases, but increase solubility and permselectivity in CO2-based separations. In systems with an ordered nanostructure, CO2 permeability was 5.2 Barrers and CO2/N2 separation selectivity was 27. For membranes with the same chemical composition, but lacking an ordered hexagonal nanostructure, permeability rose slightly to 6.9 Barrers, but CO2/N2 separation selectivity dropped to 21. A similar trend was also observed for CO2/CH4 separation. In separations with gases other than CO2, no permselectivity differences were observed between the two membranes. A model explaining these trends is also presented.