Synthesis, Characterization, Membrane Fabrication and Gas Transport Behavior of Liquid Crystal Polymer Materials

A variety of liquid crystalline (LC) materials have been examined as potential membrane separation materials. The order present in the LC phases has measurable effects on solute sorption, diffusivity, permeability, and selectivity, and can thus be used to tune the transport and separation of different species. The current work has focused on polymer dispersed liquid crystal (PDLC), linear butadiene diol based side chain liquid crystalline polymer (LCP), and linear and crosslinked acrylate based LCP membranes. The focus was primarily on the separation of propylene and propane, a separation of significant industrial interest that is not easily achieved with current membrane technology.

Polysulfone (Psf) and 4-cyano-4'-octylbiphenyl (8CB) were used to fabricate polymer dispersed liquid crystal (PDLC) membranes. Permeation properties for propane and propylene through polysulfone membranes with increasing LC concentrations were measured at temperatures above and below the glass transition temperature and in several LC phases. The plasticization of PSf by 8CB increased permeability and selectivity with increasing temperatures below the Tg, and membranes with higher LC concentrations exhibited a higher mixed gas permeability and selectivity for propylene. Permeability selectivity decreased across the smectic to nematic phase transition. Overall, selectivities were low, and membrane stability was a significant problem, especially at higher pressures. Thus, several LCP systems were studies as candidates for membrane gas separations.

A side chain liquid crystalline poly(butadiene)diol with cyanobiphenyl mesogens was impregnated in a porous PTFE support for gas transport studies. Single gas sorption for propane and propylene in the LCP were investigated in the smectic A mesophase. Gas transport in the glassy state showed separation dominated by differences in gas diffusivity. Permeabilities and selectivities for propylene/propane in the liquid crystal mesophase increased with increasing temperature due to an increase in the segmental motional of the mesogenic units which facilitated solubility of propylene over propane. In addition, an increase solubility differences between propane and propylene were observed with an increase in feed pressure. Mixed gas permeability measurements resulted in an increase in selectivity both below and above the glass transition temperature due to competitive sorption of the two gases. The thermal behavior of liquid crystalline poly(butadiene)diols (PBDs) containing methoxy- or butoxy-substituted azobenzene side chains was studied. A strong dependence of the viscous and dynamic moduli of the polymer with respect to frequency and degree of modification was observed, but the results suggested that prolonged membrane stability for linear poly(butadiene)diol LCPs would be difficult to achieve. As a result, a new class of cross-linkable acrylate based side-chain LCPs was developed.

A mesogenic cyanobiphenyl based acrylate monomer, in combination with a non-mesogenic comonomers and a cross-linking agent, was used was used to fabricate stable cross-linked LCP films for membrane separation applications using an in situ free radical polymerization technique with UV initiation. To our knowledge, this is the first reported example of a crosslinked LCP membrane. Increasing the cross-linker content resulted in a decrease in mesogen order. At temperatures in the LC mesophase permeability selectivity for propylene over propane was derived from both solubility and diffusivity selectivity and was higher for the membrane with lower crosslinker content. An increase in the temperature causes a decrease in molecular ordering and consequently decreased permeability selectivity. At temperatures approaching the nematic/isotropic transition and above, the membrane with higher crosslinker content exhibited higher propylene selectivity. Mixed gas studies of propylene/propane resulted in higher selectivities compared to the single gas runs due to the decrease of propane permeability by the presence of propylene.