Abstract
Stable, cross-linked, liquid crystalline polymer (LCP) films for membrane separation applications have been fabricated from the mesogenic monomer 11-(4-cyanobiphenyl-4′-yloxy) undecyl methacrylate (CNBPh), non-mesogenic monomer 2-ethylhexyl acrylate (2-EHA), and cross-linker ethylene glycol dimethacrylate (EGDMA) using an in-situ free radical polymerization technique with UV initiation. The phase behavior of the LCP membranes was characterized using differential scanning calorimetry (DSC) and X-ray scattering, and indicated the formation of a nematic liquid crystalline (LC) phase above the glass transition temperature. The single gas transport behavior of CO2, CH4, propane, and propylene in the cross-linked LCP membranes was investigated for a range of temperatures in the LC mesophase and the isotropic phase. Solubility of the gases was dependent not only on the condensability in the LC mesophase, but also on favorable molecular interactions of penetrant gas molecules exhibiting a charge separation, such as CO2 and propylene, 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–sotropic transition. Sorption behavior of CO2 and propylene exhibited a significant change due to a decrease in favorable intermolecular interactions in the disordered isotropic phase. 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. Mixed gas permeation measurements for propylene and propane showed close agreement with the results of the single gas permeation experiments.
Highlights
Membrane separation processes are of considerable scientific and economic interest due to their scalability and low energy requirements [1]
The ordering of liquid crystalline materials presents a unique opportunity for gas separation processes, which rely on shape and size differences of the penetrants at the atomic level
Gas permeation behavior has been studied for varying degrees of cross-linking and over a range of temperatures covering the nematic and isotropic phases, providing insight into the mechanism of transport in these liquid crystalline polymer (LCP) materials
Summary
Membrane separation processes are of considerable scientific and economic interest due to their scalability and low energy requirements [1]. We maintained the curing temperature, pressure, and cooling/heating ramp rate to mitigate any hysteresis effects Due to their intrinsic properties, liquid crystalline polymers (LCPs) are attractive in photonic, ferroelectric, and antiferroelectric applications [9,10,11]. We report on the fabrication and gas transport behavior of cross-linked acrylate-based liquid crystal elastomer membranes, which have a high degree of mechanical stability and can be fabricated into free-standing membranes To our knowledge, this is the first example of stable, freestanding acrylate LCP gas separation membranes fabricated via in-situ polymerization. Gas permeation behavior has been studied for varying degrees of cross-linking and over a range of temperatures covering the nematic and isotropic phases, providing insight into the mechanism of transport in these LCP materials
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