Block copolymer Ion gels for CO2 Separations

Abstract

Block copolymer ion gel is composed of a polymer network formed by self-assembly of triblock copolymers, and an ionic liquidIn this thesis project, the target is to study the gas separation performance of ion gels for CO2 separation, and seek ways to optimize their properties in terms of the gas separation performance and mechanical strength. Ionic liquids have shown great promise as novel CO2-separation media, largely due to their highly selective gas solubility and non-volatility. It is discovered that the polymer networks not only provides the mechanical support to the ionic liquid, but help improve the gas separation performance as well.To study the CO2 separation performance of block copolymer ion gels, model ion gel systems that comprise 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]), and a triblock copolymer with a polymerized ionic liquid mid-block was prepared.. The gas separation performance was measured on a supported ion gel membrane. It was discovered that the polymerized ionic liquid gels exhibit high gas permeability due to the high liquid fraction. Moreover, the permeation selectivity is significantly increased from that of the neat ionic liquid. Comparisons with Robeson plots also indicate very promising separation performance for ion gels. Two other ion gels formed by self-assembly of poly(styrene-b-ethylene oxide-b-styrene) (SOS) and poly(styrene-b-methyl methacrylate-b-styrene) (SMS) in [EMI][TFSA] were also examined. The separation performance of ion gels was found to be strongly dependent on the polymer mid-block. It is also desirable to enhance the mechanical properties of ion gels. A novel ion gel based on poly[(styrene-r-vinylbenzyl azide)-b-ethylene oxide-b-(styrene-r-vinylbenzylazide)] (SOS-N3) was synthesized. Such a triblock copolymer ion gel can be chemically cross-linked by high temperature annealing and UV-irradiation. After cross-linking, the mechanical strength of the gel showed significant improvement, with 400% increase in the tensile strength and almost one order of magnitude increase in toughness. The mechanical stability of the supported ion gel membranes was also enhanced. More importantly, the mass transport properties are retained after the cross-linking. Overall, block copolymer ion gels represent a promising class of materials for CO2 separation applications. Through rational choice of ionic liquid and block copolymers, the properties of ion gels can be further optimized.