Browsing by Subject "Ionic liquids"

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    Application of deep eutectic solvents and ionic liquids to hydrolase-catalyzed reactions.
    (2010-01) Gorke, Johnathan Thomas
    Hydrolases are important enzymes for stereoselective and environmentally benign synthesis. In nature, hydrolases cleave bonds with water. When used in organic solvents, these enzymes can make synthetically useful bonds through condensation and the release of a small molecule, usually water or an alcohol. Many organic solvents that preserve enzyme activity, such as toluene, can be environmentally damaging or toxic. Room temperature ionic liquids, poorly coordinating salts that are liquid at temperatures below 100 degrees C, are a potential alternative to organic solvents for hydrolase-catalyzed reactions because of their low volatility, moderate polarity, and recyclability. However, many commonly used ionic liquids are orders of magnitude more expensive than conventional organic solvents and may also cause adverse environmental effects if released into aquatic environments. We demonstrate that ionic liquids are effective solvents for the lipase-catalyzed polymerization of epsilon-caprolactone and other poly(hydroxyalkanoates) and are effective in enhancing the electrical conductivity of carotenoid-containing polymers produced enzymatically. However, we found that they were not as effective as toluene for enzyme catalysis, and strove to find better alternative solvents for biotransformations. We discovered that deep eutectic solvents, mixtures of ammonium or metal salts such as choline chloride and hydrogen bond donors such as urea or glycerol, were exceptional low-cost, biodegradable alternatives to organic solvents for hydrolase-catalyzed reactions. These physical mixtures may be thought of as ionic liquids, because they share similar physical properties to those solvents. Though they are composed of potential denaturants such as urea or halide anions, deep eutectic solvents stabilize enzymes. This stabilization is likely due to a preference for intra-solvent hydrogen bonding compared to enzyme-solvent hydrogen bonding. Deep eutectic solvents enhanced enzyme activity for a number of lipases either as pure solvents for reactions such as transesterification or polyesterification; or as additives in aqueous reactions such as epoxide ring opening or ester hydrolysis. We have preliminary evidence that deep eutectic solvents may induce a conformational change in enzymes that can alter reaction rates. These changes appear to be distinct from those caused by denaturing.
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    Block copolymer-based ion gels as solid polymer electrolytes
    (2012-10) Zhang, Sipei
    Block copolymer-based (BCP) ion gels are a class of interesting solid polymer electrolytes (SPEs) in electrochemical applications. This thesis aims to systematically study the mechanical and electrical properties of BCP-based ion gels formed by the selfassembly of ABA triblock copolymers in an ionic liquid, and find ways to enhance the properties of the gels, in order to formulate optimal designs in terms of the triblock for applications to electrochemical devices. Two particular target applications are organic transistors and electrochemical capacitors, due to the very large specific capacitance (on the order of F/cm2) of these electrolytes and therefore low voltage operation and potentially desirable energy storage. To study the effect of the BCP on the properties of ion gels, BCPs with different midblocks and end-block lengths were prepared, and the viscoelastic and electrical properties of the ion gels were investigated over large composition and temperature ranges. The gels were formed by the self-assembly of poly(styrene-b-methyl methacrylate-b-styrene) (SMS) and poly(styrene-b-ethylene oxide-b-styrene) (SOS) in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]). The end-blocks associate into cross-links, while the midblocks are well-solvated by this ionic liquid. In terms of viscoelastic properties, it was found that the plateau modulus of the gels depends primarily on concentration and the molecular weight of the mid-block, while high temperature behavior is controlled by the length of the end-blocks. A body-centered cubic (BCC) structure was observed at elevated temperatures only for gels with short end-blocks due to end-block pull-out from the cross-linking cores, while the relaxation of the end-blocks are within the cores for gels with long end-blocks. In terms of electrical properties, the double-layer capacitance of the gels was found to be fairly insensitive to polymer content and identity, whereas the ionic conductivity varies significantly especially at polymer concentrations of more than 20 wt%. It was also found that the presence of the end-blocks primarily obstructs the ion paths without much effect on ion number density. In terms of materials design, a flexible, low molecular weight mid-block is desirable. Generally, there is a trade-off between ionic conductivity and shear modulus for this type of gels. To enhance the mechanical properties of the gels, a novel ion gel based on poly[(styrene-r-vinylbenzyl azide)-b-ethylene oxide-b-(styrene-r-vinylbenzyl azide)] (SOS-N3) with chemically cross-linkable end-blocks was prepared. The gel with 10 wt% polymer goes through two transitions as temperature increases: solid (physically crosslinked network) --> liquid --> solid (chemically cross-linked network). The modulus and ionic conductivity was found to remain fairly constant after chemical cross-linking, while the toughness is more than 8 times higher. This demonstrates a promising approach to improve the mechanical properties of a moderately dilute gel without interfering with the high ionic conductivity. Overall, BCP-based ion gels are promising SPEs for transistor and capacitor applications. Through judicious selection of the triblocks, the properties of the gels can be tuned to fulfill different requirements.