Spin-Polarized Local Probes of Soft Matter and Free Radical Chemistry

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My research involves using spin-polarized, local probes (muons and 8Li+) and the corresponding magnetic resonance techniques (muon spin resonance (muSR) and beta-detected NMR (beta-NMR), respectively) to study dynamics in soft matter and the structure and dynamics of free radicals. Implanted radioisotopes have broad applications in chemistry and materials science as a low-density, very-high-signal-to-noise in situ detectors of local environments and dynamics. muSR and beta-NMR offer as much as 10-orders-of-magnitude improvement in sensitivity over conventional magnetic resonance techniques through the combination of high polarization and beta-decay anisotropy. The sensitivity of these techniques allows me to study systems not amenable to more conventional research tools. I will use implanted 8Li+ and beta-NMR to study depth-dependent properties in polymers. I will determine (1) the length-scale of enhanced dynamics near the free surface in poly(methyl methacrylate); (2) the length-scale of entropy-driven segregation in thin films made up of mixtures of low and high molecular-weight polystyrene (PS) and (3) how the dynamics of phenyl rings near a buried interface in PS change during the process of making a welded joint. I will also use beta-NMR to study the microscopic process of Li+ diffusion in thin films of the polymer electrolyte poly(ethylene oxide) (PEO). I will determine how the 8Li+ hopping parameters depend on the concentration of the salt lithium bis(trifluoromethane)sulfonimide (LiTFSI) and on the film thickness. Implanting muons into materials can produce muoniated radicals, which I use as 100% spin-polarized labels in soft matter systems. I propose to use muSR to study the dynamics and local environment of cholesterol within model bilayers composed of phospholipids such as 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol and the dynamics of the acyl chains in bilayers composed of DPPC and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-choline (POPC), which contains a C=C bond in one chain. In both systems I will determine how the dynamics of the spin probe varies with composition of the bilayer and temperature. Antioxidants are molecules that react with free radicals, which can cause cellular damage. An important question is what parts of a molecule are responsible for the antioxidant behaviour. I will determine where muonium (Mu), a light isotope of hydrogen, reacts with an antioxidant by determining the structures of the resulting radicals. I will study the reaction of Mu with epigallocatechin gallate (EGCG), an antioxidant that is found in high concentrations in green tea, and determine how structural modifications of EGCG, such as altering the stereochemistry, affect the antioxidant properties.