Self-diffusion and spatially heterogeneous dynamics in supercooled liquids near Tg

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Mark Ediger of the University of Wisconsin Madison is supported by the Experimental Physical Chemistry Program to carry out experiments that will measure fundamental aspects of translational motion in low molecular weight supercooled liquids. In experiments using thermally programmed desorption and forward recoil spectrometry, the self-diffusion coefficients of four supercooled liquids of varying fragility will be measured very near the glass transition temperature, a range in which there are currently very few experimental results. The systems selected for study are glycerol, salol, ortho-terphenyl, and decalin, ranging from one of the least fragile to one of the most fragile single component organic glass formers identified in the literature. Self-diffusion outcomes on these systems will be combined with the PI's recent data for tris-naphthylbenzene (TNB). For TNB, neutron reflectivity will be used to determine the distance over which molecules must move before translational motion becomes diffusive. The technique of small angle neutron scattering will be used to directly detect regions of heterogeneous dynamics and to determine their size. As well, neutron reflectivity will be used to determine the influence of nearby interfaces on dynamics near the glass transition temperature. All of these experiments will be performed using vapor deposition to prepare thin bilayers of normal and deuterated glass formers. Experimental outcomes will place strong constraints on models of molecular motion near the glass transition temperature, and will be used to test these models. The results will also increase understanding of the origin of spatial heterogeneities in dynamics.Understanding transport properties near the glass transition temperature is important for polymers and other technologically important materials that are disordered, or amorphous. Outcomes are expected to advance the ability to manipulate properties of polymers and metallic glasses, to provide fundamental insights into the collapse of small polymer structures prepared by lithography, and to be essential for the development of practical nanoscale structures and devices involving amorphous materials. As well, proteins, foams, and granular materials all exhibit glassy dynamics whose understanding will be enhanced by these studies on supercooled liquids.