Local Polymer Dynamics in Melts and Glasses

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9732483 Ediger Many important properties of synthetic polymers are controlled by the local segmental dynamics of the polymers. These include the glass transition temperature(Tg), the temperature dependence of viscous flow, the diffusion coefficients of small molecules, and the rates of physical ageing. Optical experiments and NMR spectroscopy will be used to investigate the relationship between local segmental dynamics and these important properties. One project will explore the molecular basis for physical ageing in polystyrene and polycarbonate. The goal is to understand why different observables age differently and how ageing times below Tg are related to equilibrium relaxation times above Tg. The prediction of long term properties of polymer glasses depends upon the answers to these questions. A second project explores spatially heterogeneous dynamics in polymer melts and their influence on the diffusion coefficients of low molecular weight species. It is important to understand why these heterogeneous regions are more important in some melts than others so that technologically important diffusion coefficients can be accurately predicted. The lifetimes of these regions will be explored because this determines how larger length scale motions will be influenced by these regions. A third project uses variable-field NMR spectroscopy to study polyethylene and atactic polypropylene melts. These two simple systems are ideal for exploring the relationship between local dynamics and viscous flow, and for understanding the polymer motions which are responsible for the glass transition. It is expected that these results will contribute to an understanding of the industially important flow properties of branched polyethylene. A fourth project uses molecular dynamics trajectories to test the interpretation of 2D NMR and neutron spin echo experiments. This investigation may result in more reliable microscopic inferences being drawn from these experimental ob servables. %%% The utility of polymers in our society is based upon the wide range of properties which can be obtained by changing the chemical identity of the repeat units, e.g., some structures produce rubbery materials while others produce hard plastics. This proposal investigates how some important properties are controlled by chemical structure. This fundamental understanding will be applied to some problems of industrial interest in the proposed work and will suggest strategies for solving a broader range of practical problems. ***