Cooperative motion is widely accepted as being important for dynamics of glass-forming liquids, yet the length scales over which cooperative motion occurs are poorly understood. Experiments are proposed and computer simulations to address cooperative motion in polymers. While cooperative motion is thought to be important for the dynamics of all liquids near their glass transition, polymers o.er some unique opportunities for probing cooperative motion. Monte Carlo and Molecular Dynamics simulations will be utilized to directly measure the distribution of sizes and shapes of cooperatively relaxed regions, with an emphasis on determining measures that can be applied to real experiments. Dielectric experiments measuring the distribution of segmental relaxation times in miscible polymer blends will be used to estimate the temperature dependence of the cooperative size, for each blend component. Experiments that systematically vary polymer structure (chain length and side groups) will also provide less direct measures of the temperature dependence of cooperative size in single-component polymer melts. Rheology experiments and Molecular Dynamics simulations will explore the connection between segmental dynamics and chain dynamics, in parallel studies. This connection will allow our models for segmental dynamics in miscible blends to be extended to predict the terminal dynamics, including the blend viscosity, with no additional parameters. The intellectual merit of this research will be an improved understanding of dynamics in polymers and their miscible blends, with particular emphasis on the role of cooperative motion and the connection between segmental and chain dynamics. The broader impacts of the proposed research are threefold. On the one hand, the model of miscible blend rheology will find immediate pragmatic use in the plastics industry, where polymer blends are vital for plastics recycling and for metals replacement in the transportation industry. The latter enables weight to be reduced without sacrificing strength, thereby saving fuel. Simultaneously, understanding of cooperative motion will have far-reaching consequences in developing an understanding of liquid state dynamics that goes far beyond polymers. And finally the proposed research will create learning opportunities for graduate and undergraduate students. In addition to the involvement of both undergraduate and graduate students in the experimental research, the PIs make extensive use of their growing knowledge of polymer physics in recruiting, advising and teaching at both the undergraduate and graduate levels.
