Collaborative: Investigation of the Consequences of Cooperative Motion in Polymers and their Miscible Blends

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Cooperative motion is widely accepted as being important for dynamics of glass-formingliquids, 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 liquidsnear their glass transition, polymers o.er some unique opportunities for probing cooperativemotion. Monte Carlo and Molecular Dynamics simulations will be utilized to directly measurethe distribution of sizes and shapes of cooperatively relaxed regions, with an emphasison determining measures that can be applied to real experiments. Dielectric experimentsmeasuring the distribution of segmental relaxation times in miscible polymer blends willbe used to estimate the temperature dependence of the cooperative size, for each blendcomponent. Experiments that systematically vary polymer structure (chain length and sidegroups) will also provide less direct measures of the temperature dependence of cooperativesize in single-component polymer melts. Rheology experiments and Molecular Dynamicssimulations will explore the connection between segmental dynamics and chain dynamics, inparallel studies. This connection will allow our models for segmental dynamics in miscibleblends to be extended to predict the terminal dynamics, including the blend viscosity, withno additional parameters. The intellectual merit of this research will be an improved understandingof dynamics in polymers and their miscible blends, with particular emphasis onthe 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 modelof miscible blend rheology will find immediate pragmatic use in the plastics industry, wherepolymer blends are vital for plastics recycling and for metals replacement in the transportationindustry. The latter enables weight to be reduced without sacrificing strength, therebysaving fuel. Simultaneously, understanding of cooperative motion will have far-reachingconsequences in developing an understanding of liquid state dynamics that goes far beyondpolymers. And finally the proposed research will create learning opportunities for graduateand undergraduate students. In addition to the involvement of both undergraduate andgraduate students in the experimental research, the PIs make extensive use of their growingknowledge of polymer physics in recruiting, advising and teaching at both the undergraduateand graduate levels.