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

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.