(680a) Molecular Simulations of the Influence of Interfaces on the Dynamics of Polymers

After multiple decades of work, it is widely accepted that interfaces in a polymer severely modify both the dynamics and the thermodynamics of polymers and other glass-forming materials. This is a problem not only in the design of thin films, where the material is confined in one dimension, but also in polymer nanocomposites, which can have a complicated confining geometry in multiple dimensions. Despite extensive work in the area, there are many outstanding issues that remain poorly understood. In this talk I will summarize our work where we attempt to understand the dynamics of polymers in a variety of confining geometries. Under simple planar confinement, we find dramatic changes in the dynamics associated with the glass transition, and we find a slight reduction in the entanglement density as the films become thinner than the characteristic size of the polymers. These results are compared to the dynamics of polymers within polymer nanocomposites where the nanoparticle loading approaches random close packing. In such a composite, interfaces are ubiquitous within the material and dominate the behavior relative to the bulk dynamics of the polymer. In these materials, we provide a molecular view of the spatial variations in the polymer dynamics relative to the bulk material. We expect the insights provided from our simulations to impact a variety of membrane and thin film technologies.