APS -APS March Meeting 2017 - Event - Confinement and Interfacial Effects on the Dynamics of Polymer Nanocomposites and Ultra-Thin Films

摘要

Changes in the dynamics of confined polymer materials relative to thebulk polymer involve a complex convolution of effects, including theform and scale of confinement, as well as the interfacial interactionstrength and surface roughness, leading to a seemingly intractabledegree of complexity in describing these changes. In this talk, wereview the results of molecular dynamics simulations of confinementeffects on polymer dynamics in both ultra-thin films and nanocomposites.We show that all observed changes to the polymer dynamics can beunderstood in a unified way. In particular, we quantitatively describethe change in dynamics based on how the collective motion isperturbed. These changes are ultimately parameterized in terms of thehigh-temperature activation free energy, leading to the almostparadoxical finding that changes in glass formation are controlled bythe changes to activation barriers in the high temperature limit. Wealso consider the peculiar case of strongly interacting interfaces,where experiments often report small or no changes in the glasstransition temperature, $T_g$. We find that when interfacialinteractions exceed the polymer-polymer interactions, a distinctrelaxation that is slower than the main $alpha$-relaxation emerges,arising from an adsorbed ``bound'' polymer layer near the interface.This bound layer ``cloaks'' the interfacial interactions, so that thedynamics of the matrix polymer is largely unaffected. Consequently,$T_g$ defined from the temperature dependence of the routinely measured thermodynamics or the polymer matrix relaxation can nearly independentof interfacial interaction strength.