Energy barriers and cooperative motion at the surface of freestanding glassy polystyrene films

TL;DR

This study uses molecular dynamics simulations to explore how surface relaxation, energy barriers, and cooperative motion vary with depth in freestanding glassy polystyrene films, revealing depth-dependent activation barriers and reduced heterogeneity at the surface.

Contribution

It provides new insights into the depth-dependent relaxation dynamics and cooperative behavior at the surface of glassy polystyrene films through detailed simulation analysis.

Findings

Relaxation times follow a power law relation with bulk times.

Activation barriers vary with depth from the surface.

Dynamical heterogeneity decreases at the interface.

Abstract

We investigate the near-surface relaxation of freestanding atactic \glsdesc{ps} films with molecular dynamics simulations. As in previous coarse-grained simulations, relaxation times for backbone segments and phenyl rings are linked to their bulk relaxation times via a power law coupling relation. Variation of the coupling exponent with distance from the surface is consistent with depth-dependent activation barriers. We also quantify a reduction of dynamical heterogeneity at the interface which can be interpreted in the framework of cooperative models for glassy dynamics.