# Collective Motion in the Interfacial and Interior Regions of Supported Polymer Films and its Relation to Relaxation

## TL;DR

This study uses molecular dynamics simulations to compare collective motion in the interfacial and interior regions of supported polymer films, revealing similar extents of collective motion in both regions and linking it to configurational entropy.

## Contribution

It challenges the assumption that interfacial regions have significantly different collective motion, showing similar behavior throughout the film and connecting it to the Adam-Gibbs theory.

## Key findings

- Collective motion magnitude is similar in interfacial and interior regions.
- Both regions differ from bulk material in collective motion.
- Extent of collective motion relates to the film's configurational entropy.

## Abstract

To understand the role of collective motion in the often large changes in interfacial molecular mobility observed in polymer films, we investigate the extent of collective motion in the interfacial regions of a thin supported polymer film and within the film interior by molecular dynamics simulation. Contrary to commonly stated expectations, we find that the extent of collective motion, as quantified by string-like molecular exchange motion, is similar in magnitude in polymer-air interfacial layer to that in the film interior, and both are generally distinct from the bulk material. This finding is consistent with Adam-Gibbs description of the segmental dynamics within mesoscopic film regions where the extent of collective motion is related to the configurational entropy of the film as whole rather than a locally defined extent of collective motion or configurational entropy.