Depth-dependent interplay of dynamical heterogeneity and chain dynamics at the surface of glass-forming polymers

TL;DR

This study uses molecular dynamics simulations to explore how depth-dependent mobility gradients at the surface of glass-forming polymers influence chain dynamics, revealing contrasting effects near the surface and in the interior.

Contribution

It provides a detailed, spatially resolved understanding of how interfacial mobility gradients affect chain relaxation and heterogeneity in polymer films, supported by the Heterogeneous Rouse Model.

Findings

Surface acceleration suppresses Rouse scaling exponents to as low as 0.4.

Bulk heterogeneity enhances Rouse scaling exponents, consistent with the HRM.

A linear gradient of the minimum scaling exponent separates surface and bulk regimes.

Abstract

Polymer thin films exhibit pronounced interfacial mobility gradients that modify chain relaxation, yet how these gradients govern chain-scale dynamics across depth remains incompletely understood. Using molecular dynamics simulations of freestanding glass-forming polymer films, we resolve how depth-dependent variations in segmental relaxation shape chain dynamics across a wide range of displacement scales. Near the free surface, accelerated segmental mobility suppresses Rouse-regime scaling exponents to values as low as gamma = 0.4, reflecting transient localization induced by interfacial mobility gradients rather than topological entanglement. In contrast, the film interior exhibits enhanced Rouse scaling exponents consistent with predictions of the Heterogeneous Rouse Model (HRM), indicating that bulk dynamic heterogeneity compresses the Rouse regime. Mapping the minimum scaling exponent gamma_min across depth reveals a linear gradient that separates the bulk-like enhancement regime from the surface-induced suppression regime of chain dynamical scaling. Together, these results demonstrate that bulk and interfacial dynamic heterogeneity modify chain relaxation in opposite ways and establish Rouse scaling as a sensitive, spatially resolved probe of glassy dynamical heterogeneity and interfacial dynamical gradients in polymers.