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

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.

💡 Research Summary

This study uses large‑scale molecular dynamics simulations to dissect how depth‑dependent dynamical heterogeneity influences chain‑scale motion in freestanding glass‑forming polymer films. By varying temperature around the glass transition, chain length, and film thickness, the authors obtain spatially resolved segmental relaxation times τ_α(z) and chain‑center‑of‑mass mean‑square displacements ⟨Δr²(t;z)⟩. From these they extract the Rouse‑regime scaling exponent γ(z) defined by ⟨Δr²⟩∝t^γ. Near the free surface (within roughly 5 nm) segmental mobility is dramatically accelerated, leading to a pronounced reduction of γ to values as low as 0.4. This suppression is not caused by topological entanglement; instead the authors attribute it to “transient localization” induced by the steep interfacial mobility gradient, whereby fast surface segments couple to slower underlying layers and temporarily constrain the entire chain.

In contrast, the film interior exhibits a modest but systematic increase of γ above the classical Rouse value of 0.5, reaching 0.6–0.7. This enhancement matches predictions of the Heterogeneous Rouse Model (HRM), which posits that bulk‑like spatial variations in segmental relaxation compress the Rouse regime when different sub‑domains relax on distinct time scales. Thus bulk dynamic heterogeneity effectively “stretches” the chain’s motion, opposite to the surface‑induced suppression.

Mapping the minimum exponent γ_min(z) across the film reveals an almost linear gradient that cleanly separates a surface‑dominated suppression zone from an interior‑dominated enhancement zone. The position of the crossover shifts slightly inward with increasing temperature and outward with longer chains, providing a quantitative marker for the depth at which interfacial effects give way to bulk‑like heterogeneity. Because γ(z) can be measured experimentally through surface‑sensitive techniques such as fluorescence recovery after photobleaching, optical tweezers, or X‑ray reflectivity, the authors propose γ as a highly sensitive, spatially resolved probe of both interfacial mobility gradients and bulk dynamic heterogeneity in polymer thin films.

Overall, the paper demonstrates that (1) accelerated surface dynamics lower the Rouse scaling exponent via transient localization, (2) bulk heterogeneity raises the exponent by compressing the Rouse regime, and (3) the depth‑dependent γ profile offers a practical diagnostic for designing polymer coatings, nanocomposites, and electronic devices where surface and interior dynamics must be simultaneously controlled.