Depth-Resolved Chain Dynamics and Interfacial Effects in Freestanding and Supported Glass-Forming Polymer Films

Polymer dynamics near interfaces exhibit profound deviations from bulk behavior, driven by confinement and interfacial mobility gradients. Using molecular dynamics simulations, we investigate glass-forming polymer films in both freestanding and supported configurations to understand how these gradients modulate rheological properties and cooperative motion across multiple length and time scales. We focus on how spatial variations in glass transition temperature (Tg) and segmental relaxation time influence chain-level dynamics. In freestanding films, enhanced surface mobility leads to suppressed chain relative to segmental mobility, reflected by sub-Rouse scaling behavior. This suppression arises from transient localization effects driven by interfacial mobility gradients, distinct from entanglement physics. In contrast, the film interior exhibits elevated scaling exponents consistent with predictions of the Heterogeneous Rouse Model (HRM), where dynamic heterogeneity enhances chain mobility. We extend this framework to understand supported films, where substrate interactions reshape the interplay Tg gradients and chain connectivity, revealing a depth-dependent crossover from surface-dominated to substrate-dominated relaxation. Together, these results establish a unified picture of how bulk and interfacial dynamic heterogeneities jointly alter polymer dynamics, and they provide new insights for designing thin-film, nanocomposite, and surface-engineered materials with tailored dynamic and mechanical properties.