Molecular, polymeric, colloidal, and other classes of liquids can exhibit very large, spatially heterogeneous alterations of their dynamics and glass transition temperature when confined to nanoscale domains. Considerable progress has been made in understanding the related problem of near-interface relaxation and diffusion in thick films. However, the origin of “nanoconfinement effects” on the glassy dynamics of thin films, where gradients from different interfaces interact and genuine collective finite size effects may emerge, remains a longstanding open question. Here, we combine molecular dynamics simulations, probing 5 decades of relaxation, and the Elastically Cooperative Nonlinear Langevin Equation (ECNLE) theory, addressing 14 decades in timescale, to establish a microscopic and mechanistic understanding of the key features of altered dynamics in freestanding films spanning the full range from ultrathin to thick films. Simulations and theory are in qualitative and near-quantitative agreement without use of any adjustable parameters. For films of intermediate thickness, the dynamical behavior is well predicted to leading order using a simple linear superposition of thick-film exponential barrier gradients, including a remarkable suppression and flattening of various dynamical gradients in thin films. However, in sufficiently thin films the superposition approximation breaks down due to the emergence of genuine finite size confinement effects. ECNLE theory extended to treat thin films captures the phenomenology found in simulation, without invocation of any critical-like phenomena, on the basis of interface-nucleated gradients of local caging constraints, combined with interfacial and finite size-induced alterations of the collective elastic component of the structural relaxation process.

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Polymers in the nanoscale vicinity of interfaces exhibit a broad range of alterations in their dynamics and glass-formation behavior. A major goal in the study of these effects is to understand their strong apparent dependence on chemistry, measurement time scale, and metrology. Here we employ molecular dynamics simulations of thin freestanding polymer films over a range of thicknesses and polymer backbone stiffnesses to probe these dependences. Results suggest that a chemistry-and metrology-dependent onset of strong nanoconfinement may play an important role in chemical and metrological variations in the apparent strength of nanoconfinement effects. Beyond this onset, we find that the activation barrier for relaxation is subject to a simple temperature-insensitive rescaling near a surface at low temperatures, leading to a fractional power law decoupling relationship between thin film and bulk dynamics. We show that a generic two-barrier model of the glass transition can parsimoniously describe much of this phenomenology, with the "onset" of strong interface effects on dynamics related to a crossover in dominance from a high-temperature barrier to a lowtemperature barrier. We suggest that variation of this onset time scale and temperature may play an important role in system-tosystem and measurement-to-measurement variations in the observed strength of interfacial effects on dynamics and glass formation. These results may also explain why simulations at relatively short time scales commonly report effects of a magnitude comparable to experiments at much larger time scales.

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The extent to which altered glass formation behavior in block copolymers and layered polymers results from local compositional mixing vs from longer-ranged dynamical correlations has long been unresolved. Here, we perform molecular dynamics simulations at model polymer–polymer interfaces to understand the relative roles of these mechanisms. Results indicate a crossover from the high-χ regime, where near-interface T g alterations are driven purely by dynamical gradient effects, to the low-χ regime, where both local mixing and dynamical correlations play a role. Observed T g gradient ranges are asymmetric, exceed the range of composition gradients, and modestly grow with decreasing χ. These results provide new insight into the design of nanostructured polymers with targeted local dynamics. They also emphasize an apparent dichotomy between many studies, including ours, pointing toward T g gradient ranges on the order of 10 nm, and a second set of studies reporting on longer ranges up to hundreds of nanometers.