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.

The origin of stretched exponential relaxation in supercooled glass-forming liquids is one of the central questions regarding the anomalous dynamics of these fluids. The dominant explanation for this phenomenon has long been the proposition that spatial averaging over a heterogeneous distribution of locally exponential relaxation processes leads to stretching. Here, we perform simulations of model polymeric and small-molecule glass-formers in the isoconfigurational ensemble to show that stretching instead emerges from a combination of spatial averaging and locally nonexponential relaxation. The results indicate that localities in the fluid exhibiting faster-than-average relaxation tend to exhibit locally stretched relaxation, whereas slower-than-average relaxing domains exhibit more compressed relaxation. We show that local stretching is predicted by loose local caging, as measured by the Debye–Waller factor, and vice versa. This phenomenology in the local relaxation of in-equilibrium glasses parallels the dynamics of out of equilibrium under-dense and over-dense glasses, which likewise exhibit an asymmetry in their degree of stretching vs compression. On the basis of these results, we hypothesize that local stretching and compression in equilibrium glass-forming liquids results from evolution of particle mobilities over a single local relaxation time, with slower particles tending toward acceleration and vice versa. In addition to providing new insight into the origins of stretched relaxation, these results have implications for the interpretation of stretching exponents as measured via metrologies such as dielectric spectroscopy: measured stretching exponents cannot universally be interpreted as a direct measure of the breadth of an underlying distribution of relaxation times.