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Nanoscale confinement alters thedynamics of glass-forming liquidsin films of order 100 nm in thickness. A common hypothesis for theorigin of this long range posits that interfacial dynamics propagateinto the film via cooperatively rearranging regions (CRRs). However,the precise nature of the dynamic interface remains uncertain, andits identification with CRRs has yet to be firmly established. Basedon results from coarse-grained molecular dynamics simulations of afreestanding polymer film, here we show that an apparent qualitativediscrepancy between computational and several experimental measuresof the interfacial dynamic length scale results from a differencein definition and not from a difference in underlying behavior ofexperimental and simulated systems. We then show that the computationaldefinition exhibits a direct correspondence with the expected behaviorof CRRs as predicted by the Adam–Gibbs theory, and we suggestthat it should be possible to extract this length scale from experimentalmeasurements. [ABSTRACT FROM AUTHOR]

We employ molecular dynamics simulations of nanolayered polymers to systematically quantify the dependence of T g nanoconfinement effects on interfacial energy and the "softness" of confinement. Results indicate that nanoconfined T g depends linearly on interfacial adhesion energy, with a slope that scales exponentially with the ratio of the bulk Debye-Waller factors ⟨ u 2 ⟩ of the confined and confining materials. These trends, together with a convergence at low interfacial adhesion energy to the T g of an equivalent freestanding film, are captured in a single functional form, with only three parameters explicitly referring to the confined state. The observed dependence on ⟨ u 2 ⟩ indicates that softness of nanoconfinement should be defined in terms of the relative high frequency shear moduli, rather than low frequency moduli or relaxation times, of the confined and confining materials.