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.

Nanoscale confinement alters the dynamics of glass-forming liquids in films of order 100 nm in thickness. A common hypothesis for the origin of this long range posits that interfacial dynamics propagate into the film via cooperatively rearranging regions (CRRs). However, the precise nature of the dynamic interface remains uncertain, and its identification with CRRs has yet to be firmly established. Based on results from coarse-grained molecular dynamics simulations of a freestanding polymer film, here we show that an apparent qualitative discrepancy between computational and several experimental measures of the interfacial dynamic length scale results from a difference in definition and not from a difference in underlying behavior of experimental and simulated systems. We then show that the computational definition exhibits a direct correspondence with the expected behavior of CRRs as predicted by the Adam–Gibbs theory, and we suggest that it should be possible to extract this length scale from experimental measurements.

Evidence suggests that the fragility (m) of glass formation both underpins and is sensitive to nanoconfinement effects on the glass transition. Here we present data indicating that nanoconfinement-induced changes in m of freestanding films emerge from a dominance of finite-size-driven fragility suppression over interfacial fragility enhancement.