Despite decades of research on the effects of nanoconfinement on the glass transition temperature T, apparent discrepancies between pseudothermodynamic and dynamic measurements of these effects have raised questions regarding the presence of long-ranged interfacial dynamic gradients in glass-forming liquids. Here we show that these differences can be accounted for based on disparities in these methods' weightings over local T's within an interfacial gradient. This finding suggests that a majority of experimental data are consistent with a broad interfacial dynamic interphase in glass-forming liquids.

Nanoscale confinement has been shown to alter the glass transition and associated mechanical and transport properties of glass-forming materials. Inspired by expected interrelations between nanoconfinement effects, cooperative dynamics in supercooled liquids, and the "fragility" (or temperature-abruptness) of the glass transition, it is commonly expected that nanoconfinement effects on T should be more pronounced for more fragile glass formers. Here we employ molecular dynamics simulations of glass formation in the bulk and under nanoconfinement of model polymers in which we systematically tune fragility by several routes. Results indicate that a correlation between fragility and the strength of nanoconfinement effects is weak to modest at best when considering all systems but can appear to be stronger when considering a subset of systems. This outcome is consistent with a reanalysis of the Adam-Gibbs theory of glass formation indicating that fragility does not necessarily track in a universal way with the scale of cooperative motion in glass-forming liquids. Finally, we find that factors such as composition gradients or variability in measurement sensitivity to different parts of the dynamic gradient have the potential to significantly confound efforts to identify trends in T-nanoconfinement effects with variables such as fragility, emphasizing the importance of employing diverse data sets and multiple metrologies in the study of this problem.

Linear bead‐spring chain models have been widely employed to study the glass‐formation of polymer melts due to their apparent resistance to homogenous crystallization. In this article, we present simulation data demonstrating that, in contrast to their bulk behavior, widely used bead‐spring models are subject to rapid heterogenous nucleation and crystallization when exposed to rigid crystalline walls. We propose a modified bead‐spring model, employing the finitely extensible nonlinear elastic bond, and show that it exhibits robust glass‐formation in the presence of crystalline walls. This new model will enable study of supercooled polymers in the presence of rigid, atomistically structured walls, with application to study of nanoscale confinement effects on the glass transition. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2013, 52, 134–140