We perform molecular dynamics simulations of a model bead-spring polymer with and without the addition of molecular diluents. Simulations employ an attractive bead-spring model polymer, extended from the earlier model of Kremer and Grest 1,2 , that has been extensively employed to study polymer glass formation behavior [3][4][5][6][7][8] . Within this model, non-bonded interactions are given by a 12-6 Lennard Jones potential:12 6

When geometrically confined to the nanometer length scale, a condition in which a large portion of the material is in the nanoscale vicinity of interfaces, polymers can show astonishing changes in physical properties. In this investigation, we employ a unique noncontact capillary nanoshearing method to directly probe nanoresolved gradients in the rheological response of ultrathin polymer films as a function of temperature and stress. Results show that ultrathin polymer films, in response to an applied shear stress, exhibit a gradient in molecular mobility and viscosity that originates at the interfaces. We demonstrate, via molecular dynamics simulations, that these gradients in molecular mobility reflect gradients in the average segmental relaxation time and the glass-transition temperature.

High-throughput simulations reveal a universal onset of particle localization in diverse glass-forming liquids.

It has been known for 50 years that polymers exhibit chain normal mode decoupling upon approach to the glass transition, with chain dynamics exhibiting a weaker temperature dependence than segmental dynamics. Inspired by Sokolov and Schweizer’s suggestion that this thermorheological complexity is a consequence of dynamic heterogeneity in the supercooled state, we generalize the Rouse model to account for a distribution of segmental mobilities. The heterogeneous Rouse model (HRM) predicts chain translational normal mode decoupling as a manifestation of diffusion/relaxation decoupling (Stokes–Einstein breakdown)a consequence of differences in how normal modes average over a distribution of segmental mobilities. Molecular dynamics simulations agree with theoretical predictions, with the HRM found to quantitatively predict deviations from Rouse scaling of the translational friction coefficient based on the observed degree of Stokes–Einstein breakdown.

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.