Surface Diffusion Is Controlled by Bulk Fragility across All Glass Types

Glass Transition of the Surface Monolayer of Polystyrene Films with Different Film Thicknesses and Supporting Surfaces

“…These changes can cause severe modifications to the local compressibility and molecular arrangements . Separately, there are tendencies for the chain ends, which possess more free volume than chain segments do, to segregate to the free surface and for the density of chain entanglement near the surface to be smaller than that in the bulk. ,− All of these factors can lead to enhancements in the near-surface mobility ,, and reductions in the T g of thin polymer films ( T g film ). − ,,, On the other hand, the surface chemistry ,,− and stiffness of the substrate supporting the films, and the near-substrate chain conformations , can affect the dynamics of the nearby polymer. Combinations of the free surface and substrate effects can produce a variety of thickness dependences of the thin film dynamics as found in experiments. ,,,, …”

“…27 Separately, there are tendencies for the chain ends, which possess more free volume than chain segments do, to segregate to the free surface 3 and for the density of chain entanglement near the surface to be smaller than that in the bulk. 3,28−31 All of these factors can lead to enhancements in the near-surface mobility 15,23,32 and reductions in the T g of thin polymer films (T g film ). [1][2][3]5,6,8 On the other hand, the surface chemistry 1,16,33−35 and stiffness 36 of the substrate supporting the films, and the near-substrate chain conformations 37,38 can affect the dynamics of the nearby polymer.…”

Combined Mixing and Dynamical Origins of Tg Alterations Near Polymer–Polymer Interfaces

“…All of this leaves open the question of the origin of the disparity between the larger T g gradient range reported by Baglay et al , (and also possibly implied by earlier fluorescence T g measurements in some high-molecular-weight films) and the shorter range observed here and in other simulations, reported by Christie et al , and Li et al in the experiment, and implied by the long history of block copolymer T g measurements . More recent work by the Roth group has added to this mystery, reporting on very long-ranged T g gradients near substrates at extremely low grafting densities .…”

“…Perhaps more profoundly, the type of roughly additive range enhancement between composition and dynamical gradients discussed above seemingly cannot account for the extraordinarily long-ranged gradients (>100 nm) reported by Baglay et al , However, our findings are qualitatively consistent with a number of other experimental findings. They are consistent with spatially resolved T g gradients probed in block copolymers, in terms of both the approximate range of the gradient and the asymmetry of the T g gradient. , In the high-χ limit, they are consistent with recent measurements of free surface diffusion in small molecules in terms of the range of the dynamical gradient (several nanometers exponential decay range) and the implied functional form of the gradients (exponential T g gradients here, double exponential diffusion rate gradients there). , This suggests that the difference between our findings and the behavior observed by Baglay et al is not simply a matter of the differing time scales typically accessed by experiments vs simulations.…”

Compositional trends in surface enhanced diffusion in lead silicate glasses

“…In this section, we discuss how the predicted bulk and surface diffusion from this work can be extrapolated from MD cooling rates to experimentally relevant cooling rates, and how the extrapolated values are related to the earlier observed experimental trends of diffusion with fragility. [9,17,28] In order to compare our results to those analyzed from experiments, it is necessary to estimate D values for a glassy state similar to those measured in experiments, which are far more relaxed than those obtained from our MD quench process. This estimation can be done by extrapolation using the approach developed by Chen et al in Ref.…”