Deeper penetration of surface effects on particle mobility than on hopping rate in glassy polymer films

Abstract

Free surfaces in glassy polymer films are known to induce surface mobile layers with enhanced dynamics. Using molecular dynamics simulations of a bead-spring model, we study a wide variety of layer-resolved structural and dynamical properties of polymer films equilibrated at a low temperature. Surface enhancement on thermally induced particle hopping rate is found to terminate abruptly only about 5 particle diameters from the free surface. In contrast, enhancement on the net motions of particles measured at longer time scales penetrates at least 2 particle diameters deeper. The diverse penetration depths show the existence of a peculiar sublayer, referred to as the innersurface layer, in which surface enhanced mobility is not caused by more frequent particle hops but instead by a reduced dynamic heterogeneity associated with diminished hopping anti-correlations. Confinement effects of the free surface thus provide a unique mechanism for varying the dynamic heterogeneity and hopping correlations while keeping the hopping rate constant. Our results highlight the importance of correlations among elementary motions to glassy slowdown and suggest that dynamic facilitation is mediated via perturbations to the correlations rather than the rate of elementary motions. arXiv:1810.00192v1 [cond-mat.soft]

Metastable Polymer Adsorption Dictates the Dynamical Gradients at Interfaces

“…The results in Figure also show that the interfacial dynamical length scale of PS well exceeds the static scale of the adsorption layer immobilized on top of the substrates, leading to a strong contrast between the long dynamic transferring distance and the limited thickness of the regions with perturbated density at the interface. Similar decoupling of the dynamic and static length scales has been found in ordinary molecular glasses , and simulated particle and short-chain systems; ,,− however, the range of the dynamical gradient in these systems is generally limited within 10 nm, much smaller than the range for polymers, which can exceed 100 nm (see Figure ). This means that the flexibility and connectivity of the long chain structure, or, namely, the conformational feature of adsorbed polymers (i.e., “trains”, “loops”, and “tails”), play a role in transferring the interfacial dynamics.…”

Configuration-tree theoretical calculation of the mean-squared displacement of particles in glass formers

“…[13,14], our calculation covers the full range of time scales, spanning both the time window before and that after the plateau and hence providing a complete picture of the glassy dynamics. The theoretical result is then benchmarked against the 'distinguishable particle lattice model' (DPLM) [15], which is a lattice glass model realizing a dynamic facilitation mechanism motivated by glassy film simulation observations [16]. Satisfactory agreement has been achieved -including the position of the MSD plateau -for various particle densities and over a wide range of temperatures by adjusting only two parameters of the model, which can further be fixed by the diffusion coefficient D of the particles resulting in no additional free parameter.…”

“…Going beyond related calculations in [13,14], our calculation covers the full range of time scales, spanning both the time window before and that after the plateau and hence providing a complete picture of the glassy dynamics. The theoretical result is then benchmarked against the 'distinguishable particle lattice model' (DPLM) [15], which is a lattice glass model realizing a dynamic facilitation mechanism motivated by glassy film simulation observations [16]. Satisfactory https://doi.org/10.1088/1742-5468/ab39d7 J. Stat.…”

Surface mobility gradient and emergent facilitation in glassy films

“…Besides, the observed dynamical gradients penetrate much deeper than the particle density gradients. 16,17 Impacts on confinement effects by polymer chain connectivity 18 and a possible non-equilibrium nature of film samples 19 are also interesting questions.…”