Local-Average Free Volume Correlates with Dynamics in Glass Formers

Abstract

Glass formers exhibit a pronounced slowdown in dynamics, accompanied by progressive heterogeneity as they approach the glass transition. There is intense debate over whether the dramatic slowdown is caused by dynamical heterogeneity and whether the enhanced dynamical heterogeneity originates from structural causes. However, the connection between dynamical heterogeneity and the spatial distribution of the single-particle free volume (a purely static structural quantity) was found to be rather weak, which raises the question of whether dynamic heterogeneity has a purely structural origin. Here, by introducing the concept of local-average free volume, we present numerical evidence that long-time dynamic heterogeneity shows significantly enhanced correlation with the average local free volume over a length scale of a few neighboring shells. Our results resolve the long-standing controversy about whether free volume plays an important role in particle rearrangements associated with the activated hopping relaxation. The concept of “local average” can be applied to other local structural descriptors to better correlate with dynamic heterogeneity in glass-forming liquids.

Theory of the Effects of Specific Attractions and Chain Connectivity on the Activated Dynamics and Selective Transport of Penetrants in Polymer Melts

“…Therefore, we deduce that the value of Δ r p /Δ r X relative to unity is directly related to the nonmonotonic behavior predicted for Δ r m,c , β F el,p , and τ α,p /τ 0 as a function of βε mp . As an important reminder, recall that Δ r X is a structural quantity while Δ r p is a dynamical property, so the above results further reinforce the core thesis of the ECNLE , and SCCH , theories that structure determines dynamics, consistent with recent experimental, − machine learning, and simulation − studies.…”

Self-Consistent Theory for Structural Relaxation, Dynamic Bond Exchange Times, and the Glass Transition in Polymeric Vitrimers

“…Given that factors such as bond exchange, different relaxation rates of normal Kuhn segment and cross-linkers, heterogeneous distribution of cross-linkers, etc. can introduce significant dynamic heterogeneity , on the alpha relaxation dynamics, an extension of SC-ECNLE theory to include dynamic heterogeneity effects is of high interest. Significant progress in this direction was recently made in the ECNLE theory framework for homopolymers melts .…”

Penetrant shape effects on activated dynamics and selectivity in polymer melts and networks based on self-consistent cooperative hopping theory

“…Another topic is the role of external stress, 41 which decreases the elastic effects significantly while the local cage barrier is affected much less. Other possible directions include how to control the selectivity of nonspherical penetrants in dynamic bond forming vitrimers and associating polymer matrices, [59][60][61][62] and whether dynamic heterogeneity [63][64][65][66][67] effects can modify penetrant shape effects on diffusion selectivity. Work is underway in these directions.…”