Mutual Information in Molecular and Macromolecular Systems

TL;DR

This paper demonstrates that mutual information analysis of molecular dynamics simulations offers new insights into dynamical heterogeneity and secondary relaxation processes in glass-forming liquids and polymers, revealing structural and mechanistic details.

Contribution

It introduces MI-based analysis as a novel tool to study relaxation and heterogeneity in molecular systems, providing clearer evidence of secondary relaxation and particle clustering behaviors.

Findings

MI reveals particle clustering with different mobility in molecular liquids.

MI provides sharper evidence of Johari--Goldstein secondary relaxation.

Both heterogeneity and relaxation maxima involve rotation-translation coupling.

Abstract

The relaxation properties of viscous liquids close to their glass transition (GT) have been widely characterised by the statistical tool of time correlation functions. However, the strong influence of ubiquitous non-linearities calls for new, alternative tools of analysis. In this respect, information theory-based observables and, more specifically, mutual information (MI) are gaining increasing interest. Here, we report on novel, deeper insight provided by MI-based analysis of molecular dynamics simulations of molecular and macromolecular glass-formers on two distinct aspects of transport and relaxation close to GT, namely dynamical heterogeneity (DH) and secondary Johari--Goldstein (JG) relaxation processes. In a model molecular liquid with significant DH, MI reveals two populations of particles organised in clusters having either filamentous or compact globular structures that exhibit different mobility and relaxation properties. In a model polymer melt, MI provides clearer evidence of JG secondary relaxation and sharper insight into its DH. It is found that both DH and MI between the orientation and the displacement of the bonds reach (local) maxima at the time scales of the primary and JG secondary relaxation. This suggests that, in (macro)molecular systems, the mechanistic explanation of both DH and relaxation must involve rotation/translation coupling.