Mutual Information in Molecular and Macromolecular Systems

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.

Automated summary

Research indicates that analysis based on mutual information provides a deeper insight into the transport and relaxation properties of molecular and macromolecular glass formers. Near the glass transition, mutual information reveals different mobility and relaxation properties of particle clusters with filamentous or compact globular structures, especially in the context of dynamical heterogeneity and secondary Johari-Goldstein relaxation processes. Both dynamical heterogeneity and mutual information between orientation and bond displacement reach local maxima at the time scales of primary and JG secondary relaxation, indicating the involvement of rotation/translation coupling in the mechanistic explanation of both phenomena in (macro)molecular systems.