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.
Highlights
The nature of the solidification process observed at the glass transition (GT) temperature Tg by cooling supercooled viscous liquids is a topic of intense research
Building on previous studies carried out by Molecular Dynamics (MD) simulations, the present paper reports novel insight provided by mutual information (MI)-based approaches on two distinct aspects of transport and relaxation close to GT, namely dynamical heterogeneity [23,24]
We argued that the role of the barrier is twofold [26]: (i) it is responsible for a partial averaging of the dynamical heterogeneity (DH) at the JG relaxation time scale, giving rise to a peculiar double-peaked time evolution of the nonGaussian parameter (NGP) with two local maxima located at the JG and primary relaxation time scales, respectively, and (ii) it slows down the bond reorientation and, jointly, the monomer translation, suggesting a roto-translation coupling, which, has been recently reported in glassforming systems with strong JG relaxation [60]
Summary
The nature of the solidification process observed at the glass transition (GT) temperature Tg by cooling supercooled viscous liquids is a topic of intense research. As outlined by Kubo in his seminal paper on adiabatic linear response theory, a formal treatment of the adiabatic nonlinear response in terms of a series expansion involving two-, three-, four-, and higher-order time correlation functions may be developed [10] These expansions are exceedingly difficult to translate into a useful, experimentally verifiable form and are nowadays believed to be not a viable approach for most transport processes and irreversible relaxation processes [11]. Unlike the Pearson correlation coefficient, the MI evaluation requires knowledge of the distributions of random variables If these are not known, their estimation is a delicate (and sometimes uncertain) procedure leading to higher computational cost [13,15]. Details about the model systems and numerical methods are found elsewhere [23,24,25,26]
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