Characteristics of the Johari-Goldstein process in rigid asymmetric molecules

TL;DR

This study uses molecular dynamics simulations to identify and characterize the Johari-Goldstein secondary relaxation process in rigid diatomic molecules, highlighting its properties and relation to structural relaxation in glassy materials.

Contribution

It demonstrates that the Johari-Goldstein process involves all parts of the molecule and exhibits specific temperature, frequency, and pressure dependencies, distinguishing it from trivial intramolecular motions.

Findings

The JG process merges with alpha relaxation at high temperatures.

The relaxation strength changes with vitrification.

The JG process is sensitive to volume, pressure, and aging.

Abstract

Molecular dynamics simulations were carried out on a Lennard-Jones binary mixture of rigid (fixed bond length) diatomic molecules. The translational and rotational correlation functions, and the corresponding susceptibilities, exhibit two relaxation processes, the slow structural relaxation (alpha dynamics) and a higher frequency secondary relaxation. The latter is a Johari-Goldstein (JG) process, by its definition of involving all parts of the molecule. It shows several properties characteristic of the JG process - (i) merging with the alpha relaxation at high temperature; (ii) a change in temperature-dependence of the relaxation strength on vitrification; (iii) a separation in frequency from the alpha relaxation that correlates with the breadth of the structural dispersion; and (iv) sensitivity to volume, pressure, and physical aging - that can be used to determine whether a secondary relaxation in a real material is an authentic JG process, rather than trivial motion involving intramolecular degrees of freedom. The latter has no connection to the glass transition, whereas the JG relaxation is closely related to structural relaxation, and thus can provide new insights into the phenomenon.