Molecular dynamics simulations of the Johari-Goldstein relaxation in a molecular liquid

TL;DR

This study uses molecular dynamics simulations to explore the Johari-Goldstein relaxation in a molecular liquid, revealing its characteristics in liquid and glassy states and its relation to structural relaxation.

Contribution

It provides detailed insights into the JG relaxation process, including its independence from local density and its evolution into alpha-relaxation in liquids.

Findings

JG relaxation involves large-angle jumps in glassy states.

The JG relaxation time is independent of local density.

In liquids, JG motion evolves into structural relaxation.

Abstract

Molecular dynamics simulations (mds) were carried out to investigate the reorientational motion of a rigid (fixed bond length), asymmetric diatomic molecule in the liquid and glassy states. In the latter the molecule reorients via large-angle jumps, which we identify with the Johari-Goldstein (JG) dynamics. This relaxation process has a broad distribution of relaxation times, and at least deeply in the glass state, the mobility of a given molecule remains fixed over time; that is, there is no dynamic exchange among molecules. Interestingly, the JG relaxation time for a molecule does not depend on the local density, although the non-ergodicity factor is weakly correlated with the packing efficiency of neighboring molecules. In the liquid state the frequency of the JG process increases significantly, eventually subsuming the slower alpha-relaxation. This evolution of the JG-motion into structural relaxation underlies the correlation of many properties of the JG- and alpha-dynamics.