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

Abstract

Molecular dynamics simulations 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 glassy 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 nonergodicity factor is weakly correlated with the packing efficiency of neighboring molecules. In the liquid state the intensity of the JG process increases significantly, eventually subsuming the slower α relaxation. This evolution of the JG motion into structural relaxation underlies the correlation of many properties of the JG and α dynamics.