Participation in the Johari-Goldstein Process - Molecular Liquids versus Polymers

TL;DR

This study uses molecular dynamics simulations to compare Johari-Goldstein relaxation in molecular liquids and polymers, revealing limited participation in molecular glasses and uniform participation in polymers.

Contribution

It demonstrates the contrasting participation in JG relaxation between molecular liquids and polymers, highlighting the 'islands of mobility' in molecular glasses.

Findings

Limited JG participation in molecular glasses at low temperatures

Universal JG participation in polymers when segments are mobile

Validation of 'islands of mobility' concept in molecular glasses

Abstract

We show using molecular dynamics simulations that simple diatomic molecules in the glassy state exhibit only limited participation in the Johari-Goldstein (JG) relaxation process. That is, with sufficient cooling local reorientations are essentially frozen for some molecules, while others continue to change their orientation significantly. Thus, the "islands of mobility" concept is valid for these molecular glass-formers; only near the glass transition temperature does every molecule undergo the JG process. In contrast, for a linear polymer this dichotomy in the distribution of JG relaxation strengths is absent - if any segments are changing their local orientation, all segments are.