Repetition and pair-interaction of string-like hopping motions in glassy polymers

TL;DR

This study uses molecular dynamics simulations to analyze string-like hopping motions in glassy polymers, revealing that these motions become repetitive and reversible at low temperatures, with structural relaxation driven by breaking these repetitions through pair interactions.

Contribution

It uncovers the high reversibility of string-like motions in glassy polymers and links structural relaxation to pair-interactions disrupting these repetitive motions.

Findings

String-like motions become highly reversible at low temperatures.

Particle hops are reversed with over 73% probability at low temperature.

Structural relaxation is governed by breaking of hopping repetitions.

Abstract

The dynamics of many glassy systems are known to exhibit string-like hopping motions each consisting of a line of particles displacing one and other. By using molecular dynamics simulations of glassy polymers, we show that these motions become highly repetitive back-and-forth motions as temperature decreases and do not necessarily contribute to net displacements. Particle hops which constitute string-like motions are reversed with a high probability, reaching 73% and beyond at low temperature. Structural relaxation rate is then dictated not by a simple particle hopping rate but instead by the rate at which particles break away from hopping repetitions. We propose that disruption of string repetitions and hence also structural relaxations are brought about by pair-interactions between strings.