Isomorph Invariant Dynamic Mechanical Analysis: A Molecular Dynamics Study

TL;DR

This study uses molecular dynamics simulations to validate isomorph theory in dynamic mechanical analysis of glass-forming systems, showing that scaled loss moduli collapse onto a single curve across different densities and temperatures.

Contribution

It demonstrates the applicability of isomorph theory to dynamic mechanical properties of glasses, introducing methods to identify isomorphic temperatures and analyzing force decorrelation effects.

Findings

Loss moduli collapse when scaled by isomorph theory

Two methods for determining isomorphic temperatures are compared

Isomorph invariance holds for a wide range of densities and temperatures

Abstract

We simulate dynamic mechanical analysis experiments for the Kob-Andersen binary Lennard-Jones system. For this, the SLLOD algorithm with time-dependent strain rates is applied to give a sinusoidally varying strain at different densities and temperatures. The starting point is a temperature scan at a fixed reference density. Isomorph theory predicts that for other densities corresponding temperatures can be identified at which the mechanical properties are unchanged when scaled appropriately. We determine the isomorphically equivalent temperatures by analysing how particle forces change upon scaling configurations to the new density. Loss moduli expressed in suitable reduced units are compared for isomorphic state points. When plotted against the unscaled temperatures, these reduced loss curves are observed to collapse indicating the validity of isomorph theory for dynamic mechanical analysis experiments. Two different methods to determine isomorphic temperatures are considered. While one of them breaks down for the largest density rescalings considered in this study, the other one is still applicable in this region. The decorrelation of force vectors upon rescaling is investigated as a possible origin of this effect. Our results demonstrate that the simplification of the phase diagram entailed by isomorph theory for a wide class of system is relevant also for the mechanical properties of glasses.