Shear modulus and shear-stress fluctuations in polymer glasses

TL;DR

This study uses molecular dynamics simulations to analyze how the shear modulus and its fluctuations in polymer glasses vary with temperature and sampling time, revealing a peak in fluctuations at the glass transition.

Contribution

It introduces a detailed analysis of shear modulus fluctuations and their relation to stress relaxation in polymer glasses across different temperatures and timescales.

Findings

The average shear modulus decreases continuously with temperature.

Fluctuations in shear modulus peak sharply at the glass transition.

The shear modulus can be understood through stress relaxation modulus G(t).

Abstract

Using molecular dynamics simulation of a standard coarse-grained polymer glass model we investigate by means of the stress-fluctuation formalism the shear modulus $\mu$ as a function of temperature $T$ and sampling time $\Delta t$. While the ensemble-averaged modulus $\mu(T)$ is found to decrease continuously for all $\Delta t$ sampled, its standard deviation $\delta \mu(T)$ is non-monotonous with a striking peak at the glass transition. Confirming the effective time-translational invariance of our systems, $\mu(\Delta t)$ can be understood using a weighted integral over the shear-stress relaxation modulus $G(t)$. While the crossover of $\mu(T)$ gets sharper with increasing $\Delta t$, the peak of $\delta \mu(T)$ becomes more singular. % It is thus elusive to predict the modulus of a single configuration at the glass transition.