Shear modulus and shear-stress fluctuations in polymer glasses

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 $\hat{I}¼$ as a function of temperature $T$ and sampling time $\hat{I}”t$. While the ensemble-averaged modulus $\hat{I}¼(T)$ is found to decrease continuously for all $\hat{I}”t$ sampled, its standard deviation $\hat{I}´\hat{I}¼(T)$ is non-monotonous with a striking peak at the glass transition. Confirming the effective time-translational invariance of our systems, $\hat{I}¼(\hat{I}”t)$ can be understood using a weighted integral over the shear-stress relaxation modulus $G(t)$. While the crossover of $\hat{I}¼(T)$ gets sharper with increasing $\hat{I}”t$, the peak of $\hat{I}´\hat{I}¼(T)$ becomes more singular. % It is thus elusive to predict the modulus of a single configuration at the glass transition.

5 pages, 6 figure, published at PRL