Shear-stress fluctuations and relaxation in polymer glasses

Abstract

We investigate by means of molecular dynamics simulation a coarse-grained polymer glass model focusing on (quasi-static and dynamical) shear-stress fluctuations as a function of temperature T and sampling time $\Delta t$. The linear response is characterized using (ensemble-averaged) expectation values of the contributions (time-averaged for each shear plane) to the stress-fluctuation relation ${\mu }_{sf}$ for the shear modulus and the shear-stress relaxation modulus $G(t)$. Using 100 independent configurations we pay attention to the respective standard deviations. While the ensemble-averaged modulus ${\mu }_{sf}(T)$ decreases continuously with increasing T for all $\Delta t$ sampled, its standard deviation $\delta {\mu }_{sf}(T)$ is non-monotonous with a striking peak at the glass transition. The question of whether the shear modulus is continuous or has a jump-singularity at the glass transition is thus ill-posed. Confirming the effective time-translational invariance of our systems, the $\Delta t$-dependence of ${\mu }_{sf}$ and related quantities can be understood using a weighted integral over $G(t)$. This implies that the shear viscosity $\eta (T)$ may be readily obtained from the $1/\Delta t$-decay of ${\mu }_{sf}$ above the glass transition.

arXiv comment21 pages, 21 figures, submitted to PRE

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Cited by in corpus (1)

• Shear-stress fluctuations in free-standing polymer films