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Register a new userShear-stress fluctuations and relaxation in polymer glasses
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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.
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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.
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