Shear-stress fluctuations and relaxation in polymer glasses

• Published in: arXiv.org 2017-11
• Main Authors: Kriuchevskyi, I, Wittmer, J P, Meyer, H, Benzerara, O, Baschnagel, J
• Format: Article
• Language: English
• Subjects: Computer simulation; Dependence; Glass; Ill posed problems; Molecular dynamics; Polymers; Shear modulus; Shear stress; Shear viscosity; Stress relaxation; System effectiveness; Variation
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1711.00725

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.