Relaxation dynamics in supercooled oligomer liquids: From shear-stress fluctuations to shear modulus and structural correlations
L Klochko and J Baschnagel and JP Wittmer and AN Semenov, JOURNAL OF CHEMICAL PHYSICS, 151, 054504 (2019).
Static and dynamical properties of a model glass-forming oligomer liquid are analyzed using molecular dynamics simulations. The temperature and system size effects are assessed for the affine shear modulus mu(A), the quasistatic shear modulus mu(sf) (obtained using the stress-fluctuation relation), and the shear relaxation modulus G(t). It is found that while both mu(A) and mu(sf) are nearly independent of the system size, their variances show significant system size dependence, in particular, below the glass transition temperature T-g. It is also shown that the standard deviation of the shear modulus, delta mu(sf)(T), exhibits a pronounced peak at T approximate to T-g whose position is nearly independent of the system volume V. Moreover, the whole function delta mu(sf)(T) is nearly the same for different system sizes above the glass transition. We propose a theory which quantitatively predicts delta mu(sf)(T) at T greater than or similar to T-g and explains both its independence of V and its peak near T-g. It is also established that below T-g the variance of the affine modulus follows the standard power law, delta mu A2 proportional to 1/V, while delta mu(sf) shows anomalously a slow decrease with V as >delta mu sf2 proportional to 1/V alpha with alpha < 1. On this basis, it is argued that the studied glass-forming systems must show long-range structural correlations in the amorphous state.
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