Relaxation dynamics in supercooled oligomer liquids: From shear-stress fluctuations to shear modulus and structural correlations

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 μ_A, the quasistatic shear modulus μ_sf (obtained using the stress-fluctuation relation), and the shear relaxation modulus G(t). It is found that while both μ_A and μ_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, δμ_sf(T), exhibits a pronounced peak at T ≈ T_g whose position is nearly independent of the system volume V. Moreover, the whole function δμ_sf(T) is nearly the same for different system sizes above the glass transition. We propose a theory which quantitatively predicts δμ_sf(T) at T ≳ 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, $δμA2∝1/V$⁠, while δμ_sf shows anomalously a slow decrease with V as $δμsf2∝1/Vα$ with α < 1. On this basis, it is argued that the studied glass-forming systems must show long-range structural correlations in the amorphous state.