Relaxation moduli of glass-forming systems: temperature effects and fluctuations

L Klochko and J Baschnagel and JP Wittmer and AN Semenov, SOFT MATTER, 17, 7867-7892 (2021).

DOI: 10.1039/d1sm00778e

Equilibrium and dynamical properties of a two-dimensional polydisperse colloidal model system are characterized by means of molecular dynamics (MD) and Monte Carlo (MC) simulations. We employed several methods to prepare quasi-equilibrated systems: in particular, by slow cooling and tempering with MD (method SC-MD), and by tempering with MC dynamics involving swaps of particle diameters (methods Sw-MD, Sw-MC). It is revealed that the Sw-methods are much more efficient for equilibration below the glass transition temperature T-g leading to denser and more rigid systems which show much slower self-diffusion and shear-stress relaxation than their counterparts prepared with the SC-MD method. The shear-stress relaxation modulus G(t) is obtained based on the classical stress-fluctuation relation. We demonstrate that the alpha-relaxation time tau(alpha) obtained using a time-temperature superposition of G(t) shows a super-Arrhenius behavior with the VFT temperature T-0 well below T-g. We also derive novel rigorous fluctuation relations providing isothermic and adiabatic compression relaxation moduli in the whole time range (including the short-time inertial regime) based on correlation data for thermostatted systems. It is also shown that: (i) the assumption of Gaussian statistics for stress fluctuations leads to accurate predictions of the variances of the fluctuation moduli for both shear (mu(F)) and compression (eta(F)) at T greater than or similar to T-g. (ii) The long-time (quasi-static) isothermic and adiabatic moduli increase on cooling faster than the affine compression modulus eta(A), and this leads to a monotonic temperature dependence of eta(F) which is qualitatively different from mu(F)(T) showing a maximum near T-g.

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