Isochoric deformation of molecular glasses

Abstract

Brittleness of molecular glasses precludes studying their behavior under large deformations; thus, the latter has only been studied for polymeric glasses. This leaves the question of which observed effects are generic features of glass and which are specifically polymeric. To address this, MD simulations have been used to study the deformation of model glass-forming systems, including Lennard-Jones binary mixtures and single-component dumbbells. Simulations of undeformed and deformed materials are performed at constant volume. In the case of anisotropic deformations, both uniaxial and shear, a significant increase in hydrostatic stress is observed—behavior not predicted by current constitutive models. As a result of the hydrostatic stress contribution, in uniaxial extension, the post-yield flow stress decreases with the strain rate, in contrast to what is observed under constant-pressure deformation. In addition to the stress, the decay of the self-scattering correlation function and, for the dumbbells, the orientational autocorrelator are monitored to assess the effect of deformation on molecular mobility. A narrowing of the relaxation spectrum to single-exponential is observed in the stress flow regime under uniaxial compression, which challenges the prevailing hypothesis that the presence of domains of varying mobility is the source of the relaxation spectrum. Analysis of the potential energy minima, known as the inherent structures, has been performed for the systems with and without deformation. It is shown that the inherent structure energy serves as a descriptor that unifies the dependence of the main relaxation time on both the temperature and the deformation.