Deformation of molecular glasses

Unlike polymeric glasses, the molecular glasses are too brittle to be subjected to large anisotropic deformation experiments. This is unfortunate because deconvoluting the glass proper effects from the polymeric ones is of fundamental significance for developing the notoriously lacking theory of the glassy state. Molecular simulations do not suffer from such limitations. MD simulation of large deformations in uniaxial extension and compression have been performed on a set of model systems of molecular glass, including the Lennard-Jones binary mixture and the single component dumbbells. In addition to the stress and strain, the molecular mobility in the post-yield flow regime has been investigated by observing the decay of the self-scattering correlation function and, for the dumbbells, of the orientational autocorrelator. There are significant differences as compared to the polymeric systems: under constant volume deformation a post-yield softening occurs in extension but not in compression, the relaxation spectrum in the flow state in compression becomes single-exponential. The first finding challenges existing constitutive models of glass and the second raises questions about the mobility domains as a source of the broad relaxation spectrum. Implications for the current models of glass will be discussed.