Matthew L. Wallace

Rigidity transition in polymer melts with van der Waals interaction.

We study the onset of rigidity near the glass transition (GT) in a short-chain polymer melt modelled by a bead-spring model, where all beads interact with Lennard-Jones potentials. The properties of the system are examined above and below the GT. In order to minimize high-cooling-rate effects and computational times, equilibrium configurations are reached via isothermal compression. We monitor quantities such as the heat capacity CP, the short-time diffusion constants D, the viscosity eta , and the shear modulus; the time-dependent shear modulus Gt is compared with the shear modulus mu obtained from an externally applied instantaneous shear. We give a detailed analysis of the effects of such shearing on the system, both locally and globally. It is found that the polymeric glass displays long-time rigid behavior only below a temperature T1 , where T1 < TG. Furthermore, the linear and nonlinear relaxation regimes under applied shear are discussed.

Microstructure and dynamics of a polymer glass subjected to instantaneous shear strain

The application of instantaneous shear deformations to a polymer glass modifies the energy landscape of the glass in non-trivial ways. Using molecular dynamics simulations on a freely jointed chain model, we investigate the effect of the strain on the dynamical heterogeneities in the glassy system. The resulting behaviour can be separated into two regimes: elastic for small shear deformations, and plastic for large deformations. Dynamic heterogeneity in the system tends to diminish with deformation. This increased homogenization can be seen, for instance, through changes in the distribution of particle mobilities, both in space and in time, as the glass relaxes following an affine deformation. We are able to directly correlate changes in the overall diffusion in the system with local configurations of mobile and immobile particles. The effect of the deformation on the ageing process is briefly addressed, as we present some new insight into the local dynamics of the polymer chains.