AuthorR.C. Picu and M.C. Pavel
TitleFast relaxation modes in model polymeric systems
AbstractThe fast B-stress relaxation modes in a model polymeric melt are investigated by means of non-equilibrium molecular dynamics simulations. The stress is computed on the atomic level by accounting for both bonded and non-bonded interatomic interactions. The system evolution is traced during the loading and the relaxation periods, and the mechanisms of stress production are identified. Stress relaxation takes place by several modes, each corresponding to specific atomic scale structural changes. The B relaxation corresponds to the return to isotropy of the atomic distribution in the neighborhood of a representative atom, and encompasses a quasi-elastic mode, (B1), and a slower mode (B2). A diffusion-like process governs the B2 mode. The B1 mode is non-exponential and accounts for roughly 50% of the total atomic scale stress drop during relaxation. The B2 mode is exponential, leads to a smaller stress drop but takes longer to complete compared to B1. Both B modes involve only local structural changes and their time constants are independent of the molecular weight of the chains. These are simple thermally activated processes which do not involve cooperative relaxations and whose temperature dependence may be described by an Arrhenius equation. Furthermore, it is shown that the time constant for the exponential mode, B2, may be derived from equilibrium simulations based on the fluctuation dissipation theorem and a continuum model of diffusion in the neighborhood of a representative atom. This shows that the non-equilibrium system is in the linear response regime during this relaxation regime.