Entropic Character of the Atomic Level Stress in Polymeric Melts

Abstract

The entropic character of the atomic level stress in polymeric melts and the stress optical coefficient are studied in model systems by the use of equilibrium and nonequilibrium molecular dynamics. The atomic level stress is defined in intrinsic coordinates, a mobile frame tied to the generic bond. The global stress σ is obtained in the global coordinate system by summing up the contributions due to the intrinsic stress corresponding to each atom in the population. The atom-based global stress is proportional to an average measure of bond orientation (P2), with the proportionality constant σ/P2 being related to the macroscopic stress optical coefficient (SOC). The proportionality constant may be expressed in terms of intrinsic quantities which, in turn, are computable from equilibrium simulations. The model reproduces most experimentally observed properties of the SOC. The ratio σ/P2 is chain length and deformation rate independent in the melt and becomes rate dependent in the glassy state. The dependence...