Intrinsic Distribution and Atomic Level Stress in Polymeric Melts

Abstract

The relationship between stress production and relaxation on one hand and the local structure on the other is studied in model polymeric melts by the use of equilibrium and nonequilibrium molecular dynamics. The analysis is performed in the intrinsic coordinate systema mobile frame tied to the generic bond. The variation of the intrinsic distribution of interacting neighbors about a representative atom, g̃, with density and temperature is investigated above and below the glass transition. It is shown that g̃ captures close packing effects and the buildup of structure upon transition, similar to the radial distribution function g(r). When computed from the nearest nonbonded neighbors, the intrinsic distribution is nonuniform due to steric shielding. Neighbors at distances larger than a covalent bond length from the representative atom, however, lead to a uniform distribution g̃. Thus, the steric shielding effect generates a nonzero intrinsic deviatoric stress in both equilibrium and nonequilibrium systems, while longer range interactions do not contribute to deviatoric stress production. Consequently, each intrinsic frame carries a nonhydrostatic stress (induced by both bonded and nonbonded interactions) which, upon rotation in the global coordinate system, contributes to the global stress in the melt. A preferential orientation of intrinsic frames (induced, for instance, by the deformation of the fluid) generates therefore a deviatoric global stress. In nonequilibrium simulations, the intrinsic distribution is seen to be independent of the deformation of the fluid. Furthermore, when computed from chain inner atoms, the intrinsic distribution is also chain length independent. This implies, in turn, that intrinsic stresses are deformation and chain length independent. The relevance of these observations to stress relaxation in polymeric melts is discussed.