Transient rheology of a polyethylene melt under shear

Using nonequilibrium molecular dynamics simulation, we have studied the res ponse of a C-100 model polymer melt to a step change from equilibrium to a constant, high shear rate flow. The transient shear stress of the model pol ymer melt exhibits pronounced overshoot at the strain value predicted by th e reptation model, in striking similarity to melts of longer, entangled pol ymer governed by reptation motion. Ar the maximum of shear stress overshoot , the molecular orientational order and the alignment angle are found to be midway between those characteristic of Newtonian flow and full alignment w ith the flow. The Doi-Edwards theory is found to be applicable but only by taking into account the shear-rate-dependence of the terminal relaxation ti me. We further analyze the molecular origins of such behavior in short poly mer chains by decomposing the total stress into the contributions from vari ous molecular interactions. [S1063-651X(99)08911-4].