Nonmonotonic Stress Overshoot in Entangled Polymer Melts under Two-step Shear

Shear stress overshoot in entangled polymer rheology is a hallmark of transient dynamics, but its microscopic origin remains under debate. Using molecular dynamics simulations, we investigate a two-step shear protocol consisting of successive startup shears separated by a waiting period, with the first shear interrupted before the overshoot. In the homogeneous flow, the GLaMM theory captures the stress response during the first shear, but fails to reproduce the nonmonotonic dependence of the second stress overshoot (σ_2max) on the waiting time. Contrary to the prediction of a nonmonotonic normal stress component σ_yy during the waiting period, our simulations show that σ_yy, like the tube segment orientation (S_xy), the contour length of the primitive chain (L), and the entanglement number per chain (Z), relaxes monotonically toward equilibrium. At the strain corresponding to σ_2max, both the tube segment orientation and the entanglement number per chain exhibit a nonmonotonic dependence on the waiting time that closely mirrors the behavior of σ_2max, indicating that both factors play significant roles in governing σ_2max. Our findings are consistent with the interpretation of Ianniruberto and Marrucci [ACS Macro. Lett.2014, 3, 552] for orientation effects and with the viewpoint of Wang et al. [Macromolecules2013, 46, 3147] for entanglement effects, although the two explanations are rooted in distinct physical pictures. These results provide new insights into the stress responses of entanglement polymer fluids and underscore the need for a more unified theoretical framework.