Origin of Stress Overshoot under Start-up Shear in Primitive Chain Network Simulation

Birefringence measurement demonstrates that the segment orientation of entangled polymers overshoots on start-up of fast shear [Pearson et al. J. Rheol. 1989 33, 517-535]. The stress-optical rule holds for those polymers, so that the overshoot of orientation results in the overshoot of shear stress. On the other hand, an opposite result was deduced from the recent molecular dynamics simulation for bead-spring chain [Lu et al. ACS Macro Lett. 2014 3, 569-573]: the evolution of segment orientation does not overshoot but the chain stretch induces the stress overshoot, even at the shear rate γ̇ smaller than the reciprocal Rouse time, 1/τ_R. In this study, we performed the primitive chain network simulation to find that our simulation reproduces the overshoot of both stress and orientation and the chain stretch exhibits a slight, monotonic increase but no overshoot. Our result is thus fully consistent with the experiment.