Stress overshoot behavior in polymer nanocomposites filled with spherical nanoparticles under steady shear flow via molecular dynamics simulation

Stress overshoot is one important nonlinear rheological phenomenon under the start-up shear for polymer nanocomposites (PNCs). However, the origin mechanism of this nonlinear behavior is still unclear, which is closely dependent on the variation of microstructure response to the shear strain. In this work, the nonequilibrium molecular dynamics (NEMD) simulation is employed to investigate the transient behavior of structure and stress response to the shear strain in PNCs. First, the stress overshoot behavior is observed, which is consistent with the experimental result. By performing the stress decomposition, the stress overshoot behavior occurs in both polymer matrix and nanoparticles (NPs). Meanwhile, by calculating the overshoot amount and the linear correlation between the shear stress and the chain orientation, the extent of the stress overshoot nearly keeps unchanged with increasing the NPs loading while it rises with improving the shear rate or polymer-NP interaction. In addition, the contribution of NPs to the overshoot amount gradually rises with increasing the NPs loading, shear rate or polymer-NP interaction. Notably, several characterized topological structures (such as the chain orientation, chain stretching, and polymer-NP networks) all exhibit an overshoot behavior, which can be correlated with the stress behavior qualitatively. These results clearly indicate that the dominant structural mechanism resulting in the stress overshoot is the chain orientation. In summary, this work provides a fundamental understanding on the stress overshoot behavior of PNCs on the molecular level.