Challenging the Chain Retraction Hypothesis of the Tube Model: Large-Scale Molecular Dynamics Simulations*

The chain retraction hypothesis of the tube model for nonlinear polymer rheology has been challenged by the recent small-angle neutron scattering experiment [Phys. Rev. X 7, 031003 (2017)]. In this work, we further examine the microscopic relaxation mechanism of entangled polymers after a large step uniaxial extension by using large-scale molecular dynamics simulation. We show that the unique structural features associated with the chain retraction mechanism of the tube model are not computationally observed, confirming the previous experimental results. In particular, molecular dynamics simulation allows us to directly and unambiguously determine the evolution of the radius of gyration tensor after large step deformation. Contrary to the prediction of the tube model, the radius of gyration in the perpendicular direction to stretching increases monotonically towards its equilibrium value throughout the stress relaxation in our simulation. These findings provide important observations for improving our understanding of nonlinear rheology of entangled polymers.