Evolution of Polymer Melt Conformation and Entanglement under High-Rate Elongational Flow

Using molecular dynamics (MD) simulations, this study explores the fluid properties of three polymer melts with the same number of entanglements, Z, achieved by adjusting the entanglement length N_e, while investigating the evolution of polymer melt conformation and entanglement under high-rate elongational flow. The identification of a master curve indicates consistent normalized linear viscoelastic behavior. Surprising findings regarding the steady-state viscosity at various elongational rates (Wi_R>4.7) for polymer melts with the same Z have been uncovered, challenging existing tube models. Nevertheless, the study demonstrates the potential for normalizing the steady-state elongational viscosity at high rates (Wi_R>4.7) by scaling with the square of the chain contour length. Additionally, the observed independence of viscosity on the elongational rate at high rates suggests that higher rates lead to a more significant alignment of polymer chains, a decrease in entanglement, and a stretching in contour length of polymer chains. Molecular-level tracking of tagged chains further supports the assumption of no entanglement under rapid elongation, emphasizing the need for further research on disentanglement in polymer melts subjected to high-rate elongational flow. These results carry significant implications for understanding and predicting the behavior of polymer melts under high-rate elongational flow conditions.