(602h) Elucidating the Molecular Rheology of Entangled Polymeric Fluids Via Direct Comparison of NEMD Simulations and Model Predictions

The startup and steady shear flow properties of an entangled, monodisperse polyethylene liquid (C₁₀₀₀H₂₀₀₂) were investigated via virtual experimentation using nonequilibrium molecular dynamics. The simulation results for transient and steady-state rheological functions were directly compared with several versions of the tube model including the DEMG, MLD, GLaMM, and Rolie-poly models. These comparisons demonstrated that all the studied tube models performed poorly under startup conditions for shear rates higher than the inverse Rouse time of the system due to inaccurate evolution equations for the tube orientation and tube stretch variables. Nevertheless, the factorization of the stress tensor into tube segmental orientation and tube stretch contributions appeared to be reasonable for shear rates up to the inverse disentanglement time of the liquid. At higher shear rates, the entanglement dynamics (i.e., flow-induced disentanglement) began to influence the stress relaxation significantly. A simple modification to the stress expression was proposed to incorporate the entanglement dynamics into tube-based models.

The comparison of transient shear viscosity with the dynamic responses of key variables of the tube model, including the tube segmental orientation and tube stretch, revealed that the stress overshoot and undershoot in steady shear flow of entangled liquids are essentially originated and dynamically controlled by the shear component of the tube orientation tensor, rather than the tube stretch, over a wide range of flow strengths.