Individual Molecular Dynamics of an Entangled Polyethylene Melt Undergoing Steady Shear Flow: Steady-State and Transient Dynamics

Abstract

The startup and steady shear flow properties of an entangled, monodisperse polyethylene liquid (C ₁₀₀₀H ₂₀₀₂) were investigated via virtual experimentation using nonequilibrium molecular dynamics. The simulations revealed a multifaceted dynamical response of the liquid to the imposed flow field in which entanglement loss leading to individual molecular rotation plays a dominant role in dictating the bulk rheological response at intermediate and high shear rates. Under steady shear conditions, four regimes of flow behavior were evident. In the linear viscoelastic regime ( $\dot{\gamma }<{\tau }_d^{-1}$ ), orientation of the reptation tube network dictates the rheological response. Within the second regime ( ${\tau }_d^{-1}<\dot{\gamma }<{\tau }_R^{-1}$ ), the tube segments begin to stretch mildly and the molecular entanglement network begins to relax as flow strength increases; however, the dominant relaxation mechanism in this region remains the orientation of the tube segments. In the third regime ( ${\tau }_R^{-1}<\dot{\gamma }<{\tau }_e^{-1}$ ), molecular disentangling accelerates and tube stretching dominates the response. Additionally, the rotation of molecules become a significant source of the overall dynamic response. In the fourth regime ( $\dot{\gamma }>{\tau }_e^{-1}$ ), the entanglement network deteriorates such that some molecules become almost completely unraveled, and molecular tumbling becomes the dominant relaxation mechanism. The comparison of transient shear viscosity, ${\eta }^{+}$ , with the dynamic responses of key variables of the tube model, including the tube segmental orientation, $S$ , and tube stretch, $\lambda$ , revealed that the stress overshoot and undershoot in steady shear flow of entangled liquids are essentially originated and dynamically controlled by the $S_{xy}$ component of the tube orientation tensor, rather than the tube stretch, over a wide range of flow strengths.