Abstract
The startup and steady shear flow properties of an entangled, monodisperse polyethylene liquid (C1000H2002) 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 (), orientation of the reptation tube network dictates the rheological response. Within the second regime (), 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 (), 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 (), 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, , 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 component of the tube orientation tensor, rather than the tube stretch, over a wide range of flow strengths.
Highlights
The study of flow properties of polymeric solutions and melts has a rich history of perplexing the physicists and engineers who have endeavored to understand and model the many and varied physical responses of these complex fluids to an imposed flow field
Many continuum level theories have proven capable of describing gross rheological data in the linear and weakly nonlinear viscoelastic flow regimes, most of these have not been able to provide a quantitative description of the flow properties of solutions and melts at high flow strength
P(Lpp ) shifts to the right and becomes wider; the distribution continues to follow a Gaussian distribution. These results suggest that, by the end of this flow regime the system loses about 30% of its entanglements, the nature of the entanglement network does not change radically
Summary
The study of flow properties of polymeric solutions and melts has a rich history of perplexing the physicists and engineers who have endeavored to understand and model the many and varied physical responses of these complex fluids to an imposed flow field. Many continuum level theories have proven capable of describing gross rheological data in the linear and weakly nonlinear viscoelastic flow regimes (i.e., at low to intermediate values of the strain rate relative to a characteristic relaxation time of the fluid), most of these have not been able to provide a quantitative description of the flow properties of solutions and melts at high flow strength. There are many possible reasons one could cite to explain this state of dysfunction, but the overall reason is abundantly clear: for polymeric fluids experiencing strong flow conditions, all of the physical and dynamical phenomena occurring within these materials have not been understood and accounted for in the prevailing mathematical models. For much of the 20th century, rheologists had little in the way of small length and time scale information to guide attempts at improved mathematical modeling
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