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Polymers 2019, 11(3), 476; https://doi.org/10.3390/polym11030476

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

Materials Research and Innovation Laboratory (MRAIL), Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996, USA
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Received: 29 January 2019 / Revised: 20 February 2019 / Accepted: 7 March 2019 / Published: 12 March 2019
(This article belongs to the Special Issue Theory and Simulations of Entangled Polymers)
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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 ( γ ˙ < τ d 1 ), orientation of the reptation tube network dictates the rheological response. Within the second regime ( τ d 1 < γ ˙ < τ 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 ( τ R 1 < γ ˙ < τ 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 ( γ ˙ > τ 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, η + , with the dynamic responses of key variables of the tube model, including the tube segmental orientation, S , and tube stretch, λ , revealed that the stress overshoot and undershoot in steady shear flow of entangled liquids are essentially originated and dynamically controlled by the S x y component of the tube orientation tensor, rather than the tube stretch, over a wide range of flow strengths. View Full-Text
Keywords: entangled polymer melts; linear polymers; nonequilibrium molecular dynamics simulations; steady and startup shear flows entangled polymer melts; linear polymers; nonequilibrium molecular dynamics simulations; steady and startup shear flows
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Nafar Sefiddashti, M.H.; Edwards, B.J.; Khomami, B. Individual Molecular Dynamics of an Entangled Polyethylene Melt Undergoing Steady Shear Flow: Steady-State and Transient Dynamics. Polymers 2019, 11, 476.

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