Structure and Dynamics of Dilute Polymer Solutions under Shear Flow via Nonequilibrium Molecular Dynamics

1999 ◽  
Vol 32 (17) ◽  
pp. 5660-5672 ◽  
Author(s):  
C. Aust ◽  
M. Kröger ◽  
S. Hess
Keyword(s):  
Molecular Dynamics ◽  
Shear Flow ◽  
Polymer Solutions ◽  
Nonequilibrium Molecular Dynamics ◽  
Structure And Dynamics ◽  
Dilute Polymer Solutions

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

Polymers ◽  
2019 ◽  
Vol 11 (3) ◽  
pp. 476 ◽  
Author(s):  
Mohammad Nafar Sefiddashti ◽  
Brian Edwards ◽  
Bamin Khomami
Keyword(s):  
Molecular Dynamics ◽  
Shear Flow ◽  
Relaxation Mechanism ◽  
Steady Shear ◽  
Nonequilibrium Molecular Dynamics ◽  
Dynamical Response ◽  
Rheological Response ◽  
Wide Range ◽  
Steady Shear Flow ◽  
Entanglement Network

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.

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