In-plane and out-of-plane rotational motion of individual chain molecules in steady shear flow of polymer melts and solutions

Edwards, Carl N.; Sefiddashti, Mohammad Hadi Nafar; Edwards, Brian J.; Khomami, Bamin

doi:10.1016/j.jmgm.2018.03.003

Cited by 17 publications

(26 citation statements)

References 41 publications

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“…The Siepmann–Karaboni–Smit (SKS) united-atom potential model [19] was used to quantify the energetic interactions between the atomistic components of the polyethylene liquid. This is the same potential model employed in many other prior simulation studies [1,2,3,8,10,11,12,13,14,17,18,20,21,22,23] to represent energetic interactions between either -CH 3 for the end-groups of the chains or -CH 2 - groups for interior carbon atoms along the chain backbone. (Please refer to one of the references cited above for a detailed discussion of the SKS model equations and parameters.…”

Section: Simulation Methodologymentioning

confidence: 99%

“…The relaxation time of the thermostat was set equal to 100 times the long time step. These time steps are longer than those used in many of the prior studies [1,2,3,4,8,11,12,13,14,17,18,21,22]; however, a series of test simulations were performed at various Wi to ensure that the new (longer) time steps produced statically equivalent results as the prior (shorter) time steps. Furthermore, these time steps have been used successfully in recent NEMD studies of planar elongational flows of entangled polyethylene melts [20,23].…”

Section: Simulation Methodologymentioning

confidence: 99%

“…This new phenomenon has been observed via nonequilibrium molecular dynamics (NEMD) simulations of molten polyethylenes in the unentangled and moderately-entangled molecular-weight regimes (i.e., polyethylenes ranging up to C 700 H 1402 ) [1,2,3,4,8,9,10,11,12,13,14,15]. This unexpected observation from atomistic simulations has already been hypothesized to explain some of the difficulties that manifest in flow models for high strain-rate flows [16,17,18].…”

Section: Introductionmentioning

confidence: 97%

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

2019

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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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Section: Simulation Methodologymentioning

confidence: 99%

Section: Simulation Methodologymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 97%

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

2019

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“…The simulated fluid is a monodisperse, linear polyethylene C 1000 H 2002 melt under planar extensional flow (PEF) at elongation rates expressed in terms of the Deborah number De ≡ τ R (where τ R = 137 ns is the Rouse relaxation time of the liquid [50] and˙ is the applied strain rate), in the range of 0 De 60. NEMD simulations were performed in the NV T statistical ensemble at 450 K and the experimental density of 0.766 g/cm 3 using the Siepmann-Karaboni-Smit (SKS) united-atom potential model [51], which has been used by many research groups to study the quiescent and nonequilibrium structure and dynamics via MD and MC of liquid and solid phase alkanes and PEs (e.g., [38,[52][53][54][55][56]). NEMD simulations were performed using the p-SLLOD equations of motion with a Nosé-Hoover thermostat [57][58][59][60][61] implemented within the large-scale atomic/molecular massively parallel simulator (LAMMPS) computing environment developed at Sandia National Laboratory (see [62]).…”

Section: Methodsmentioning

confidence: 99%

Flow-induced crystallization of a polyethylene liquid above the melting temperature and its nonequilibrium phase diagram

Sefiddashti

Edwards

Khomami

2020

Phys. Rev. Research

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Manufacturing of plastics is typically performed via flow processing of a polymer melt. Semicrystalline polymers, such as polyethylene (PE), play a critical role in the global plastics markets, but fundamental understanding of how process flow conditions affect their properties is lacking. Nonequilibrium molecular dynamics of a PE liquid above its melting point revealed reversible transitions from coiled, biphasic, stretched, pretransitional, and crystalline phases with applied stress in elongational flow. A nonequilibrium phase diagram was developed for the thermodynamic state space.

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“…Hence there has been a growing body of work aimed at atomistic or coarse-grained simulations of polymeric fluids. These include, atomistic Non-Equilibrium Molecular Dynamics (NEMD) [8][9][10][11][12] , coarse-grained NEMD [13][14][15][16] , Dissipative Particle Dynamics (DPD) [17][18][19][20][21][22][23][24][25] , and Slip-Link (SL) and Slip-Spring (SS) simulations [26][27][28][29] .…”

mentioning

confidence: 99%

High-fidelity scaling relationships for determining dissipative particle dynamics parameters from atomistic molecular dynamics simulations of polymeric liquids

Sefiddashti

Boudaghi-Khajehnobar

Edwards

et al. 2020

Sci Rep

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An optimized Dissipative Particle Dynamics (DPD) model with simple scaling rules was developed for simulating entangled linear polyethylene melts. The scaling method, which can be used for mapping dimensionless (reduced units) DPD simulation data to physical units, was based on scaling factors for three fundamental physical units; namely, length, time, and viscosity. The scaling factors were obtained as ratios of equilibrium Molecular Dynamics (MD) simulation data in physical units and equivalent DPD simulation data for relevant quantities. Specifically, the time scaling factor was determined as the ratio of longest relaxation times, the length scaling factor was obtained as the ratio of the equilibrium endto-end distances, and the viscosity scaling factor was calculated as the ratio of zero-shear viscosities, each as obtained from the MD (in physical units) and DPD (reduced units) simulations. The scaling method was verified for three MD/DPD model liquid pairs under several different nonequilibrium conditions, including transient and steady-state simple shear and planar elongational flows. Comparison of the MD simulation results with those of the scaled DPD simulations revealed that the optimized DPD model, expressed in terms of the proposed scaling method, successfully reproduced the computationally expensive MD results using relatively cheaper DPD simulations.

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In-plane and out-of-plane rotational motion of individual chain molecules in steady shear flow of polymer melts and solutions

Cited by 17 publications

References 41 publications

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

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

Flow-induced crystallization of a polyethylene liquid above the melting temperature and its nonequilibrium phase diagram

High-fidelity scaling relationships for determining dissipative particle dynamics parameters from atomistic molecular dynamics simulations of polymeric liquids

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