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Title: Nonequilibrium molecular dynamics study of ring polymer melts under shear and elongation flows: A comparison with their linear analogs

Abstract

We present detailed results for the structural and rheological properties of unknotted and unconcatenated ring polyethylene (PE) melts under shear and elongation flows via direct atomistic nonequilibrium molecular dynamics simulations. Short (C{sub 78}H{sub 156}) and long (C{sub 400}H{sub 800}) ring PE melts were subjected to planar Couette flow (PCF) and planar elongational flow (PEF) across a wide range of strain rates from linear to highly nonlinear flow regimes. The results are analyzed in detail through a direct comparison with those of the corresponding linear polymers. We found that, in comparison to their linear analogs, ring melts possess rather compact chain structures at or near the equilibrium state and exhibit a considerably lesser degree of structural deformation with respect to the applied flow strength under both PCF and PEF. The large structural resistance of ring polymers against an external flow field is attributed to the intrinsic closed-loop configuration of the ring and the topological constraint of nonconcatenation between ring chains in the melt. As a result, there appears to be a substantial discrepancy between ring and linear systems in terms of their structural and rheological properties such as chain orientation, the distribution of chain dimensions, viscosity, flow birefringence, hydrostatic pressure, themore » pair correlation function, and potential interaction energies. The findings and conclusions drawn in this work would be a useful guide in future exploration of the characteristic dynamical and relaxation mechanisms of ring polymers in bulk or confined systems under flowing conditions.« less

Authors:
; ;  [1]
  1. Department of Chemical Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798 (Korea, Republic of)
Publication Date:
OSTI Identifier:
22598952
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Rheology; Journal Volume: 60; Journal Issue: 4; Other Information: (c) 2016 The Society of Rheology; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; 42 ENGINEERING; BIREFRINGENCE; COMPARATIVE EVALUATIONS; COMPUTERIZED SIMULATION; CORRELATION FUNCTIONS; COUETTE FLOW; ELONGATION; LIMITING VALUES; MOLECULAR DYNAMICS METHOD; NONLINEAR PROBLEMS; POLYETHYLENES; RINGS; SHEAR; STRAIN RATE; TOPOLOGY; VISCOSITY

Citation Formats

Yoon, Jeongha, Kim, Jinseong, and Baig, Chunggi, E-mail: cbaig@unist.ac.kr. Nonequilibrium molecular dynamics study of ring polymer melts under shear and elongation flows: A comparison with their linear analogs. United States: N. p., 2016. Web. doi:10.1122/1.4954246.
Yoon, Jeongha, Kim, Jinseong, & Baig, Chunggi, E-mail: cbaig@unist.ac.kr. Nonequilibrium molecular dynamics study of ring polymer melts under shear and elongation flows: A comparison with their linear analogs. United States. doi:10.1122/1.4954246.
Yoon, Jeongha, Kim, Jinseong, and Baig, Chunggi, E-mail: cbaig@unist.ac.kr. 2016. "Nonequilibrium molecular dynamics study of ring polymer melts under shear and elongation flows: A comparison with their linear analogs". United States. doi:10.1122/1.4954246.
@article{osti_22598952,
title = {Nonequilibrium molecular dynamics study of ring polymer melts under shear and elongation flows: A comparison with their linear analogs},
author = {Yoon, Jeongha and Kim, Jinseong and Baig, Chunggi, E-mail: cbaig@unist.ac.kr},
abstractNote = {We present detailed results for the structural and rheological properties of unknotted and unconcatenated ring polyethylene (PE) melts under shear and elongation flows via direct atomistic nonequilibrium molecular dynamics simulations. Short (C{sub 78}H{sub 156}) and long (C{sub 400}H{sub 800}) ring PE melts were subjected to planar Couette flow (PCF) and planar elongational flow (PEF) across a wide range of strain rates from linear to highly nonlinear flow regimes. The results are analyzed in detail through a direct comparison with those of the corresponding linear polymers. We found that, in comparison to their linear analogs, ring melts possess rather compact chain structures at or near the equilibrium state and exhibit a considerably lesser degree of structural deformation with respect to the applied flow strength under both PCF and PEF. The large structural resistance of ring polymers against an external flow field is attributed to the intrinsic closed-loop configuration of the ring and the topological constraint of nonconcatenation between ring chains in the melt. As a result, there appears to be a substantial discrepancy between ring and linear systems in terms of their structural and rheological properties such as chain orientation, the distribution of chain dimensions, viscosity, flow birefringence, hydrostatic pressure, the pair correlation function, and potential interaction energies. The findings and conclusions drawn in this work would be a useful guide in future exploration of the characteristic dynamical and relaxation mechanisms of ring polymers in bulk or confined systems under flowing conditions.},
doi = {10.1122/1.4954246},
journal = {Journal of Rheology},
number = 4,
volume = 60,
place = {United States},
year = 2016,
month = 7
}
  • Nonequilibrium molecular dynamics simulations have been performed in order to compare the characteristics of planar Couette, planar elongation, uniaxial stretching, and biaxial stretching flows in simple fluids at different strain rates. After deriving the periodic boundary conditions for general flow fields and introducing some methodological improvements for elongation flow calculations we simulated the combination of shear and shear-free flows as well. We found that even at high strain rates where simple fluids exhibit strong non-Newtonian behavior (shear-thinning) it is a reasonable approximation to consider the two planar flows to be rotationally equivalent. This is because in planar Couette flow themore » in-plane normal stress difference of simple fluids is approximately zero even far from equilibrium. Similarly to planar Couette flow, the trace of the pressure tensor and the internal energy vary approximately as function of the 3/2 power of the strain rate in shear free flows. However, the individual diagonal elements of elongation flow pressure tensors deviate considerably from this approximation. In the extension direction the pressure seems to have a minimum in terms of the strain rate in every shear-free flow. We have discussed the implications of these results. {copyright} {ital 1995} {ital American} {ital Institute} {ital of} {ital Physics}.« less
  • We simulate both microscopic and macroscopic shear flows in two space dimensions using nonequilibrium molecular dynamics and smooth-particle applied mechanics. The time-reversible {ital microscopic} equations of motion are isomorphic to the smooth-particle description of inviscid {ital macroscopic} continuum mechanics. The corresponding microscopic particle interactions are relatively weak and long ranged. Though conventional Green-Kubo theory suggests instability or divergence in two-dimensional flows, we successfully define and measure a finite shear viscosity coefficient by simulating stationary plane Couette flow. The special nature of the weak long-ranged smooth-particle functions corresponds to an unusual kind of microscopic transport. This microscopic analog is mainly kinetic,more » even at high density. For the soft Lucy potential which we use in the present work, nearly all the system energy is potential, but the resulting shear viscosity is nearly all kinetic. We show that the measured shear viscosities can be understood, in terms of a simple weak-scattering model, and that this understanding is useful in assessing the usefulness of continuum simulations using the smooth-particle method. We apply that method to the Rayleigh-Benard problem of thermally driven convection in a gravitational field.« less
  • Identical particle trajectories can result from driven shear flows of two different types: (i) thermostatted flows, simulating a nonequilibrium steady state, and (ii) adiabatic flows, in which the irreversible heating associated with viscous work is not extracted from the system. This trajectory isomorphism applies to shears of hard particles, such as hard disks and spheres. Here we simulate such isomorphic shear flows. We also discuss the associated instantaneous Lyapunov spectra, which are not isomorphic. We extrapolate the dissipative hard-disk spectra to the large-system limit. {copyright} {ital 1998} {ital The American Physical Society}
  • Doubts about the validity of the nonequilibrium molecular dynamics (NEMD) methods of computing shear viscosity have persisted, partly because of the apparent disagreement (approx.25%) between NEMD and equilibrium Green--Kubo (GK) results for the Lennard-Jones system near its triple point. This region of the phase diagram near the melting line is the so-called ''molasses'' regime where the tail of the shear-stress autocorrelation function is quite large, deviating from ''exponential'' decay at a level of about 10%. In order to see whether the effects of the ''molasses tail'' might be obscuring a more profound difference between NEMD and GK results, we havemore » carried out independent NEMD and GK calculations for a state in the LJ fluid far away from this troublesome molasses region, namely at a temperature twice critical and a density between the triple and critical points. We find the NEMD and GK results for the linear shear viscosity to be in good agreement.« less