Atomistic simulation of crystallization of a polyethylene melt in steady uniaxial extension

The Journal of Chemical Physics

2010

Self Cite

Rheological and structural studies of liquid decane, hexadecane, and tetracosane under planar elongational flow using nonequilibrium molecular-dynamics simulations J. Chem. Phys. 122, 184906 (2005) We present a detailed analysis of the configurational temperature ͑T conf ͒ for its application to polymeric materials using nonequilibrium molecular dynamics ͑NEMD͒ and nonequilibrium Monte Carlo ͑NEMC͒ methods. Simulations were performed of linear polyethylene liquid C 78 H 158 undergoing shear and elongational flows. At equilibrium, T conf is equal to the set point temperature of the simulation. An aphysically large decrease in T conf is observed in the NEMD simulations for both flows, especially at strong flow fields. By analyzing separately the individual contributions of the different potential interaction modes to the configurational temperature, it is found that the bonded modes ͑which constitutes almost 99.5% of the total͒ dominate the total T conf over the nonbonded ones; i.e., bond-stretching ͑Ϸ86.5%͒, bond-bending ͑Ϸ11.8%͒, bond-torsional ͑Ϸ1.2%͒, nonbonded intermolecular ͑Ϸ0.4%͒, and intramolecular ͑Ϸ0.1%͒ Lennard-Jones. The configurational temperature of the individual modes generally exhibits a nonmonotonic behavior with the flow strength and a dramatic change beyond a critical value of flow strength; this is mainly attributed to the dynamical effect of strong molecular collisions occurring at strong flow fields. In contrast, no such behavior is observed in the NEMC simulations where such dynamical effects are absent. Based on the principal physical concept of the configurational temperature, which represents the large-scale structural characteristics of the system, we propose to exclude the dynamical effects exhibited by the individual interaction modes, in obtaining a physically meaningful T conf as the configurational entropy of the system should not be affected by such factors. Since ͑a͒ the main difference between equilibrium and nonequilibrium states lies in the change in the overall ͑global͒ structure ͑represented by the bond torsional and nonbonded modes͒, and ͑b͒ the local, very short structure ͑represented by the bond-stretching and bond-bending modes͒ is barely changing between equilibrium and nonequilibrium states and its contribution to the total system configurational entropy is negligible compared to the large-scale structural changes, in order to accurately describe the structural changes occurring at nonequilibrium states by use of the configurational temperature, we further propose that only the contributions from the bond-torsional and nonbonded modes to ⌬T conf between equilibrium and nonequilibrium states should be taken into account to generate a physically meaningful ⌬T conf . Applying the above hypothesis to the analysis of the simulation data, good agreement between the NEMD and NEMC simulations ͑and between NEMD simulations for different flows͒ is observed. Furthermore, the configurational temperature obtained in such way is found to match remarkably well with the heat capacity...

Section: Nemc Simulations Of Polymeric Fluidssupporting

confidence: 86%

Section: Discussionmentioning

confidence: 68%

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Analysis of the configurational temperature of polymeric liquids under shear and elongational flows using nonequilibrium molecular dynamics and Monte Carlo simulations

Baig

The Journal of Chemical Physics

2010

Self Cite

“…Indeed, the majority of this work involves realistic atomistic and united-atoms models of PE [33,[36][37][38][39][40][41][42][43][44][45][46], which are known to be quite accurate for both the liquid and solid phases due to the simplicity of the interatomic forces. Notable of the NEMD and NEMC studies of FIC are those of Baig and Edwards [40,41,47] and Rutledge and co-workers [36,39,[44][45][46]. These works illustrated important crystal properties and nucleation mechanisms of PE morphologies under shear and extensional flows; however, all of these simulations of FIC were performed on relatively low molar mass PEs with few entanglements under quiescent conditions, and each ranged from 15%-35% undercooling; i.e., below the crystalline melting point.…”

Section: Introductionmentioning

confidence: 99%

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

Sefiddashti

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.

“…Furthermore, virtual experimentation can be conducted on pure systems (e.g., strictly monodisperse, linear macromolecular melts) under ideal conditions without regard to instrument compliance or inertia. NEMD simulations also provide rheological, optical, and spectroscopic data in physical units that can be directly compared with experimental data [34][35][36][37][38] , in contrast to simulations of coarse-grained models (as discussed below). The primary limitations on what can be achieved are (1) the accuracy of the atomic potential model of the polymeric liquid and (2) the vast computational resources necessary to simulate these highly entangled macromolecular systems possessing up to 10 8 degrees of freedom which must be tracked over multiple disengagement times.…”

mentioning

confidence: 99%

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

Sefiddashti

Boudaghi-Khajehnobar

et al. 2020

Sci Rep

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.