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Atomistic Molecular Dynamics Simulation of Polydisperse Linear Polyethylene Melts
Well-relaxed atomistic configurations of polydisperse, linear polyethylene (PE) melts, obtained with the end-bridging Monte Carlo algorithm, have been subjected to detailed molecular dynamics simulations in both the canonical (NVE) and microcanonical (NVT) ensembles. Three different systems have been investigated, characterized by mean molecular lengths C 24, C78, and C156, and by the same polydispersity index I of about 1.09. Results are presented for the static and (mainly) dynamic properties of these melts at P ) 1 atm and T ) 450 K. The diffusion coefficient D, determined for various chain lengths, N, is in very good agreement with experimentally measured values. The friction coefficient D is extracted from D by invoking the Rouse model; it is seen to increase from a relatively small value characteristic of short alkanes to a chain-length-independent plateau, reached in a region of N ) 60-80. The friction coefficient τ is also obtained by analyzing the decay of the time autocorrelation function for the normal modes Xp at various chain lengths; the values thus extracted are consistent with those obtained from D for N above 40. Although the decay of the autocorrelation function of the end-to-end vector is very well described by the Rouse model, individual Rouse modes show some deviation from theoretical predictions. Even for chains sufficiently long to be in the asymptotic regime, only the first two normal modes fully conform to Rouse theory in terms of their squared amplitudes and correlation times. Zeroshear viscosities computed from D values by means of the Rouse model are in excellent agreement with available experimental data for N ) 90.
Systematic time-scale-bridging molecular dynamics applied to flowing polymer melts
Physical Review E, 2009
We present a novel thermodynamically guided, low-noise, time-scale bridging, and pertinently efficient strategy for the dynamic simulation of microscopic models for complex fluids. The systematic coarse-graining method is exemplified for low-molecular polymeric systems subjected to homogeneous flow fields. We use established concepts of nonequilibrium thermodynamics and an alternating Monte-Carlo-molecular dynamics iteration scheme in order to obtain the model equations for the slow variables. For chosen flow situations of interest, the established model predicts structural as well as material functions beyond the regime of linear response. As a by-product, we present the first steady state equibiaxial simulation results for polymer melts. The method is simple to implement and allows for the calculation of time-dependent behavior through quantities readily available from the nonequilibrium steady states.
ms2: A molecular simulation tool for thermodynamic properties
Computer Physics Communications, 2011
This work presents the molecular simulation program ms2 that is designed for the calculation of thermodynamic properties of bulk fluids in equilibrium consisting of small electro-neutral molecules. ms2 features the two main molecular simulation techniques, molecular dynamics (MD) and Monte-Carlo. It supports the calculation of vapor-liquid equilibria of pure fluids and multi-component mixtures described by rigid molecular models on the basis of the grand equilibrium method. Furthermore, it is capable of sampling various classical ensembles and yields numerous thermodynamic properties. To evaluate the chemical potential, Widom's test molecule method and gradual insertion are implemented. Transport properties are determined by equilibrium MD simulations following the Green-Kubo formalism. ms2 is designed to meet the requirements of academia and industry, particularly achieving short response times and straightforward handling. It is written in Fortran90 and optimized for a fast execution on a broad range of computer architectures, spanning from single processor PCs over PCclusters and vector computers to high-end parallel machines. The standard Message Passing Interface (MPI) is used for parallelization and ms2 is therefore easily portable onto a broad range of computing platforms. Auxiliary feature tools facilitate the interaction with the code and the interpretation of input and output files. The accuracy and reliability of ms2 has been shown for a large variety of fluids in preceding work.
The molecular dynamics (MD) simulation technique is a powerful tool for the investigation of multicomponent liquids and solids. A realistic description of such systems relies on the quality of the effective potential with which the interactions between the atoms are modelled in a MD simulation. We propose a fitting scheme to derive effective potentials from ab initio simulations. This scheme is used to parametrize a new potential for silica. In a second case study, MD simulations are used to investigate crystallization in an AlNi alloy, elucidating the crystal growth mechanism on an atomistic scale.
2008
The molecular dynamics (MD) simulation technique is a powerful tool for the investigation of multicomponent liquids and solids. A realistic description of such systems relies on the quality of the effective potential with which the interactions between the atoms are modelled in a MD simulation. We propose a fitting scheme to derive effective potentials from ab initio simulations. This scheme is used to parametrize a new potential for silica. In a second case study, MD simulations are used to investigate crystallization in an AlNi alloy, elucidating the crystal growth mechanism on an atomistic scale.
Scientific Reports, 2020
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 end-to-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 differ...
Quantitative Study of Polymer Dynamics Through Hierarchical Multi-scale Dynamic Simulations
The long time dynamics of polymeric materials has been extensively studied in the past through various experimental techniques and computer simulations. While computer simulations typically treat generic, simplified models, experiments deal with specific chemistries. In this letter we present a hierarchical approach that combines atomistic and coarse-grained simulations to quantitatively study polymer dynamics. As an example we predict diffusion coefficients of atactic polystyrene melts of molecular weights relevant to polymer processing (up to 50kDa) without any adjustable parameter and compare the results to experiment.
Novel procedure for thermal equilibration in molecular dynamics simulation
2009
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