Satoru Kuwajima

Molecular-dynamics simulation of moisture diffusion in polyethylene beyond 10 ns duration

Molecular-dynamics (MD) simulation of the diffusion of water in amorphous polyethylene has been performed. In order to obtain information about the fundamental diffusion behavior of water, the constant particle number, constant pressure, and constant temperature (NPT) MD was employed for three model systems differing in the number of water molecules and methylene group in the polyethylene chains. Time duration beyond 10 ns showed that the water molecules came together forming water clusters of various sizes. Although the self-diffusion coefficient Dself estimated from the mean-square displacement curve scattered considerably in the order from 10−8–10−5 cm2/s, Dself for the water cluster is generally one order lower in magnitude than that of a single water. However, the linearity of the mean-square curve for the larger system which contained 4000 carbon atoms was very good, and Dself obtained for ten water molecules up to 11 ns duration showed 7.1×10−7 cm2/s which was in good agreement with the experimenta...

The extended Ewald method: A general treatment of long‐range electrostatic interactions in microscopic simulations

The Ewald method, which is known as a method for evaluating electrostatic interactions in periodic systems, is extended to nonperiodic systems. A general way of applying the Ewald method to nonperiodic systems is described and a detailed application is demonstrated for the treatment of ions immersed in dipolar solvent molecules. As a preliminary test, electric fields produced by point dipoles forming a simple cubic lattice are calculated, both by the new Ewald treatment and by the conventional cutoff (truncation) method. These test calculations not only demonstrate the advantage of the new Ewald method but also reveal inherent problems associated with the cutoff method.

Molecular dynamics simulation of supercritical carbon dioxide fluid with the model potential from ab initio molecular orbital calculations

Abstract The properties of supercritical fluid carbon dioxide were predicted satisfactorily by molecular dynamics simulations using a model potential from ab initio calculations. The model potential was obtained from the interaction energies of dimers calculated at the MP2/6-311 + G(2df)-level. The pressures and self-diffusion coefficients calculated by the simulations for the NVT ensembles reproduce the experimental values.

Computing the rotational viscosity of nematic liquid crystals by an atomistic molecular dynamics simulation

Abstract An atomistic simulation method is proposed for the computation of the rotational viscosity of nematic liquid crystals. The method is a non-equilibrium molecular dynamics (MD) in which the director of nematic systems is forced to rotate by an artificial force termed aligning force. A test calculation is presented for trans -4-( trans -4- n -butylcyclohexyl) cyclohexylcarbonitrile (CCH4) and very good agreement between the simulation and experiment is demonstrated.

Molecular dynamics simulation of water diffusion in atactic and amorphous isotactic polypropylene

Molecular dynamics (MD) simulation of the diffusion of water in both atactic polypropylene (aPP) and amorphous isotactic polypropylene (iPP) models has been performed. Constant particle number, constant pressure, and constant temperature (NPT) MD was employed for both systems consisting of three polypropylene chains of 500 monomer units with ten water molecules. The density observed for the iPP model after full equilibration was slightly greater than that for the aPP model, but predicted x-ray scattering curves indicated that both systems were completely amorphous structures. In the aPP model, time duration up to 11 ns showed that the water molecules dynamically came together forming a water cluster with various sizes and again departed to a smaller size cluster or a single water sometimes. In contrast, the clustering was remarkable in the iPP model and maintained its aggregated structure over several nanoseconds. The self-diffusion coefficient, Dself, estimated from the mean-square displacement curve for...

Novel method to estimate solubility of small molecules in cis-polyisoprene by molecular dynamics simulations

A novel method to predict gas solubility in cis-1,4-polyisoprene is developed using molecular dynamics (MD) simulations under constant particle number, constant pressure and constant temperature (NPT) conditions. Analogous to the experimental sorption technique, the binary-phase model constructed of gas/polymer was prepared. In order to maintain external pressure of the whole system during long NPT-MD runs, the vapor phase was filled with virtual liquid which has no interaction with the gas molecules and has only a repulsive interaction with the polymer. After attaining equilibration of the system, the solubility of oxygen and carbon dioxide in the polymer phase were estimated in the temperature range from 273 K to 373 K by counting the number of gas molecules inside the polymer phase. The average solubility linearly increased with the increase in the external pressure, indicating that Henry’s Law was satisfied. The solubility coefficient obtained from the present method showed good agreement with the exp...

Permeability of gases in rubbery polymer membrane: application of a psuedo-nonequilibrium molecular dynamics simulation

The permeation behavior of carbon dioxide in a melt cis-polyisoprene (cis-PI) membrane has been studied by a psuedo-nonequilibrium molecular dynamics (MD) simulation. The three-phase model, gas/cis-PI-membrane/vacuum, was constructed. The adoption of the virtual liquid (VL) molecules which have no interaction with gas molecules and also the artificial potential walls between the interface enabled the permeation simulation by making the concentration slope of the gas molecules. Time enhancement of the gas molecules which go across the membrane showed a linear relation after attaining a steady state. The permeation coefficient was directly obtained from the time lag analogous to the experiment at 373 K.

Molecular-dynamics evaluation of fluid-phase equilibrium properties by a novel free-energy perturbation approach: Application to gas solubility and vapor pressure of liquid hexane

A novel free-energy perturbation method is developed for the computation of the free energy of transferring a molecule between fluid phases. The methodology consists in drawing a free-energy profile of the target molecule moving across a binary-phase structure built in the computer. The novelty of the method lies in the difference of the definition of the free-energy profile from the common definition. As an important element of the method, the process of making a correction to the transfer free energy with respect to the cutoff of intermolecular forces is elucidated. In order to examine the performance of the method in the application to fluid-phase equilibrium properties, molecular-dynamics computations are carried out for the evaluation of gas solubility and vapor pressure of liquid n-hexane at 298.15 K. The gas species treated are methane, ethane, propane, and n-butane, with the gas solubility expressed as Henrys constant. It is shown that the method works fine and calculated results are generally in good agreement with experiments. It is found that the cutoff correction is strikingly large, constituting a dominant part of the calculated transfer free energy at the cutoff of 8 A.

Molecular Dynamics Studies of Amorphous Poly(Tetrafluoroethylene)

Abstract Force field parameters for amorphous poly(tetrafiuoroethylene) (PTFE) and perfluoroalkanes for molecular simulations are developed using four perfluoroalkanes (C3F8, n-C4F10, n-C5F12 and n-C6F14) as reference molecules. Molecular dynamics calculations of the amorphous PTFE are performed under constant temperature and constant pressure conditions. Surprisingly, more than 6 ns calculations are needed to equilibrate the system. The calculated density and the structure factor are in good agreement with experiment. The concentration of the trans conformation of the backbone corresponds closely to that estimated from a Boltzmann distribution. The determined force field parameters are confirmed to reproduce the realistic amorphous structure of PTFE. The molecular motions of the backbones are investigated by tracing the time evolutions of the dihedral angles and several types of the conformational changes are observed.