Molecular dynamics simulation study of elastic properties of HMX

Journal ArticleAtomistic simulations were used to calculate the isothermal elastic properties for b-, a-, and d-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The room-temperature isotherm for each polymorph was computed in the pressure interval 0≤p≤10.6 GPa and was used to extract the initial isothermal bulk modulus Ko and its pressure derivative using equations of state employed previously in experimental studies of the b-HMX isotherm

Temperature dependent shear viscosity coefficient of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX ): a molecular dynamics simulation study

Journal ArticleEquilibrium molecular dynamics methods were used in conjunction with linear response theory and a recently published potential-energy surface [J. Phys. Chem. B 103, 3570 (1999)] to compute the liquid shear viscosity and self-diffusion coefficient of the high explosive HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) over the temperature domain 550-800 K. Predicted values of the shear viscosity range from 0.0055 Pa *s at the highest temperature studied up to 0.45 Pa *s for temperatures near the melting point

Molecular dynamics simulations of HMX crystal polymorphs using a flexible molecule force field

Journal ArticleMolecular dynamics simulations using a recently developed quantum chemistry-based atomistic force field [J. Phys. Chem. B 103 (1999) 3570] were performed in order to obtain unit cell parameters, coefficients of thermal expansion, and heats of sublimation for the three pure crystal polymorphs of octahydro-l,3,5,7-tetranitro-l,3,5,7-tetrazocine (HMX). The predictions for β-, α-, and δ-HMX showed good agreement with the available experimental data

Bidirectional mapping between self-consistent field theory and molecular dynamics

Copyright (2007) AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Journal of Chemical Physics 127 and may be found at http://dx.doi.org.proxy.lib.uwaterloo.ca/10.1063/1.2776261A bidirectional mapping scheme that bridges particle-based and field-based descriptions for polymers is presented. Initial application is made to immiscible homopolymer blends. The forward mapping (upscaling) approach is based on the use of molecular dynamics simulations to calculate interfacial density profiles for polymer molecular weights that can be readily relaxed using standard simulation methods. These profiles are used to determine the optimal, effective interaction parameter that appears in the one-parameter self-consistent field theory treatment employed in the present work. Reverse mapping from a field representation to a particle-based description is accomplished by the application of a density-biased Monte Carlo method that generates representative chain configurations in the blend using statistical weights derived from fields obtained from self-consistent field theory.This work was supported by the Los Alamos National Laboratory Exploratory Research component of the Laboratory Directed Research and Development program. This work was carried out under the auspices of the National Nuclear Security Administration of the U.S. Department of Energy at Los Alamos National Laboratory under Contract No. DE-AC52–06NA25396

Effective Soft-Core Potentials and Mesoscopic Simulations of Binary Polymer Mixtures

Mesoscopic molecular dynamics simulations are used to determine the large scale structure of several binary polymer mixtures of various chemical architecture, concentration, and thermodynamic conditions. By implementing an analytical formalism, which is based on the solution to the Ornstein-Zernike equation, each polymer chain is mapped onto the level of a single soft colloid. From the appropriate closure relation, the effective, soft-core potential between coarse-grained units is obtained and used as input to our mesoscale simulations. The potential derived in this manner is analytical and explicitly parameter dependent, making it general and transferable to numerous systems of interest. From computer simulations performed under various thermodynamic conditions the structure of the polymer mixture, through pair correlation functions, is determined over the entire miscible region of the phase diagram. In the athermal regime mesoscale simulations exhibit quantitative agreement with united atom simulations. Furthermore, they also provide information at larger scales than can be attained by united atom simulations and in the thermal regime approaching the phase transition.Comment: 19 pages, 11 figures, 3 table

Classical Density Functional Study on Interfacial Structure and Differential Capacitance of Ionic Liquids near Charged Surfaces

We have implemented a generic coarse-grained model for the aromatic ionic liquid [CnMIM+][Tf2N-]. Various lengths for the alkyl chain on the cation define a homologous series, whose electric properties are expected to vary in a systematic way. Within the framework of a classical density functional theory, the interfacial structures of members of this series are compared over a range of surface charge densities, alkyl chain lengths, and surface geometries. The differential capacitance of the electric double layer, formed by ionic liquids against a charged electrode, is calculated as a function of the surface electric potential. A comparison of planar, cylindrical, and spherical surfaces confirms that the differential capacitance increases and varies less with surface potential as the surface curvature increases. Our results are in qualitative agreement with recent atomistic simulations

Screening Methodology for the Efficient Pairing of Ionic Liquids and Carbonaceous Electrodes Applied to Electric Energy Storage

A model is presented that correlates the measured electric capacitance with the energy that comprises the desolvation, dissociation and adsorption energy of an ionic liquid into carbonaceous electrode (represented by single-wall carbon nanotubes). An original methodology is presented that allows for the calculation of the adsorption energy of ions in a host system that does not necessarily compensate the total charge of the adsorbed ions, leaving an overall net charge. To obtain overall negative (favorable) energies, adsorption energies need to overcome the energy cost for desolvation of the ion pair and its dissociation into individual ions. Smaller ions, such as BF4 −, generally show larger dissociation energies than anions such as PF6 − or TFSI−. Adsorption energies gradually increase with decreasing pore size of the CNT and show a maximum when the pore size is slightly greater than the dimensions of the adsorbed ion and the attractive van der Waals forces dominate the interaction. At smaller pore diameters, the adsorption energy sharply declines and becomes repulsive as a result of geometry deformations of the ion. Only for those diameters where the adsorption reaches maximum values is the adsorption energy sufficiently negative to balance the positive dissociation and desolvation energies. We present for each ion (and ionic liquid) what the most adequate electrode pore size should be for maximum capacitance

Inhibition of Hotspot Formation in Polymer Bonded Explosives Using an Interface Matching Low Density Polymer Coating at the Polymer–Explosive Interface

In order to elucidate how shocks in heterogeneous materials affect decomposition and reactive processes, we used the ReaxFF reactive force field in reactive molecules dynamics (RMD) simulations of the effects of strong shocks (2.5 and 3.5 km/s) on a prototype polymer bonded explosive (PBX) consisting of cyclotrimethylene trinitramine (RDX) bonded to hydroxyl-terminated polybutadiene (HTPB). We showed earlier that shock propagation from the high density RDX to the low density polymer (RDX ? Poly) across a nonplanar periodic interface (sawtooth) leads to a hotspot at the initial asperity but no additional hotspot at the second asperity. This hotspot arises from shear along the interface induced by relaxation of the stress at the asperity. We now report the case for shock propagation from the low density polymer to the high density RDX (Poly ? RDX) where we find a hotspot at the initial asperity and a second more dramatic hotspot at the second asperity. This second hotspot is enhanced due to shock wave convergence from shock wave interaction with nonplanar interfaces. We consider that this second hotspot is likely the source of the detonation in realistic PBX systems. We showed how these hotspots depend on the density mismatch between the RDX and polymer and found that decreasing the density by a factor of 2 dramatically reduces the hotspot. These results suggest that to make PBX less sensitive for propellants and explosives, the binder should be designed to provide low density at the asperity in contact with the RDX. Based on these simulations, we propose a new design for an insensitive PBX in which a low density polymer coating is deposited between the RDX and the usual polymer binder. To test this idea, we simulated shock wave propagation from two opposite directions (RDX ? Poly and Poly ? RDX) through the interface matched PBX (IM-PBX) material containing a 3 nm coating of low density (0.48 g/cm3) polymer. These simulations showed that this IM-PBX design dramatically suppresses hotspot formation