Mark L. Elert

Heteroatom effects in heterocyclic ring chain polymers

Abstract Recent experimental results for the heterocyclic ring chain polymers, such as polythiophene and polypyrrole, have been most frequently interpreted in terms of their carbon backbone structure. Previous theoretical studies have assumed that the heteroatom in these systems has negligible effect on the π-band structure of the chain, except for effects on the σ-bond structure. We demonstrate, using concepts similar to those of the Su-Schrieffer-Heeger model, that the converse is true. The dominant effect of the heteroatom on the π-band structure is via the direct interaction of the heteroatom p-orbital lone pair with the carbon backbone π-band structure.

Molecular dynamics study of shock-induced chemistry in small condensed-phase hydrocarbons

Molecular dynamics simulations using an empirical bond order potential have been performed to investigate shock-induced chemistry in solid acetylene, ethylene, and methane. Acetylene was found to undergo significant polymerization reactions for flyer plate impact speeds above 10 km/s. These conditions are similar to those which would be experienced upon planetary impact of comets, which are known to contain condensed-phase acetylene. Ethylene exhibits similar reactivity above 15 km/s. Methane undergoes hydrogen abstraction reactions at flyer plate impact speeds of 16–20 km/s and produces hydrocarbon chains at higher impact speeds. The latter results are significant for elucidating the fate of atmospheric methane upon cometary or meteor impact, and for predicting the initial reaction steps in the reactivity of methane ices in the high-pressure, high-temperature interiors of Neptune and Uranus.

Conformation and electronic structure of heterocyclic ring chain polymers

Abstract We present a theoretical study of the conformation and electronic properties of the polypyrrole, polythiophene and polyfuran chain polymer systems. These results are compared with experimental results and other theoretical studies, as well as with our previous model studies indicating the importance of the heteroatom in the π-band structure of these systems.

Conformation and Electronic Properties of Helical Cis-Polyacetylene

Abstract Recent experimental evidence indicates that cis-polyacetylene may exist in a helical conformation in addition to the well-known planar form. We report electronic structure calculations which demonstrate that planar cis-polyacetylene is indeed marginally unstable towards helix formation, but that the potential energy curve is essentially flat over a wide range of single-bond twist angles. This result is consistent with the fact that both the planar and the helical isomers are found experimentally. We employ a tight-binding model to investigate the band structure changes associated with helix formation.

Effects of nanoscale voids on the sensitivity of model energetic materials

Because of its importance in designing safer, more reliable explosives the shock to detonation transition in condensed phase energetic materials has long been a subject of experimental and theoretical study. This transition is thought to involve local hot spots which represent regions in the material which couple efficiently to the shock wave leading to a locally higher temperature and ultimately initiation. However, how at the atomic scale energy is transferred from the shock front into these local ``hot spots`` remains a key question to be answered in studies of the predetonation process. In this paper the authors report results of molecular dynamics simulations that suggest that even nanometer scale defects can play an important role in the shock to detonation transition.

SIMULATIONS OF CHEMICALLY-SUSTAINED SHOCK FRONTS IN A MODEL ENERGETIC MATERIAL

Tersoff-like potentials together with molecular dynamics calculations are used to simulate the detonation of an energetic two-dimensional semi-infinite molecular solid. The resulting shock front produced in this system exhibits four separate regions whose distinct interfaces between the regions move at constant and different velocities. The properties of the resulting shock front are independent of the initiation conditions. This model predicts a critical minimum impact velocity for a chemically-sustained shock front.

Nanoscale Molecular Dynamics Simulaton of Shock Compression of Silicon

We report results of molecular dynamics simulation of shock wave propagation in silicon in [100], [110], and [111] directions obtained using a classical environment‐dependent interatomic potential (EDIP). Several regimes of materials response are classified as a function of shock wave intensity using the calculated shock Hugoniot. Shock wave structure in [100] and [111] directions exhibit usual evolution as a function of piston velocity. At piston velocities 1.25< vp < 2.75 km/s the shock wave consists of a fast elastic precursor followed by a slower plastic front. At larger piston velocities the single overdriven plastic wave propagates through the crystal causing amorphization of Si. However, the [110] shock wave exhibits an anomalous materials response at intermediate piston velocities around vp ≃ 1.75 km/s which is characterized by the absence of plastic deformations.

Molecular dynamics investigation of the effects of variation in energy release on detonation initiation

The amount of energy released in the detonation of an energetic material clearly influences the properties of the detonation, such as peak temperature and detonation front velocity. Using a model diatomic system which has previously been shown to produce realistic detonation properties, we have performed molecular dynamics simulations in which the exothermicity of the chemical reaction supporting the detonation was systematically varied. The minimum energy release necessary to support a chemically sustained shock wave was determined for this model system, as well as the dependence of front velocity, reaction zone temperature, and density on the magnitude of energy release.

MOLECULAR DYNAMICS SIMULATIONS OF AN ANOMALOUS RESPONSE OF DIAMOND TO SHOCK COMPRESSION

We performed molecular dynamics simulations of shock wave propagation in diamond in the [110] crystallographic direction and observed an anomalous response of the material. This regime is characterized by absence of plastic deformation in the intermediate interval of shock wave intensities between shear‐deformation and overdriven rehybridization shock wave regimes.

Molecular Dynamics Studies of Orientation Dependence of Shock Structure in Solids

Molecular dynamics (MD) simulations using empirical potentials have proven to be an efficient tool for study at the lattice level of non‐equilibrium phenomena in solids under shock loading. Such anisotropic properties of a single crystal as elastic constants, slip directions and especially shear stresses might significantly affect the internal structure of a shock wave in different directions of high strain rate uniaxial shock compression. Specifically, the mechanisms of lattice deformation, plasticity, and relaxation of shear stresses, structure of the elastic precursor, as well as shock‐induced chemistry are found to depend crucially on the direction of shock propagation.