Edward M. Kober

Thermal decomposition of RDX from reactive molecular dynamics

We use the recently developed reactive force field ReaxFF with molecular dynamics to study thermal induced chemistry in RDX [cyclic-[CH(2)N(NO(2))](3)] at various temperatures and densities. We find that the time evolution of the potential energy can be described reasonably well with a single exponential function from which we obtain an overall characteristic time of decomposition that increases with decreasing density and shows an Arrhenius temperature dependence. These characteristic timescales are in reasonable quantitative agreement with experimental measurements in a similar energetic material, HMX [cyclic-[CH(2)N(NO(2))](4)]. Our simulations show that the equilibrium population of CO and CO(2) (as well as their time evolution) depend strongly of density: at low density almost all carbon atoms form CO molecules; as the density increases larger aggregates of carbon appear leading to a C deficient gas phase and the appearance of CO(2) molecules. The equilibrium populations of N(2) and H(2)O are more insensitive with respect to density and form in the early stages of the decomposition process with similar timescales.

Carbon Cluster Formation during Thermal Decomposition of Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine and 1,3,5-Triamino-2,4,6-trinitrobenzene High Explosives from ReaxFF Reactive Molecular Dynamics Simulations

We report molecular dynamics (MD) simulations using the first-principles-based ReaxFF reactive force field to study the thermal decomposition of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) at various densities and temperatures. TATB is known to produce a large amount (15-30%) of high-molecular-weight carbon clusters, whereas detonation of nitramines such as HMX and RDX (1,3,5-trinitroperhydro-1,3,5-triazine) generate predominantly low-molecular-weight products. In agreement with experimental observation, these simulations predict that TATB decomposition quickly (by 30 ps) initiates the formation of large carbonaceous clusters (more than 4000 amu, or approximately 15-30% of the total system mass), and HMX decomposition leads almost exclusively to small-molecule products. We find that HMX decomposes readily on this time scale at lower temperatures, for which the decomposition rate of TATB is about an order of magnitude slower. Analyzing the ReaxFF MD results leads to the detailed atomistic structure of this carbon-rich phase of TATB and allows characterization of the kinetics and chemistry related to this phase and their dependence on system density and temperature. The carbon-rich phase formed from TATB contains mainly polyaromatic rings with large oxygen content, leading to graphitic regions. We use these results to describe the initial reaction steps of thermal decomposition of HMX and TATB in terms of the rates for forming primary and secondary products, allowing comparison to experimentally derived models. These studies show that MD using the ReaxFF reactive force field provides detailed atomistic information that explains such macroscopic observations as the dramatic difference in carbon cluster formation between TATB and HMX. This shows that ReaxFF MD captures the fundamental differences in the mechanisms of such systems and illustrates how the ReaxFF may be applied to model complex chemical phenomena in energetic materials. The studies here illustrate this for modestly sized systems and modest periods; however, ReaxFF calculations of reactive processes have already been reported on systems with approximately 10(6) atoms. Thus, with suitable computational facilities, one can study the atomistic level chemical processes in complex systems under extreme conditions.

Application of the energy gap law to the decay of charge transfer excited states, solvent effects

Abstract Values of non-radiative decay rate constants (knr) and emission energies (Ecm) have been obtained for Os(Phen3)2+ in a series of solvents and the results are consistent with the energy gap law. For hydroxylic solvents like water or methanol related studies suggest the existence of strong, specific contributions to the vibrational trapping energy of the solvent.

A molecular dynamics simulation study of the pressure-volume-temperature behavior of polymers under high pressure.

Isothermal compression of poly (dimethylsiloxane), 1,4-poly(butadiene), and a model Estane (in both pure form and a nitroplasticized composition similar to PBX-9501 binder) at pressures up to 100 kbars has been studied using atomistic molecular dynamics (MD) simulations. Comparison of predicted compression, bulk modulus, and U(s)-u(p) behavior with experimental static and dynamic compression data available in the literature reveals good agreement between experiment and simulation, indicating that MD simulations utilizing simple quantum-chemistry-based potentials can be used to accurately predict the behavior of polymers at relatively high pressure. Despite their very different zero-pressure bulk moduli, the compression, modulus, and U(s)-u(p) behavior (including low-pressure curvature) for the three polymers could be reasonably described by the Tait equation of state (EOS) utilizing the universal C parameter. The Tait EOS was found to provide an excellent description of simulation PVT data when the C parameter was optimized for each polymer. The Tait EOS parameters, namely, the zero-pressure bulk modulus and the C parameter, were found to correlate well with free volume for these polymers as measured in simulations by a simple probe insertion algorithm. Of the polymers studied, PDMS was found to have the most free volume at low pressure, consistent with its lower ambient pressure bulk modulus and greater increase in modulus with increasing pressure (i.e., crush-up behavior).

Phase segregation of diblock copolymers in nanopore geometries

The self-assembly of a diblock copolymer melt in a confined regular geometry with a given pore size is studied using self-consistent field theory. For a particle in a polymer domain, we obtain the interaction potential as a function of the distance from the polymer interface. For a given concentration of particles of a certain size and separation, we find that microphase segregation is sensitive to the characteristic length scales of the geometry. In particular, novel polymer morphologies arise when the size of the pores and the distance between them are comparable to the diblock polymer radius of gyration Rg. Confinement can result in morphologies not allowed in the bulk. However, if the pore size is much larger than Rg, the effects are then limited to the vicinity of the pore surface.

95Mo and 183W NMR studies of triply bonded dinuclear M(III) and related MC (M = Mo or W) complexes

95Mo and 183W NMR data are reported for a series of dinuclear triply bonded M(III) (M = Mo or W unless specified) and related [MCR]3+ complexes. Complexes of general formulae M2L6 (L = [OCMe3]− or [NMe2]−, M = Mo, L = [CH2CMe3]−, [CH2SiMe3]−, [OCH2CMe3]− or [OCHMe2]− and M2(OCHMe2)6L2 (L = C5H5N, 12Me2PCH2CH2PMe2 or 12MeNHCH2CH2NHMe) exhibit very deshielded resonances in the δ-2430–3695 (M = Mo) and δ-4196–4736 (M = W) regions. The novel M2(O2CMe)4(CH2CMe3)2 complexes exhibit resonances which are shielded relative to the above complexes and related M2(O2CR)4 complexes. The M(CXMe3)(CH2XMe3)3 (X = C or Si) complexes, which contain formal MC triple bonds, exhibit resonances at δ 1400 and 1845 (M = Mo), and δ 2867 and 3613 (M = W), respectively. The tungsten alkylidyne complexes exhibit septet 183W resonances due to coupling of 183W and CH2XMe−3 protons (2JW-H = 10.5 Hz). The magnitude of the nuclear shielding in these related species increases in the order: [MM]4+ (σ2π4δ2) < [MM]6+ (σ2π4) < [MM]6+ (π4δ2) ∼ [MC]3+ < [MN]3+. The nuclear deshielding appears to be related to the extreme covalency of the multiple bond in which the metals are involved.

Equation of state and Hugoniot locus for porous materials: P-α model revisited

Foams, porous solids and granular materials have a characteristic Hugoniot locus that for weak shocks is concave in the (particle velocity, shock velocity)-plane. An equation of state (EOS) that has this property can be constructed implicitly from a Helmholtz free energy of the form Ψ(V,T,φ)=Ψs(V,T)+B(φ) where the equilibrium volume fraction φeq is determined by minimizing Ψ, i.e., the condition ∂φΨ=0. For many cases, a Hayes EOS for the pure solid Ψs(V,T) is adequate. This provides a thermodynamically consistent framework for the P-α model. For this form of EOS the volume fraction has a similar effect to an endothermic reaction in that the partial Hugoniot loci with fixed φ are shifted to the left in the (V,P)-plane with increasing φ. The equilibrium volume fraction can then be chosen to match the concavity of the principal Hugoniot locus. An example is presented for the polymer estane. A small porosity of only 1.4 percent is required to match the experimental concavity in the Hugoniot data. This type of...

Morphology of diblock copolymers in porous media

We study the self-assembly of a diblock copolymer melt confined within a porous medium with a prescribed regular two-dimensional geometry using self-consistent field theory. We find that the morphology of the polymer sensitively depends on the characteristic length scales of the porous material and the polymer radius of gyration (Rg). When the pore size is much larger than Rg, the polymer self-assembly is affected only locally close to the contact with the pore surface. However, when the size of the pores and the distance between them is comparable to the diblock characteristic length, novel morphologies appear and the polymer structure changes according to the constraints imposed by the porous material. We develop an interaction potential for the solid particle and copolymer, and show how this provides an understanding of the qualitative feature of the morphologies.

Ordering and reverse ordering mechanisms of triblock copolymers in the presence of solvent.

Self-consistent field theory is used to study the self-assembly of a triblock copolymer melt. Two different external factors (temperature and solvent) are shown to affect the self-assembly. Either one or two-step self-assembly can be found as a function of temperature in the case of a neat triblock melt, or as a function of increasing solvent content (for non-selective solvents) in the case of a triblock-solvent mixture. For selective solvents, it is shown that increasing the solvent content leads to more complicated self-assembly mechanisms, including a reversed transition where order is found to increase instead of decreasing as expected, and re-entrant behavior where order is found to increase at first, and then decrease to a previous state of disorder.

MOLECULAR DYNAMICS STUDIES OF THERMAL INDUCED CHEMISTRY IN TATB

A reactive force field (ReaxFF) is used with molecular dynamics to probe the chemistry induced by intense heating (‘accelerated cook‐off’) of 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB). Large‐system simulations are desired for TATB because of the high degree of carbon clustering expected in this material. Using small, 32‐molecule simulations, we calculate the reaction rate as a function of temperature and compare the Arrhenius‐predicted activation energy with experiment. Decomposition product evolution (mainly N2, H2O, CO2 and graphitic carbon clusters) is followed using a 576‐molecule larger simulation, which also illustrates the effect of system size on both carbon clustering and reaction rate.