Unsupervised Learning-Based Multiscale Model of Thermochemistry in 1,3,5-Trinitro-1,3,5-triazinane (RDX).

The response of high-energy-density materials to thermal or mechanical insults involves coupled thermal, mechanical, and chemical processes with disparate temporal and spatial scales that no single model can capture. Therefore, we developed a multiscale model for 1,3,5-trinitro-1,3,5-triazinane, RDX, where a continuum description is informed by reactive and nonreactive molecular dynamics (MD) simulations to describe chemical reactions and thermal transport. Reactive MD simulations under homogeneous isothermal and adiabatic conditions are used to develop a reduced-order chemical kinetics model. Coarse graining is done using unsupervised learning via non-negative matrix factorization. Importantly, the components resulting from the analysis can be interpreted as reactants, intermediates, and products, which allows us to write kinetics equations for their evolution. The kinetics parameters are obtained from isothermal MD simulations over a wide temperature range, 1200-3000 K, and the heat evolved is calibrated from adiabatic simulations. We validate the continuum model against MD simulations by comparing the evolution of a cylindrical hotspot 10 nm in diameter. We find excellent agreement in the time evolution of the hotspot temperature fields both in cases where quenching is observed and at higher temperatures for which the hotspot transitions into a deflagration wave. The validated continuum model is then used to assess the criticality of hotspots involving scales beyond the reach of atomistic simulations that are relevant to detonation initiation.

Probing ultrafast shock-induced chemistry in liquids using broad-band mid-infrared absorption spectroscopy.

We probe shock-induced chemistry in two organic liquids by measuring broadband, midinfrared absorption in the 800-1400 cm-1 frequency range. To test this new method and understand the signatures of chemical reactions in time resolved vibrational spectra, we compared liquid benzene shocked to unreactive conditions (shocked to a pressure of 18 GPa for a duration of 300 ps) to nitromethane under reactive conditions (25 GPa). We see clear signatures of shock-induced chemistry that are distinguishable from the pressure- and temperature-induced changes in vibrational mode shapes. While shocked benzene shows primarily a broadening and shifting of the vibrational modes, the nitromethane vibrational modes vanish once the shock wave enters the liquid and simultaneously, a spectrally broad feature appears that we interpret as the infrared spectrum of the complex mixture of product and intermediate species. To further interpret these measurements, we compare them to reactive quantum molecular dynamics simulations, which gives qualitatively consistent results. This work demonstrates a promising method for time resolving shock-induced chemistry, illustrating that chemical reactions produce distinct changes in the vibrational spectra.

Electric double-layer structure in primitive model electrolytes: comparing molecular dynamics with local-density approximations.

We evaluate the accuracy of local-density approximations (LDAs) using explicit molecular dynamics simulations of binary electrolytes comprised of equisized ions in an implicit solvent. The Bikerman LDA, which considers ions to occupy a lattice, poorly captures excluded volume interactions between primitive model ions. Instead, LDAs based on the Carnahan-Starling (CS) hard-sphere equation of state capture simulated values of ideal and excess chemical potential profiles extremely well, as well as the relationship between surface charge density and electrostatic potential. Excellent agreement between the EDL capacitances predicted by CS-LDAs and computed in molecular simulations is found even in systems where ion correlations drive strong density and free charge oscillations within the EDL, despite the inability of LDAs to capture the oscillations in the detailed EDL profiles.

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.

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).

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 simulations of detonation instability.

After making modifications to the reactive empirical bond-order potential for molecular dynamics (MD) of Brenner et al. [Phys. Rev. Lett.70, 2174 (1993); 76, 2202(E) (1996)] in order to make the model behave in a more conventional manner, we discover that the updated model exhibits detonation instability, a first for MD. The instability is analyzed in terms of the accepted theory.

Interaction potential for atomic simulations of conventional high explosives.

In an effort to develop a chemically reactive interaction potential suitable for application to the study of conventional, organic explosives, we have modified the diatomic AB potential of Brenner et al. [Phys. Rev. Lett. 70, 2174 (1993); 76, 2202(E) (1996)] such that it exhibits improved detonation characteristics. In particular, equilibrium molecular dynamics (MD) calculations of the modified potential demonstrate that the detonation products have an essentially diatomic, rather than polymeric, composition and that the detonation Hugoniot has the classic, concave-upward form. Nonequilibrium MD calculations reveal the separation of scales between chemical and hydrodynamic effects essential to the Zel'dovitch, von Neumann, and Döring theory.

Examining the chemical and structural properties that influence the sensitivity of energetic nitrate esters.

The sensitivity of explosives is controlled by factors that span from intrinsic chemical reactivity and chemical intramolecular effects to mesoscale structure and defects, and has been a topic of extensive study for over 50 years. Due to these complex competing chemical and physical elements, a unifying relationship between molecular framework, crystal structure, and sensitivity has yet to be developed. In order to move towards this goal, ideally experimental studies should be performed on systems with small, systematic structural modifications, with modeling utilized to interpret experimental results. Pentaerythritol tetranitrate (PETN) is a common nitrate ester explosive that has been widely studied due to its use in military and commercial explosives. We have synthesized PETN derivatives with modified sensitivity characteristics by substituting one -CCH2ONO2 moiety with other substituents, including -CH, -CNH2, -CNH3X, -CCH3, and -PO. We relate the handling sensitivity properties of each PETN derivative to its structural properties, and discuss the potential roles of thermodynamic properties such as heat capacity and heat of formation, thermal stability, crystal structure, compressibility, and inter- and intramolecular hydrogen bonding on impact sensitivity. Reactive molecular dynamics (MD) simulations of the C/H/N/O-based PETN-derivatives have been performed under cook-off conditions that mimic those accessed in impact tests. These simulations infer how changes in chemistry affect the subsequent decomposition pathways.

Influence of interatomic bonding potentials on detonation properties.

The dependences of the macroscopic detonation properties of a two-dimensional (2D) diatomic (AB) molecular system on the fundamental molecular properties were investigated. This includes examining the detonation velocity, reaction zone thickness, and critical width as functions of the exothermicity (Q) of the gas-phase reaction [AB --> (1/2)(A(2) + B(2))] and the gas-phase dissociation energy (D(e)(AB)) for AB --> A + B . Following previous work, molecular dynamics (MD) simulations with a reactive empirical bond-order potential were used to characterize the shock-induced response of a diatomic AB molecular solid, which exothermically reacts to produce A2 and B2 gaseous products. Nonequilibrium MD simulations reveal that there is a linear dependence between the square of the detonation velocity and both of these molecular parameters. The detonation velocities were shown to be consistent with the Chapman-Jouguet (CJ) model, demonstrating that these dependences arise from how the equation of state of the products and reactants are affected. Equilibrium MD simulations of microcanonical ensembles were used to determine the CJ states for varying Q 's, and radial distribution functions characterize the atomic structure. The character of this material near the CJ conditions was found to be somewhat unusual, consisting of polyatomic clusters rather than discrete molecular species. It was also found that there was a minimum value of Q and a maximum value of (D(e)(AB)) for which a pseudo-one-dimensional detonation could not be sustained. The reaction zone of this material was characterized under both equilibrium (CJ) and transient (underdriven) conditions. The basic structure is consistent with the Zeldovich-von Neumann-Döring model, with a sharp shock rise and a reaction zone that extends to 200-300 Angstrom. The underdriven systems show a buildup process which requires an extensive time to approach equilibrium conditions. The rate stick failure diameter (critical width in 2D) was also found to depend on Q and (D(e)(AB)). The dependence on Q could be explained in terms of the reaction zone properties.

Intervalence Transfer at the Localized-to-Delocalized, Mixed-Valence Transition in Osmium Polypyridyl Complexes.

The mixed-valence complexes [(bpy)(2)(Cl)Os(III)(BL)Os(II)(Cl)(bpy)(2)](3+) and [(tpy)(bpy)Os(III)(BL)Os(II)(bpy)(tpy)](5+) (bpy is bipyridine; tpy is 2,2':6',2' '-terpyridine; BL is a bridging ligand, either 4,4'-bipyridine (4,4'-bpy) or pyrazine (pz)) have been prepared and studied by infrared and near-infrared measurements in different solvents. For BL = 4,4'-bpy, there is clear evidence for localized Os(II) and Os(III) oxidation states in the appearance of the expected two interconfigurational dpi --> dpi bands at Os(III) and additional, broad absorption features in the near-infrared arising from intervalence transfer (IT) transitions. For [(bpy)(2)(Cl)Os(pz)Os(Cl)(bpy)(2)](3+) and [(tpy)(bpy)Os(pz)Os(bpy)(tpy)](5+), unusually intense nu(pz) bands appear in the infrared at 1599 cm(-)(1) (epsilon = 2600 M(-)(1) cm(-)(1)) for the former and at 1594 cm(-)(1) (epsilon = 2020 M(-)(1) cm(-)(1)) for the latter. They provide an oxidation state marker and evidence for localized oxidation states. A series of bands appear in the near-infrared from 2500 to 8500 cm(-)(1) that can be assigned to a combination of interconfigurational dpi --> dpi and IT transitions. In CD(3)CN, in the mid-infrared, bands arising from nu(bpy) ring stretching modes from 1400 to 1500 cm(-)(1) are averaged for [(bpy)(2)(Cl)Os(pz)Os(Cl)(bpy)(2)](3+) or significantly perturbed for [(tpy)(bpy)Os(pz)Os(bpy)(tpy)](5+) compared to electronically isolated Os(II) and Os(III) complexes. The pyrazine-bridged complexes have properties that place them in a new class of mixed-valence molecules, Class II-III having properties associated with both Class II and Class III in the Robin and Day classification scheme. The characteristic features of this class are that oxidation states are localized because of vibrational coupling but that solvent orientational motions are uncoupled because of rapid intramolecular electron transfer.

Mid-Infrared Spectrum of [Ru(phen)(3)](2+).

Time-resolved infrared spectra in the fingerprint region (1300-1700 cm(-)(1)) are reported for the metal-to-ligand charge-transfer (MLCT) excited state(s) of [Ru(phen)(3)](2+) and [Os(phen)(DAS)(2)](2+) (phen is 1,10-phenanthroline; DAS is 1,2-bis(diphenylarsino)ethane) in acetonitrile-d(3) at 298 K. The spectra are assigned by comparison to electrochemically generated [Ru(III)(phen)(3)](3+) and [Ru(II)(phen(*)(-)())(phen)(2)](+). The data provide clear evidence for the localized description [Ru(III)(phen(*)(-)())(phen)(2)](2+) on the approximately 100 ns time scale. They also give insight into electronic distribution in the excited state, aid in the interpretation of the time-resolved resonance Raman spectrum of [Ru(phen)(3)](2+), and suggest why measuring ground- and excited-state resonance Raman spectra of phen complexes is difficult.

Model-free test of local-density mean-field behavior in electric double layers.

We derive a self-similarity criterion that must hold if a planar electric double layer (EDL) can be captured by a local-density approximation (LDA), without specifying any specific LDA. Our procedure generates a similarity coordinate from EDL profiles (measured or computed), and all LDA EDL profiles for a given electrolyte must collapse onto a master curve when plotted against this similarity coordinate. Noncollapsing profiles imply the inability of any LDA theory to capture EDLs in that electrolyte. We demonstrate our approach with molecular simulations, which reveal dilute electrolytes to collapse onto a single curve, and semidilute ions to collapse onto curves specific to each electrolyte, except where size-induced correlations arise.

Simulations on the thermal decomposition of a poly(dimethylsiloxane) polymer using the ReaxFF reactive force field.

To investigate the failure of the poly(dimethylsiloxane) polymer (PDMS) at high temperatures and pressures and in the presence of various additives, we have expanded the ReaxFF reactive force field to describe carbon-silicon systems. From molecular dynamics (MD) simulations using ReaxFF we find initial thermal decomposition products of PDMS to be CH(3) radical and the associated polymer radical, indicating that decomposition and subsequent cross-linking of the polymer is initiated by Si-C bond cleavage, in agreement with experimental observations. Secondary reactions involving these CH(3) radicals lead primarily to formation of methane. We studied temperature and pressure dependence of PDMS decomposition by following the rate of production of methane in the ReaxFF MD simulations. We tracked the temperature dependency of the methane production to extract Arrhenius parameters for the failure modes of PDMS. Furthermore, we found that at increased pressures the rate of PDMS decomposition drops considerably, leading to the formation of fewer CH(3) radicals and methane molecules. Finally, we studied the influence of various additives on PDMS stability. We found that the addition of water or a SiO(2) slab has no direct effect on the short-term stability of PDMS, but addition of reactive species such as ozone leads to significantly lower PDMS decomposition temperature. The addition of nitrogen monoxide does not significantly alter the degradation temperature but does retard the initial production of methane and C(2) hydrocarbons until the nitrogen monoxide is depleted. These results, and their good agreement with available experimental data, demonstrate that ReaxFF provides a useful computational tool for studying the chemical stability of polymers.

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.

A novel method for static equation-of state-development: equation of state of a cross-linked poly(dimethylsiloxane) (PDMS) network to 10 GPa.

Pressure-volume-temperature (PVT) equation-of-state (EOS) information for polymers and polymeric composites is valuable for predicting their response to extreme conditions. An obstacle in determining equations of state for polymeric materials is the lack of a simple, static experimental method for acquiring PVT data for solid networks and liquids at pressures greater than several kilobars. Here, we report a novel approach in determining static EOS for polymers using high-pressure diamond-anvil cells coupled with optical microscopy and image analysis. Results are presented for a cross-linked poly(dimethylsiloxane) polymer, Sylgard 184. Static isothermal results were fitted to empirical and semiempirical equations of state, including the Tait, Birch-Murnaghan, and Vinet forms. Static PV data were also converted to pseudoshock velocity-pseudoparticle velocity (U(s)-u(p)) for comparison to dynamic Hugoniot data. A linear Rankine-Hugoniot fit U(s)=s(T)u(p)+c(T) gives c(T)=1.572 km/s and s(T)=1.703. s(T) is related to the pressure derivative of the bulk modulus B(0) (') by s(T)=(B(0) (')+1)/4 and B(0) (')=5.8. A comparison of the static and shock data is given, along with an estimate of the Grüneisen parameter, and a discussion of the free volume content in the polymer network, and limitations of this novel method.