Richard H. Gee

Ionic Polymers as a New Structural Motif for High-Energy-Density Materials

Energetic materials have been used for nearly two centuries in military affairs and to cut labor costs and expedite laborious processes in mining, tunneling, construction, demolition, and agriculture, making a tremendous contribution to the world economy. Yet there has been little advancement in the development of altogether new energetic motifs despite long-standing research efforts to develop superior materials. We report the discovery of new energetic compounds of exceptionally high energy content and novel polymeric structure which avoid the use of lead and mercury salts common in conventional primary explosives. Laboratory tests indicate the remarkable performance of these Ni- and Co-based energetic materials, while DFT calculations indicate that these are possibly the most powerful metal-based energetic materials known to date, with heats of detonation comparable with those of the most powerful organic-based high explosives currently in use.

Metal-Organic Frameworks (MOFs) as Safer, Structurally Reinforced Energetics

Second-generation cobalt and zinc coordination architectures were obtained through efforts to stabilize extremely sensitive and energetic transition-metal hydrazine perchlorate ionic polymers. Partial ligand substitution by the tridentate hydrazinecarboxylate anion afforded polymeric 2D-sheet structures never before observed for energetic materials. Carefully balanced reaction conditions allowed the retention of the noncoordinating perchlorate anion in the presence of a strongly chelating hydrazinecarboxylate ligand. High-quality X-ray single-crystal structure determination revealed that the metal coordination preferences lead to different structural motifs and energetic properties, despite the nearly isoformulaic nature of the two compounds. Energetic tests indicate highly decreased sensitivity and DFT calculations suggest a high explosive performance for these remarkable structures.

Ab initio based force field and molecular dynamics simulations of crystalline TATB

An all-atom force field for 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is presented. The classical intermolecular interaction potential for TATB is based on single-point energies determined from high-level ab initio calculations of TATB dimers. The newly developed potential function is used to examine bulk crystalline TATB via molecular dynamics simulations. The isobaric thermal expansion and isothermal compression under hydrostatic pressures obtained from the molecular dynamics simulations are in good agreement with experiment. The calculated volume-temperature expansion is almost one dimensional along the c crystallographic axis, whereas under compression, all three unit cell axes participate, albeit unequally.

Molecular interactions of TATB clusters

Abstract Electronic structure calculations of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) clusters are presented. The calculated gas phase structures of TATB are close to the experimental crystal structures. Two isomers of the TATB dimer are considered. One resembles the structures found experimentally for the A–B sheet of the molecular crystal. The other, a stacked ring (‘C’) configuration, yields an eclipsed structure, while the crystal data suggest two rings displaced relative to each other. Electron correlation effects are of greater importance for the stacked C-dimer than the planar AB-dimer. Furthermore, many-body contributions to the total interaction energy are found to be of limited importance.

Ultrafast crystallization of polar polymer melts

Primary nucleation of poly(vinylidene fluoride) (pVDF) from bulk entangled amorphous polymer melts has been observed from molecular dynamics (MD) simulations. This is the first instance of spontaneous primary nucleation of an entangled bulk polymer in a MD simulation. The melt-to-crystal phase transition occurs via a three-stage process. The ordered phase is found to consist of mainly chain-folded lamella with predominantly adjacent re-entry. The observed crystal polymorphs correspond to either the well known β phase or form I polymorph or a crystal structure that does not coincide with any of the known experimental polymorphs for pVDF, but is found to be strikingly similar to the β structure but with antiparallel alignment of the dipole moments normal to the polymer chain axis. The time required for the onset of nucleation is found to decrease with the number of monomers in the polymer, while the opposite is found for the growth rate of the crystal. We find that the crystallization of the polymer is mediated by electrostatics; in fact the crystal structure spontaneously melts when the electrostatic interactions are removed.

New theoretical insight into the interactions and properties of formic acid: Development of a quantum-based pair potential for formic acid

We performed ab initio quantum-chemical studies for the development of intra- and intermolecular interaction potentials for formic acid for use in molecular-dynamics simulations of formic acid molecular crystal. The formic acid structures considered in the ab initio studies include both the cis and trans monomers which are the conformers that have been postulated as part of chains constituting liquid and crystal phases under extreme conditions. Although the cis to trans transformation is not energetically favored, the trans isomer was found as a component of stable gas-phase species. Our decomposition scheme for the interaction energy indicates that the hydrogen-bonded complexes are dominated by the Hartree-Fock forces while parallel clusters are stabilized by the electron correlation energy. The calculated three-body and higher interactions are found to be negligible, thus rationalizing the development of an atom-atom pair potential for formic acid based on high-level ab initio calculations of small formic acid clusters. Here we present an atom-atom pair potential that includes both intra- and inter molecular degrees of freedom for formic acid. The newly developed pair potential is used to examine formic acid in the condensed phase via molecular-dynamics simulations. The isothermal compression under hydrostatic pressure obtained from molecular-dynamics simulations is in good agreement with experiment. Further, the calculated equilibrium melting temperature is found to be in good agreement with experiment.

Mesoscale modeling of irreversible volume growth in powders of anisotropic crystals

Based on careful thermometric analysis, powders of micron-sized triaminotrinitrobenzene crystallites are shown to display irreversible growth in volume when subjected to repeated cycles of heating and cooling. Such behavior is counterintuitive to typical materials’ response to thermal annealing. However, through coarse-grained simulations using a mesoscale Hamiltonian, the authors quantitatively reproduce irreversible growth behavior in such powdered material. The authors demonstrate that irreversible growth happens only in the presence of intrinsic crystalline anisotropy, and is mediated by particles much smaller than the average crystallite size.

Coarse-grained model for a molecular crystal

Using the energetic material pentaerythritol tetranitrate as a specific example of molecular crystal, we describe the development of a simple coarse-graining procedure by grouping several atoms or whole functional groups into single charge-neutral beads. As compared to fully atomistic calculations the coarse-grained model speeds up simulations by more than two orders of magnitude. Yet, by adjusting only two parameters in the coarse-grained interaction, the model accurately predicts the lattice constants, sublimation energy, pressure-volume curve up to P=10GPa, and energetically the most stable facets. Computed surface and desorption energies, bulk modulus, and equilibrium morphology are reported as well.

Fractal growth in organic thin films: Experiments and modeling

Optical microscopy and atomic force microscopy were used to investigate the solidification process of the organic energetic material pentaerythritol tetranitrate thermally deposited on a silicon surface. The metastable films spontaneously undergo dendrite formation where the measured fractal dimensions indicate a diffusion-limited-aggregation mechanism. The branch growth rate was investigated as a function of temperature and fitted by a theoretical model that takes into account competing thermally activated processes of surface diffusion and molecular desorption. Consideration of the internal molecular degrees of freedom is shown to be essential for quantitative consistency between theory and experiment.

Irreversible volume growth in polymer-bonded powder systems: effects of crystalline anisotropy, particle size distribution, and binder strength

Pressed-powdered crystallites of intrinsically anisotropic materials have been shown to undergo irreversible volume expansion when subjected to repeated cycles of heating and cooling. In a previous letter publication [R. H. Gee et al., Appl. Phys. Lett. 70, 254105 (2007)], we developed a coarse-grained (micron-scale) interaction Hamiltonian for such a system and quantitatively reproduced experimentally observed irreversible growth through explicit molecular dynamics simulations. In this paper, we report (1) recent experiments with a high-density fluoropolymer binder that significantly lowers irreversible growth, (2) identification of a critical interaction parameter of our model that has a strong correlation with binder properties, (3) sensitivity of irreversible growth to the details of particle size and alignment distribution, and (4) a physical picture of irreversible growth in terms of particle displacements.