Molecular Simulation On The Formation Mechanism Of Co-crystal HMX/TATB And Morphology Of TATB

Cyclotetramethylene tetranitramine (HMX) is an important and commonly used energetic ingredient in various high performance explosives, and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is the so-called wood explosive widely used with its steady properties. Co-crystal explosive is hopeful to provide a new and more effective way for the explosive modifacation,which with unique structure and properties for explosive components are combined at the molecular level. In this study, by applying the theories of modern quantum chemistry, molecular mechanics and molecular dynamics, systematic computation and simulation are carried out to design the new explosive of HMX/TATB co-crystal using two models. In addition, a corrected attachment energy model and a "double-layer stucture" model are applied to predict the morphological importance of a crystal surface in solvent. The results are compared with the observed experimental TATB morphology grown from the solution. The outline of this dissertation is as follows:1. Different models of HMX/TATB co-crystal are established by replacing HMX with TATB on the important faces of HMX and random replacing, and molecular dynamics (MD) simulation is used to get their equilibrium structures. The X-ray powder diffraction (XRD) is simulated and the total energy was calculated based on the equilibrium structures. The results indicate that the XRD peaks of HMX/TATB co-crystal models are similar to those of the main component of HMX and some new peaks are observed compared to the XRD peaks of the pure HMX. Energy calculations show that the co-crystal structure is the most stable when the HMX molecule is substituted by TATB on the surface (011) of HMX with the lowest surface free energy and the slowest growth rate. It can be predicted that HMX molecule would be more easily substituted by TATB on the low free energy surface of HMX to obtain the stable co-crystal structure and decrease the sensitivity of HMX in the preparation of the HMX/TATB co-crystal explosive. 2. HMX/TATB (molar ratio 1:1) is designed based on crystal engineering. Crystal structure is predicted using Polymorph Predictor method. The main properties of co-crystal consisting of mechanical properties, stability, and intermolecular interaction energy of HMX dimmer, TATB dimmer and HMX/TATB are simulated through molecular dynamics methods. Simulated results indicate that the crystal structure of the HMX/TATB co-crystal may belong to the P1, P212121 or P21/c space group. According to the binding energy, the stability order of the dimers is HMX/TATB> HMX dimer> TATB dimmer, which means that the competition of the heterodimer is much bigger than the homodimer. The analysis for radial distribution function show that the interactions between two molecular components HMX and TATB consist of the electrostatic, hydrogen bonding and strong van der Waals. The new co-crystal has better mechanical properties with the moduli systematically decreased.3. The solvent effect on the crystal morphology of TATB is studied experimentally and theoretically. Molecular dynamics simulations are performed for crystal faces of TATB in contact with solvent. A corrected attachment energy model, accounting for the surface chemistry, is applied to predict the morphological importance of a crystal surface in solvent. From the solvent-effected attachment energy calculations it follows that the (01-1) face may be disappeared.In summary, the combinative methods of quantum chemistry, molecular mechanics and molecular dynamics have been applied to predict and simulate the relation between the structure and the properties of HMX/TATB co-crystal. These studies are originally innovation in the field of the explosive modification. In addition, the study of TATB crystal habit in the solvent will provide avaluable information for selecting suitable solvent to control the crystallization process of explosives.