Molecular Dynamics Simulation of Interface Interactions and Mechanical Properties of CL-20/HMX Cocrystal and Its Based PBXs

To enhance practical values and improve safety and mechanical properties of cocrystal explosives, molecular dynamics simulations were conducted to investigate the interface interactions and mechanical properties of CL-20/HMX cocrystal based polymer-bonded explosives (PBXs). Two polymers, poly(ester urethane) block copolymer (Estane 5703) and hydroxyl-terminated polybutadiene (HTPB), were respectively put along crystalline surface (1 0 0). Totally, two PBX simulation models were built. The interfacial binding energies between the crystalline surface and polymers were calculated and the binding energy between Estane 5703 and the cocrystal (1 0 0) surface is larger than that of HTPB. This indicates that the stability and compatibility of the PBX containing a small amount of Estane 5703 is better. The interface structures of CL-20/HMX cocrystal (1 0 0) surface with the two polymers were analyzed using pair correlation function (PCF). The results show that hydrogen bonds of H atoms in the cocrystal with carbonyl-O atoms in Estane 5703 and with hydroxyl-O atoms in HTPB are stronger. The elastic constants, moduli and Poisson ratio of the cocrystal and the coaystal-based PBXs were calculated based on the fluctuation analysis of production trajectories and Reuss average. Compared with the cocrystal, the mechanical properties of the PBXs containing a small amount of binder Estane 5703 or HTPB have changed apparently. It is found that the elastic constants (C-ij), tensile modulus (E), bulk (K) and shear (G) modulus all decrease, while Poisson ratio (v), Cauchy pressure (C-12-C-44) and K/G all increase. The results show that the stiffness of the PBXs system is weaker, and its elasticity and ductibility is better. And the small amount of polymer binders coating with the cocrystal makes the PBXs more insensitive. The desensitization is mainly attributed to the heat insulation and absorption of the polymers and the buffer action of the system softened by the polymers, while the bond length change for trigger bond caused by interface interactions plays a minor role.