Molecular Dynamics Simulation Study On The Structure And Properties Of CL-20 Cocrystal And Their Composite Materials

In this dissertation, molecular dynamics (MD) simulation was conducted for CL-20/HMX cocrystal, CL-20/DNB cocrystal and their composite materials using COMPASS force field with NPT ensemble at different temperatures.Firstly, MD simulation was conducted for s-CL-20 crystal, β-HMX crystal, CL-20/HMX cocrystal and its composite with the same molar ratio as in the cocrystal at different temperatures. Their maximum bond lengths (Lmax) of N-NO2 trigger bond, cohesive energy densities (CED) and binding energies (-Ebind) between HMX and CL-20 molecules as well as elastic properties were calculated. Lmax increases with rising temperature and is found to be in the order of CL-20/HMX cocrystal<CL-20/HMX composite<ε-CL-20 crystal at the same temperature. CED and-Ebind of the cocrystal decrease with increasing temperature and are all greater than those of the composite at the same temperature. These indicate the cocrystal is the most insensitive, and its safety and thermal stability is better than that of the composite. These are mainly attributed to the interactions (hydrogen bond and vdW) between CL-20 and HMX in the cocrystal are much stronger than in the composite. Furthermore, the pair correlation function g(r) reveals that the hydrogen bond interactions mainly derive from H of CL-20 and O of HMX, H of HMX and O of CL-20. Compared with ε-CL-20 and P-HMX, the stiffness of cocrystal and composite is weaker and ductibility is improved. However, CL-20/HMX cocrystal is still difficult to apply for practical use directly due to its brittleness, adding a small amount of polymer binder to prepare cocrystal-based PBXs (polymer bonded explosives) is necessary. Then 295 K MD simulation was conducted for CL-20/HMX cocrystal based PBXs by adding two polymers, poly(ester urethane) block copolymer (Estane 5703) and hydroxyl-terminated polybutadiene (HTPB) respectively. The results show 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 surface with the two polymers were analyzed using g(r). Compared with the cocrystal, the mechanical properties of the PBXs containing a small amount of binder Estane 5703 or HTPB are all improved. And the desensitization mechanism of polymer binders is also revealed.For CL-20/DNB cocrystal and its composite materials, MD simulation was conducted for DNB crystal, ε-CL-20 crystal, CL-20/DNB cocrystal and CL-20/DNB composite at different temperatures firstly, similar conclusion was obtained:CL-20/DNB cocrystal is more insensitive than its composite that mix two single explosives simply. And the calculated mechanical data show that the stiffness of CL-20/DNB cocrystal and its composite is weaker and their ductibility is better than those of s-CL-20 and DNB crystals. To enhance practical values and improve safety and mechanical property of CL-20/DNB cocrystal explosive, the relative sensitivity order of different crystalline surfaces was deduced by using the sensitivity criterion of Lmax at first, then MD simulation was conducted for CL-20/DNB cocrystal based PBXs. Two polymers, HTPB and polyethylene glycol (PEG), were respectively put along sensitive crystalline surface (001). The results show that the stability and compatibility of PBX containing PEG is better. The stiffness of the PBXs system is weaker and ductibility is better than those of the cocrystal. And the ductibility of PBX containing HTPB is better and indicating that the desensitizing effect of HTPB by improving the cocrystal explosive’s mechanical properties is better.