MD simulation study on the mechanical properties of HMX crystals and HMX/F₂₃₁₁ PBXs

Molecular simulation was applied to investigate the mechanical properties of β-octahydro- 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystals and the polymer-bonded explosives (PBXs) which were made from the HMX as a base and F₂₃₁₁ as a binder, in NVT ensemble using COMPASS force field. The PBXs' model was constituted by putting F₂₃₁₁ on crystal (001), (010) and (100) surface respectively. At 295 K, the elastic constants and the effective moduli were calculated by statistic analysis mode. Cauchy pressure C₁₂-C₄₄ was also calculated. It was shown that the rigidity was weakened and the ductibility was improved by adding small amount of F2311, and the ductibility improvement was most obvious when F2311 was put on HMX (100) crystal surface. From the temperature 245 to 445 K with the increase step of 50 K, pure HMX crystals and HMX (100)\F₂₃₁₁ PBX were allowed to dynamically evolve. It is indicated that the mechanical properties of pure HMX crystals are unchanged because thermal effect of the volume is constrained by NVT ensemble. However, the ductibility of HMX (100)\F₂₃₁₁ PBX changes according to a parabola with increasing temperature due to the improvement of F₂₃₁₁ chain moving ability and the simultaneous increment of high energy conformation ratio in this molecular chain, i.e., the increment of the molecular chain rigidity.