Atomistic studies of RDX and FOX-7 -Based Plastic-Bonded explosives: molecular dynamics simulation

Molecular dynamics simulations were carried out to study the effects of interface interactions between a crystalline structure and a plastic bonded explosive (PBX) system. In this work, the hydroxyl-terminated polybutadiene (HTPB) represents the polymer, isophorone diisocyanate (IPDI) is the diisocyanate and dioctyl adipate (DOA) is the plasticizer. Two different crystal high explosives components were used, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), which is characterized by a high chemical stability, and 1,1-diamino-2,2-dinitroethylene (FOX-7) which is known for its low sensitivity. The molecular model is composed of RDX or FOX-7 which represent the main body and PBXs which contain a small amount of polymer/plasticizer. The polymer and crystal were modeled using the atomistic classical force-field COMPASS. Interface structures of RDX and FOX-7 (hkl) crystal surface together with the HTPB-IPDI/DOA blend were predicted as well as interfacial binding energies. Bonding energy calculations have been performed in order to investigate the adhesion of the polymer to diverse atomic (hkl) planes of crystals. The (020) surface plane of RDX and (010) surface plane of FOX-7 have the largest binding energies and therefore a strongest ability to interact with the polymer was observed. NPT ensemble molecular dynamics simulation was applied to study specific mechanical properties: Poisson’s ratio and various moduli such as Young's E, bulk K and shear G, for RDX and FOX-7 -based PBXs. The investigation of mechanical properties shows that the (020) plane providing HTPB-IPDI-DOA/RDX blends is more flexible and ductile. However, for FOX-7-based blends, the three crystallographic planes that were explored present surprisingly comparable ductility.