MOLECULAR MODELING: TOWARD A REALISTIC APPROACH TO MODEL ENERGETIC MATERIALS

摘要

Models of plastic-bonded explosives were created with the aim of studying the mechanical properties and sensitivity because the latter is one of the most important problems in relation to energetic materials. Previous models proposed in the literature used short plastic chains, which are appropriate for interaction modeling. In the present work, a model with a single, long chain was built, which is more appropriate far modeling mechanical properties. The representative hydroxyl-terminated polybutadiene (HTPB)/dioctyladipate (DOA)/cyclotrimethylenetrinitramine (RDX) system zoas used (81.4 w/w% of RDX and 18.6 w/w% of the amorphous HTPB/DOA phase, with a 60/40 ratio between the polymer and plasticizer). The HTPB chain was composed of 48 trans groups, 16 cis groups, and 16 vinyl groups. Due to the length of the chain, superposing the crystalline RDX cell [cleaved at the crystalline (2 0 0), (0 2 0), and (2 1 0) planes] to the amorphous HTPB/DOA cell introduced considerable void, and therefore resulted in low density-much more so than when using models with shorter chains. A compression/minimization iterative procedure was used to converge to the optimal density. Pair distributions were calculated to verify that the procedure did not lead to abnormal changes in the RDX crystal model. Comparable energies were obtained for models built with each cleavage plane, contrary to previous work with small molecules. Long chains have lower entropy and are less able to change conformations and maximize interactions with the crystal surface. Models with densities higher than the minimum value were shown to have energy stored in two main components; i.e., the internal energy was stored mainly in the bond and torsion contributions, whereas the external energy storage was performed by van der Waals interactions. These preliminary models show the potential for studying the sensitivity of explosives through molecular modeling.