| The energetic nitramines 1,3,5,7-tetranitro-1,3,5,7-tetraazocine (HMX) and 1,3,5-trinitro-s-triazine (RDX) were studied by Raman and Fourier transform infrared and nuclear quadrupole resonance spectroscopy.; HMX has extensive polymorphism. A phase diagram was constructed for the (beta) (DBLARR) (delta)-HMX polymorph transformation up to 90,000 psig and 250(DEGREES)C using Raman spectroscopy. It was found that: (1) the slow decomposition of HMX is particle size dependent, (2) slope breaks appear and are attributable to retention of decomposition products, and (3) (delta)-HMX is stable under rocket motor conditions.; The kinetics of the (beta) (--->) (delta), (alpha) (--->) (delta), and (gamma) (--->) (delta)-HMX polymorph transitions were studied by infrared spectroscopy. The data indicate that the polymorph stability scheme originates in kinetic rather than thermodynamic factors. The similarity of the phase transition and condensed phase thermal decomposition Arrhenius data indicates that the rate controlling step in the condensed phase decomposition is the disruption of the intermolecular forces. Under rocket motor conditions, the (beta) (--->) (delta)-HMX transition is rapid enough to occur before the crystallites undergo further chemistry.; The librational motion of RDX was studied by ('14)N NQR and reveals the relative amount of intermolecular cohesion compared to (beta)-HMX. RDX is a less rigid lattice than HMX, and the intermolecular forces play less of a role on the condensed phase thermal decomposition of RDX.; Spectroscopic evidence was found for the unstable polymorph of RDX ((beta)-RDX). The molecular symmetry of (beta)-RDX (C(,3v)) is higher than the more stable modification, (alpha)-RDX (C(,s)). In solution and in the vapor phase, RDX is a very flexible molecule having overall C(,3v) symmetry similar to that in solid (beta)-RDX.; Liquefaction of HMX and RDX was studied at high heating rate. The previously uncharacterized "melt" was found to consist initially of at least 95% pure nitramine. Consistent with the conclusions above, HMX exhibits more decomposition in the condensed phase than does RDX. |
