Polymorphism of HMX

Simultaneous TGA and DTA were used by Maycock et al. to study the kinetics of the isothermal and adiabatic thermal decomposition of 8-HMX (one of the polymorphs of HMX) under helium atmosphere and reported activation energy [44] —63 KcalmoT1. The use of simultaneous DTA/TG has been reported for compatibility testing of TNT with two epoxy-and two alkyd resins paints [21, 45]. 

The polymorphs of HMX may be obtd also by heating, for example, beta HMX is converted to alpha above 102-104.5° alpha to gamma above 160-64°, gamma is metastable to 160-64° and above 160-64° to melting point delta is formed. However, these values are controversial (See Refs 15 28).More detailed info on the prqxi of HMX polymorphs is given in Refs 5, 29 30. 

The order of sensitivity of the polymorphs of HMX may be summarized as follows delta >gamma > alpha >beta. The range of values obtd using an Explosives Research Laboratory, Bruceton,... 

Titrimetric detn of RDX HMX with ferrous sulfate) 28) H.C. Cady L.C. Smith, "Studies on the Polymorphs of HMX , Los Alamos Scientific Lab Rept LAMS 2652 (1961) 29) J-T. Rogers, "Physical and Chemical Properties of RDX and HMX , Holston Defense Corp Control Rept 20-P-26A (1962) (Conf, not used as a source of info) 30) US Military Spec MIL-H-45444A (Ord) Amend 3(1962) 31) W.R. Tomlinson O.E. 

In this chapter we focus on atomistic predictions of thermophysical and mechanical properties of HMX crystals and liquid important to the development of reliable mesoscale equations of state. The outline of the remainder of the chapter is as follows In section 2 we describe briefly the philosophy and overall approach we have taken to force field development, including the results of quantum chemistry calculations for HMX and smaller model compounds that were used in the force field parameterization. The focus of section 3 is on the properties of liquid HMX, for which experimental data are completely lacking. Structural, thermal, and mechanical properties of the three pure crystal polymorphs of HMX are presented in section 4, where the results are compared to the available experimental data. At the ends of sections 3 and 4 we discuss briefly the importance of the various properties with mesoscale models of high explosives, with an emphasis on conditions relevant to weak shock initiation. We conclude in section 5, and provide our opinions (and justifications, based on our interactions with mesoscale modelers) regarding which HMX properties and phenomena should comprise the next targets for study via atomistic simulation. 

Force field validation. In addition to ensuring that the force field reproduces results of QC calculations we have compared predictions of MD simulations using this force field with the available experimental data. Gas phase MD simulations using the quantum-chemistry based force field accurately reproduced the gas phase structure of DMNA as determined from electron diffraction studies. Liquid phase MD simulations of DMNA predicted the densities and solubility parameter as well as the activation energy and correlation times associated with molecular reorientation that are in good agreement with experimental data [34], As we will show in Section 4, comparison to structural and thermal data for the three pure crystalline polymorphs of HMX support the overall validity of our formulation and parameterization. 

T-Blomquist, "The Polymorphism of HMX , ORSD Rept 1227 (Feb 1943) 4)W.E.Bachmann, "The Preparation nf OSRD Rept 1981... 

Cady, H. H. and Smith, L. C. (1962). Studies on the polymorphs of HMX. Los Alamos Scientific Laboratory LAMS-2652, Chemistry (THD-4500 17th ed.) based on Contract W-7405-ENG 36 with the U. S. Atomic Energy Commission. [278,279t, 280] Cady, H. H. and Larson, A. C. (1975). Pentaerythritrol tetranitrate II Its crystal structure and transformation to PETNI, an algorith for refinement of crystal structures with poor data. Acta Crystallogr. B, 31, 1864-9. [284]... 

Aa interesting variation of this emphasis upon the trigger linkage was offered by Kohno et al [47-49], who noted that the N-NO, distances in a group of seven nitramines (including RDX and the polymorphs of HMX) are shorter in the crystal than in the gas phase, by 0.05 - 0.08 A. This does not occur with the C-N and N-O bonds. They suggested that this compression introduces destabilizing strain into the crystal, and were able... 

Cady HH, Smith LC (1962) Studies on the polymorphs of HMX. Report LAMS-2652. Los Alamos Scientific Laboratory, Los Alamos, NM... 

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