(612h) Determination of Enthalpy of Fusion of an Energetic Material, HMX, That Undergoes Simultaneous Fusion and Decomposition

Cyclotetramethylene tetranitrate (HMX), a commonly used energetic material, undergoes a nearly simultaneous transition from melting to decomposition, which makes direct measurement of enthalpy of fusion from calorimetric study very difficult. The enthalpy of fusion reported in the literature is very high, 69.9 kJ/mol, when compared to those of chemically similar molecules. For example enthalpy of fusion of trimethylene trinitramin (RDX) is 34.5 kJ/mol. In this work, we investigate melting properties of HMX with a differential scanning calorimetry (DSC) and theories of solution thermodynamics. A series of experiments with a DSC at the heating rates 0.25â??20 K/min show the melting onset of HMX starts appearing at heating rate 5.5â??6.0 K/min. Since the melting endotherm is convoluted with the decomposition exotherm, we extend the investigation to determine enthalpy of fusion of HMX from melting point depression of HMX in presence of RDX. The enthalpy of fusion value determined from the melting point depression are further compared with the corresponding value obtained from the thermodynamic modeling of HMX solubility in solvents based on NRTL-SAC activity coefficient model. The melting point depression experiments and the solubility modeling work show that enthalpy of fusion of HMX is 31.9 ± 3.9 kJ/mol and 31.5± 9.5 kJ/mol, respectively.