Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine
(HMX) undergoes
almost a simultaneous transition from melting to decomposition, which
makes direct measurement of the heat of fusion from calorimetric studies
very difficult. The heat of fusion of HMX reported in the literature
is unusually high when compared to those of energetic materials with
similar molecular structures such as 1,3,5-trinitroperhydro-1,3,5-triazine
(RDX) and [3-nitrooxy-2,2-bis(nitrooxymethyl)propyl] nitrate. In this
work, we investigate melting properties with differential scanning
calorimetry (DSC) and theories of solubility thermodynamics. A series
of DSC experiments with heating rates 0.25–20 K/min show that
melting onset of HMX starts appearing at heating rates between 5.5–6.0
K/min. Since the melting endotherm is convoluted with the decomposition
exotherm, we determine the heat of fusion of HMX based on solubility
thermodynamics by measuring the melting point depression of HMX in
the presence of RDX. The heat of fusion obtained from the experiments
is 31.9 ± 3.9 kJ/mol. The measured heat of fusion of HMX is further
collaborated with the value calculated from a solubility thermodynamics
modeling of HMX in various solvents using the nonrandom two liquid
segment activity coefficient model, which is 31.2 ± 9.4 kJ/mol.