The thermal stability of the polymorphs of hexanitrohexaazaisowurtzitane, Part I

Abstract

Phase transitions in the α‐, β‐, γ‐, and ϵ‐polymorphs of 2,4,6,8,10,12‐hexanitrohexaazaisowurtzitane (HNIW) have been studied as a function of temperature. Described are the results of high temperature equilibrium solvation studies coupled with Fourier transform infrared spectroscopy (FTIR) for the identification of polymorphic conversion. These results are augmented by data in Part II from differential scanning calorimetry (DSC), differential thermal analysis/thermogravimetric analysis (DTA/TGA), and optical hot stage microscopy(6). The thermodynamic stability order of ϵ > γ > α‐hydrate > β is shown, with the epsilon polymorph the most thermodynamically stable phase of HNTW at room temperature. The existence of multiple α‐hydrate phases is shown to complicate the determination of the equilibrium P‐T phase diagram of HNIW.

Thermally Induced Polymorphic Transformation of Hexanitrohexaazaisowurtzitane (HNIW) Investigated by in-situ X-ray Powder Diffraction

“…The different extended orientations of the nitro groups relative to the five-or six-membered rings of HNIW leads to different molecular packing models and different numbers of molecules in a cell, which defines several probable polymorphs of HNIW. So far, five different polymorphs (α, β, γ, ε, and ζ) have been reported and identified [10][11][12][13][14]. Four of the polymorphs (α, β, γ, and ε) exist at ambient pressure and temperature.…”

Kinetics of Polymorphic Transitions in Energetic Compounds

“…The authors supposed that this was due to uneven contact of the powdered sample with the sample pan as a result of changes in morphology and density of the crystals in the course of the PT. A similar fine structure was observed in DSC curves for the PT ε → γ in HNIW [11] and α → β in 1,1-diamino-2,2-dinitroethene (FOX-7) [12], but an unambiguous explanation of the nature of these phenomena was not given in these reports. It is important to note that separate peaks in the DSC curves cannot be attributed to PTs in separate crystals: in all cases the number of peaks forming the fine structure does not exceed several tens, whereas the samples used contained many thousands of crystals.…”

“…As a result, the τ distribution density is discrete, and unlike quasi-homogeneous PTs (Figure 1a), the process proceeds in a stepwise mode (Figures 1b, 2 The stepwise PT kinetics is a rather regular phenomenon. In particular, under nonisothermal conditions it appears as multiple minima in the DSC curves of some samples of HNIW [11], HMX [10] and FOX-7 [12,32]. As noted in [12], the "reaction profile […] is actually a superposition of nucleation events for individual crystallites.…”

Polimorfismo: caracterização e estudo das propriedades de uma fase cristalina

“…Kholod et al (2007) descreveram que a identificação dos polimorfos do CL-20 também poderia ser conduzida com FT-IR ou Raman. Foltz et al (1994) estudaram transições de fase cristalina em polimorfos α, β, γ e ε do CL-20 em função da temperatura, descrevendo os resultados do estudo do equilíbrio de soluções em alta temperatura em ligação com FT-IR, permitindo a identificação da transição polimórfica. O estudo do crescimento de cristal e solvatação foi conduzido em amostras de CL-20 imersas em bis(2-flúor-2,2-dinitroetil)formol (FEFO) aquecido, sendo que o aumento da temperatura entre 35 e 80ºC resulta na dissolução dos polimorfos na sequência β, α, γ e ε. O estudo revelou que solventes com grupos hidroxila (i.e., alcoóis, água) estabilizam preferencialmente a fase polimórfica α, retardando sua conversão para a fase mais estável ε. Todavia, a ordem de solubilidade dos polimorfos não estava em concordância com a ordem de estabilidade de α-hidrato > ε > α-seco > β > γ.…”