Probing Crystal Growth of ε- and α-CL-20 Polymorphs via Metastable Phase Transition Using Microscopy and Vibrational Spectroscopy

Abstract

This article presents the evidence of possible route to the formation of ε-and α-polymorphic phases of 2, 4,6,8,10,4,6,8,10, studied through CL-20 solution using reverse and normal precipitation method. Reverse precipitation with instant addition facilitated with the opportunity to track the crystal phases from their immediate formation to end of phase stabilization. Precipitation under apparent conditions to achieve α-or ε-phases, showed initial occurrence of metastable β-phase and subsequent transformation to the intended stable phases. The β-phase showed sufficiently longer stability while under specified conditions for ε-than in a hydrated medium set to obtain the α-phase.Transformation of fine needle shaped β-CL-20 crystals to uniform diamond shaped α-or bipyramidal ε-habit had been observed to pass through an equilibrium state of dissolution and reprecipitation. This work also elaborates the effect of crystallization methodology on conversion time. Vibrational spectroscopy and microscopic techniques were employed to track the time dependent polymorphic conversions. Drastic reduction in β → ε conversion time, from 160 minutes to 10 minutes could have been affected by using ultra dispersed seed crystals. We thus also demonstrated a hazard free non-grinding method to prepare ε-CL-20 with particle size <10 µm through precipitation and their effect on thermal stability & mechanical sensitivity.

Rapid and High‐Yielding Formation of CL‐20/DNDAP Cocrystals via Self‐Assembly in Slightly Soluble‐Medium with Improved Sensitivity and Thermal Stability

“…It is well known that CL-20 can be crystallized into four different polymorphs under ambient temperature and pressure conditions, including α-, β-, γ-, and ɛ-, while the hemihydrate α-form is preferentially obtained even small amounts of water present in the reaction medium. [36] However, by adding ɛ-CL-20 and DNDAP to pure water after a few days, we found unexpectedly that some cocrystals precipitated out. To optimize the reaction conditions, three slightly soluble-solvents: H 2 O, H 2 O/methyl acetate, and isopropanol were selected as the reaction medium.…”

Evaluation of a CL‐20/TATB Energetic Co‐crystal

“…3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 include the a-phase as diamonds, b-phase as needles, and e-phase as bipyramidal crystals [19,20]. Recrystallization of CL-20 matches well with pure a-phase and confirms what has been reported for recrystallized a-CL-20 [18,19]. SEM imaging shows that 1 adopts a unique crystal morphology when compared to the physical mixture of recrystallized components.…”

“…Ghosh et al [19] reported that a-CL-20 was distinguishable in FTIR spectroscopy by the presence of a low intensity hydrate peak at about 3700 cm À1 and a triplet-like feature near 745 cm À1 (739, 750, and 763 cm À1 ). CL-20 recrystallized from DMSO/Water agreed well with the reported spectra and retained a sharp hydrate peak at 3693 cm À1 and a triplet at 739, 750, and 763 cm À1 found in Figure 5.…”

Study on the Crystallization Rates of β‐ and ϵ‐form HNIW in in‐situ Raman Spectroscopy and FBRM

“…Hexanitrohexaazaisowurtzitane (HNIW) is one of the high energy materials (HEMs) that have been developed to obtain an improved performance and low sensitivity. HNIW performs well because it has a higher oxygen balance, density, and detonation velocity than other energetic nitramines such as RDX and HMX . HNIW has five polymorphs because of different molecular structures.…”