Solution-mediated polymorphic transformation of CL-20: An approach to prepare purified form epsilon particles

The solution-mediated polymorphic transformation (SMPT) of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) from form beta to form epsilon was investigated in ethyl acetate-chloroform mixed solvent. The transformation process was revealed by using on-line tools including attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy and in-situ Raman spectroscopy combined with off-line analysis of the solid phase using optical microscopy and powder X-ray diffraction (PXRD). Thermodynamic driving force of the SMPT process was evaluated through solubility determination of the two forms. The results demonstrate that the SMPT process consists of three stages, which are the dissolution of form beta, the nucleation and the growth of form e, and the rate-determining step is the nucleation and growth of form e. The transformation experiments were also performed under different solvent compositions, solid loadings, percentages of seeds and particle sizes of seeds to give deeper insights into the SMPT process. Besides, the transformation kinetics were further investigated by Avrami-Erofeev model to access the effect of temperature on the SMPT process. This study enriches the research of SMPT mechanism for CL-20 and provides a good guidance for obtaining particles with superior crystal form purity. (C) 2018 Elsevier B.V. All rights reserved.