Study On The Mechanisms Of Polymorphic Transformation And Solvate Formation For CL-20

The sensitivity of energetic material to mechanical stimuli have become increasingly significant during storage,transport and handling processes.Several approaches have been proposed to enhance the safety of energetic material,such as the improvement of crystal form purity and the development of novel multi-component crystals.Hexanitrohexaazaisowurtzitane(CL-20)is the most powerful commercially available explosive and has been successfully applied in military industry,whereas the high cost and sensitivity limit its further application.In this study,CL-20 was selected to investigate the polymorphic transformation and solvate formation mechanism based on the combination of experimental research,structural analysis and theoretical calculation.The content of this research is as follows:Firstly,the thermodynamic properties of ε-CL-20 were investigated.Solubility of ε-CL-20 in eight kinds of pure solvents and four groups of binary solvent systems was determined using the isothermal gravimetrical method from 288.15 K to 313.15 K under0.1 MPa.The experimental solubility data was correlated by four thermodynamic models,and the modified Apelblat model gives the best correlation results.Moreover,the apparent thermodynamic properties for ε-CL-20 dissolution were also estimated under the experimental conditions.The results confirm that the apparent standard dissolution enthalpy and entropy of ε-CL-20 are related to solvent systems.Secondly,the solution-mediated polymorphic transformation of CL-20 from form βto form ε was investigated in ethyl acetate + chloroform binary solvent system using on-line tools combined with off-line analysis.The results reveal that the solutionmediated polymorphic transformation process consists of three stages,which are the dissolution of form β,the nucleation and the growth of form ε,and the rate-determining step is the nucleation and growth of form ε.The effects of solvent composition,temperature,solid loading,percentage of seeds and particle size of seeds on the transformation process were analyzed from the perspective of thermodynamics and kinetics.Besides,the transformation kinetics were further investigated by AvramiErofeev model.This study provides a good guidance for the control of polymorphic transformation and the preparation of form ε particles with superior crystal form purity.Thirdly,the structural characteristics and formation mechanism of CL-20 solvates were investigated.Six novel CL-20 solvates were obtained from solvate screening experiment.Crystal structures were elucidated to analyze the molecular conformations,intermolecular interactions and packing patterns.The solvate formation was found to be a multi-point molecular recognition and self-assembly process through C-H···O synthons.The incorporation of solvent molecules does not improve the packing efficiency.Hirshfeld surface analysis confirms that numerous C-H···O interactions are formed between host and guest molecules,and more efficient hydrogen bonding networks are established in CL-20 solvates than in polymorphs,which becomes the driving force for solvate formation.These findings provide a promising potential in multi-component crystal design of CL-20 using crystal engineering strategies.Last but not least,the isothermal ternary phase diagrams of CL-20 in six binary solvent systems were established using slurry and gravimetrical methods to give deeper insights into the phase behaviors of CL-20.The effects of solvent activity and temperature on the solution-mediated phase transformation from CL-20 solvate to formε were discussed based on the information from ternary phase diagrams.Meanwhile,the thermal stability of CL-20 solvates was evaluated.In-situ monitoring of the solidstate phase transformation during the desolvation process was performed,and the structural similarity,kinetic and thermodynamic factors play important roles in determining the polymorphs obtained after desolvation.The phase behaviors of CL-20 can be predicted and controlled exactly to obtain the desired products during the crystallization process.