Thermodynamic Study On The Formation Of CL-20 Based Cocrystal Explosives

The formation of energetic cocrystal explosives has a significant regulatory effect on the performance of single compound explosives.It has become a research hotspot in the field of energetic materials for nearly ten years.Among them,CL-20?hexanitrohexaazaiso-wurzitane?based cocrystal explosives are the most widely studied and most important class of cocrystal explosives.However,its research work mainly focuses on the screening of cocrystal components and preparation methods,structural characterization and performance testing,theoretical simulation of structure and properties,etc.There are few thermodynamic studies on co-crystallization.As a result,it cannot give pre-judgment for the formation of the cocrystal explosives and the theoretical basis for the preparation method selection and process parameter adjustment,which greatly limits the application and development of CL-20-based cocrystal explosives.This paper took the CL-20/1,4-DNI?1,4-dinitroimidazole?and CL-20/FOX-7?1,1-diamino-2,2-dinitroethylene?cocrystal explosives as the research object,the ternary phase diagram of two cocrystal explosives were drawn by measuring the solubility under different conditions to determine the co-crystal region.The influence of solvent and temperature on the thermodynamics of co-crystallization were analyzed,and the thermodynamic parameters of the co-crystallization process were obtained.The CL-20/FOX-7 cocrystal was prepared by solvent/non-solvent method,and its structure and properties were characterized.The main research work is as follows:?1?Study on the solubility properties of the cocrystal component 1,4-DNI.The solubility of1,4-DNI in different solvents at different temperatures was determined by gravity method,and the?_disH^?,?_disS^?and?_disG^?of 1,4-DNI were estimated based on mathematical models of solubility and temperature.The crystal morphology of 1,4-DNI in four solvents was studied by the cooling crystallization.The results show that the solubility of 1,4-DNI increases monotonously with increasing temperature.The solubility values in different solvents are acetone>ethyl acetate>methanol>ethanol.At room temperature,the dissolution is an endothermic and non-spontaneous process.The crystal quality use ethyl acetate and acetone as the crystallization solvent of 1,4-DNI is better than that in methanol and ethanol,and the yield and purity are also higher.?2?Thermodynamic study on the formation of CL-20/1,4-DNI cocrystal.The solubility data of co-crystal components at different temperatures in different solvents were determined by using high performance liquid chromatography methods.The ternary phase diagram of CL-20/1,4-DNI/solvent was plotted,and the effects of solvent and temperature on each region in the ternary phase diagram were analyzed.The measured solubility data were fitted using the corresponding mathematical model to obtain the thermodynamic parameters of each system,such as the solubility product K_sp,the complexation constant K₁₁ and the reaction free energy?G⁰.The results show that the cocrystal region in the acetone solution is the largest and the symmetry is better than that of ethyl acetate and methanol.The calculation of thermodynamic parameters also confirms that the acetone solvent system is more favorable to the formation of the CL-20/1,4-DNI cocrystal.The ternary phase diagram and thermodynamic parameters in acetone solvent at different temperatures show that lowering the crystallization temperature is more favorable for the precipitation of the CL-20/1,4-DNI cocrystal.?3?Thermodynamics of CL-20/FOX-7 cocrystal formation and its preparation and characterization.The ternary phase diagram of CL-20/FOX-7/DMSO at 25°C was determined by high performance liquid chromatography.The thermodynamic parameters of the system were obtained by fitting the solubility data.The results indicate that the formation of CL-20 and FOX-7 cocrystal is thermodynamically feasible.Due to the strong intramolecular and intermolecular hydrogen bonds of FOX-7,a fast solvent nonsolvent method was used to obtain CL-20/FOX-7 cocrystal sample by the temperature effect and non-solvent effect.Scanning electron microscopy?SEM?results show that the CL-20/FOX-7cocrystal shape is regular,the surface is flat and has no defects;the particle is uniform with the size of about 10?m.The overall morphology is significantly different from the raw materials.By comparative analysis of X-ray diffraction?XRD?spectroscopy,it can be seen that there is obvious discrepancy between the diffraction peaks of sample and co-crystal components,resulting in a new diffraction peak.Analysis of the infrared absorption spectrum?IR?also indicates that there are two components in the sample,where in the molecular conformation of CL-20 is the same as that of the?-CL-20.The peak position is shifted due to the intermolecular interaction between the components.The results of differential scanning calorimetry?DSC?and thermogravimetry?TG?show that the CL-20/FOX-7 cocrystal sample suffers a step weight loss and the weight loss rate is higher than those of the two raw materials.The thermal decomposition peak temperature is between the highest decomposition peak temperatures of the two raw materials,and its thermal performance is significantly improved.