Exploring The Competition Mechanism Of Cocrystallization And Separated Crystallization Of CL-20 Series Explosives

Hexanitrohexaazaisowurtzitane?CL-20?,as one of the most powerful explosives at present,exhibited an excellent oxygen balance,detonation velocity,detonation pressure and thermal stability.Therefore,it was considered to be the most potential replacements for HMX,the current main military explosive.However,the high impact sensitivity and production costs were seriously limited its application.Significantly,cocrystallization was employed in energetic materials and it provided a new opinion for reducing the sensitivity of CL-20.Recently,some excellent CL-20 based cocrystals have been prepared successfully,but the neglect of the cocrystal formation mechanism made the slow development of explosive cocrystals.In this article,we carried on the studies of the formation mechanism and the design theory of cocrystals.The main contents of this work were as follows:1.The SEM,XRD qualitative and quantitative analysis methods were used to explore the formation process of CL-20/HMX cocrystal systematically.The experimental results shown that the formation process of CL-20/HMX cocrystal experienced a significant separated crystallization of CL-20 and HMX and the thermodynamic equilibrium process of the translation from separated crystallization to cocrystallization.While the presence of H₂O molecule could not affect the formation results of cocrystals but its formation process.Additionally,the tiny nucleus produced by the secondary nucleation were proved to be the key factor of the transitions from separately crystalized CL-20 and HMX to CL-20/HMX cocrystal.Finally,the reliability of the formation mechanism was further confirmed by the transition mechanism experiments and thermodynamic calculations.2.By investigating the formation process of CL-20/TNT cocrystal under different solvent systems and crystallization rates,it was found that changing the solvent systems and the crystallization rate could adjust the nucleation order of the solutes,which could affect the formation process of cocrystal.The nucleation concentration and the metastable zone width were proved to be the key factors that change the nucleation order of the solutes.3.Based on above formation mechanism of cocrystals,it could be concluded that the nucleation order was the main factor in the formation process of cocrystal.The cocrystal was preferred to nucleation of its coformers was the most ideal nucleation order.The secondary nucleation rate promoted the conversion of the separated crystallization to cocrystal phase under the conditions of the coformers were preferred to nucleation.Based on this conclusion,we designed and obtained a fast crystallized CL-20/TNP cocrystal by rotary evaporator solvent method.The crystal structure of this cocrystal was characterized by PXRD,FT-IR,SSNMR and TG-DSC methods.This study further revealed the influencing factors of cocrystal formation in solution,which could help us to understand the formation mechanism of cocrystals better.4.In addition,we also attempted to prepare cocrystals under high temperature conditions,but the experimental conditions might limit the experimental process to obtain ideal cocrystals.It will be our next step in the study of cocrystals.