To explore the difference in thermal stability of different CL-20 polymorphs, the initial thermal decomposition of four CL-20 polymorphs(α-, β-, γ-, and ε-)were depicted using longtime ReaxFF MD simulations at relative low temperatures of 600—900K, which are close to the conventional thermal analysis experiments. The dynamic responses of varied CL-20 polymorphs under heating were characterized by the initial conformer transformation and thermal decomposition kinetics. The results show that under relative mild thermal stimulation contidions, conformer conversion dominates the initial responses of four CL-20 polymorphs. The molecular conformer stability of varied CL-20 polymorphs ranks as γ>β≈α>ε. Under heating conditions, ε-CL-20 converts to γ or ζ conformers, and γ-CL-20 converts to ζ conformer. The above conformational transition phenomenons agree broadly with experimental phase diagram of Russell et al. Conformer conversion from ε to γ occurring very early in ε-CL-20 polymorph induces its fast initiation of decomposition reactions. In contrast, γ-CL-20 is rather stable, and its conformer transformation and decomposition lags behind ε-CL-20. Once the decomposition initiates, the overall decomposition rate of γ-CL-20 polymorph is higher than that of ε-CL-20 polymorphs with increasing of thermolysis duration or temperatures. In terms of the reaction kinetics of weight loss and major bond cleavage during the entire thermolysis process, the thermal stability of ε-CL-20 polymorph is higher than that of γ-CL-20, which agrees with Nedelko's TG experiments. This work suggests that ReaxFF MD simulations may be served as a computational prediction method for temperature phase diagram of polymorph conversion of energetic materials.
