Difference in the Thermal Stability of Polymorphic Organic Crystals: A Comparative Study of the Early Events of the Thermal Decay of 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) Polymorphs under the Volume Constraint Condition

Abstract

Polymorphism is universal in organic crystals like conventional energetic materials (EMs), and it may cause a difference in thermal stability, one of the most important properties of EMs. Nevertheless, deep insights into the differences of polymorphic EMs are lacking. 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) is the most powerful EM commercialized already and possesses three polymorphs stabilized at common condition, which exhibit many differences in properties and performances. However, the underlying mechanism responsible for these differences remains unclear. In this work, a self-consistent charge density functional tight binding scheme and molecular dynamics simulations are combined to reveal the difference in the decay mechanism of the three polymorphs of CL-20, with considerations of an extreme of volume constraint and two heating types. The lower thermal activity of ε-CL-20 is distinguished from β- and γ-CL-20 at a relatively low temperature of 1000 K, as only ε-CL-20 does not decompose in the time scale of the simulation of 20 ps. The lower thermal activity of ε-CL-20 is partly responsible for its lower impact sensitivity. Two relatively high temperatures of 1500 and 2000 K cannot differentiate their decay activities. Moreover, the unimolecular N–N breakage governs the first steps of the thermal decay of all of the three polymorphs. Five types of bond cleavages, including the NO2 partitions from the 5- and 6-membered rings, the cleavage of C–N bonds of the 5- and 6-membered rings, and the C–C bond breakage, are observed in the decay. Interestingly, all of the five types of ignitions are not be observed in any case. Besides, we find that the low temperature disfavors the formation of the stable products of N2 and CO2. These results of the thermal decay of polymorphic EMs are expected to give deep insights into the complicated sensitivity mechanism of EMs.

The role of electric field on decomposition of CL‐20/HMX cocrystal: A reactive molecular dynamics study

“…8,9 It is of particular importance for CL-20/HMX as it was predicted to have higher detonation velocity with the same sensitivity, when compared with pure β-HMX, thus may replace HMX. 9 The hotspot theory shows that thermal decomposition is prior to the hotspot formation 10 and the ignition mechanism actually represents the sensitivity mechanism. 11 This motivated researchers to gain an insight into the underlying mechanism of decreased impact sensitivity by investigating thermal decomposition of the cocrystals at atomic level.…”

Molecular Dynamics Simulations of the Thermal Decomposition of 3,4-Bis(3-nitrofurazan-4-yl)furoxan

“…This is attributed to the fact that the generation of NO 2 increases the number of molecules, thus enhancing the entropy effect of the system and changing the original decomposition law. 38 It is worth mentioning that no isomerization of CONO was found.…”

Molecular Forcefield Methods for Describing Energetic Molecular Crystals: A Review

“…Reactivity is the key property for energetic crystals, which is highly related to the detonation and sensitivity of EMs. Theoretical methods—including Reactive FFs [ 97 , 98 , 99 , 100 ], AIMD [ 101 , 102 , 103 , 104 , 105 , 106 , 107 ], and DFTB-MD [ 108 , 109 , 110 , 111 , 112 , 113 , 114 ]—were utilized to reveal reaction mechanism of energetic crystals, and ReaxFF is the most efficient. The reactive FF is a unique type of molecular FF, which can describe not only the interactions between particles and their movements, but also the chemical bond formation and rapture, namely chemical reactions, in the process.…”