Understanding metastable phase transformation during crystallization of RDX, HMX and CL-20: experimental and DFT studies

Abstract

Multiphase growth during crystallization severely affects deliverable output of explosive materials. Appearance and incomplete transformation of metastable phases are a major source of polymorphic impurities. This article presents a methodical and molecular level understanding of the metastable phase transformation mechanism during crystallization of cyclic nitramine explosives, viz. RDX, HMX and CL-20. Instantaneous reverse precipitation yielded metastable γ-HMX and β-CL-20 which undergo solution mediated transformation to the respective thermodynamic forms, β-HMX and ε-CL-20, following 'Ostwald's rule of stages'. However, no metastable phase, anticipated as β-RDX, was evidenced during precipitation of RDX, which rather directly yielded the thermodynamically stable α-phase. The γ→β-HMX and β→ε-CL-20 transformations took 20 and 60 minutes respectively, whereas formation of α-RDX was instantaneous. Density functional calculations were employed to identify the possible transition state conformations and to obtain activation barriers for transformations at wB97XD/6-311++G(d,p)(IEFPCM)//B3LYP/6-311G(d,p) level of theory. The computed activation barriers and lattice energies responsible for transformation of RDX, HMX and CL-20 metastable phases to thermodynamic ones conspicuously supported the experimentally observed order of phase stability. This precise result facilitated an understanding of the occurrence of a relatively more sensitive and less dense β-CL-20 phase in TNT based melt-cast explosive compositions, a persistent and critical problem unanswered in the literature. The crystalline material recovered from such compositions revealed a mixture of β- and ε-CL-20. However, similar compositions of RDX and HMX never showed any metastable phase. The relatively long stability with the highest activation barrier is believed to restrict complete β→ε-CL-20 transformation during processing. Therefore a method is suggested to overcome this issue.

TGA, DSC and DFT Studies of TKX‐50, ABTOX and Their Key Precursors

“…Initially, conformational search was performed on I–VI because of their flexibility by means of their geometric orientations and energies . The conformational changes associated with the rotations of substituent groups along ‐C‐C‐ bond and other orientations were analyzed by manual alterations at wb97xd/6‐311++g(3df,3pd) method in gas phase . Couple of unique conformers of I and II were obtained and they are presented in Figure .…”

“…One of the unique energetic materials of this series is diaminoazobistetrazine (DAAT) having highest positive heat of formation (about +800 kJ mol −1 ) among the reported HEMs. This compound also shows remarkable insensitivity towards external stimuli with the theoretical performance level exceeding that of the currently used powerful military explosives such as RDX, HMX, CL‐20 etc . Researchers from USA reported the synthesis of powerful energetic materials such as CL‐20 .…”

Structures, Elasticity, and Sensitivity Characteristics of ɛ‐CL‐20 under High Pressure from First‐Principles Calculations

“…Therefore, it is necessary and meaningful to explore the pressure behaviors for energetic materials. Except for studying the phase transitions and decomposition mechanism of CL‐20 under pressure, early study has made a clear knowledge about the structural characterization of a high‐pressure phase of CL‐20 through experiments . Gump et al investigated the structural changes of ϵ‐CL‐20 under high pressure, showing that the unit cell volume decreased under compression and no phase was observed up to 6.3 GPa .…”

Polymorphic phase transitions: Macroscopic theory and molecular simulation

“…Crystal to crystal phase transitions are of considerable scientific interest and industrial importance. The interest spans numerous fields and application domains that include Earth sciences [1,2], materials science [3][4][5], biomineralisation [6][7][8], explosives [9], and pharmaceuticals [10,11]. For pharmaceuticals, drug substances and formulation excipients can exist in different solid forms, with particular form(s) having advantages over others in terms of efficacy, ease of manufacture, or stability on storage [12][13][14][15][16][17].…”