Enhanced Intermolecular Hydrogen Bonds Facilitating the Highly Dense Packing of Energetic Hydroxylammonium Salts

The energy and performance of energetic materials can be improved by increasing their crystal packing density. Thus, we propose a strategy involving salification with hydroxylammonium cations (HA⁺) to increase the packing coefficients (PCs) and packing densities of energetic ionic salts (EISs). Structural analyses and theoretical calculations of the observed EISs indicate that the strong intermolecular hydrogen bonds (HBs) between HA⁺ and anions are primarily responsible for the increase in EIS density. Such strong HBs usually exist in HA⁺-based energetic salts and rarely in other EISs but are absent in energetic crystals with neutral molecules. Such HBs induce high PCs and relatively high crystal packing densities by compensating for the relatively lower molecular density of HA⁺ compared with other cations. Moreover, in combination with HBs in common explosives, we find a simple dependence showing that the shorter the strongest HB corresponds to the higher PC, suggesting that the strongest HB can be regarded as a simple indicator of PC. This study proposes that enhancing intermolecular HBs is the main strategy to increase compactness because H atoms usually exist in currently available energetic materials

A Novel Spherulitic Self-Assembly Strategy for Organic Explosives: Modifying the Hydrogen Bonds by Polymeric Additives in Emulsion Crystallization

A novel strategy to prepare spherical crystals of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) has been developed by introducing polymeric additives into the antisolvent emulsion crystallization. The spherical crystals are induced by modifying the intermolecular hydrogen bonds network of LLM-105. The results show that the concentration of polyvinylpyrrolidone (PVP), which acts as a polymeric additive, is a crucial factor to get quite different morphologies of LLM-105 crystal products. X-like shaped crystals have been produced in the absence of PVP. In contrast, spherical crystals have been obtained in the presence of PVP. Importantly, LLM-105 spherulites with a mean particle size of 78.0 μm can be obtained by adding a proper amount of PVP, which has a narrow size distribution (CV = 31.2). In addition, time-resolved morphological evolution processes of X-like shaped and spherical crystals have been performed. Meanwhile, 1H NMR experiments also have been conducted to understand the intermolecular hydrogen bonds between LLM-105 molecules and the polymer. Inspired by the result of the above experiments, an LLM-105 spherulitic formation mechanism has been proposed. Furthermore, LLM-105 spherulites exhibit more excellent mechanical and safety properties. It is suggested that these spherulites have a great potentiality in the military application. Therefore, this polymer-induced spherical crystallization method is significantly important for the design and fabrication of organic small molecules with spherical shapes

Characterization and Properties of a Novel Energetic–Energetic Cocrystal Explosive Composed of HNIW and BTF

A novel 1:1 cocrystal explosive of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW) and benzotrifuroxan (BTF) has been prepared. The structure of this cocrystal was characterized by single crystal X-ray diffraction (SXRD). Properties of the cocrystal including thermal decomposition and detonation performance were studied. Further, the cocrystal explosive is predicted to display superior detonation power compared to BTF

Mechanical Anisotropy of the Energetic Crystal of 1,1-Diamino-2,2-dinitroethylene (FOX-7): A Study by Nanoindentation Experiments and Density Functional Theory Calculations

The mechanical anisotropy of the wavelike π-stacked energetic crystal of 1,1-diamino-2,2-dinitroethylene (FOX-7) is investigated by nanoindentation experiments and density functional theory (DFT) calculations. The FOX-7 crystal exhibits distinct mechanical anisotropy when indented on different faces. The elastic modulus and hardness of the (020), (−101), and (002) faces change in a decreasing order. The indentation on the (020) face induces the largest depth and the highest pile-up around all three edges of the indenter without causing crack formation. By contrast, the indentations on the (−101) and (002) faces are similar and induce a small indentation depth, low pile-up with a small distribution, and crack formation. Mechanical anisotropy is essentially determined by the wavelike π stacking of FOX-7 along the (020) face with the support of intermolecular hydrogen bonds; i.e., the molecular orientations and intermolecular spaces along different faces vary distinctly. This is also supported by the DFT calculations on uniaxial compression and shear sliding. In this work, the nature of the wavelike π stacking responsible for the low impact sensitivity of FOX-7 is discussed and compared with that of other explosives with different packing structures

High-Yielding and Continuous Fabrication of Nanosized CL-20-Based Energetic Cocrystals via Electrospraying Deposition

Energetic cocrystals, especially CL-20-based cocrystals, have attracted a wide range of attention due to their low sensitivity and impressive detonation performance. In this study, a series of nanosized CL-20-based energetic cocrystals were successfully fabricated by electrospray deposition. For CL-20/TNT nanococrystals, the influence of different solvents on the morphology and crystal structure of as-prepared cocrystals were investigated. The results showed that all the electrosprayed CL-20/TNT samples were partial formation of cocrystals and particles obtained from ketone had smaller size than those obtained from ethyl solvents. In contrast, electrosprayed CL-20/DNB nanococrystals had completely formed the cocrystal structure proved by DSC and PXRD. Moreover, the terahertz (THz) result confirmed the formation of intermolecular hydrogen bonds. Additionally, we have fabricated the CL-20/TNB cocrystals for the first time by using electrospray method. The PXRD and DSC results confirmed the formation of this novel energetic cocrystal. Expectedly, all the electrosprayed nanosized CL-20-based cocrystals exhibited visible reduced impact sensitivity compared with raw CL-20. The electrospray can thus offer a flexible and versatile approach for continuous and high-yielding synthesis of nanosized energetic cocrystals with preferable safety performance, and provide an efficient screening to quickly distinguish whether two energetic materials can form a cocrystal

Structure–Property Relationship in Energetic Cationic Metal–Organic Frameworks: New Insight for Design of Advanced Energetic Materials

Understanding the structure–property relationship in a material is of great importance in materials science. To study the effect of ligand backbones and anionic groups on the properties of energetic cationic metal–organic frameworks (CMOFs) and to disclose their structure–property relationships, we designed and synthesized a series of CMOFs based on either 4,4′-bi-1,2,4-triazole (btrz) or its azo analogous, 4,4′-azo-1,2,4-triazole (atrz) as ligand, and either perchlorate [ClO4–] or nitroformate [C­(NO2)3–, NF–] anion as extra-framework anion. Surprisingly, the effect of ligand backbones on the CMOFs is inverse that of the backbones on traditional energetic compounds, while the effect of the anionic groups follows the traditional group law. We found that btrz-based CMOFs exhibit higher densities and better chemical and thermal stabilities than those of their corresponding atrz-based CMOFs, although btrz has a lower density and a lower stability than atrz. In particular, the density of btrz-Fe is more than 0.11 g cm–3 higher than that of its atrz-based analogue (atrz-Fe). Moreover, the decomposition temperature of btrz-Zn (363 °C) is 80 °C higher than that of atrz-Zn, even higher than that of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), making it a potential heat-resistant explosive. The effect mechanisms were also discussed according to the experimental results. This investigation is significant for understanding the structure–property relationship in energetic CMOFs. Moreover, it also brings about new design rules for future high-performance energetic materials