Delay Mechanism of β→δ Phase Transition of Cyclotetramethylene Tetranitramine in Polymer Bonded Explosive Formulations by Heat Conduction Obstacle

MD-DFT Calculations on Dissociative Absorption Configurations of FOX-7 on (001)- and (101)-Oriented Crystalline Parylene Protective Membranes

Crystalline poly-para-xylylene (parylene) has the potential for use as a protective membrane to delay the nucleation of explosives by separating the explosives and their decomposition products to decrease the explosive sensitivity. Here, molecular dynamics (MD) and density functional theory (DFT) techniques were used to calculate the dissociative adsorption configurations of 1,1-diamino-2,2-dinitroethylene (FOX-7) on (001)- and (101)-oriented crystalline parylene membranes. Based on the results of the calculations, this work demonstrates that the -NO2-π electrostatic interactions are the dominant passivation mechanism of FOX-7 on these oriented surfaces. FOX-7 can dissociatively adsorb on oriented parylene membranes due to the interactions between the LUMO of the toluene (or methyl) groups on parylene and the HOMO of the -NO2 (or -NH2) groups on FOX-7. The formation of a new intermolecular H-bond with the ONO group leads to FOX-7 decomposition via intramolecular C-NO2 bond fission and nitro-to-nitrite rearrangement. The most likely adsorption configurations are described in terms of the decomposition products, surface active groups of parylene, binding behaviors, and N charge transfer. Importantly, the (001)-oriented parylene AF8 membrane is promising for use as a protective membrane to passivate the high-energy -NO2 bonds during the dissociative adsorption of FOX-7. This study offers a new perspective on the development of protective membranes for explosives.

Core@Double-Shell Structured Energetic Composites with Reduced Sensitivity and Enhanced Mechanical Properties

A novel core@double-shell (CDS) 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) based energetic composite was constructed with an inner nano-1,3,5-triamino-2,4,6-trinitrobenzene (nano-TATB) shell and outer polydopamine (PDA) shell fabricated via a facile ultrasonic method and a simple immersion method, respectively. The inner nano-TATB shell was chosen to reduce the sensitivity of HMX while maintaining explosion performance; the outer PDA shell was adopted to enhance the interfacial interaction between explosive crystals and polymer binder. The uniform PDA coating resulted in the increased β-δ phase transition temperature of HMX from 197.0 to 212.8 °C. Because of the perfect and compact nano-TATB coating on the surface of the HMX particles, the impact sensitivity  was significantly decreased for the HMX@TATB@PDA particles (10 J), in comparison with the physical mixture with an equivalent composition (5 J). Polymer-bonded explosives (PBXs) based on CDS structured particles were designed and characterized in comparison with their core@single-shell (CSS) counterparts or physical mixtures. Due to the strong chemical and physical interfacial interaction, PBXs based on CDS structured particles displayed improved mechanical strength and roughness, storage modulus, as well as creep resistance.

Core-Shell Structured HMX@Polydopamine Energetic Microspheres: Synergistically Enhanced Mechanical, Thermal, and Safety Performances

The solid⁻solid phase transition, poor mechanical properties, and high sensitivity has impeded further practical applications of 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) based polymer bonded explosives (PBXs). To address these issues together, a facile and effective route was employed to achieve a coating of polydopamine (PDA) on the surface of explosive crystals via in situ polymerization of dopamine. Additionally, PBXs based on HMX@PDA microcapsules were prepared with a fluoropolymer as polymer binder. Improved storage modulus, static mechanical strength and toughness, and creep resistance has been achieved in as-prepared PDA modified PBXs. The β-δ phase transition temperature of as-obtained PBXs based on conventional HMX (C-HMX)@PDA was improved by 16.3 °C. The friction sensitivity of the C-HMX based PBXs showed a dramatic drop after the PDA coating. A favorable balance proposed in this paper among thermal stability, mechanical properties, and sensitivity was achieved for C-HMX based PBXs with the incorporation of PDA.