Selective Morphological and Polymorphic Control of CL-20 Thin Films Using Meniscus-Guided Coating

Abstract

Controlling the morphology and crystal structure of energetic materials (EMs) is crucial for predictable performance and stability under operational and accidental scenarios. In this work, an EM of interest, hexanitrohexaazaisowurtzitane (CL-20), was crystallized as thin films using a meniscus-guided coating technique termed solution shearing. We observed four distinct morphological regimes within the parameter space, as well as regions with selective formation of the β and γ polymorphs of CL-20. The thin films ranged in thickness from 79 nm to 2.2 μm and in surface coverage from 36.2 to 99.9%. Thermal annealing of films at 180 °C promotes a β-to-γ phase transformation for films with thicknesses >500 nm. Water vapor annealing of films results in a phase transition to α-hydrate. We were unable to attain the ε polymorph in our thin films across all processing conditions, including dissolution and recrystallization, which preferentially forms the β polymorph. Therefore, we show that thin-film confinement stabilizes the β polymorph of CL-20 over the ε polymorph. The results of this work have important implications for EM composites as interfacial recrystallization of the stable bulk ε polymorph may result in the formation of the β polymorph as a thin film at the interface, changing the EM stability. We show that the solution shearing technique provides a promising platform for the study of the interfaces between energetic crystals and polymers in composites.

Preparation of Polydopamine Functionalized HNIW Crystals and Application in Solid Propellants

“…Thermogravimetric differential scanning calorimetry (TG-DSC) tests were performed to obtain the thermal properties of HNIW@PDA, as displayed in Figure 10 and Table 3 . The sharp exothermic peak at 242.9 °C represents the thermal decomposition peak temperature ( T P ) of HNIW [ 13 ]. T P of HNIW crystals subjected to coating in air and oxygen for 3, 6, and 9 h is similar to that of pure HNIW, with no significant decrease observed.…”

“…Polymers 2024, 16, x FOR PEER REVIEW 9 of 15 temperature (TP) of HNIW [13]. TP of HNIW crystals subjected to coating in air and oxygen for 3, 6, and 9 h is similar to that of pure HNIW, with no significant decrease observed.…”

“…The construction of a core-shell structure with HNIW as the core and an inert layer as the shell is an effective way to inhibit the phase transformation and reduce the impact sensitivity [ 13 , 14 ]. Materials such as waxes, thermoplastic polymers, and graphite/graphene are commonly associated with the ability to form smooth and flexible films that prevent the formation and propagation of “hot spots” [ 15 , 16 ].…”

“…50-2045). Peaks at 12.5°, 12.7°,13.8°, 15.7°, 16.3°, 17.7°, 19.9°, 21.5°, 21.9°, 22.2°, 22.6°, 23.6°, 24.2°, 25.7°, 27.8°, 28.3°, 28.7°,…”