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

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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.