(692d) Utilizing Meniscus Guided Coating Techniques to Obtain Polymorphic Control and Large Area Single Crystalline Domains in Energetic Material Thin Films

Energetic materials (EMs) are a class of materials that contain a significant amount of stored energy in the chemical bonds within their molecules. This energy is intended to be released upon an initiation event (i.e. impact, friction, shock, spark, heat) for a variety of applications including construction, mining, space exploration, pyrotechnics, and in the military. Undesired initiation can lead to catastrophic and detrimental failures; therefore, these materials require the utmost level of fundamental understanding in terms of predictability and stability. Producing these materials in crystalline form stabilizes the periodic structure of the EM and allows for more consistent material properties (i.e. impact stability, detonation velocity). However, organic crystalline structures can be crystallized in different packing structures, termed polymorphs. Polymorphs exhibit varying material properties due to differences in their crystal structure (i.e. density, morphology); therefore, it is crucial to develop a fundamental understanding of how polymorphs form and how they can interconvert for energetic material stability.

In other fields (i.e. semiconductors, pharmaceuticals), meniscus guided coating has been utilized to obtain polymorphic control in small organic molecule thin films. A specific type of meniscus guided coating, termed solution shearing, sandwiches a solution containing the monomer between a heated substrate and a coating blade that moves translationally at a set speed. Varying the temperature and the coating speed influences evaporation and deposition rates, allowing for the formation of metastable polymorphs. Through solution shearing in a variety of coating regimes, we have crystallized large, aligned, millimeter to centimeter wide crystal domains of two energetic materials of interest. Furthermore, we have utilized Raman spectroscopy to demonstrate polymorphic control for these two EMs. Finally, we have utilized optical microscopy to observe recrystallization of the metastable polymorph of one of the EMs formed via solution shearing, and we utilized in situ grazing incidence x-ray diffraction (GIXD) to study what type of recrystallization (i.e. polymorphic interconversion, Ostwald’s ripening) is observed for thin films of this material.