Controlling Energetic Crystal Morphology Using Tailored Polymeric Additives

While much emphasis is placed on chemical structure when deploying a compound for a given application, the shape in which the compound crystallizes, its morphology, has also proven influential in the control of properties that span fields from pharmaceuticals to energetic materials. Outside of processability, crystal morphology is vital for energetic materials because of its ties to explosive sensitivity and performance, stemming from the influence of morphology on the crystal packing density. Morphology control is commonly accomplished by altering crystallization conditions (e.g., solvent, temperature, and supersaturation) in order to alter the relative growth rates of crystal faces. Less commonly employed to control the morphology is the use of additives. Herein, a methodology for selecting additives to control the morphology of explosives is investigated based on targeting interactions present in cocrystals. Specifically, 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX) was used as a model compound; three polymers were selected and successfully implemented as soluble, face-selective shape modifiers for HMX, a structurally similar analogue 3,3,7,7-tetrakis(difluoramino)octahydro-1,5-dinitro-1,5-diazocine (HNFX), and nitroguanidine (NQ). These results demonstrate morphology modification in both desirable (needle-to-block) and undesirable (block-to-needle) directions and illustrate a general design approach for crystal shape engineering applicable to energetic materials.