Polymer-directed crystallization of HMX to construct nano-/microstructured aggregates with tunable polymorph and microstructure

Nano-/microstructured energetic materials (EMs) often display a unique combination of low sensitivity and high stability, but their structural control across multiple length scales has proven difficult. Here, a polyvinyl pyrrolidone (PVP) polymer-directed crystallization method is developed to construct high explosive 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX) polycrystalline aggregates with controllable molecular-scale structure (polymorph) and crystal-scale aggregation microstructures. It was found that the polymorph and microstructural characteristics of aggregates show a strong dependence on the initial supersaturation, PVP concentration, and feeding rate. In particular, the morphology and the phase composition of the aggregates could be systematically tuned from delta-phase star-like aggregates to gamma-phase flake spherulites by simply varying the HMX concentration. A slower feeding rate could lead to more compact spherulites with a well-defined Maltese-cross extinction pattern. According to process analysis technology (PAT) and Hansen solubility parameter (HSP) analysis, it was proposed that the preferential absorption of antisolvent and strong interaction between PVP and HMX could disrupt the local solution chemistry, lower the level of supersaturation and increase the induction time, resulting in regulation in the molecular assembly pathway of HMX. Moreover, the HMX spherulitic aggregates are much less sensitive to impact and friction than the reported gamma-HMX as well as raw HMX. This work provides a fresh way for the design and preparation of high explosive polycrystalline aggregates with well-defined multi-scale architectural features and reduced mechanical sensitivity.