Imparting high polymorph transition resistance to cyclotetramethylene-tetranitramine via spherulitic aggregation enabled crystal shape constraint

Abstract

The high melting explosive (HMX), i.e., cyclotetramethylene-tetranitramine, is plagued by issues such as structural instability, increased sensitivity, and unwanted deflagration at elevated temperatures, which are strongly linked to the microstructural damage generated by its β → δ polymorphic transition around 160–185 °C. An extension of the β form thermal stability is therefore desired. Herein, a new strategy to regulate such β → δ transition via particle-level aggregation is demonstrated. Using polyvinyl pyrrolidone as a growth additive, this study shows that single-crystalline β-HMX can evolve into a unique spherulitic morphology with fine control over particle sizes and compactness. The phase transition is then found sensitive to the spherulitic aggregation while highly correlating with the decrease in particle size. Remarkably, the β-HMX spherulites exhibit an onset transition temperature of up to 205 °C, corresponding to a maximum 20 °C increment as compared with the highest value achieved by its single crystalline counterpart. Mechanistic investigation reveals that the suppressed β → δ transition origins from the restricting of anisotropic expansion of the subunit crystallites in spherulitic aggregates, induced by their unusual radial packing mode. The proposed strategy is expected to be translated to other polymorphic explosives, hence bringing a fresh perspective to future high-performance energetic materials development.