The structural diversity of hybrid qy-HMX crystals with constraint of 2D dopants and the resulted changes in thermal reactivity

In this paper, the 2D (two-dimensional) high nitrogen triaminoguanidine-glyoxal polymer (TAGP) has been used to dope HMX crystals. Various constraint TAGP-doped HMX crystals (qy-HMX) with higher density and better stability have been obtained due to strong constraint interactions between dopant and HMX molecules. The structure and thermal reactivity properties of qy-HMX are unexpectedly largely affected by a slight change in the way of co-crystallization/precipitation under the same precursor conditions. In particular, the density of qy-HMX could be largely changed in the range of 0.94 g cm⁻³ to 2.12 g cm⁻³ due to varied doping state, depending on the crystallization conditions. These qy-HMX crystals are more thermally stable than HMX, showing either no polymorphic transition or increased transition temperature. As a typical example, the polymorphic transition of qy-HMX-211 with doping amount of 21.1 wt% was excluded due to formation of a completely new more stable phase of HMX. The qy-HMX-341 has the highest heat release (1869 J g⁻¹), even higher than that of HMX. The average E_a (activation energy) for the polymorphic transition of qy-HMX-341 is about 334.1 kJ mol⁻¹, following a random chain scission model (L2), whereas it is 222 kJ mol⁻¹ for HMX. The E_a for thermal decomposition of qy-HMX-341 is 215.7 kJ mol⁻¹, which is 39.8 kJ mol⁻¹ lower than that of HMX. Therefore, the qy-HMX crystal has high potential to be used as a promising high explosive to replace HMX for various applications in the future.