Stabilization of ε-CL-20 crystals by a minor interfacial doping of polydopamine-coated graphene oxide.

• Various modified CL-20 crystalline composites with graphene/polydopamine as the dopants have been prepared; • These CL-20 based composites have improved thermal stability, and lower mechanical sensitivity; • Some of the abovementioned CL-20 based composites have even higher density with exclusion of polymorphic transition; Hexanitrohexaazaisowurtzitane (CL-20) is a well-known high energy density nitramine, but it is of high sensitivity and has the problem of polymorphic transition. In order to reduce the sensitivity of ε-CL-20 and improve the stability of its crystal structure, the polydopamine (PDA)-coated graphene oxide (GO) was used to dope ε-CL-20 crystals by strategy of in-situ coating followed with a solvent-non-solvent crystallization process. It has been shown that modified CL-20 crystals with the best performances are of polygon shape with a smooth surface and with the average diameter of 16 μm, much smaller than that of raw ε-CL-20 (140 μm). In addition, the doping can either improve the polymorphic transition temperature up to 19.0 °C or exclude such a transition due to formation of completely new crystal phase with better stability, depending on the type and content of the dopants. The density of a certain GO@PDA modified CL-20 is even slightly higher than that of pristine ε-CL-20 due to better assembling of the molecules under the nucleation of the dopants. CL-20/PDAG-2 does not show any polymorphic transition peak. Moreover, the XRD spectrum of CL-20/PDAG-2 also demonstrates that it is a completely new crystal phase better than the other four reported ones in terms of thermal stability. [ABSTRACT FROM AUTHOR]