Recrystallization of CL-20 and HNFX from Solution for Rigorous Control of the Polymorph Type: Part II, Experimental Studies

Abstract

The recrystallization of CL-20 and HNFX to form different polymorphs was investigated by employing various experimental analyses, including micro-crystallization under myriad conditions, novel means of crystallization, analysis of polymorphs using powder diffraction, and Rietveld analysis. In the investigation all three groups of solvents: polar protic, non-polar aprotic, and dipolar aprotic solvents were employed. These results are reported in two parts with the experimental results reported here and the mathematical modeling results reported in Part I of this paper. The conventional crystallization techniques included combinations of solvent and anti-solvent system (including high molecular weight polymer melts), combination of solvents, the use of habit modifiers, and seeding. Other recrystallization techniques, which were also investigated included recrystallization under UV, under microwaves, upon freeze drying, upon annealing, in the presence of high molecular weight polymers, and under different hydrodynamic and hence different micro-mixing conditions. As will be seen in Part I of this paper (this issue) our computations suggest (in agreement with earlier studies) that multiple polymorphs with various densities for both CL-20 and HNFX are possible. However, the recrystallization studies reported here revealed only one type of polymorph for HNFX, i.e., the Ci R-3 polymorph. On the other hand, CL-20 gave rise to indeed multiple polymorphs which changed with the crystallization conditions, especially with the solvent/anti-solvent systems utilized. An X-ray based method was used to determine the types of polymorphs and also the percentages of the polymorphs generated upon crystallization. The presence of multiple polymorphs or polymorph impurities in CL-20 may have significant ramifications in the sensitivity and other ultimate properties of energetic grains containing CL-20. On the other hand the existence of only one low-density polymorph for HNFX significantly limits the application areas for HNFX.