Molecular Inclusion of Small Aging Products into the Hexanitrohexaazaisowurtzitane (CL-20) Lattice: Part II, Polymorph Dependence

In Part I of this two-part series, we conducted accelerated aging studies of beta CL-20 thin films deposited on glass surfaces in different environments (N-2, air, air/water) and analyzed the samples with attenuated total reflectance infrared (ATR-IR) spectroscopy. We observed a polymorph transformation to alpha CL-20, accompanied by lattice incorporation of water and other small guest molecules when samples were aged in air/water. Comparison to simulated IR spectra identified NO2 (possibly interacting with a carbon species), HNCO, N2O, and CO2 as potential molecular inclusions. For these molecules, we assessed the thermodynamic feasibility of lattice incorporation into alpha, beta, and epsilon CL-20 and the accompanying local stress due to lattice expansion in Part II, reported here. We found that the insertion of guest molecules (with the exception of water), into beta and epsilon CL-20 is thermodynamically and kinetically unfavorable, explaining the absence of ATR-IR features corresponding to molecular inclusions in beta CL-20 prior to polymorph conversion. We hypothesize that water facilitates the transformation to alpha CL-20, which allows the insertion of small degradation products into the CL-20 lattice. We predict a larger stability of epsilon CL-20 compared to beta CL-20 towards polymorph transformation to alpha CL-20, including in the presence of water.