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

Abstract

In Part I of this two‐part series, we conducted accelerated aging studies of β CL‐20 thin films deposited on glass surfaces in different environments (N2, air, air/water) and analyzed the samples with attenuated total reflectance infrared (ATR‐IR) spectroscopy. We observed a polymorph transformation to α 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 α, β, and ϵ 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 β and ϵ CL‐20 is thermodynamically and kinetically unfavorable, explaining the absence of ATR‐IR features corresponding to molecular inclusions in β CL‐20 prior to polymorph conversion. We hypothesize that water facilitates the transformation to α CL‐20, which allows the insertion of small degradation products into the CL‐20 lattice. We predict a larger stability of ϵ CL‐20 compared to β CL‐20 towards polymorph transformation to α CL‐20, including in the presence of water.

Molecular Inclusion of Small Aging Products into the Hexanitrohexaazaisowurtzitane (CL‐20) Lattice: Part I, Infrared Spectra

“…This was similar to hydration model 3, but two, instead of one, void spaces in the α CL‐20 unit cell were occupied by a guest molecule. The same modified model 3 was also employed in Part II [51] of this two‐part series, where the energetic feasibility of molecular inclusion in different CL‐20 polymorphs was investigated. Here, modified model 3 was used to discuss and compare structural attributes of the guest/host complexes to other work [18, 22–24] that does not consider mixed water/guest complexes.…”