Characterization of α- and β‑RDX Polymorphs in Crystalline Deposits on Stainless Steel Substrates

The highly energetic material (HEM) hexahydro-1,3,5-trinitro-s-triazine, also known as RDX, has two stable conformational polymorphs at room temperature: α-RDX (molecular conformation of −NO2 groups: axial–axial–equatorial) and β-RDX (molecular conformation of −NO2 groups: axial–axial–axial). Both polymorphs can be formed by deposition on stainless steel substrates using spin coating methodology. α-RDX is the most stable crystal form at room temperature and ambient pressure. However, β-RDX, which has been reported to be difficult to obtain in bulk form at room temperature, was readily formed. Reflection–absorption infrared spectroscopy measurements for RDX-coated stainless steel substrates provided spectral markers that were used to distinguish between the conformational polymorphs on large surface areas of the substrates. Raman microspectroscopy was employed to examine small areas where the intensity was proportional to the height of the structures of RDX. Spectral features were interpreted and classified by using principal component analysis (PCA). The results from these spectral analyses provided good correlation with the values reported in the literature. Conditions to generate predominantly β-RDX crystalline films as a function of the spin coating rotational speed on these substrates were obtained. PCA was also applied to predict percentages of polymorphs present in experimental samples. Applications of the results obtained suggest the modification of existing vibrational spectroscopy based spectral libraries for defense and security applications. Understanding the effects of polymorphism in HEMs will result in the attainment of higher confidence limits in the detection and identification of explosives, especially at trace or near trace levels.