Ruiz-Caballero, José L.

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    Preparation and study of thin RDX films standards on various substrates deposited by spin coating technology
    (2017-05) Ruiz-Caballero, José L.; Hernández-Rivera, Samuel P.; College of Arts and Sciences - Sciences; Mina Camilde, Nairmen; Santana Vargas, Alberto; Infante Castillo, Ricardo; Department of Chemistry; Rodríguez Solís, Rafael A.
    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 crystallization upon deposition on stainless steel (SS) and gold (Au) substrates using a 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 (RAIRS) measurements of RDX-coated substrates provided spectral markers that were used to distinguish between the conformational polymorphs on large surface areas of the substrates. Raman spectroscopy was employed to examine small areas where the intensity is proportional to the height of the structures for RDX. Spectral features were interpreted and classified by using principal component analysis (PCA). The results of these spectral analyses provided good correlation with the values reported in the literature. Optimized conditions to generate nearly only β-RDX crystalline films using increasing rotational speeds on these substrates were obtained. PCA was also applied to predict percentages of polymorphs present in experimental samples. On the other hand, the optical properties for crystalline films of the highly energetic material (HEM) previously cited as RDX, deposited on gold (Au) and stainless steel (SS) substrates are also presented. The optical properties obtained in the present study correspond to thin film samples of predominantly β-RDX polymorph. The infrared spectroscopic intensities measured showed significant differences in the -RDX crystalline films deposited on the two substrates with respect to the calculated real part of refractive index. The β-RDX/Au crystalline films have a high dynamic response, which is characterized by the asymmetric stretching mode of the axial nitro groups, whereas for the β-RDX/SS crystalline films, the dynamic response was mediated by the –N–NO2 symmetric stretch mode. This result provides an idea of how the electric field vector propagates through the β-RDX crystalline films deposited on the two substrates. Applications for the results obtained suggest the modification of existing vibrational spectroscopy-based spectral libraries for defense and security applications. Understanding the effects of polymorphism of HEMs will result in the attainment of higher confidence limits in the detection and identification of explosives especially at trace or near trace levels.