Characterization of α-and β-RDX Polymorphs in Crystalline Deposits

Jun 9, 2016 - Environmental Engineering Program, Vice-Rector for Research, ECCI University, Bogota, D. C., Colombia. •S Supporting Information...
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Characterization of #- and #-RDX Polymorphs in Crystalline Deposits on Stainless Steel Substrates Amanda M Figueroa-Navedo, José L Ruiz-Caballero, Leonardo C Pacheco-Londoño, and Samuel P. Hernández-Rivera Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00078 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 11, 2016

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Characterization of α- and β-RDX Polymorphs in

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Crystalline Deposits on Stainless Steel Substrates

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Amanda M. Figueroa-Navedo, José L. Ruiz-Caballero, Leonardo C. Pacheco-Londoño and

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Samuel P. Hernández-Rivera*

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ALERT-II DHS Center of Excellence for Explosives Research

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Department of Chemistry, University of Puerto Rico-Mayagüez, Mayagüez, PR, 00681 USA

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ABSTRACT: The highly energetic material (HEM) hexahydro-1,3,5-trinitro-s-triazine, also

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known as RDX, has two stable conformational polymorphs at room temperature: α-RDX

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(molecular conformation of –NO2 groups: axial-axial-equatorial) and β-RDX (molecular

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conformation of –NO2 groups: axial-axial-axial). Both polymorphs can be formed by deposition

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on stainless steel substrates using spin coating methodology. α-RDX is the most stable crystal

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form at room temperature and ambient pressure. However, β-RDX, which has been reported to

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be difficult to obtain in bulk form at room temperature, was readily formed. Reflection-

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absorption infrared spectroscopy measurements for RDX-coated stainless steel substrates

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provided spectral markers that were used to distinguish between the conformational polymorphs

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on large surface areas of the substrates. Raman microspectroscopy was employed to examine

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small areas where the intensity was proportional to the height of the structures of RDX. Spectral

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features were interpreted and classified by using principal component analysis (PCA). The

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results from these spectral analyses provided good correlation with the values reported in the

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literature. Conditions to generate predominantly β-RDX crystalline films as function of the spin

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coating rotational speed on these substrates were obtained. PCA was also applied to predict

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percentages of polymorphs present in experimental samples. Applications of the results obtained

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suggest the modification of existing vibrational spectroscopy based spectral libraries for defense

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and security applications. Understanding the effects of polymorphism in HEMs will result in the

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attainment of higher confidence limits in the detection and identification of explosives,

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especially at trace or near trace levels.

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1. INTRODUCTION

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The highly energetic material (HEM) studied in this work was the cyclic nitramine hexahydro-

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1,3,5-trinitro-s-triazine, which is commonly known as cyclonite or Research Department

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Explosive (RDX). This HEM is classified as a secondary explosive, and it is widely used in

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military applications as one of the main ingredients of plastic bonded explosives (PBX: Semtex,

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C2, C3, and C4), and other explosives formulations. RDX has been widely studied in its room

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temperature stable crystalline α-polymorphic form (α-RDX). However, several studies discuss

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the characterization of the β-metastable form of RDX (β-RDX) under standard growth

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conditions, which involve drop-casting techniques at room temperature and atmospheric

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pressure.1-5 The formation of β-RDX follows Ostwald’s step rule, where the least stable

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polymorph is formed first due to a very small change in Gibbs free energy between both

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crystalline forms.6 This rule states that kinetics takes a prominent role over thermodynamics and

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that the kinetically favored species is formed first.7,8 Concentrations below the solubility

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parameters increases the probability of obtaining the metastable form. Therefore, it has proven

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difficult to obtain only β-RDX without simultaneous formation of α-RDX on the substrate of

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interest, because there is very little change in the rates of formation of these polymorphs.1

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Nevertheless, the β-RDX polymorph was obtained with drop-cast crystallization studies by

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Goldberg and Swift, who demonstrated consistent growth of β-RDX on glass substrates using

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various solvents.1 Mercado et al. studied the characterization of RDX nanoparticles deposited by

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an aerosol jet technique, where they also found the coexistence of room temperature phases of

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RDX (β and α).9 Spectroscopic measurements of the polymorphic forms of RDX have been used

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to follow its transition from α-RDX using temperature,10 concentration,11 mechanical

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contact,2,3,5,12-14 and specific solvent crystallization.1,14 Polymorphism within energetic materials

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(both primary and secondary) is a main cause of concern for trace detection and quantification

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due to the change in physical properties inherent to polymorphism.

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Various studies on the polymorphism of RDX show that there are five phases: α, β, γ, δ, and

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ε.2-5,13-18 Of these five forms, only α-RDX and β-RDX exist at room temperature and ambient

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pressure. In the α-RDX crystalline form, the orientations involving two of the –NO2 groups are

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in axial positions and the third group is in an equatorial position (AAE), imparting CS symmetry

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for the molecular conformer when in its static equilibrium position. However, in the β-RDX

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polymorph, the orientations of the three –NO2 groups are all axial (AAA), leading to a higher

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C3V symmetry for its equilibrium conformation. Regarding crystal morphology, Hultgren first

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described α-RDX crystals as orthorhombic.19 In contrast, McCrone described the β-polymorph as

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needle-like.15 Using IR vibrational spectroscopy, Karpowicz demonstrated that RDX assumes an

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(AAA) conformation in both liquid solution and its vapor phase, but that the room temperature

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bulk solid phase assumed the (AAE) conformation in crystalline form.16 More recent studies by

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Torres et al. described the β-polymorph as structures resembling islands as well as scattered

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particles.11 They described the α-RDX polymorph as well defined crystals. In contrast, Goldberg

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and Swift described both α/β-RDX polymorphs as assuming a variety of different morphologies.1

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To investigate these apparent contradictions, it is important to use basic aspects of nucleation

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theories. An ideal thin film growth on a surface involves smooth, layered growth throughout the

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surface. This two-dimensional (2D) growth is called Frank-van der Merwe mode. The growth of

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3D islands throughout the surface is called Volmer-Weber mode. There is a third type of growth,

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called Stranski-Krastanov mode, that implies a wetting monolayer below the 3D structures.20

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One possible explanation for the formation of Volmer-Weber mode structures originates from

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mass transfer limitations, where a major challenge in shearing solution is the prevention of

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crystal defects, which often plays a major role in void formation and dendritic growth.7,8 The

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formation of islands with defined or void centers provides indication of abrupt growth.21

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Evidence of possible Stranski-Krastanov nucleation has been reported by atomic force

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microscopy (AFM) measurements for dihexylterthiophene (DH3T), which exhibited island-like

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structures.22 However, this evidence to support this type of growth mechanism has been highly

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debated.21-23 AFM measurements are required to characterize the morphological aspects of the

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films prepared where a general idea of the polymorphs within the substrate is known. However,

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AFM measurements can provide evidence of Stranski-Krastanov growth if a monolayer or thin

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film is present.17 Furthermore, Raman spectra should confirm polymorphism in small areas of

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the structures generated. In contrast, Fourier transform infrared (FT-IR) spectroscopy should

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allow assessment of larger sample areas. Therefore, RDX conformational polymorphs within

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these crystal structures can be discovered by FT-IR measurements and confirmed by Raman

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spectra. In this work, the area studied by FT-IR was > 3,000 times larger than the area studied by

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Raman microspectroscopy. Thermal Raman microspectroscopy was used to study drug

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polymorphism and is commonly used in the pharmaceutical industry.24,25 An important aspect of

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this study involved the methodology developed for samples preparation since these were needed

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to address the required morphological AFM evidence for the presence of thin films. Various

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thermal inkjet methodologies have provided major advances in the control of morphology and

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polymorphism within thin films.20,26

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In this study, a methodology for growing thin crystalline films by spin coating27 was

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developed, where the α/β-RDX polymorph distribution was studied as a function of angular

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velocity (ω) of deposition coating at room temperature and atmospheric pressure. The

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explanation for this behavior lies in the theories of spin coating, as well as the theory of creating

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a metastable polymorph. Spin coating is governed by the magnitude of shear stress (τ) applied to

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the deposited solution at increasing ω values.27 Most of the solution is instantly ejected during

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this process, where the air flow on top of the substrate applies the τ, which increases by

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increasing ω according to the following equation.27

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, =  /    = [  ] /      π / /

π

/ /





(1)

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where τ is represented in planar polar coordinates (r, z), υ is the viscosity of air, f is the rotational

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speed or frequency (revolutions/min, rpm), and µ represents the dry airflow as controlled by the

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spin-coater. The aforementioned equation states that shearing stress depends on viscosity and

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surface tension of the solvent, as well as the rotational speed applied. This equation agrees with

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Stranski-Krastanov growth, since it relies on the formation of a “solid skin” on the surface due to

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an increased solvent evaporation process with the spin coating method. In addition, Capes and

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Cameron discussed the parameters to form a metastable polymorph in lieu of the room stable

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form of paracetamol: seeding of a saturated solution, obtaining the metastable form from the

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melting process, and isolating and drying the crystals removing as much remaining solvent as

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possible.28 Therefore, a qualitative hypothesis for this method would explain that the particle

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size and height of the film decreases while increasing f, which can also imply that lower sample

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concentration would be present. In addition, identification of remnant solvent traces would

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support the Stranski-Krastanov theory.

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Principal component analysis (PCA) is one of the most frequently used multivariate analysis

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techniques for chemometrics in a broad range of industrial and research applications.29 PCA is

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predominantly used for evaluating sources of variation within a data set. These variations may be

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easily distinguished in the original data or they may not be as obvious to the analyst. The main

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purpose of applying PCA in the analysis of FT-IR spectra is to classify the changes in the

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vibrational signatures of the compounds of interest in an organized manner, via a graphical

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representation called scores plot or principal components (PCs) plot.29 The changes obtained

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could be due to shifts in maximum wavenumber location, a decrease in relative intensity of the

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bands, or the emergence or disappearance of signals, among others. The first PC contains the

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highest variance and the following PCs decrease in percentage of the variance captured. This

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variance can be examined using an orthogonal projection of the original data in the form of PCs,

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when these are plotted as scores.30 A simple example to determine if the PCs do not express

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correlation is the capture of spectral noise or any other type of interference.31 Consequently,

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these factors can be visualized in the scores plot or PC plot (PC1 vs. PC2, for example), where a

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noisy PC spectrum may indicate the presence of interferences from ambient conditions (carbon

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dioxide, water vapor, or contaminants) in FT-IR spectroscopic data. In this case, preprocessing

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routines can be applied to the data prior to performing the multivariate analyses. If little or no

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spectral noise were present, preprocessing options would be unnecessary, resulting in a faster for

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the spectral analysis in obtaining good results and the corresponding scores plots. These plots are

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important because they allow the analyst to extract additional information from these data and

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formulate tools to explain why certain properties extracted as classes from the original spectra

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are alike or different. When dealing with vibrational spectra, the PCs can be compared to a

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“ground truth” by adding a reference spectrum of the sample to the original spectra used in the

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PCA regression to determine where the variations originate.

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2. EXPERIMENTAL

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2.1 Materials. The solvents used in this work: acetone (CH3COCH3, 98% w/w), methanol

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(HPLC grade), and isopropyl alcohol (IPA, 99% w/w), were purchased from Sigma-Aldrich

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Chemical Co. (Milwaukee, WI, USA) and used without further purification. Water used was

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doubly distilled and deionized (18.0 M ohms) to render it equivalent to HPLC grade water. RDX

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was synthesized according to the Bachmann method with minor modifications.31 Stainless steel

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(SS) substrates (1.0 x 1.0 in2) were purchased from Stainless Supply, Inc. (Monroe, NC, USA).

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2.2 Substrate Preparation. The substrates were cleaned by sonication in acetone

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followed by rinsing with distilled water and IPA. Once the substrates were cleaned, vibrational

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chemical maps using Raman microspectroscopy (InVia, 532 nm; Renishaw, Inc., Hoffman

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Estates, IL, USA) were obtained to determine the degree of cleanliness of the substrates’

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surfaces. The chemical maps of the substrates showed that these evidenced: (a) no presence of

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RDX; and (b) no solvent remaining after the drying process. The RDX coatings on hydrophobic

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SS substrates (RDX/SS) were prepared from concentrated stock solutions of approximately 0.2

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M RDX in acetone (working solution), according to the methodology discussed by Hikal where

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homogeneity on quartz substrates was established.32 This solvent choice did not necessarily

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guaranteed that homogeneity would persist on the hydrophobic SS substrates. Various previously

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reported RDX deposition methods have used metallic substrates for samples and standards

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preparation as well as for fundamental studies.33-35 Dilute solutions of 49 ppm (mg L-1) were

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prepared from the working solution to evaluate the reported findings on the influence of analyte

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concentration on the occurrence of polymorphism in RDX.7

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2.3 Crystal Growth by Spin Coating. A model 6800 spin coater system (Specialty

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Coating Systems, Inc., Indianapolis, IN, USA) was used to develop the methodology for RDX

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deposition on SS substrates. Depositions on the substrates were made at 20 °C and for f values

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from 3 k to 8 k rpm at 1 k rpm increments. The ramp increase time, dwell time, and ramp

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decrease time were set at 2, 60, and 5 s, respectively. The interior of the spin coater chamber was

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purged from dust particles by filtered dry air. Six substrates and replicas were prepared by

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depositing 100 µL of the working RDX solution on various SS substrates by spin coating using

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as f was gradually increased.

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2.4 Instrumentation. FT-IR spectra were recorded on an evacuable benchtop

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interferometer (IFS-66 v/s, Bruker Optics, Billerica, MA, USA) equipped with a DTGS detector

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and a KBr beamsplitter with the aid of a reflection accessory (A-513/Q, Bruker Optics) in which

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the angle of incidence could be varied from: 13° to 85°. FT-IR spectral data were acquired and

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analyzed using OPUS™ software suite (v. 7.2.139.1294, Bruker Optics). The optimum incidence

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angle chosen for RAIRS depends on the type of metal where the analyte is adsorbed.36 The

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grazing angle is the value of the incidence angle where the intensity of reflected light increases

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the most with respect to a phase change (δ) when using polarized light parallel and perpendicular

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to the substrate.36 All measurements were made at an angle of incidence of 80°, which although

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it is not the optimum incidence angle for reflectance-absorption infrared spectroscopy (RAIS)

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measurements as seen in SI-1, it was a much more convenient and reproducible value. The

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reference spectra which included the β-RDX spectra used for the last section of this study were

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also carried out at 80° incidence.35 The interferograms were collected at a 4 cm-1 nominal

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spectral resolution. Sample areas of 1.0 x 1.0 in2 were analyzed in RAIS mode in the spectral

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range 400–1600 cm-1 with 32 background and sample scans and a scanner velocity set to 4.0

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kHz.

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Raman spectra of the various crystal structures formed on the SS surface were obtained using

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a Raman microspectrometer (RM2000; Renishaw, Inc.) equipped with a Leica DM/LM

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microscope (100× objective) in the 100–3200 cm-1 region. The spectral parameters included 10 s

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scans with 8 accumulations per measurement. A 632 nm laser (Melles-Griot, IDEX Corp.,

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Carlsbad, CA, USA) was employed as the excitation source with a power of 0.23 mW reaching

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the sample. The laser beam diameter was measured at 5.2 µm with the 100× objective. Mapping

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measurements were made in an model Xplora Raman microspectrometer (Horiba Jobin-Yvon

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SAS, Edison, NJ, USA) using a CCD detector, 100× objective and a 638 nm solid state diode as

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the laser excitation source with 1.10 mW of laser power at the sample. These spectral parameters

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included 10 s scans with 3 accumulations and 9 µm of linear scan area with steps of 2 µm.

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An AFM (model CP-II; Veeco Instruments, Inc., Plainview, NJ, USA) was employed to

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characterize the morphology and topology of the coatings prepared. The cantilever was used in

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non-contact mode with a natural frequency of 75.5 kHz. The imaging software, Image

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Processing and Data Analysis Software, v. 2.1.15 (Veeco Instruments, Inc.) provided with this

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equipment was employed for roughness calculations.

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3. RESULTS AND DISCUSSION

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3.1 FT-IR RAIS Spectra. Representative RAIS spectra of thin RDX films on highly

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polished SS substrates prepared by spin coating at increasing f values are shown in Figure 1. The

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main differences observed between the two RDX phases present were the spectral markers

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related to the position of one of the –NO2 groups. The prominent feature corresponding to the

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asymmetric –NO2 stretch in β-RDX is the broad peak that appears in the 1592-1600 cm-1 region.

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However, for α-RDX, this prominent peak unfolds into additional peaks, 1535 cm-1 (tentatively

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assigned to νas NO2eq asymmetric stretch in α-RDX), 1581 and 1582 cm-1 (tentatively assigned as

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νas NO2ax asymmetric stretch in α-RDX; 1600 cm-1 in β-RDX). These vibrational modes

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assignments are based on the values reported by Millar, et al.2, Dreger and Gupta4, and Infante-

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Castillo, et al.10,37 Other prominent vibrational modes at 1279 cm-1 (tentatively assigned as νN-

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N-O2 stretch in α-RDX; 1270 cm-1 β-RDX), 1390 cm-1 (tentatively assigned as γ-CH2 out of

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plane bending in α-RDX; 1377 cm-1 in β-RDX), 1535 cm-1 (tentatively assigned as νas NO2eq

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asymmetric stretch in α-RDX) are also present. These spectra were compared to a reference

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standard obtained from a deposition of the 0.2 M RDX solution that formed only the α-

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polymorph (labeled “Alpha” in Figure 1). Further evidence of the coexistence of α/β-RDX

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phases was investigated by Raman microspectroscopy. However, the differences in FT-IR

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spectra were evaluated by PCA.

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Figure 1. RAIS spectra of thin RDX films on SS substrates as a function of f show the

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appearance of the –NO2 equatorial peak at higher rotational frequencies (marked by red arrows).

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3.2 PCA of RAIS Spectra. The RAIS spectral region of 1500–1650 cm-1 was used for

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PCA regression analysis, since this region is highly important in the spectroscopic differentiation

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between the two polymorphs. Figure 2 displays the scores plot for baseline corrected RAIS

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spectra. First derivative preprocessing was applied to the spectral data to eliminate variations

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within the spectra for shifts in the baseline. The figure shows the spectral variation of the RDX

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deposited at each f, where a general increase in f is correlated with PC2 in the multivariate

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analysis. PC2 also reflects a general clustering at low (3 k-5 k rpm) and high (6 k-8 k rpm) f

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values. This is indicative of a major change detected by FT-IR that differentiates these two broad

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classification parameters.

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Figure 2. PCA scores plot for RAIS spectra of RDX deposited on SS substrates showing a

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marked difference between low f values (3 k, 4 k, and 5 k) and high f values (6 k, 7 k, and 8 k).

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The spectra obtained for depositions at 7 k rpm and 8 k rpm point to the direction of

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coexistence of α/β-RDX. This was correlated with the PCA loadings plot using vibrational

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modes at 1039 cm-1 (tentatively assigned to νN-C-N), 1390 cm-1 (tentatively assigned as γ-CH2

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out of plane bending in α-RDX), 1437 cm-1 (βCH2 in-plane bending), and 1535 cm-1 (νas NO2eq

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asymmetric stretch in α-RDX). The spectra reveal similar peaks at 1270 cm-1, with no

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broadening of the peak (νN-N-O2 stretch). For 6 k rpm, intense bands were located at 1326 cm-1

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(νN-N-O2 stretch) and 1446 cm-1 (tentatively assigned to βCH2 in-plane bending) compared to

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other f values.6 PC1 correlates with the increasing intensity of the axial –NO2 vibrational mode,

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which is characteristic of the presence of β-RDX.

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Figure 3. Comparison of RAIS spectra of α/β-RDX polymorphs with PC1 and PC2 of the

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loadings plot for the PCA model in the asymmetric stretch spectral region: 1500 to 1650 cm-1.

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3.3 PCA of Raman Spectra. Raman microspectroscopy is often described as a

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complementary tool to RAIRS mode FT-IR spectroscopy because it is able to focus on particles

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and structures on the micron scale with higher spatial resolution. This technique was employed

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to examine the polymorphism of RDX within the various crystal structures detected. Raman

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mapping measurements were taken in steps of 2 nm to examine and confirm the presence of each

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polymorph on the substrates. Fiber-like structures, as well as islands and crystals (prisms), were

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examined for Raman activity under ×100 magnification. The region studied was 100–3200 cm-1.

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A wide variation in intensities is noted in each of the colored coded spectra shown in Figure 4.

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These intensities were directly dependent on the topology of the structure examined. The

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variations are illustrated in Figures SI-2 and SI-3 of the Supporting Information section. Spectral

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data were preprocessed with baseline correction to avoid affecting the PCs. Figure 4 shows three

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regions that can be used to distinguish the polymorphs: Region A: the ring breathing modes at

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800–900 cm-1: the most prominent spectral feature in this region corresponds to the ring-

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breathing mode for the β-polymorph was observed at 881 cm−1. The corresponding mode for the

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α-polymorph was located near at 866 cm−1.10,11,37 Region B: asymmetric –N-O2 stretch region at

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1600 cm-1, and Region C: CH symmetric stretch region at 2800–3050 cm-1.6 Notably, the

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spectrum at 7 k rpm falls within the region of the α-RDX polymorph in Figure 4. In addition,

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another spectrum of the sample prepared at 7 k rpm has to be assigned as belonging to the β

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conformer of RDX.

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Figure 4. Raman spectra of RDX crystals deposited on SS substrates at various f values.

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Examples of RDX polymorph identification by Raman spectra are shown in Figure 5. One

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representative spectra per rpm is shown in normalized intensity values. Alpha and beta RDX

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spectra were found at 7 k rpm and representative spectra of both polymorphs formed at 7 k are

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shown. The α-RDX form can be recognized for the samples prepared at f values of 3 k (red), 6 k

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(violet), and 7 k alpha (orange). The β-RDX form was present at depositions made at f values of

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4 k (navy blue), 5 k (green), 7 k beta (lighter blue), and 8 k (sky blue). The most noticeable

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region for differentiating between the two polymorphs is the C-H stretch region from 2850 to

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3100 cm-1, where α-RDX deposits show three peaks at 2953, 3009, and 3081 cm-1, while β-RDX

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shows two at 2996 and 3084 cm-1. The ring-breathing mode located at 881 cm-1 for the β-RDX

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polymorph (886 cm-1 for the α-RDX polymorph) agree precisely with literature value.1,9-11,37

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Furthermore, the –N-O asymmetric stretching mode shows a broad peak at 1600 cm-1 for the β-

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form, indicating the presence of slightly non-totally equivalent axial nitro group substituents.1, 9-

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11,37

These vibrational modes have been extensively documented in the literature, and there is

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good agreement between our results and those of Infante-Castillo et al. and others.1-3,9-11,18,37

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Figure 5 provides evidence of the presence of the solvent used for depositing the RDX samples:

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acetone. Evidence of the presence of acetone in α-RDX spectra in Figure 5, was also based on

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the –CH vibrational marker at 2926 cm-1.38 Vibrational markers from the solvent (acetone) were

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found. These can be appreciated in Figure 5 after normalizing intensity values in the secondary

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“y” axis. Notice that an increased presence of solvent at 2927 cm-1 is evidence of a solvent-

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mediated transformation from β-RDX to α-RDX.

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Figure 5. Raman spectra (638 nm) for RDX deposits examined in the CH-stretch region showing

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two peaks for β-RDX (4 k and 6 k)) and three peaks for the α-RDX polymorph (3 k and 7 k).

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Acetone Raman shift signature is clearly shown at 2927 cm-1.

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Raman and IR spectra were then added to the same matrix for analysis by direct

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standardization in the region from 1500 to 1600 cm-1. To account for intensity variations within

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the spectra, centroid scaling, standard normal variate, and first derivative were applied as

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preprocessing steps to the modified matrix. The scores plot obtained after model modification is

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shown in Figure 6, where the IR and Raman spectra are correlated for f values (3 k and 4 k) and

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for high f values (5 k, 6 k, 7 k, and 8 k) with respect to PC1 for RDX. The β-RDX reference

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spectra at a concentration of 49 ppm was added for comparison of the –NO2 vibrational mode in

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RAIS. When compared to the reference IR spectra for α/β-RDX, low f values favors the β-RDX

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polymorph.

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Figure 6. Correlation between IR and Raman spectra. PC1 favors grouping β-RDX at low f

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values.

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3.4 AFM. The purpose of characterizing the RDX crystal structures formed on SS substrates

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by AFM was to study the surface topology and morphology for both polymorphs. The Raman

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spectra shown in Figures 4 and 5 show marked intensity fluctuations, before normalization. High

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intensities were obtained for the prism structures near the center of the substrates at 5 k rpm. The

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next most intense Raman spectra also represented prism structures, which were near the center

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and the corner of the substrate at 8 k rpm. As seen in Figure 7, the height of these crystal

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structures was as high as 550 nm following deposition at 8 k rpm on a 10x10 µm2 region.

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Figure 7. Surface structures formed at 8 k rpm: (a) topological AFM imaging; (b) height profile

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along white line on the surface; and (c) white light micrograph of RDX/SS with both prisms and

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fibers.

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Fiber like structures in the shape of dendrites were found for the RDX deposits at 3 k, 4 k,

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and 5 k rpm (Supporting Information). The RDX structures with lower intensity found on the

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substrates were fibers, which were found on the RDX/SS samples formed at 3 k rpm (Supporting

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Information). The characterization of the fibers deposited at 3 k rpm is shown in Figure 8, where

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the white line along the length of the dendrite (longitudinally across one of the fibers) shows the

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surface roughness. Surface roughness was calculated using the filter kernel for a Gaussian filter,

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where the average roughness (Ra) and root-mean-square roughness (Rq) were calculated by:

329

330

 =



 

& =



  ∑ % ∑ %  , ! −  #$  



∑  ∑ (  ) ' * *  , ! #$  



(2)

(3)

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where +,,-! represents surface topography, +./0 is the average surface roughness and the 1, 2

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values denote the pixels in directions x and y. The maximum default pixel values for the

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software were 34 = 35 = 256.

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Interestingly, the observations for alternating Raman intensities corresponded to the height

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and roughness of the structures analyzed. The prismatic structures provided stronger Raman

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intensities than the fiber structures. The average roughness for the fibers was 16.6 nm in the

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longitudinal direction. A transversal line was also traced along the surface of the crystal

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structures. The positions of the fibers are evident from the height profiles shown in Figure 8,

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where the average surface roughness was 33.8 nm.

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One possible explanation for the presence of polymorphs after the spin coating process at a

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given f value may be the mechanical influence of nucleation during the coating process. This has

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been discussed in detail for the pharmaceutical applications of crystal growth and polymorphism

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induced by pressure and temperature.24,25 In addition, solvent choice and stress by mechanical

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conversion are also known to induce polymorphism.1,2,11,24 Recent studies have classified the

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crystalline structure of α-RDX as orthorhombic.39 However, this study established that the β-

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polymorph predominantly forms on the smooth SS surface at low f values as well as specific

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solubility parameters at room temperature and ambient pressure using an optimized spin coating

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methodology. With the information gathered from the scores for PCA using FT-IR, the

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percentage of α-RDX and β-RDX was calculated using the following equation:

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% α−RDX =

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where the term “nk” represents the score value for the spectra selected for evaluation at each f

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value, “a” represents the average of the scores for α-RDX spectra, and “b” the average of the

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scores for β-RDX spectra.38 The results are listed in Table 1, where shear stress (ττ) is expressed

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as a ratio with respect to the f value of 3 k rpm. A clear tendency to increase the percentage of α-

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RDX polymorph present on the substrate as f increased was found. This trend may be influenced

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by the solvent choice (acetone). There is evidence of coexistence and rate of crystallization in

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drop casting techniques due to solvent-mediation.1,7,8,20

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Table 1. Spectral tests for coexistence of α- and β-RDX polymorphs with increase in f value

;