Contrast enhancement for the recovery of obliterated serial numbers in

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Contrast enhancement for the recovery of obliterated serial numbers in different polymers by correlated Raman imaging of strain, phonon-lifetime and strain-induced anisotropy Cédric Parisien, Gitanjali Kolhatkar, Andreas Dörfler, Frank Crispino, André Lajeunesse, and Andreas Ruediger Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b01621 • Publication Date (Web): 03 Jul 2019 Downloaded from pubs.acs.org on August 28, 2019

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Analytical Chemistry

Contrast enhancement for the recovery of obliterated serial numbers in different polymers by correlated Raman imaging of strain, phonon-lifetime and strain-induced anisotropy Cédric Parisien†, Gitanjali Kolhatkar†, Andreas Dörfler†‡*, Frank Crispino§, André Lajeunesse§ and Andreas Ruediger†* †Nanophotonics and Nanoelectronics group, Énergie-Matériaux-Télécommunication Research center, Institut National de la recherche scientifique, 1650 boulevard Lionel-Boulet, Varennes (Québec), Canada, J3X1S2 ‡ Department of Applied Sciences and Mechatronics, Munich University of Applied Sciences, Lothstrasse 34, 80335 Munich, Germany §

Laboratoire de recherche en criminalistique, Université du Québec à Trois-Rivières, 3351 boulevard Des Forges, Trois-Rivières (Québec), Canada, G9A5H7 ABSTRACT: In the field of forensic science, we have recently introduced Raman imaging is a promising non-destructive technique to efficiently recover obliterated serial numbers in polycarbonate. The present study is extending the investigation towards different polymers for the reconstruction of abraded information by Raman spectroscopy. Samples of polyethylene, nylon and nylatron, which are mainly used in items such as firearms, banknotes and package materials, are investigated by monitoring the vibrational modes which are most susceptible to peak shifts, changes in the full width at half maximum (FWHM) and peak intensity ratios. In all cases, the most affected peak depends on the polymer’s 3D structure and displays a ~1 cm-1 shift and a broadening above ~2 cm 1, as well as a relative intensity change of over 50%, more than enough for a successful recovery through confocal imaging. Depending on the polymer’s structural arrangement, either of the three contributions prevails for the strongest contrast. The propagation of the plastic deformations is mainly affected by the Young’s modulus of the material, due to a change in its elasticity. The shift, the width, and the relative intensity of the Raman peaks being three independent parameters, they can be correlated to enhance the contrast and thus to accelerate the image acquisition or to enhance statistical significance.

In the past decade, industry has been systematically replacing metals and alloys by polymers with tunable properties while at the same time reducing production costs. Due to their versatile properties, polymers are the ideal contenders and are becoming more and more widely used. They find many applications, among which automobile parts and firearms.1 Safety regulations may require those items to bear a serial number. To ensure that this serial number remains readable throughout the item’s lifespan, the marking needs to induce a strong visual contrast. To do so, several techniques such as engraving or laser ablation can be used, but the most commonly employed is stamping. 2–4 In addition to the visual contrast, the marking induces plastic deformations, resulting in local structural changes and residual mechanical strain in the polymer’s microstructure.1 However, physical abrasion can make the serial number partially or completely unreadable. 2,3,5 Forensic

scientists need a reliable technique to recover the obliterated information in polymers. Available techniques such as the magneto-optical method6 or chemical etching,2,3,7 that either require magnetic ordering or rely on acids, cannot be applied to polymers. Other methods such as heat treatment, relief polishing or chemical swelling have been demonstrated as feasible but destructive, unreliable and unreproducible as the parameters affecting the recovery cannot be controlled with sufficient precision.1,2,6,8–10 In previous work, the potential of Raman spectroscopy to reconstruct serial numbers in polycarbonate was demonstrated.11 Raman spectroscopy is a well-established vibrational spectroscopy technique that relies on light’s inelastic scattering.12–15 Variations in the Raman peak position or full width half-maximum (FWHM) indicate the presence of residual strain13,16 or local structural changes affecting the phonon lifetime,13,15 respectively. Changes of

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peak intensity ratios are attributable to changes of the relative orientations of polarizability tensors thus reflecting changes in local preferential orientations of polymer chains or subunits. Due to their high propagation depth of up to a millimeter, plastic deformations induced by the marking of polymers remain present even after superficial abrasion and can be monitored by Raman imaging to non-destructively reconstruct obliterated serial numbers. In addition, Raman spectroscopy does not require any sample pretreatment.14,15 While polycarbonate, a transparent and tough material, is widely used for automobile parts or bulletproof windows, other polymers such as polyethylene or nylon are also commonly used for other applications.1,13 The propagation of the plastic deformation depends on the material’s properties such as structure, hardness, ductility, strength, (which in return might depend on additives). Additives that affect the absorbance will in turn influence the imaging capability of Raman spectroscopy.1,12,14 In this work we investigate the effect of the polymer properties on the ability of Raman imaging to reconstruct erased information. By comparing samples of polyethylene, nylon and nylatron that have been marked and abraded under similar conditions, we demonstrate that mechanical properties of polymers, in particular the screening length for plastic deformations, have a direct impact on the required pixel density, which in return determines the image acquisition time. As polyethylene is mainly used for banknotes and packaging1,12,13 while nylon and nylatron are the main components of firearms, this study holds the promise to diverse applications in forensic science.

EXPERIMENTAL SECTION Sample preparation. A 0.5 cm × 0.6 cm capital letter ‘H’ was manually stamped with a pressure of ~170 MPa into samples of polyethylene, nylon and nylatron. The depth of the marks was measured to be ~230 µm in polyethylene, ~120 µm in nylon and ~85 µm in nylatron. The right half of each letter was then homogeneously abraded using a mechanical milling machine from Swiss Varnamo with a depth of ~320 µm for the polyethylene sample, ~180 µm for the nylon and ~135 µm for the nylatron. The abrasion depth was adapted for each sample according to the depth of the mark in order to make the ‘H’ invisible to the naked eye in the obliterated region, choosing the abrasion depth to be approximately 150% of the profile depth. Raman measurements. Confocal Raman measurements were performed at room temperature using an Olympus BX41 confocal microscope connected to an iHR-320 Horiba Scientific spectrometer coupled with a thermoelectrically cooled, back illuminated deep depleted CCD Synapse™ camera. A linearly polarized Cobolt Blues™ 25 mW diode-pumped solid state (DPSS) TEM 00 laser operating at 473 nm and a grating of 2400 lines per millimeter were employed. The laser was focused on the sample through a 0.25 NA 10× objective, creating a focal spot of 2.3 µm diameter. To avoid thermal degradation

and local thermal expansion of the sample, a laser power of 8.4 mW was used. The spectral resolution of the hardware under these experimental conditions was ~1 cm-1, which was further enhanced by numerical fitting algorithms under a custom made Julia code for a fast, automated treatment of spectra. The results of the numerical procedure were confirmed by OriginTM software.

Figure 1. Chemical structure of the monomeric unit of (a) polyethylene and (b) nylon. The corresponding typical Raman spectra recorded in a strain-free region are presented in (c) and (d), respectively.

RESULTS AND DISCUSSION Raman response to plastic deformations in polymers. The chemical structures of polyethylene and nylon are illustrated in Figure 1(a) and (b), respectively.17 The carbon (C) atoms are covalently bound, forming a rigid structure.17 The density of polyethylene is determined by the density of ramifications.1,13,17 In our case, the polyethylene sample has a high density and is therefore essentially semi-crystalline, while still presenting amorphous regions.17 The chains composing polyethylene are bound together by electrostatic interactions and van der Waals bonds.17,18 Nylon on the other hand is a fibrous polymer. Each fiber is composed of chains of covalently bound diamines (NH) and dicarboxylic acids (C=O) resulting in an amide polymeric structure.1,13,18 The chains are linked by hydrogen bonds, which makes this polymer stronger and more resistant.13,17,18 The typical Raman spectra of polyethylene and nylon acquired in strain-free regions are presented Figure 1 (c) and (d), respectively. The labeled peaks can be attributed to specific chemical bonds within the monomeric units, as listed in Table 1.18–20 Nylatron consists of nylon fibers reinforced by a molybdenum disulfide resin, and thus has the same chemical structure and Raman spectrum as nylon.21 To identify which bonds are sensitive to the stress induced by the stamping, the Raman shift, the FWHM of each peak as well as their relative intensity was monitored across the visible part of the ‘H’, following the procedure described in Ref. [11]. We emphasize that this is a necessary step for each new material under investigation to maximize the image contrast. Similarly to polycarbonate, the Raman bonds are differently affected by stress, as


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Analytical Chemistry though the trend is not as pronounced on the nylon sample. For both peaks, the area under the curve also changes, confirming that this effect is independent of the one reflected by the FWHM.

summarized in Table 1. In polyethylene, peak P1 displays a tensile residual strain as indicated by a shift towards lower wavenumbers,22,23 while peaks P4, P6 and P7 depict a shift towards higher wavenumbers due to a compressive residual strain.22,23 In both nylon and nylatron, peaks N3 and N8 undergo a tensile residual strain whereas peaks N5 and N10 reveal a compressive residual strain. Overall, the most susceptible peaks were P1 and N10 (Figure 2 (a) and (b), respectively), which are centered at 1069 cm-1 and 1645 cm-1 respectively in the stress-free region and shift by up to 1 cm-1 under stress. Unlike polycarbonate, 11 those bonds do not correspond to the links with the lowest dissociation energies.1,13 In polyethylene, P1 corresponds to the C-C bond that links a semi-crystalline region to an amorphous region.19 Due to the 3D arrangement and the van-der-Waals interactions, the semi-crystalline regions will deform less under stamping.19 Therefore, the most susceptible bond is located at the interface with an amorphous region.17,19,24 Likewise, in nylon and nylatron, N10 is attributed to the carbonyl (C=O) bond that links the fibers together,18,20 as stress will mainly modify the distance between the polymer chains before affecting the chains themselves.17 In addition, the FWHM of both P7 and N3 increases by ~2 cm-1 in the marked region due to a change in the local structure.1,13,25,26 The region over which the peaks were monitored is illustrated in Figure 3. Moreover, the intensity ratio of the Raman peaks also varies over the stamped region. A change in this parameter indicates the preferential local reorientation of chains or subunits in strained areas, also known as strain-induced anisotropy.27 The intensity ratios of P7/P2 and N3/N4 showed the largest changes in polyethylene and nylon, respectively. These peaks intensity ratios depicted changes from ~ 0.8 to 0.45 for the former, and ~2.8 to 2.2 for the latter, even

Figure 2. Peak shifts, FWHM and peak intensity ratio variations across a marked region of (a) polyethylene and (b) nylon. Spectral imaging of the obliterated letter. All three samples are similar to the polycarbonate sample,11 where the left part of the ‘H’ was unaltered and remains visible while the right part was completely obliterated. Comparing the different materials reveals that even though the same pressure was applied during the marking process, the depth of the resulting mark varies, in agreement with the respective hardnesses.1,28 Variations in the shift and FWHM of peaks P1 and P7, as well

Table 1. Raman peaks composing the spectrum of polyethylene and nylon.18–20 Raman peak

Identification

P1

Anti-symmetric C-C stretching C-C stretching Symmetric C-C stretching C-C twisting C-C twisting CH2 bending CH2 bending CH2 bending C-CO bending Anti-symmetric C-C stretching C-C stretching N-H wagging CH2 twisting C-CH2 bending C-CH bending C-CH2 bending C-N stretching C=O stretching

P2 P3 P4 P5 P6 P7 P8 N1 N2 N3 N4 N5 N6 N7 N8 N9 N10

Chemical bond energy (kJ/mol) 348

Peaks in the absence of stress (cm-1) 1062

Strain observed

348 412 348 412 412 412 412 348 348

1130 1168 1293 1368 1418 1441 1460 953 1063

Compressive Compressive Compressive -

348 388 412 348 412 348 305 743

1130 1235 1298 1335 1383 1441 1474 1630

Tensile Compressive Tensile Compressive


Tensile


as N10 and N3 were monitored over the abraded area to reconstruct the obliterated section of the letter in polyethylene, nylon, and nylatron, as depicted in Figure 3. For all polymers, the maps obtained by monitoring the variation in the peak position expose an H-shaped strained region (purple region on polyethylene, and yellow on nylon and nylatron) indicated by a shift of ~1 cm-1, consistent with the values described in Figure 2. Likewise, the spectral images of the FWHM reveal a peak broadening of ~2 cm-1 for polyethylene and nylon, and ~4 cm-1 for nylatron, respectively over an H-shaped area (yellow region) due to a degradation of the local ordering. The regions displaying local structural changes (phonon lifetime through FWHM) provide a strong contrast, different from observations in amorphous polycarbonate.11 Furthermore, by imaging the intensity ratio of P7/P2 and N3/N4, we were able to recover the obliterated portion of the letter (dark purple region). In the case of polyethylene, a stronger contrast is obtained by monitoring the FWHM and peak intensity ratio, than the peak shift. We will refer to this later in terms of optimized experimental conditions as the imaging of the FWHM or the peak intensity ratio may have different requirements for the equipment. The peak shift, FWHM and peak intensity ratio maps demonstrate the successful recovery of the obliterated information and attest to the potential of this technique independently of the polymer types and properties. The detection of changes in the peak intensity ratios is relatively little demanding in terms of experimental conditions as it is technically sufficient to discriminate one peak against another while

the detection of peak shifts and changes of the FWHM in the aforementioned range requires a spectral resolution of the hardware of approximately 1 cm-1 that can then be enhanced numerically through peak fitting routines for the analysis we describe. The width and clarity of the visual contrast of the reconstructed section of the ‘H’ vary between the polymers. In polycarbonate, the ‘H’ shaped strained area has a width of ~1 mm, similar to the initial mark. 11 In polyethylene, the strained area appears narrower, with a value of ~0.5 mm. In nylon and nylatron, the strained region is even narrower with values of ~0.1 mm. This tendency can be linked to the Young’s modulus and therefore the elasticity of the polymers. In addition, some horizontal stripes can be observed in the peak shift maps for all three polymers. This effect is explained by a temperature drift of the spectrometer during the measurements, which took about 24 hours per sample. Since the peak shift is extremely small when compared to the absolute wavenumber, these tiny, slowly changing fluctuations with a magnitude of below 0.5 cm-1 can be seen in the image. The observed pattern agrees with the scan direction along horizontal lines. The fact that both the FWHM and peak intensities appear unaffected is consistent with this observation.

Figure 3. Spectral images of the peak shift, FWHM and peak intensity ratio of (a) polyethylene, (b) nylon and (c) nylatron. The left part corresponds to a picture of the sample while the right part presents the spectral image over the obliterated region, showing the successful recovery of the abraded section of the ‘H’.


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Analytical Chemistry The yield stress, defining the threshold of plastic deformation shows a linear dependence on Young’s modulus in a variety of polymers so that we use this correlation for simplicity at the moment.29,30 As a result, polyethylene, which has the lowest Young’s modulus (1.2 GPa) will depict a more contrasted plastic deformation than polycarbonate (2.5 GPa) and nylon (3.4 GPa).1,13,17,18 Moreover, nylatron is a reinforced nylon, and is therefore tougher. 21 The use of a resin to maintain the fibers together allows for a higher propagation of the deformation.1 Enhanced acquisition and analysis. The hyperspectral image of our investigation can in principle be acquired in two different ways: 1) a pixel-by-pixel imaging procedure where a complete, high resolution Raman spectrum is registered at each location as in our case, 2) a stack of wide-field images at different Raman shifts. The second technique, while holding the promise of much better throughput, relies on the availability of tunable Raman line filters with a cut-off precision of less than a wavenumber, which are not yet available despite substantial progress with filters based on volume Bragg gratings in recent years. As a consequence, the pixel density in our technique is a crucial parameter for both the signal-to-noise ratio as well as for the total integration time. Following basic algorithms of optical character recognition (OCR), a letter requires 5 × 7 = 35 pixels for identification. Given that the exact location of an obliterated serial number is unknown, this range should be enlarged to e.g. 10 × 15 = 150 pixels. These pixels extend over the area of the letter, for which we assume 4 mm height. So, in terms of necessary OCR pixel density, we would require 4/7 mm per pixel in height (for this calculation we do not take into consideration the extra pixels around the obliterated letters) and about the same size in width. Each pixel should therefore ideally have a size of roughly 600 µm. This requirement is challenged by the confocal Raman setup. In order to collect the largest possible signal for minimum integration times, it is necessary to collect a large solid angle for the largest possible total scattering cross-section, so that the numerical aperture of the microscope objective should be chosen as large as possible. As an immediate consequence, the focal spot size decreases to the size of the Airy disc, in our case ~2 µm in diameter. This means that if we

chose the pixel size of our scan to meet the OCR requirements of 600 µm, our surface would be heavily undersampled following the Nyquist-Shannon theorem. Explicitly, we would have probed an area of roughly 3 µm2 (focal spot area) in an image pixel of 360,000 µm2. For 99.9992% of the image pixel area, we would not actually have information. It is impossible to technologically reconcile these contradicting demands, high photon collection efficiency on one side and large focal spots on the other. Since the integration time linearly scales with the number of pixels, it is also not a viable option to increase the number of pixels to the level, which would avoid under-sampling. Scanning an area of e.g. 0.6 cm by 3 cm (area where the obliterated number is expected) with a resolution of the Airy disc would take close to a week for the image acquisition for an optimistic integration time of 1 second per pixel. To this end, we will have to investigate the ideal pixel density, which has to be large enough to be sure that we are detecting the strained regions and small enough to provide realistic image acquisition times. Our analysis shows that the spatial resolution needed to reconstruct obliterated information is inversely proportional to the Young’s modulus of the polymer and thus, as outlined above, needs to be adapted to the material under study. As previously mentioned, the zone for which residual strain can be detected in nylon and nylatron has a width of 100 µm, the pixel size should be chosen accordingly. Using the most affected parameter between the shift and the FWHM, a minimal spatial resolution of 100 dpi was required to retrieve the obliterated information on polycarbonate, while a successful reconstruction was achieved with values of 200 dpi on polyethylene, and 300 dpi on nylon and nylatron. This exercise, while requiring a series of preliminary experiments, is highly rewarding as the total acquisition time scales quadratically with the linear pixel density. Statistical upgrade of the approach. To quantify the observation of two distinct populations of unstrained/undistorted and strained/distorted areas in each spectral image, histograms of the pixel values were created and fitted using two Gaussian distributions of identical standard deviation σ.11,31 The results are summarized in Table 2. For comparison, the results obtained on polycarbonate are also included. For all

Table 2. Statistical analysis parameters obtained on the spectral images of the different polymers. Polymer Polycarbonate11 Polyethylene Nylon Nylatron

Shift FWHM Shift FWHM Shift FWHM Shift FWHM

Peak values without deformation (cm-1) 1236.69 28.57 1441.35 28.62 1630.96 15.51 1630.71 15.93

Peak values with deformation (cm-1) 1235.69 30.50 1441.60 30.00 1631.63 16.65 1631.88 18.94


σ 0.09 0.37 0.22 0.42 0.20 0.39 0.34 0.81

Separation (σ) 11.39 2.53 3.20 3.29 3.30 2.97 3.48 3.71


polymers, the pixel values are distributed between two populations separated by at least 2.5σ for both the peak shift and the FWHM.31 Even though the peak shift, FWHM, and peak intensity ratio variations originate from the same deformation, they described three different physical effects and remain independent from one another.1,12,13,22,25 The first one depends on the phonon deformation potential while the second is inversely proportional to the lifetime of the associated optical phonon.13,22,25 The change in peak intensity ratio originates from strain induced anisotropy. As a consequence, the polarizability tensors of all bonds in the respective region will also experience an anisotropy leading to some peaks increasing and other peaks decreasing in intensity. An alternative interpretation of broken bonds leading to a reduced population of Raman oscillators cannot account for increases in Raman peak intensities and was therefore not considered any further even though we cannot exclude a contribution. We will now demonstrate how to enhance the spectral image contrast correlating the parameters two by two. We used the peak intensity ratio and the FWHM as they

gave the highest individual contrasts. For this purpose, a 2D histogram of the FWHM, and peak intensity ratio of each individual pixel is plotted for a given Raman peak, as depicted in Figure 4(a). The density map thereby obtained shows at least two distinguishable populations, as indicated by the red and green ellipses, each representing the 2σ threshold of 2D Gaussian probability density fit. The red ellipse corresponds to the undeformed region of the sample, while the green ellipse is attributed to the deformed region. Using the red ellipse as a threshold, we were able to reconstruct the obliterated portion of the letter (Figure 4). This threshold value was determined by optimizing the separation between the undeformed and deformed regions while providing a high signal-to-noise ratio. In the maps shown in Figure 4, the black and white dots correspond to the pixels outside and inside the red ellipse, respectively. The 2D Gaussian fit provides the threshold values. In the case of all three samples, the deformed region obtained through this correlation is broader and more defined than when the shift, FWHM or peak intensity ratio are imaged individually.

Figure 4. Density plot, FWHM, peak intensity ratio, and correlated binary images for (a) polyethylene, (b) nylon, and (c) nylatron. In the density plot, black histograms show the individual distributions and the color gradient specifies normalized probability density.


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CONCLUSIONS In summary, we demonstrate the potential of Raman imaging for the successful reconstruction of obliterated information in polymers with different properties. We present a comparison between three different polymers, namely polyethylene, nylon and nylatron, onto which a letter was cold stamped with equivalent pressure (~170 MPa) and abraded on the right side. For all three materials, we were able to identify at least one Raman peak that undergoes a peak shift of up to 1 cm-1 between the strained and unstrained region, a broadening of ~24 cm-1 in the presence of local structural changes, and a set of peaks that undergo a substantial change in peak intensity ratio. The propagation of plastic deformations was related to the materials’ properties, mainly to their Young’s modulus, which defines the spatial resolution required for a successful reconstruction. The contrast between the deformed region and the background can be enhanced by correlating the peak shift and the FWHM. This study should assist in the transfer of this technique to forensic and other analytical laboratories focusing on strain imaging in polymers.

AUTHOR INFORMATION

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Corresponding Author * Email: [email protected] (A.R.); [email protected] (A.D.)

ACKNOWLEDGMENT We thank C. Muehlethaler (UQTR) for fruitful discussions. A.R. gratefully acknowledges an NSERC discovery grant {RGPIN-2014-05024], an FRQNT team grant [2019PR-256964] and a strategic partnership grant [5062892017; 506953-17]. G.K. is grateful for an FRQNT postdoctoral scholarship.

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de Smith, M. J. Statistical Analysis Handbook: A Comprehensive Handbook of Statistical Concepts, Techniques and Software Tools; John Wiley & Sons, 2010.

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Contrast enhancement for the recovery of obliterated serial numbers in

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