Contrast Enhancement for the Recovery of Obliterated Serial Numbers in Different Polymers by Correlated Raman Imaging of Strain, Phonon Lifetime, and Strain-Induced Anisotropy

Abstract

In the field of forensic science, we have recently introduced Raman imaging as a promising nondestructive technique to efficiently recover obliterated serial numbers in polycarbonate. The present study is extending the investigation toward 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 and 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, any 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.

Hyperspectral Raman imaging and multivariate statistical analysis for the reconstruction of obliterated serial numbers in polymers

“…As shown in previous studies, [ 3,4 ] hyperspectral Raman imaging allows for observable contrast between strained and unstrained areas, as a result of mechanical stress introduced by stamping of the character, in different polymers. Exploitable variations of signals such as the peak shift which is correlated to the energy and interatomic distance of the molecular bond, the FWHM corresponding to the phonon lifetime and local ordering of the material, and the intensity ratios, thus far attributed to strain‐induced anisotropy, have mainly been considered individually for reconstruction purposes.…”

“…This value was chosen in order to facilitate comparison between polymers and is based on previous experiments, which aimed to correlate Young's modulus (i.e., elasticity and strain propagation) and spatial resolution. [ 4 ] The spectral ranges and acquisition times per spectrum for each polymer are indicated in Table S1.…”

“…[ 3,4 ] As they provide information on the vibration modes of a given molecular bond, [ 9,10 ] these signals can be used to create contrast when imaging a sample that has sustained partial strain, despite being obliterated. [ 3,4 ]…”

“…Although the method has shown success for recovering serial numbers in multiple polymers, such as polycarbonate, [ 3 ] polyethylene, nylon, and nylatron, [ 4 ] it still faces certain challenges. In our previous work, we primarily relied on the extraction of peak shifts directly related to strain in the sample [ 3,4 ] and considered additional variations of relative intensity ratios and FWHM (phonon lifetime) to enhance the image contrast.…”

Chemical Composition and Strain at Interfaces between Different Morphologies in Block Copolymer Thin Films

“…Peak shifts have been related to strain in other polymer systems. Tensile strain has been found to cause a red shift as a result of increasing the bond length during strain. − Similarly, compressive strain can cause a blue shift due to the decreasing bond length. In flat areas (A), chains are in a compressed state because the film thickness, t (85 nm), is less than the equilibrium lamella period, L 0 (230 nm).…”