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

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 shift and a broadening above ∼2 cm, 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.