Quantitative diagnostics of ancient paper using THz time-domain spectroscopy

An innovative method to quantitatively assess the state of degradation of paper in a non-destructive way has been developed by using THz time-domain spectroscopy (THz-TDS). The method relies on the observation that the hydrolytic and oxidative degradation of cellulose in paper is accompanied with rearrangement of the hydrogen bonds (H-bond) network of cellulose polymers. This gives clear fingerprints in the THz spectral profiles. THz-TDS is a suitable approach to study low-energy vibrational properties of biological materials in a non-destructive way. The THz photon energy range (about 1-40 meV) is particularly suitable to probe the H-bonds between molecules. However, the quantitative assessment of the state of preservation of artworks poses significant challenges, in particular when measurements on low refractive index and thin samples, such as ancient documents and drawings, must be carried out [1]. In the present study, we have successfully solved this problem obtaining a precise determination of the absorption by cellulose fibers in the 0.2-3.5 THz (7-117cm-1) range in single freely standing paper sheets. This has been achieved by developing a new procedure to remove from the experimental signals the spurious interference effects generated by the Fabry-Pérot resonances in the sheet. By using this approach, the THz absorption coefficient of cellulose fibers in ancient and modern samples artificially aged has been obtained. The THz absorption spectra are explained in terms of absorption peaks of the cellulose crystalline phase superimposed to a background contribution due to a disordered H-bonds network [2]. The complex evolution of the spectra as a function of natural and artificial aging is explained with a reduction up to 30% of the H-bond density in the cellulose polymer networks and a parallel increasing of the sample crystallinity evident in the peaks at about 2.1 and 3.1 THz. The comparison with density functional theory ab-initio calculations suggests that the observed THz spectral changes could be related not only to the recrystallization of amorphous regions, but also to the progressive degradation of the external surfaces of crystalline domains. These results demonstrate a significant potential in the non-destructive analysis of the structural modifications of cellulose and paper artifacts by aging, resulting, at a macroscopic scale, in a progressive weakening of the mechanical properties of paper [3]. [1] T. Trafela, M. Mizuno, K. Fukunaga, and M. Strlic, Appl. Phys. A, 111, 2013, 83-90. [2] E. Scarpellini, M. Ortolani, A. Nucara, L. Baldassarre, M. Missori, R. Fastampa, and R. Caminiti, J. Phys. Chem. C 120, 2016, 24088-24097. [3] L. Teodonio, M. Missori, D. Pawcenisc, J. ?ojewskac, F. Valle, Micron 91, 2016, 75-81.