Terahertz (THz) time-domain spectroscopy and 3D THz pulsed imaging have been explored with regard to polymer materials, both commodity and historic polymers. A systematic spectroscopic study of a wide range of different polymer materials showed significant differences in their spectra. Polyolefins and polystyrenes generally exhibit lower absorption than other examined polymers, various cellulose derivates, poly(vinyl chloride), poly(methyl methacrylate), polyamide, hard rubber and phenol formaldehyde resin, the last of these exhibiting the most intense absorption over the entire range, 0.15-4.2 THz. It was also examined how the presence of plasticisers in poly(vinyl chloride), the presence of fillers in polypropylene, and the degree of branching in polyethylene and polystyrene affect the spectra; inorganic fillers in polypropylene affected the absorption most. With 3D THz pulsed imaging, features in polymer objects were explored, appearing either as integral parts of the material (coatings and pores in foams) or as a consequence of physical deterioration (cracks, delamination). All of these features of various complexities can be successfully imaged in 3D. Terahertz technology is thus shown to have significant potential for both chemical and structural characterisation of polymers, which will be of interest to heritage science, but also to the polymer industry and development of analytical technologies in general.
"With respect to point-like analysis, imaging techniques can provide a rapid indication of the heterogeneous degradation and condition of plastic objects. More in detail, both spectroscopy imaging methods, including those based on the analysis of the NIR radiation reflected and absorbed by a surface  , and structural imaging methods, based on THz techniques , have proved to be effective tools for monitoring degradation. In turn, fluorescence imaging, based on the analysis of the spectral   and lifetime  properties of the emission, has been used for the assessment of the conditions of cultural heritage and specifically paintings and sculptures, but has not been applied to the analysis of plastics or historical objects. "
[Show abstract][Hide abstract] ABSTRACT: Complementary use of fluorescence multispectral imaging and fluorescence lifetime imaging (FLIM) for the assessment of the degradation of a historical design object made in acrylonitrile–butadiene–styrene (ABS) is demonstrated. Model samples of ABS, exposed to UV irradiation to simulate photodegradation, have been analysed with fluorescence spectroscopy and fluorescence lifetime spectroscopy. Results demonstrate rapid modifications of the picosecond and nanosecond fluorescence lifetimes following short irradiation times, which are accompanied with significant modifications in fluorescence excitation-emission spectra. Changes in spectra and lifetimes in samples are ascribed to the accumulation of α,β unsaturated aldehydes and α,β diketones and the loss of volatile organic components (including acetophenone). Complementary analyses of samples with Fourier transform infrared spectroscopy suggest that modifications in fluorescence are correlated with photo-oxidative degradation of ABS. Fluorescence lifetime and fluorescence multispectral images of the Grillo telephone (1965) made in ABS were acquired. Analysis of images allowed the rapid mapping of differences in fluorescence which are attributed to heterogeneous degradation on the object surface. Implications for the assessment, monitoring and conservation of objects in ABS and other polymers are suggested.
[Show abstract][Hide abstract] ABSTRACT: THz spectroscopy in the time domain was explored in combination with multivariate data analysis, for quantitative determination of chemical and mechanical properties of historic paper, such as lignin content, tensile strength, and ash content. Using partial least squares (PLS) regression, it was shown that quantitative prediction of the material properties is possible, which indicates the potential of THz spectroscopy for chemical characterisation of complex organic materials of natural origin. In addition, the results demonstrate that THz spectra and PLS loading weights for lignin content differ significantly, which leads to the conclusion that THz spectra of composite macromolecular materials do not represent sums of spectra of the individual components. This supports the premise that THz spectra reflect intermolecular interactions.
The study was carried out using 250 historical paper samples from the sixteenth century to present. Although the measurements were performed in vacuum to improve the quality of spectra, THz spectroscopy is in principle non-destructive. This research therefore reinforces the role of THz spectroscopy in characterisation of valuable historic materials, where invasive analysis is often not possible.
Applied Physics A 04/2013; 111(1). DOI:10.1007/s00339-012-7525-y · 1.70 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this study, the effect of thermal and photochemical accelerated degradation on the deterioration pathways of cellulose diacetate was evaluated. The degradation induced under controlled experimental conditions was carefully monitored in order to investigate the correlation between specific degradation mechanisms and the visible signs of deterioration that are often encountered on cellulose acetate museum objects. In the time interval studied, thermally degraded samples did not show visible discoloration or noticeable vinegar smell, but appeared severely distorted. Samples degraded under UVA radiation did not show any significant sign of deterioration, while samples degraded under UVB and UVC radiation appeared discolored and released the typical vinegar odor. Samples degraded under UVB radiation were also brittle and prone to crazing. By monitoring the samples as they deteriorated over time, new insights into the degradation mechanisms were obtained. During thermal degradation, the experimental results showed that plasticizer diffusion through the polymer matrix is affected by the progressive loss of plasticizer that occurred during the degradation experiment. During photo-degradation experiments, yellowness induced by UVC and UVB radiation was found to be reversible by subsequent exposure to UVA radiation, while the depth of the degradation appeared to be one of the key factors that led to brittleness and the appearance of crazing.
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