Evaluation of Fossil Amber Birefringence and Inclusions Using Terahertz Time-Domain Spectroscopy

Abstract

Using a cross-polarization transmission geometry, stress maps for the normalized birefringence and intrinsic stress direction of polymeric materials may be obtained using terahertz nondestructive evaluation. The analysis method utilizes a deconvolution method to determine the arrival times and amplitude of the cross-polarized terahertz pulses through a birefringent material. Using amber (a naturally occurring polymer) as a material of interest, stress maps show that inclusion-free Lebanese amber samples behave as classic uniaxial birefringent (photoelastic) materials whose principal stress directions, as inferred in the terahertz spectral range, agree well with visible photoelasticity measurements. Since amber samples, depending upon their source, may be either transparent or opaque to visible light, comparing birefringence measurements in the visible and terahertz spectral ranges cross-validates the stress measurements, thereby establishing a strong and unique stress analysis methodology for visibly opaque samples. While the material of interest for this paper is amber, the method is generally applicable for any terahertz-transparent polymer. The cross-polarization experimental configuration enables stress levels within the amber matrix to be visualized while also outlining highly localized regions of stress surrounding inclusions. Birefringence stress maps clearly show localized increases in stress magnitude and directional changes surrounding inclusions.

Amber extract as a bio‐additive to poly(lactic acid) films: Multimethod analysis of crystallinity and stability

“…On the second heating, we recorded two glass transition temperatures, 54 and 97°C, without apparent enthalpy relaxation or thermal event overlap. There were no melting or crystallization peaks for AbE, which confirms that it maintained its amorphous behavior recorded for amber in the literature 8,38,40–42 …”

“…As such, it suffered various kinds of stress (heating, polishing, pressure treatments, and solvent extraction). In polarized light, the different colors represent various levels of stress 38 …”

“…There were no melting or crystallization peaks for AbE, which confirms that it maintained its amorphous behavior recorded for amber in the literature. 8,38,[40][41][42] At the first heating, all the PLA films presented endothermic events. It was to be expected, as the PLA used has a high degree of crystallinity (X c = 39%).…”

“…In polarized light, the different colors represent various levels of stress. 38 The spherulite growth since 0 min in the PLA/AbE an sample suggests that some structures worked as nucleating agents. In addition, one might think that the annealing process affects the interaction between the amber extract and PLA.…”