Terahertz Birefringence and Anisotropic Absorption Characteristic of Ordered Polymer Structures for Simultaneous Orientation Measurement in Crystalline and Amorphous Regions

Strain-induced crystallization (SIC) is one of the most significant phenomena in semicrystalline polymers. During the stretching process, ordered structures occur in the crystalline and amorphous regions, accompanied by a complex evolution process. The ordered structure of the crystalline region is usually a perfect crystal, while that of the amorphous region mainly consists of locally ordered chain segments. The simultaneous characterization of ordered structures in both crystalline and amorphous regions faces significant challenges due to differences in the scale of the ordered structure. This research proposed a method for simultaneously measuring ordered structures of poly(ethylene terephthalate) (PET) in both crystalline and amorphous regions based on terahertz time-domain spectroscopy (THz-TDS) with extreme sensitivity to the long-range ordered structure. The models between orientation and birefringence and absorption anisotropy in the THz band were established. The evolution of the refractive index and absorption coefficient of samples at different draw ratios was investigated. The results showed that birefringence was proportional to the orientation of trans conformations, and the orientation–birefringence model was validated in the THz band. Then, two characteristic absorption bands, ranging within 1.3–2.3 and 2.3–3.0 THz, in the absorption spectra of stretched PET are closely related to the content and orientation of trans conformation in the amorphous and crystalline regions of PET. Finally, the mechanism of strain-induced anisotropy of the refractive index and absorption coefficient was elucidated, which originates from the orientation of the molecular chain during necking and undergoes rapid changes once necking occurs. These insights deepen the understanding of the refractive index and absorption in the THz frequency range and provide an effective method for simultaneously characterizing the ordered structures in the amorphous and crystalline regions.