Non-destructive evaluation of thermally induced structural transformation in PET using terahertz time-domain spectroscopy

In this study, terahertz time-domain spectroscopy (THz-TDS) was used to inspect the thermally induced structural transformation of polyethylene terephthalate (PET). PET specimens with different structural states, including variations in free volume and crystallinity, were prepared by controlling aging time and temperature. The THz-TDS system was utilized in transmission mode to examine these thermally induced structural changes in PET specimens. The structural transformation of PET specimens was analyzed using the frequency-domain absorption coefficient obtained by Fourier transformation of time-domain THz signals. The free volume state of PET specimens was evaluated in the low-frequency region of 0.1 to 0.3 THz, while the crystallinity was examined in the high-frequency region of 0.8 to 1.6 THz. Differential scanning calorimetry (DSC) measurements were performed to validate the free volume and crystallinity states of PET specimens analyzed by THz waves, showing a correlation greater than 95 %. In addition, tensile tests were conducted to analyze the effects of PET thermal structural transformation on mechanical properties. The developed THz inspection technique is expected to enable prediction of the structural state of polymer products through a real-time, non-contact, and non-destructive inspection method. • Low-frequency THz absorption coefficient is correlated with free volume in PET. • High-frequency THz absorption coefficient is correlated with crystallinity in PET. • Structural transformations of PET affect its electromagnetic properties. • Free volume and crystallinity of PET can be monitored in situ using terahertz waves.