Real-Time Monitoring of Structural Changes in Poly(lactic acid) during Thermal Treatment by High-Speed Terahertz Time-Domain Spectroscopy for Nondestructive Inspection

Thermally induced structural changes in polymeric materials are usually accompanied by a change of vital properties such as mechanical stability. We demonstrate an emerging technique for real-time monitoring of thermally induced structural changes of poly(lactic acid) (PLA). The technique is based on terahertz time-domain spectroscopy (THz-TDS) because spectra of the dielectric function of PLA in the terahertz frequency range reflect macroscopic molecular vibration modes which are strongly linked to the PLA morphology and structure. By comparison with conventional X-ray diffraction data, we confirm a strong correlation between the polymer structure and the spectra of the dielectric function. We achieve real-time monitoring of two essential thermally induced structural changes: the cold crystallization process from amorphous phase to the crystalline alpha-phase and the glass transition process. The proposed technique is useful for understanding the structural changes in polymeric materials induced by thermal treatments and holds promise for nondestructive inspection of polymeric products in a contactless manner.