Evaluation of relative content and crystallization behavior of homogeneous crystals in poly (lactic acid) by terahertz spectroscopy

Poly (lactic acid) (PLA) is a typical polymer with homogeneous polymorphism. Its different crystal forms have their own crystallization paths, which endow it with special properties. Therefore, it is crucial to study the structure and crystallization behavior of PLA to better understand its properties. Here, we investigate PLA films obtained under different crystallization conditions by terahertz (THz) spectroscopy. The alpha ', alpha and their mixed crystal forms of PLA exhibit completely different spectral feature. Based on the frequency of THz characteristic peaks, we further determine the relative contents of alpha ' form and alpha form in PLA with mixed crystals, the validity of which is verified by comparing X-ray diffraction (XRD). Furthermore, the change of absorption intensity of different PLA films is significantly different with crystallization time, mainly manifested as the change rate of glassy PLA crystallized at 80 degrees C is faster than that of molten PLA. We believe that this is because the crystallization of the alpha form of PLA at 80 degrees C is at a disadvantage, which reduces the speed of conformational adjustment. The results show that THz spectroscopy can not only characterize the crystal changes of polymers, but can also be an effective method to study the kinetics of polymer crystallization.

Automated summary

Poly(lactic acid) (PLA) is a polymer with homogeneous polymorphism, and its different crystal forms exhibit distinct spectral features. The structure and crystallization behavior of PLA are crucial for understanding its properties. In this study, we used terahertz (THz) spectroscopy to investigate PLA films obtained under different crystallization conditions. We determined the relative contents of different crystal forms in PLA and studied the kinetics of polymer crystallization. The results demonstrate that THz spectroscopy is an effective method for characterizing polymer crystallization and studying its kinetics.