Detailed molecular movements during poly(l-lactic acid) cold-crystallization investigated by FTIR spectroscopy combined with two-dimensional correlation analysis

Abstract

A good understanding of the detailed molecular movements of the poly(L-lactic acid) (PLLA) cold-crystallization is significant for academic investigation and commercial application. In this article, the cold-crystallization process of PLLA was successfully investigated via temperature-dependent FTIR spectroscopy combined with the perturbation correlation moving-window two-dimensional (PCMW2D) spectroscopy and generalized two-dimensional correlation (2D) spectroscopy. PCMW2D reveals that the cold-crystallization of PLLA undergoes two processes: Process I (60.8–73.7 °C) and Process II (91.1–104.0 °C), which is consistent with the results of differential scanning calorimetry (DSC) measurement. The method of estimating the regularization enthalpy of function groups (CH₃, CO, and CCO–O) during both Process I and Process II is established based on van't Hoff plots. The results show that the enthalpy of the regularization of chain segments is decreased with the increasing mobility. The sequential order of the groups' movement in both Process I and Process II is successfully investigated by the generalized 2D correlation analysis, which has great significance for understanding the detailed microdynamics mechanism of the PLLA cold-crystallization. Process I is the relaxation process, which undergoes 2 steps and Process II is the primary cold-crystallization process, which has 5 steps.