Effect of annealing time and temperature above and below the Tg of poly(L-lactide)

Polylactic acid (PLA) is a biodegradable polyester obtained by renewable resources, such as sugarcanes and corn starch. Low price, good processability and excellent mechanical properties are just some of the factors that have made PLA one of the most produced and studied bioplastic in recent years, accounting in 2020 for a fifth of the global bioplastics production capacities [1]. In this work the effects on amorphous quenched poly(L-lactide) of annealing time (ta) and temperature (Ta), far above and slightly below the glass transition (Tg), were investigated by the combination of time-resolved Fourier transform infrared spectroscopy (TR-FTIR), able to follow the sample micro-structural transformation directly, and differential scanning calorimetry (DSC), which highlight the effects of thermal treatment in a subsequent scan. As regards TR-FTIR experiments, the attention was focused on the bands at 1265 cm−1, relative to the νas C–O–C + δ CH of the high energy gg conformation [2], and at 1195 cm−1, relative to the νas C–O–C + ras CH3 whose intensity is affected by the formation of α’ crystalline phase, respectively. In all the experiments the 1195 cm-1 band intensity, after an induction time (tonset) in which the PLLA remains in the amorphous state, increased because of the cold crystallization. During the induction time at Ta>Tg, the 1267 cm-1 band intensity decreases exponentially (Figure 1A) evidencing a transformation of high energy gg conformations in low energy gt conformations in PLLA liquid state. Such a reorganization, occurring at taTg, is not due to the crystallization process neither to an alpha relaxation of glass transition. Differently, the experiments at TaTg in devitrified liquid state. From the relaxation time at different Ta, an activation energy 370±20 J mol-1 of the transformation occurring in liquid state was evaluated. As a results of the annealing, the DSC curves show an endothermic peak between Tg and the cold crystallization whose intensity increases with ta (figure 2). This phenomenon could be associated to the recovery of the liquid relaxation occurred at lower temperature and, then, tentatively and preliminary assigned to a liquid-liquid transition. However, because of the inherent difficulty to investigate the observed subtle phenomena, further studies are in progress by using flash DSC. Figure 1 Intensity variation of 1265 cm-1 and 1195 cm-1 band as a consequence of annealing conducted at Ta > Tg (A) and Ta > Tg (B). Evolution of the endothermic peak for sample annealed at for different times (C). [1] N. Becker, L. Mundzeck and A. Siebert-Raths, Biopolymers facts and statistics 2021, 2021. [2] P. Pan, B. Zhu, T. Dong, K. Yazawa, T. Shimizu, M. Tansho, Y. Inoue, J. Chem. Phys, 2008, 129, 184902.