New insights into cold crystallization behavior of annealed poly(L-lactide): a combined calorimetric and spectroscopic study

In a research based on temperature modulate FTIR, my research group has observed a non-reversing transformation of poly(L-lactide) (PLLA) occurring above Tg and before the crystallization [1]. This finding suggested that PLLA may evolve in the liquid state towards a more stable structure, characterized by a decrease of high-energy gg conformers concentration. The relaxation of the initial liquid into the more stable state was obtained by annealing the sample few degrees above the Tg. As a consequence of this work, the effects of annealing either above and below the glass transition temperature of amorphous quenched poly(L-lactide) were further investigated by the combination of time-resolved Fourier transform infrared spectroscopy (TR-FTIR), differential scanning calorimetry (DSC) and fast scanning calorimetry (FSC). The synergic use of all these techniques enables us to emphasize the presence of transformations characterized by slight variations of material properties or by low-intensity signals that are intrinsically difficult to identify and describe. Moreover, although DSC and FSC are the standard technique to provide information about the material thermodynamic states and thermal transitions, time-resolved FTIR (TR-FTIR) and variable temperature FTIR (VT-FTIR) allow the investigation at microstructural level of the conformational states involved in the sample transformation. As regards TR-FTIR experiments, we followed the band at 1267 cm−1, relative to the νas C–O–C + δ CH of the high energy gg conformation characteristic of the amorphous state [2], and the 1195 cm−1 absorption, relative to the νas C–O–C + ras CH3, whose intensity is related to the α’ crystalline phase [3]. All the experiments exhibit an induction time in which the 1195 cm-1 band intensity remains constant up to the crystallization onset (tonset) while the 1267 cm-1 band shows an exponential decay. Then, PLLA crystallization takes place, evidenced by the simultaneous increase of the α’ crystalline phase and the decrease of gg conformations. These results let us infer a hierarchic conformational rearrangement in the liquid state in which gg conformers transform into more stable gt conformers before crystallization. From the determination of the characteristic time of the phenomena observed at different annealing temperature, an activation energy of the conformational relaxation of 370±20 J mol-1 and crystallization onset of 400±20 J mol-1 were calculated. The thermal properties of PLLA during the annealing above Tg were investigated by DSC. By increasing the annealing time, as reported in literature [4] [5] [6], the cold crystallization shifts at lower temperatures. An activation energy of 350±40 J mol-1 was calculated for this transformation. Interestingly, a low-intensity endotherm appeared at about 350 K (supercooled liquid), indicating the presence of a transformation in the liquid phase. FSC was therefore performed to obtain an in-depth analysis of the liquid phase by suppressing the cold crystallization during the heating scan. In accordance with DSC, the presence of a wide and low-intensity endothermic peak in the region between the Tg and the Tm was observed in all FSC experiments. This peak was then attributed to the enthalpic recovery of the sample as a result of the conformational relaxation directly observed by TR-FTIR. References: [1] V. Di Lisio, E. Sturabotti, I. Francolini, A. Piozzi, and A. Martinelli, “Effects of annealing above Tg on the physical aging quenched PLLA studied by modulated temperature FTIR”, Journal of Polymer Science Part B: Polymer Physics, vol. 57, pp. 174–181, 2019. [2] P. Pan, B. Zhu, T. Dong, K. Yazawa, T. Shimizu, M. Tansho, Y. Inoue, “Conformational and microstructural characteristics of poly(L-lactide) during glass transition and physical aging”, The Journal of Chemical Physics, vol. 129, 184902, 2008. [3] V. Di Lisio, “Setup of Modulated Temperature FTIR technique to investigate thermal transitions of polymers”, [Doctoral Dissertation, Sapienza University of Rome], 2018. [4] H. Tsuji, M. Sawada, “Accelerated crystallization of poly(L‐lactide) by physical aging”, Journal of Applied Polymer Science, vol. 116, pp. 1190-1196, 2009. [5] B. Na, S. Zou, R. Lv, M. Luo, H. Pan, Q. Yin, “Unusual cold crystallization behavior in physically aged poly(L-lactide)”, The Journal of Physical Chemistry B, vol. 115, pp. 10844–10848, 2011. [6] R. Androsch, M. L. Di Lorenzo, “Kinetics of crystal nucleation of poly(L-lactic acid)”, Polymer, Vol. 54, pp. 6882-6885, 2013.