Multistep Nucleation Mechanism of Poly(L-lactide) Revealed by Nanocrystallization in Low-Pressure Carbon Dioxide

Despite extensive efforts devoted to the polymer crystallization, the microscopic mechanisms of crystal nucleation remain elusive, which primarily arises from the difficulty of monitoring the nuclei formation involving small length and time scales. With the help of low-pressure carbon dioxide, which ensures slow crystallization with ultrahigh nuclei density, the crystal nucleation of melt-quenched poly(L-lactide) under high supercooling was investigated by infrared spectroscopy, atomic force microscopy, calorimetry, and X-ray diffraction. It was found that the nucleation proceeded via a multistep pathway through two types of metastable intermediate phases including mesophase and then intermediately ordered structure, eventually transforming into the nascent crystalline phase. The results indicated that both intermediates were not the phase-mixed structures containing crystals but were formed and evolved gradually by continuous structural optimization that involves conformational adjustment and chain packing arrangement. These intermediates possessed distinct intermediate orders as compared with those of the liquid and the crystalline phase. The mesophase had a lower order and exhibited amorphous nature to some extent, whereas the intermediately ordered structure possessed a higher order bearing less amorphous chain sequences and improved chain packing. Moreover, the mesophase was stable with respect to the liquid and metastable compared with the subsequent intermediately ordered structure, which in turn had a lower thermodynamic stability than that of the emerging crystals. Therefore, the multistep nucleation would be energetically favorable and is consistent with the Ostwald's step rule. The present results shed light on the crystal nucleation mechanisms under large supercooling and have implications for understanding the crystallization of synthetic polymers.

Automated summary

Despite extensive efforts in studying polymer crystallization, the microscopic mechanisms of crystal nucleation are still not well understood. In this study, the crystal nucleation of poly(L-lactide) under high supercooling was investigated using various techniques. The results revealed a multi-step nucleation pathway involving the formation of metastable intermediate phases. These intermediate phases possessed distinct intermediate orders and gradually transformed into the nascent crystalline phase. The findings provide insights into the crystal nucleation mechanisms under large supercooling and have implications for understanding the crystallization of synthetic polymers.