Transparent, Heat-Resistant, Ductile, and Self-Reinforced Polylactide through Simultaneous Formation of Nanocrystals and an Oriented Amorphous Phase

Abstract

Imparting polylactide (PLA) films with great heat resistance, transparency, and balanced mechanical performance simultaneous is a big challenge in academia and industry due to mutual exclusiveness of properties. In this work, a newfound strategy that combines elongational stress field and thermal field is proposed to reorganize the crystalline and amorphous structure of PLA films. It is discovered that the oriented PLA films with high draw ratios form a unique structure of the oriented amorphous phase restricted by the oriented nanosized crystal during thermal treatment at an appropriate temperature. Superb stiffness–toughness balance of the film is achieved as the elongation at break increases from 16 to 91.9% after the thermal treatment at 150 °C, while the yield strength increases from 50.0 to 76.0 MPa. It is worth noting that the PLA film also has excellent transparency, i.e., a transmittance as high as 86.6% (550 nm), owing to crystallites formed on the nanometer scale. Excellent heat resistance is also obtained, with the onset decline temperature increasing from 63.7 (for the unstructured casting film) to 77.1 °C due to increased crystallinity. Furthermore, a new understanding of the orientation of the amorphous phase promoted by the confinement effect to the toughening of PLA is obtained, and a phenomenal model for the thermal-treatment-induced toughening effect is proposed. This work could provide a significant guidance for the facile preparation of a PLA film with promising comprehensive properties of toughness–stiffness balance, high transparency, and good heat resistance.