Integration of Stiffness, Ductility, Heat Resistance, and Transparency for Polylactide Films by Manipulation of Amorphous Chain Networks and Oriented Nanocrystals

Developing a promising and scalable strategy for simultaneously improving ductility, stiffness, heat resistance, as well as transparency of polylactide (PLA) films is of great significance but remains a big challenge. In this work, the manipulation of the amorphous chain network and oriented nanocrystals simultaneously reconciles mechanical properties, heat resistance, and transparency of isotropic PLA films via the "biaxial orientation-confined crystallization" process. The biaxial extensional stress with a high strain rate induces the oriented homocrystallites (HCs), which can act as physical cross-links, and strengthens the amorphous chain entanglement network. Subsequent confined crystallization is utilized to promote the perfection of oriented HCs and simultaneously induce the formation of stereocomplex crystals (SCs), further strengthening the chain entanglement and restricting chain relaxation. Thus, superior stiffness–ductility balance is achieved as the elongation at break and yield strength increase from 12.5% and 56.2 MPa to 106.1% and 84.2 MPa, respectively. Moreover, the newly formed oriented SCs also impart excellent heat resistance, with a remaining storage modulus of 11.5 MPa at 190 °C and a certain dimensional stability in the hot oil (170 °C). Furthermore, the nanocrystals also impart PLA films with good transparency (>90% at 550 nm). This work could provide significant guidance for the facile and efficient fabrication of high-performance PLA films, promoting sustainable development in polymer film manufacturing.