Toughening of poly(lactide acid) with low crystallinity through biaxial poststretching

Poly(lactide acid) (PLA) is an eco-friendly, biocompatible and biodegradable polymer that has been extensively explored in diverse applications ranging from biomedical devices to food packages. One well-known limitation of PLA in the applications is its low mechanical toughness and small stretchability. In this work, we have conducted systematic studies of the effects of the thermo-mechanical processing on the mechanical behaviors of PLA with extremely low crystallinity (less than 3%). It is found that a freshly quenched PLA after hot pressing can be quite stretchable and tough. However, after only 2 days of aging at room temperature, the PLA becomes very brittle. We attribute such aging-induced ductile-to-brittle transition of the PLA to its plastic deformation mode changing from shearing to crazing. To increase the toughness and stability of the PLA, we apply poststretching onto the PLA. Our results have shown that after poststretching, both the toughness and stretchability of aged PLA can be greatly increased. Moreover, the mechanical properties of the PLA after poststretching become more aging resistant.