Tensile Deformation above the Glass Transition Temperature to Construct a Crystalline Microfibrillar Superstructure for Achieving Supertough, Ultrastrong, and Transparent Neat Polylactide

Notwithstanding that obvious progresses in toughening and strengthening polylactide (PLA) materials have been achieved, there still exists a strong challenge in finding a simple and low-cost approach to improve the impact toughness, tensile strength, and ductility for neat PLA while still maintaining its transparency and full biodegradability with no addition of any petroleum-based elastomers. In this work, neat PLA with superhigh impact strength, ultrastrong tensile strength, high heat resistance, and high transparency can be prepared by simply prestretching PLA at a temperature slightly above the glass transition temperature to prestrains of 150% and above; for example, at the prestrain of 400%, the notched Izod impact strength, Young′s modulus, ultimate tensile strength, elongation at break, and tensile toughness reach up to 358 kJ/m², 2.7 GPa, 133 MPa, 42%, and 45 MJ/m³, respectively. To the best of our knowledge, such a comprehensive enhancement of the mechanical properties, especially such high an impact strength for neat bulk PLA alone (358 kJ/m², about 188 times of that for unannealed PLA) has not been reported in the literature. Various measurements of WAXD, SAXS, SALS, SEM, and POM disclose that the produced crystalline microfibrillar superstructures composed of highly oriented nanofibrils containing PLA interlocked shish-kebab microstructures are responsible for the achieved excellent mechanical properties. This work provides a milestone key to guiding the structural design for industrial high-performance neat PLA bulk materials, thus expanding its wide applications especially in the fields where high transparency and exceptional supertoughness and ultrastrong tensile strength are required.