Self-nanofibrillation strategy to an unusual combination of strength and toughness for poly(lactic acid)

Abstract

Despite various efforts made to toughen poly(lactic acid) (PLA), currently developed routes (e.g., blending and chain modification) are prominently hampered by economical concerns and strength–ductility trade-off dilemma. Here we disclose an additive-free, one-step approach to create numerous PLA nanofibrils with a high structural integrity and self-reinforcing effect. Specifically, adequately crystallized PLA was partially melted below the melting point and refined into tiny crystalline blocks by intensive extrusion. The chain networks crosslinked by shear-aligned crystals with high melt strength were stretched at drawing ratios as high as 20, conferring extremely ordered alignment in the stretching flow and straightforward nanofibrillation by PLA itself. The perfectly aligned nanofibrils featured an ultralow diameter approaching 80 nm and perfect alignment, contributing to direct formation of compact shish-kebabs with ordered chain packing and enhanced thermal stability. The unique structural features allowed self-nanofibrillar PLA to achieve an impressive combination of high strength and toughness, reaching a remarkable increase of 36%, 4077% and 3976% in tensile strength, elongation at break and toughness (63.7 MPa, 325.8% and 101.9 MJ m⁻³) compared to normal PLA, respectively. The profound morphology control, the simplicity of manufacturing, together with the versatility of multiple property improvements, should make the self-nanofibrillation strategy useful to guide the design of high-performance PLA with industrial feasibility.