Fabrication of strong and tough PLLA/PDLA composites via constructed stereocomplex crystal nanofibrils under an intense shear extensional flow field

• The PLLA/PDLA composites achieved significantly improve in mechanical properties. • The oriented SC nanofibrillar structures were achieved under a high shear-extensional flow field. • 3.The tensile stress can be effectively transferred from matrix to SC nanofibrils. • The SC nanofibrils promote the crystallization of PLLA. The intrinsic friability of poly(L-lactic acid) (PLLA) significantly restricts its practical application. In this study, a facile melt-processing approach was developed to concurrently improve both the strength and toughness of PLLA by introducing oriented stereocomplex crystal (SC) nanofibrillar structures under a high shear extensional flow field through blending with poly(D-lactic acid) (PDLA). Highly oriented SC nanofibrils owning a distinct "pearl-necklace" morphology were formed at low PDLA concentrations (2–4 wt%), exhibiting an average diameter of 70 nm. These structures with increasing PDLA content (6–10 wt%) evolved into a more densely packed shish-kebab architecture. These hierarchical crystalline structures not only acted as efficient nucleating agents but also formed a rigid three-dimensional reinforcing framework within the PLLA matrix. Consequently, the tensile strength increases by 84% (from 63.6 to 117.0 MPa), and the elongation at break improves by 6.5-fold (from 6.0% to 39.4%) compared to neat PLLA. This work offers noble perspectives on the structure–property relationships of PLLA/PDLA blends, highlighting the potential for tailoring material performance through precise control of crystalline morphology.