Non-equimolar stereocomplex polylactide fibers strengthen and toughen poly(L-lactic acid) via interfacial stereocomplexation while retaining transparency.

In this work, stereocomplex PLA (SC-PLA) fibers with various PLLA/PDLA mass ratios were fabricated and subsequently incorporated into the poly(L-lactic acid) (PLLA) matrix to prepare all-PLA composites. By deviating from the stoichiometric ratio toward PDLA excess, in situ interfacial stereocomplexation was triggered, converting the initially weak fiber-matrix interface into a robust stereocomplex (SC)-mediated interface. Remarkably, although the PDLA-rich SC-PLA fibers (LD12) exhibited the lowest intrinsic SC crystallinity (35.1%), the corresponding composite showed the highest SC content and significantly enhanced birefringence under polarized light microscopy, together supporting the formation of additional SC crystallites preferentially associated with the fiber-matrix interphase. Dynamic mechanical analysis revealed that the glass transition temperature of the PLLA/LD12 composite was 4.4 °C higher than that of neat PLLA, representing the largest increase among the composites and indicating pronounced restriction of PLLA segmental mobility consistent with SC-mediated molecular anchoring. Consequently, the enhanced interfacial anchoring facilitated efficient stress transfer from the matrix to the fibers, promoting both crazing and shear yielding during tensile deformation. As a result, the PLLA/LD12 composite simultaneously achieved enhanced toughness and strength. Relative to neat PLLA, the elongation at break and fracture energy increased by 285.7% and 418.5%, respectively, while the tensile strength increased by 23.1%. Moreover, because of the small refractive-index contrast between the SC-PLA fibers and the PLLA matrix, all composite films maintained high transparency (>88% transmittance at 750 nm). This work provides a new strategy for designing strong, tough, transparent, all-PLA composites through stereocomplex-mediated interfacial regulation.