Tailoring polylactide toughness: Synergistic effect of D-isomer content and biaxial stretching on mesophase evolution and toughening mechanism

This study investigates the synergistic effects of D -isomer content and biaxial stretching on the mechanical, thermal, and microstructural evolution of polylactide (PLA). Commercial PLA grades with 2 %, 4 %, and 12 % D -isomer content were biaxially stretched up to 3.5 × 3.5 and characterized. Increasing D -isomer content reduced stereoregularity, entanglement density, and glass transition temperature, thereby lowering strength. Biaxial stretching induced a brittle-to-ductile transition, accompanied by formation of a locally ordered mesophase whose content and morphology depended strongly on both D -isomer content and stretch ratio. Notably, PLA with 2 % D -isomer achieved 146.6 % elongation at break at a 3 × 3 stretch ratio, with mesophase content reaching 40 %. In contrast, grades with higher D -isomer content required greater stretch ratios for lower maximum elongations (∼80–84 %). Analyses revealed that smaller spherical mesophase domains in low D -isomer PLA promote microfibril-rich crazes that stabilize crack propagation. A novel toughening mechanism is proposed: biaxial stretching converts entanglements into a spherulitic mesophase, which nucleates controlled, energy-dissipative crazing. This strategy provides a practical route for tailoring PLA performance for advanced applications. • A non-oriented mesophase is achieved solely via high-speed biaxial stretching. • Mesophase content and morphology are governed by D -isomer and stretch ratio. • A novel toughening mechanism of entanglement-to-mesophase conversion is proposed. • Strength and toughness are simultaneously enhanced in biaxially stretched PLA. • Low D -isomer PLA achieves 40 % mesophase and 146.6 % elongation at break.