An Interfacial Stereocomplexation Strategy for Tailoring the Mechanical Properties and Hydrolysis Resistance of Poly(lactic acid)

This study demonstrates that interfacial stereocomplexation in poly(L-lactic acid) (PLLA)/poly(D-lactic acid) (PDLA) alternating multilayered structures offers a simple approach to tuning the mechanical properties and hydrolysis resistance of poly(lactic acid) (PLA). PLLA/PDLA multilayered sheets with various layer numbers were fabricated by sequential compression molding at a moderate temperature, enabling selective formation of stereocomplex crystals (SCs) while suppressing thermal degradation. Enzymatic etching visualized the internal morphology and confirmed that the interfacial area between PLLA and PDLA increased with increasing layer number. X-ray diffraction measurements further revealed that only SCs were formed across all layer numbers, and SC crystallinity increased as the number of layers increased. This enhanced SC content contributed to the development of interfacial anchoring structures, allowing the tensile modulus and strength to exceed rule-of-mixtures predictions by more than 15 and 17%, respectively, for high-layer-number sheets. Furthermore, hydrolysis tests revealed that amorphous structures, such as mobile amorphous fraction (MAF) and rigid amorphous fraction (RAF), are critical factors governing the degradation behavior. Above the polymer’s glass transition temperature, Tg, MAF reorganized into α-crystals via cold crystallization, generating RAF that accelerated hydrolysis due to increased chain defects and enhanced water diffusivity. In high-layer-number sheets, high SC content suppressed RAF formation and significantly reduced the hydrolysis rate. In contrast, below Tg, restricted molecular mobility limited structural rearrangement, resulting in negligible dependence of degradation rate on layer number. These findings demonstrate that interfacial stereocomplexation not only reinforces PLA mechanically but also controls high-temperature hydrolysis pathways by regulating structural changes.