Promotion of Stereocomplex Crystallization by Oligomeric Lactic Acid in PLLA/PDLA Blends Under Static and Stretching Conditions

ABSTRACT Stereocomplex crystallization enhances the thermal and mechanical performance of poly(lactic acid) (PLA), but its formation in poly( l ‐lactic acid)/poly( d ‐lactic acid) (PLLA/PDLA) blends is kinetically hindered by insufficient interchain diffusion. Blending with a flexible second component promotes stereocomplexation, yet the mechanism remains unclear due to coupled factors like miscibility, intermolecular interactions, and chain mobility. This study introduces non‐crystallizable oligomeric lactic acid (OLA) into equimolar PLLA/PDLA to regulate homocrystal (HC) and stereocomplex crystallite (SC) competition under static and uniaxial stretching. OLA is highly miscible and shares the same lactate backbone with PLA, thus allowing the pure plasticization effect to be isolated. DSC, POM, and in situ WAXS reveal crystallization kinetics, morphology, and strain‐induced structural evolution. OLA reduces the cold crystallization temperatures, accelerates crystallization, and promotes spherulitic growth by enhancing chain mobility. Under static conditions, increasing OLA shifts the crystalline structure from HC‐ to SC‐dominated. During stretching above T g , OLA lowers the onset strain for crystallization of both phases. However, with increasing strain, oriented HC growth dominates, weakening the SC‐promoting effect compared with static crystallization. Tensile tests show that OLA improves room‐temperature ductility but reduces modulus and strength. These results show oligomeric plasticization effectively promotes SC crystallization and provide guidance for processing SC‐PLA materials.