Multiscale Structural Evolutions and Deformation Mechanisms of Stereocomplexed and Homocrystalline Poly(lactic acid)s during Stretching: Correlating Thermomechanical Properties with Polymorphic Structures

Stereocomplex (SC) crystallization has been an effectiveway toimprove the physical properties of polymers. Poly(lactic acid) (PLA)is a representative stereocomplexable polymer, and PLA-based SC materialspresent a higher strength and modulus than the common homocrystallineone. However, physical origins for the better mechanical propertiesof SC materials remain unclear. In this work, we prepared PLAs withdifferent initial crystal polymorphs [SC, homocrystal (HC), and SC/HCmixture] and investigated their multiscale structural evolutions duringuniaxial stretching at different temperatures. The PLA with initialSC crystals (denoted as SC-PLA) displays a higher strength, modulus,and work of extension at fracture than the PLA with initial HC (HC-PLA),as a result of a distinct deformation mechanism. During stretching,SC-PLA shows less crystal fragmentation and its plastic deformationis driven by a lamellar rotation mechanism until failure. Meanwhile,the strain-induced oriented HC in SC-PLA helps to strengthen the polymernetwork. However, HC-PLA shows a typical fibrillation process whileaccompanied by severe cavitation upon stretching, which hinders thedevelopment of plasticity. The more effective stress transfer proposedfor SC-PLA can partially explain its improved mechanical resistanceover HC-PLA. In addition, mixing high amounts of HC with SC leadsto a similar deformation behavior to that of HC-PLA. This study haselucidated an underlying origin for better mechanical resistance ofSC materials from the chain and lamellar level.