Promoting the formation of stereocomplex crystal in PLLA/PDLA by crosslinking restricted phase separation and solid-state stretching induced orientation

Stereocomplex crystal (SC) in poly( l -lactide) (PLLA)/poly( d -lactide) (PDLA) can significantly improve the heat resistance and mechanical performance of poly(lactic acid). In this work, a selective degradation method is developed to etch the PLLA/PDLA blend using proteinase K, and a two-phase co-continuous structure is observed directly using scanning electron microscope. Meanwhile, the evolution of the microphase morphology, inter-chain interaction, chains orientation, and crystallization of un-crosslinked and crosslinked blends during solid-state stretching, as well as the formation of SC during heating/annealing are comprehensively investigated. Results showed that the phase separation in the stretched blend could be drastically restricted by crosslinking; during stretching a blue-shift of 2995 cm −1 band shows up for both crosslinked and un-crosslinked blends as a result of the dipole-dipole coupling interaction between CH 3 and CH 3 , while the hydrogen bond interaction between CH 3 and C O is absent; only mesophase or homocrystal (HC) is formed in the stretching temperature range of 60∼105 °C; the critical strain for HC formation is advanced and the orientation of chains is enhanced by crosslinking under the same stretching condition. In the last, benefiting from the greatly reduced microphase size (namely enlarged interfacial area between PLLA and PDLA) and enhanced chains orientation, the crystallization rate of SC is accelerated during annealing, and the ultimate crystallinity of SC is elevated in the crosslinked blend.