Abstract:
By controlling the molar ratio of
D-enantiomer to
L-enantiomer of the lactide, a series of di-stereoblock polylactides (di-sb-PLA) bearing similar block sequences between poly(
D-lactide) (PDLA) and poly(
L-lactide) (PLLA) were synthesized by ring-opening polymerization of
L-lactide (
L-LA) using hydroxyl terminated PDLA (HO-PDLA) as a macromolecular initiator. Micro/nanofibers were then fabricated by electrospinning di-sb-PLAs from their dichloromethane (DCM) and hexafluoroisopropanol (HFIP) mixtures. The thermal and mechanical properties, fiber morphology and crystallization properties of di-sb-PLAs were analyzed. The results of DSC showed that di-sb-PLA formed complete stereocomplex(sc) crystals with the melting point higher than 215 ℃, and no homocrystal was formed. The crystallization ability of sc was decreased with an increase in length of each block. It is worth to note that sc of stereoblock PLA was formed on molecular-scale without post-processing compared with PLA homopolymers, which enhanced the heat resistance and improved the elongation at break of solution-cast film to 250% without losing tensile strength, especially for di-sb-PLAs that has higher molecular weight. After electrospinning, the as-spun micro/nanofibers of di-sb-PLA were homogeneous and smooth, in which PLA molecules were almost amorphous. A large amount of sc crystals were then formed by cold crystallization during heating-up process, and the melting points of sc crystals in these fibers were increased by 8 ℃ as compared with the melt-quenched samples. Moreover, the results of
in-situ ATR-FT-IR at variable temperature disclosed that the formation of sc crystals was related to the interactions between side methyl groups and carbonyl groups in PLA. The results hold promise for the further application of di-sb-PLA.