Favorable formation of stereocomplex crystals in long-chain branched poly(L-lactic acid)/poly(D-lactic acid) blends: impacts of melt effect and molecular chain structure

A feasible approach is proposed to promote the formation of the stereocomplex crystal (S-c) in long-chain branched Poly(L-lactic acid)/Poly(D-lactic acid) (LCBPLA/PDLA) blends with hydrogen bond interactions. The synergistic effect of PDLA and long-chain branches significantly increases the crystalline ability of S-c from 20.1 to 30.4% due to the improved intermolecular crystal nucleation/growth. The frequency-independent loss tangent appeared at a PDLA concentration of 5.0 wt%, indicating a transition from the liquid-like to solid-like viscoelastic and the formation of a network composed of long-chain branches and the reserved S-c crystallites. Changing the processing temperatures from 190 to 230 degrees C seems to induce different melting behavior of S-c with diverse topological conformations so as to act as a template to attract PLLA molecule chains to perform the crystallization behavior. In situ wide-angle X-ray diffraction analysis reveals that a higher cooling temperature (less than 220 degrees C) significantly contributes to the S-c with more integrated structures and the H-c-to-S-c transition. The polarizing optical microscope results show that increasing the PDLA content and long-chain branched points significantly promote the spherulite growth and nuclei density of S-c, increasing the S-c size from 56.47 to 94.13 mu m and nuclei density from 52.2 to 98.3%.