Handedness of Twisted Lamella in Banded Spherulite of Chiral Polylactides and Their Blends

Abstract

Banded spherulite resulting from lamellar twisting due to the imbalanced stresses at opposite fold surfaces can be formed by isothermal crystallization of chiral polylactide and its blends with poly(ethylene glycol) (PEG). Using a polarized light microscope, the handedness of the twisted lamella in banded spherulite is determined. With the same growth axis along the radial direction as evidenced by wide-angle X-ray diffraction (WAXD) for isothermally crystallized samples at different temperatures, the twisted lamellae of chiral polylactides (poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA)) display opposite handedness. The split-type Cotton effect on the Co=O stretching motion of vibrational circular dichroism (VCD) spectra helps determine the helix handedness (i.e., conformational chirality). The results indicate that the conformational chirality can be defined by the molecular chirality through intramolecular chiral interactions. Moreover, the preferred sense of the lamellar twist in the banded spherulite corresponds to the twisting direction identified by the C-O-C vibration motion of VCD spectra, reflecting the role of intermolecular chiral interactions in the packing of polylactide helices. Similar results are obtained in the blends of chiral polylactides and poly(ethylene glycol) (PEG, a polymer compatible with polylactide), indicating that the impact of chirality is intrinsic irrespective of the specific crystallization conditions. In contrast to the chiral polylactides, the spectrum of the crystalline stereocomplex that associates PLLA and PDLA shows VCD silence. The spectroscopic results are in line with the morphological observations. No banded spherulites are observed in the stereocomplex crystallites due to the symmetric packing of mirror L and D-chain conformations in the fold surfaces and the crystallites core.