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 C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.

Elastic flow instabilities and macroscopic textures in graphene oxide lyotropic liquid crystals

“…be interpreted as vorticity banding (i.e., with bands stacked in the global vorticity direction imposed by the shear), as the observed combination of MC and SB appears similar to the radially stacked bands seen in PLIs of the well-known banded spherulites 42 . Detailed consideration of banded spherulites and related structures has been helpful in interpreting our PLIs.…”

Generalizing the effects of chirality on block copolymer assembly

“…Namely, the induced VCD signals are intrinsic VCD signals that are attributed to asymmetric packing of the chiral PCGs. In contrast to the chiral PLAs in the amorphous state (31), the VCD signals in the absorption band of C-O-C region become significant, reflecting that the chiral PCGs indeed have stronger intermolecular chiral interactions than chiral PLAs. While this VCD signal is not a result of strong interactions induced by crystallinity, its appearance is suggestive of strong chainchain interactions that reflect a chiral symmetry, such as would be the case of interactions between helically distributed side groups that drive cholesteric order chiral polymers (28).…”

“…S21). According to a previous study of crystallized chiral PLAs, the VCD signals in the absorption bands of the C-O-C vibration are silent in the amorphous state, with strong VCD signals appearing only upon crystallization (31). In the case of crystalline PLLA, this signal was interpreted as evidence of strong interactions between groups on distinct chains whose geometry reflects a chiral symmetry, for example, interactions between helically distributed side groups on parallel stems.…”

“…Thus, we examined the optical activities of C-O-C vibration by VCD to assess the intermolecular chiral interactions because the transition dipole moment of this group in nearly parallel to the backbone of chiral PCG (SI Appendix, Fig. S7) (31,32). The VCD signals that probe C-O-C vibration, in the wavelength range of 1,100-1,300 cm − 3 , are shown from chiral PCG homopolymers in Fig.…”

Self-Assembly of Chiral Menthol Molecules from a Liquid Film into Ring-Banded Spherulites

“…However, the racemic stereocomplex did not form banded spherulites because of the symmetric packing of L-and D-chain conformations. 36 Xu et al studied birefringent banded polymer spherulites. They stated that the driving force of lamellar twisting is associated with the unbalanced surface stresses.…”