Hierarchically Diminishing Chirality Effects on Lamellar Assembly in Spherulites Comprising Chiral Polymers

Three-dimensional interior structures of poly (lactic acid) (PLA) spherulites have been examined in crystallization with poly(ethylene oxide). A high concentration of water-soluble PEO is used as diluent to obtain spherulitic morphologies of PLA in thick enough films (20-50 mu m) for interior observation. Two different but closely related spherulite patterns (dendritic and spiral banded) of both PLA enantiomers have been studied in details, after removal of PEO by water etching. The interior of both curved dendrites and spiral banded spherulites are not connected by continuous twisting, as conventional models proposed; instead, the ridge band is composed of multiple polycrystals in parallel alignment and the valley band is also composed of polycrystalline branching lamellae. At increased film thickness, the molecular chirality may not be entirely responsible for the lamellar bending sense (i.e., clockwise or counterclockwise) in all hierarchical crystal structures. Film thickness also contributes to curvature sense and banding. The approaches taken and results presented here are ground breaking for interpreting the mechanisms of periodic patterns that can be observed in spherulites.