Coexisting straight, radial, and banded lamellae on the six corners of hexagon-shaped spherulites in poly( L -lactide)

A novel six-petal spherulite morphology composed of a combination of a central hexagonal core, hexagon-shaped ring bands, and six fibrous stalks is discovered in a low-molecular-weight poly( L -lactide acid) (LM_w -PLLA) that is melt crystallized at a specific T_c = 110 °C and confi ned in thin films. The structures are analyzed by using polarized optical microscopy (POM) and atomic force microscopy (AFM). Discrete lamellae, consisting of sequenced wide and small lamellae, all in flat-on orientation with periodically up-and-down topology, are packed in a ring-banded alignment, while continuous lamellae in flat-on, tilted, or edge-on orientations are arranged in the fibrous region. On the six corners of a symmetric hexagon-banded spherulite, the radial fibrous lamellae overlap with ring-band lamellae to show a mixed combination of ring-banded and fibrous patterns. The formation of coexisting ring-banded and fibrous patterns in a mixed morphology is associated with the lamellar packing and orientation. The radial fibrous lamellae at the six corners favorably occur in the growth fronts, which are closer to the reservoir of materials of diffusing species. Superimposed on the radial lamellae, hexagon-shaped ring bands repeat themselves on the outskirts of the central hexagon nuclei. The geometry of the initial crystal is believed to be the influencing factors in final spherulite patterns.