Orientation distribution and branching mechanism of lamellar crystals inside an isotactic polystyrene spherulite

Spherulites are the most typical structures formed by cooling a melt of semicrystalline polymers. Spherulites are aggregates of plate-like polymer crystals (lamellar crystals) that are approximately 10-nm-thick and formed by repeated growth and branching. Elucidating the formation mechanism of spherulites requires an understanding of the orientation distribution and branching mechanism of lamellar crystals. However, analyzing the morphology and orientation of the lamellar crystals within spherulites has proven challenging. In addition, determining the orientation relationship of the lamellar crystals before and after branching has been deemed impossible. In this study, nanodiffraction imaging, a state-of-the-art diffraction imaging method based on scanning transmission electron microscopy, was used to directly determine the morphology and orientation of lamellar crystals inside an isotactic polystyrene spherulite. These results provide compelling evidence of higher-order structures formed by the irregular branching of lamellar crystals with a thickness of approximately 5 nm. A detailed analysis of the spherulite formation mechanism will lead to a more detailed understanding of polymer crystallization and provide valuable insights for industries that utilize semicrystalline polymers. • The formation mechanism of polymer spherulites was studied using nanodiffraction imaging. • Lamellar crystals inside a spherulite of isotactic polystyrene were visualized. • Morphologies and crystallographic orientations of lamellae revealed the radial growth of lamellae. • The small eye structure suggests formation regardless of the undercooling degree. • Lamellae bundles were formed by dense, non-crystallographic branching.