Direct AFM observation of crystal twisting and organization in banded spherulites of chiral poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

Real-time atomic force microscopy observation was carried out during crystallization in thin films of chiral poly(R-3-hydroxybutyrate-co-R-3- hydroxyhexanoate) copolymer, and the development details of single lamellae in the banded spherulites are revealed for the first time. The lamellae exhibit complicated growth behaviors: twisting, bending, backward growth, and branching. The lamellae continuously twist to show alternating edge-on and flat-on views along the radii of the spherulites. Giant screw dislocations bring forth to the birth of new lamellae. Interaction between the leading and trailing lamellae contributes to cooperative stacking of the twisting crystals. The lamellae twist before screw dislocations appear, demonstrating that screw dislocations are not causal of twisting. All the observed twisting occurs in the right-handed sense, likely resulting from the chirality of the crystal structure. Increased crystallization temperature results in decreased magnitude of lamellar twisting and bending. Models for morphological development are discussed in the context of these observations.