Shapes and Origins of Cracks and Correlations with Lamellae Assembly in Poly(L-lactic acid)

Just like ring bands in spherulites are not all of a simple type, cracks in crystallized spherulites of polymers can display a wide variety of patterns. Poly(L-lactic acid) (PLLA) upon crystallization and cooling was found to exhibit as many as four crack types of different directions and patterns, which cannot be feasibly explained simply by the directional difference in coefficients of thermal expansion. Depending on crystallization temperature (T_c), PLLA crystallizes into ringless or ring-banded spherulites; whereas, the crack patterns are dramatically different in these two types of spherulites. In ring-banded spherulites of PLLA crystallized at intermediate T_c, two uniquely different crack types are present: (1) twin-circumferential cracks coinciding with the dark-bright and bright-dark boundary, and (2) radial short-segmental voids coinciding on the bright bands in spherulites. By contrast, only circumferential cracks are present in PLLA ringless spherulites, where crack patterns are influenced by T_c's. Although all cracks in PLLA spherulite are triggered by cooling from T_c, evidence indicates that the crack patterns and ring-band types (or ringless) are highly correlated and likely both are associated with the lamellar orientation, patterns, and coarseness in spherulites.