Correlation of crack patterns and ring bands in spherulites of low molecular weight poly(l-lactic acid)

Only a single type of circular circumferential crack is conventionally reported for poly(l-lactic acid) (PLLA). In this study, PLLA samples were 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. The radial short-segmental cracks on the bright band of ring-banded spherulites may be caused by PLLA crystals of radial direction with various twisting that contract laterally upon cooling. Only circumferential cracks are present in PLLA crystallized into ringless spherulites, where concentric continuous circumferential cracks are present in the ringless spherulites at low T (c) with finer lamellae, but discontinuous and irregular circumferential cracks are present in the ringless spherulites at high T (c) with coarse lamellae. Although all cracks are triggered by cooling from T (c), all evidences indicate that the crack patterns and types are highly associated with the lamellar orientation, patterns, and coarseness in spherulites.