Lamellar morphology of disorder α'-crystals of poly(L-lactic acid)

Abstract

Crystallization of poly(l-lactic acid) (PLLA) at temperatures lower and higher than about 100–120 °C leads to the formation of α′- and α-crystals, respectively. Related to the formation of different crystal polymorphs after crystallization at different temperatures, the small-angle X-ray scattering (SAXS) patterns only show a long-period maximum after crystallization at high temperatures, when α-crystals are present. Reason for the absence of a long-period peak after low-temperature crystallization is insufficient density contrast between the formed α′-crystals and the amorphous phase but not the absence of lamellar stacks and long-range periodicity. This conclusion is based on the observation of a long period after the transformation of α′-crystals of rather low density into α-crystals of higher density, by thermal treatment, while keeping the semicrystalline morphology quasi-unchanged. Long-range periodicity and formation of lamellae in PLLA containing α′-crystals are ultimately proven by atomic force microscopy, despite a long period is not detected by SAXS.