Quiescent crystallization of poly(lactic acid) studied by optical microscopy and light‐scattering techniques

ABSTRACT The crystallization behavior of poly(lactic acid) (PLA) has been studied extensively, and this has resulted in different reported values for the nucleation densities (Ns) and crystal growth rates (Gs) for similar grades. These inconsistencies may be magnified when they are used in subsequent modeling studies. Therefore, the quiescent crystallization behaviors of three PLA grades were studied with polarized optical microscopy and small‐angle light‐scattering experiments. The Gs and Ns were determined at several isothermal crystallization temperatures with a device that provided near‐instantaneous cooling to the isothermal crystallization temperature. Two growth rate regimes, which were attributed to α and α′ crystallization with a transition around 120 °C, were observed. Avrami analysis revealed that the poly(l‐lactic acid) homopolymer crystal growth was three‐dimensional and was unaffected by the presence of stereocomplex PLA. The PLA copolymer crystals had a transition from an initial sheaflike conformation to three‐dimensional growth. Furthermore, the lamellar twisting of the homopolymer was observed at the isothermal crystallization temperature around 144 °C. These findings can be used for future modeling studies to predict material behavior in various industrial processes. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44566.