Formation of PLA Stereocomplex Crystals through Homorecrystallization and Mesophase Growth Mechanisms

The present study systematically investigates the crystallization kinetics and crystal morphology of polylactic acid (PLA) stereocomplex (SC) crystallization from the cold state in 1/1 poly l-lactic acid (PLLA) and poly d-lactic acid (PDLA) mixtures, i.e., via homorecrystallization and mesophase growth mechanisms at two different temperatures (190 and 215 °C). Thereto, homocrystals and mesophase precursors are generated at temperatures of 80, 100, and 120 °C. For the characterization of the crystalline structures, a combination of wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and microscopy techniques is employed. The study reveals that homorecrystallization is much faster than crystallization via mesophase growth and can lead to the formation of either nodular-shaped stereocomplex crystals or coarser lamellar structures, depending on the crystallization temperature. The isothermal crystallization kinetics in a temperature jump stage is characterized via in situ WAXD and modeled with a modified Hillier-type model. The combination of thermal and structural characterization techniques allows to shed light on the factors that influence the crystalline structure of SC crystals, as well as the kinetics of their formation, which can be used for the development of high-performance PLA-based materials.