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.

Cross-nucleation is defined as the nucleation of one polymorph on the surface of another polymorph of the same substance. Although the description of this particular form of heterogeneous nucleation is mainly phenomenological, recently dedicated quantitative studies are performed on several systems. In this work we propose a model framework that captures the phenomenon of cross-nucleation for a spherulitic seed-surface geometry, as well as the kinetic competition between the seed growth and the cross-nucleus formation, by the introduction of a tangential growth rate of the daughter polymorph. Regardless of the growth rate of the parent spherulite, this model describes the experimental data up-to and including the final amount of cross-nuclei on its periphery, solely based on one parameter, the cross-nucleation rate. Furthermore, a strong temperature dependency of the kinetic competition between concomitantly growing α- and β-phase isotactic polypropylene is observed and related to the previously reported anomalous behaviour of this cross-nucleating system.