Mesophase-Mediated Crystallization of Poly(l‑lactide): Deterministic Pathways to Nanostructured Morphology and Superstructure Control

The effect of the CO₂-induced mesophase on the isothermal crystallization of poly­(l-lactide) (PLLA) was investigated by infrared (IR) spectroscopy and microscopy. It was found that the crystallization rate of PLLA was significantly enhanced by the CO₂-induced mesophase, showing that the crystallization was completed even in a short period of 10⁰–10¹ s. Compared with the directly crystallized samples that showed typical spherulites with lamellae, the crystallization via CO₂-induced mesophase led to nonspherulitic (granular or featureless) morphologies consisting of nanorods, whereas the polymorphic behavior remained unaffected by the initial state, resulting in crystallized PLLA containing identical polymorphs of uniquely different nanostrutured morphologies and superstructures. The IR imaging results indicated that the formation of the equilibrium crystal was preceded by the formation of various metastable intermediate phases, including mesomorphic phase, preordering, and metastable crystal, all of which continuously evolved with time. The nucleation process proceeded via a similar pathway. In contrast to the negligible contribution of mesophase to the nucleation in direct crystallization, the CO₂-induced mesophase with extremely high nucleation density underwent disordering–reorganization into the preordering, thereby providing a tremendous number of active nucleation sites for enhancing crystallization and serving as building blocks for nanorods. Importantly, the present results highlight the decisive role of mesophase in directing the nanostructure and superstructure and support a multistep process for the crystallization (including nucleation and crystal growth) of PLLA, validating the Ostwald step rule, providing mechanistic insights into the crystallization of polymers.