Polymorphism Texture Induced by Fractional Precipitation of Poly(l-lactic acid)

Polymorphism widely exists in semi-crystalline polymers and significantly affects their physical properties. A fresh polymorphic texture of poly(l-lactic acid) (PLLA), with nested spherulite, where a small α′ spherulite composed of higher-molecular weight chains is embedded in a large α spherulite, has been recently discovered under isothermal crystallization of spin-coated films [Hu, D. et al. Chin. J. Polym. Sci. 2020, 38, 1365–1373]. Here, the formation mechanism of the nested spherulite is systematically investigated, focusing on the fractional precipitation behavior in solutions. Factors facilitating fractional precipitation promote formation of the polymorphism texture, including large molecular weight and polydispersity, poor solvent quality, and low precipitation temperature. The trends indicate that the nested spherulites originate from fractional precipitation, where high-molecular weight polymer chains locally aggregate and then crystallize into polymorphs distinct from surroundings. Insights provided by this work suggest a novel perspective for polymorphism formation and a facile strategy to tailor the polymorphs for PLLA and even other polymers.