Flash Flow-Induced Crystallization of Poly(l-lactide) under Elevated Pressure during Industrial-Scale Injection Molding Revealed by Time-Resolved Synchrotron X-ray Scattering

Poly(l-lactide) (PLLA) is a promising biodegradable alternative to petroleum-based plastics, but it exhibits slow crystallization kinetics. Understanding flow-induced crystallization under pressure (FICP) during practical polymer processing, such as injection molding, is important to tailor the crystallization and modulate the properties. Compared with the traditional "black-box" research on FICP, understanding the multistep FICP of PLLA during industrial-scale injection molding and the effect of external fields on crystallization via real-time mode is crucial for revealing the underlying mechanism. This work first pays attention to the FICP process of PLLA during industrial-scale injection molding via a homemade in situ investigation platform base-d on a highly brilliant synchrotron X-ray scattering. We find that an initial flash flow (shear time ∼0.1 s) with extremely intense flow (Weissenberg number Wi ≫ 1) induces α/α′-form and β-form precursors in the PLLA melt, and subsequent crystallization around the oriented precursors occurs under quasi-isothermal and residual-pressure conditions. In particular, the elevated packing pressure observably promotes flow-induced oriented precursors and especially the β-form nucleates preferentially, while the segmental diffusion-dominant retardant crystal growth proceeds during the following quasi-isothermal crystallization. Being composed of thicker lamellae with a higher amount, the injection-molded PLLA bars under low pressure exhibit superior mechanical strength and thermomechanical performance. The outcome of this work points out that the pressure field is of great importance in flow-induced crystallization kinetics and the final crystalline morphology, which is valuable for guiding the development of a high-performance PLLA product and expanding its applications.