Flash Flow-Induced\nCrystallization of Poly(l‑lactide) under Elevated\nPressure during Industrial-Scale\nInjection Molding Revealed by Time-Resolved Synchrotron X‑ray\nScattering

Poly(l-lactide) (PLLA) is\na promising biodegradable\nalternative\nto petroleum-based plastics, but it exhibits slow crystallization\nkinetics. Understanding flow-induced crystallization under pressure\n(FICP) during practical polymer processing, such as injection molding,\nis important to tailor the crystallization and modulate the properties.\nCompared with the traditional “black-box” research on\nFICP, understanding the multistep FICP of PLLA during industrial-scale\ninjection molding and the effect of external fields on crystallization <i>via</i> real-time mode is crucial for revealing the underlying\nmechanism. This work first pays attention to the FICP process of PLLA\nduring industrial-scale injection molding <i>via</i> a homemade\nin situ investigation platform base-d on a highly brilliant synchrotron\nX-ray scattering. We find that an initial flash flow (shear time ∼0.1\ns) with extremely intense flow (Weissenberg number <i>Wi</i> ≫ 1) induces α/α′-form and β-form\nprecursors in the PLLA melt, and subsequent crystallization around\nthe oriented precursors occurs under quasi-isothermal and residual-pressure\nconditions. In particular, the elevated packing pressure observably\npromotes flow-induced oriented precursors and especially the β-form\nnucleates preferentially, while the segmental diffusion-dominant retardant\ncrystal growth proceeds during the following quasi-isothermal crystallization.\nBeing composed of thicker lamellae with a higher amount, the injection-molded\nPLLA bars under low pressure exhibit superior mechanical strength\nand thermomechanical performance. The outcome of this work points\nout that the pressure field is of great importance in flow-induced\ncrystallization kinetics and the final crystalline morphology, which\nis valuable for guiding the development of a high-performance PLLA\nproduct and expanding its applications.