A Full-Process Perspective of Hierarchical Structure Development for Injection-Molded Isotactic Polypropylene with β-Nucleating Agent

Flow-induced crystallization of isotactic polypropylene with β-nucleating agent (iPP/β) is investigated using a homemade in situ characterization platform for injection molding, featuring millisecond-scale time-resolved synchrotron X-ray diffraction and scattering techniques (WAXD/SAXS). By combining ex situ microfocus WAXD and scanning electron microscopy (SEM) measurements, we have elucidated a full-path scenario of injection-molded iPP/β for the first time, encompassing the evolution of crystalline phases, the formation of shish and lamellae, and the “skin-core” morphology. Results indicate that the competitive growth of α-, β-, and γ-crystals occurs within every layer of injection-molded iPP/β samples, driven by the nucleation barriers of the three crystalline forms and the nucleation efficiency of the β-nucleating agent (β-NA), depending on specific processing fields. The crystallization kinetics of β-crystals in iPP/β is jointly influenced by shear intensity and duration. Shear intensity promotes the early growth of β-crystals by inducing oriented α-row nuclei, yet concurrently suppresses the nucleation activity of β-NA. Shear duration influences the relaxation of oriented structures and modulates the starting time and duration of β-NA-induced β-crystals. Furthermore, the β-NA exhibits a dual role in promoting the development of the meridional β-epitaxial kebabs and main equatorial kebabs. When β-crystals are dominant, the ductility of the samples is positively correlated with β-crystals content, accompanied by a unique secondary necking phenomenon. This work provides important insights for tailoring the crystalline composition and structural hierarchy of iPP products through processing, thereby facilitating the optimization of their end-use performance.