Extraordinary Longevity of Flow-Induced Crystallization Precursors in Isotactic Polypropylene

Flow-induced crystallization (FIC) plays a pivotal role in controlling polymer properties. However, the extraordinary longevity of FIC precursors at high temperatures has been a puzzling topic, since classical polymer dynamics would predict rapid relaxation and loss of any flow-induced structural memory. In this study, systematic experiments were designed to allow for detailed insights into the emergence, formation, and dissociation of different FIC precursor types. Specifically, thermal stability of point-like and shish-like precursors in isotactic polypropylene (iPP) above the equilibrium melting point was correlated with the flow history and their structural nuances. Nonisothermal differential scanning calorimetry (DSC) confirmed the remarkable longevity of both precursor types for more than 6 h at temperatures well above the equilibrium melting point, where the crystallization temperature (Tc) remained consistently higher than in the unsheared melt. Polarized light optical microscopy (PLOM), and confocal Raman spectroscopy showed distinct relaxation pathways: point-like nucleated morphologies gradually diminished in number, whereas shish-like nucleated fiber-like morphologies lost their orientation and disintegrated into point-like nucleated morphologies. The dissolution of shish structures generated an excess of persistent point-like nuclei, accounting for the acceleration of crystallization kinetics observed in shish-rich samples after prolonged annealing. Isothermal Avrami analysis confirmed that, although the shish-like morphology initially promoted faster nucleation, its effectiveness decreased during annealing more rapidly than the point-like morphology. A comparison with storage modulus recovery experiments suggests that shish-to-point transition is a slow structural relaxation process that continues well beyond the rapid recovery of the entanglement network. These findings demonstrate that while shish-like morphology promotes efficient nucleation in the short term, it is not the origin of the long-term thermal stability of flow-induced precursors, which results from the point-like precursors.