Flow-induced crystallization (FIC) is a typical nonequilibrium phase transition and a core industry subject for the largest group of commercially useful polymeric materials: semicrystalline polymers. A fundamental understanding of FIC can benefit the research of nonequilibrium ordering in matter systems and help to tailor the ultimate properties of polymeric materials. Concerning the crystallization process, flow can accelerate the kinetics by orders of magnitude and induce the formation of oriented crystallites like shish-kebab, which are associated with the major influences of flow on nucleation, that is, raised nucleation density and oriented nuclei. The topic of FIC has been studied for more than half a century. Recently, there have been many developments in experimental approaches, such as synchrotron radiation X-ray scattering, ultrafast X-ray detectors with a time resolution down to the order of milliseconds, and novel laboratory devices to mimic the severe flow field close to real processing conditions. By a combination of these advanced methods, the evolution process of FIC can be revealed more precisely (with higher time resolution and on more length scales) and quantitatively. The new findings are challenging the classical interpretations and theories that were mostly derived from quiescent or mild-flow conditions, and they are triggering the reconsideration of FIC foundations. This review mainly summarizes experimental results, advances in physical understanding, and discussions on the multiscale and multistep nature of oriented nuclei induced by strong flow. The multiscale structures include segmental conformation, packing of conformational ordering, deformation on the whole-chain scale, and macroscopic aggregation of crystallites. The multistep process involves conformation transition, isotropic-nematic transition, density fluctuation (or phase separation), formation of precursors, and shish-kebab crystallites, which are possible ordering processes during nucleation. Furthermore, some theoretical progress and modeling efforts are also included.
Deformation induced crystal–crystal transition of polybutene-1 (PB-1) from forms II to I at different temperatures is studied with in situ synchrotron radiation wide-angle X-ray scattering (WAXS). Analyses on the evolution of crystallinity and orientations of forms II and I during tensile deformation show that stretch accelerates the transformation from forms II to I, which is interpreted based on either a direct crystal–crystal transition or an indirect approach via an intermediate state of melt, namely a melting recrystallization process. A three-stage mechanical deformation including linear deformation, stress plateau, and strain hardening is observed in the engineering stress–strain curves, which corresponds to a process of incubation, nucleation, and gelation of form I crystals. It establishes a nice correlation between phase transition and mechanical behavior in this study.
Extension flow induced crystallization of isotatic polypropylene (iPP) has been studied with a combination of extension rheological and in situ small-angle X-ray scattering (SAXS) measurements at 140 °C. Rheological data of step extension on iPP melt are divided into before and beyond fracture strain zones in strain–strain rate space, where intermediate strains between them lead to fracture of samples. Coincidently, weak and strong accelerations of nucleation are observed in the before and beyond fracture strain zones respectively, where distinctly different features of crystallization kinetics and nucleation form occur in these two zones. The microrheological model explains the acceleration of nucleation in the “before fracture strain zone” well, while a “ghost nucleation” mechanism is proposed to interpret the strong acceleration of nucleation in the “beyond fracture strain zone”. The “ghost nucleation” is due to the displacement of initial parent point nuclei, where daughter nuclei are induced along the trails. This new mechanism explains well the acceleration of nucleation in orders of magnitude and the formation of shish in iPP melt.
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