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.

Extension-induced crystallization under nearequilibrium condition has been studied in a series of lightly cross-linked high density polyethylene (XL-HDPE) with a combination of extensional rheology and in situ synchrotron radiation small-angle X-ray scattering (SAXS) and wide-angle Xray diffraction (WAXD) measurements. According to crystal morphology and structure, four regions were defined in straintemperature space, namely "orthorhombic lamellar crystal" (OLC), "orthorhombic shish crystal" (OSC), "hexagonal shish crystal" (HSC) and "oriented shish precursor" (OSP), respectively. This indicates that flow not only induces entropic reduction of initial melt, but also modifies the free energies of the final states, which is overlooked in the classical stretched network model (SNM) for flow induced crystallization (FIC). Incorporating the free energies of various final states, a modified SNM is developed and employed to analyze strain-temperature equivalence on nucleation in FIC, which reveals that the critical nucleus thickness l* at different regions leads to a natural transition from lamellar to shish nuclei. The results suggest that classical nucleation theory is still valid for FIC under near-equilibrium condition provided that the free energy changes of initial melt and final states induced by flow are taken into account.

Extension-induced crystallization of lightly cross-linked high density polyethylene (XL-HDPE) has been studied with a combination of extensional rheology and in situ synchrotron radiation small-angle X-ray scattering (SR-SAXS) measurements, where XL-HDPE is a dynamic asymmetric system containing both cross-linked network and free chains (23 wt % gel fraction). SR-SAXS results revealed that the nucleation morphologies can be divided into four regions in strain space, namely uncorrelated oriented point-nuclei, scaffold-network nuclei, microshish nuclei, and shish nuclei. The definition of these four regions coincides nicely with the transitions in stress–strain curves, which allows us to establish a correlation between extension-induced conformations of chains and morphologies of nuclei. Even orientation of cross-linked network and free chains leads to the formation of uncorrelated oriented point-nuclei in region I, while the emergence of dynamic asymmetric nature due to disentanglement of free chain results in scaffold-network nuclei in region II. Formation of microshish in region III requires not only orientation but also stretch of chain segments, and finally nearly full extension of chain segments corresponds to shish nuclei in region IV.

Crystallization of poly(ethylene oxide) (PEO) induced by extensional flow has been studied by in-situ small-angle X-ray scattering (SAXS). A constant Hencky strain was applied to the melt with strain rates varying in 3 orders of magnitude. The evolution of the long period with crystallization time is qualitatively different for high and low strain rates. In the region of high strain rates the long period first increases, reaches a plateau, and decreases in the later stage of crystallization. In contrast, for low strain rates only a monotonic decrease is observed. Given the high orientation in the high strain rate region, we propose a localized nucleation mechanism (like row-nuclei). The initial increase of the long period for high strain rates indicates a decrease of nucleation density, which is the inverse of the long period. On the basis of the microrheological model proposed by Coppola et al., in which the flow induced free energy change under steady state flow is calculated with the DoiÀEdwards model, we develop a framework describing the nucleation after a step strain with high strain rates. By introducing the memory function, the dynamic process of relaxation is taken into account, leading to a continuous decrease of the nucleation density. Numerical fitting the model to the experimental data gives a good agreement with the terminal relaxation time τ d , the volume filling rate B, and a constant C 3 determined by surface free energy of the nuclei as fitting parameters. The results confirm the localized nucleation mechanism in highly oriented melt. Furthermore, the fitted value of C 3 indicates a drop in surface free energy of nuclei under strong flow.

The highly efficient electromagnetic (EM) wave absorbing metal-free and carbon-rich ceramics derived from hyperbranched polycarbosilazanes are presented in this contribution. The novel metal-free hyperbranched polycarbosilazanes with pendant cyano groups (hb-PCSZ-cyano) were synthesized through aminolysis reaction and subsequent Michael addition reaction, i.e., cyanoethylation reaction. As metal-free preceramic precursors, the pyrolysis of hb-PCSZ-cyano under high temperature and argon atmosphere generated carbon-rich Si−C−N multiphase ceramics. The ceramics reserve amorphous structure even at high temperature. The introduction of cyano groups in precursors leads to numerous sp 2 carbons and interface polarization in ceramics and favors the EM wave absorption performance. The minimum reflection coefficient (RC) value of Si−C−N multiphase ceramic is −59.59 dB at 12.23 GHz when the sample thickness is 2.30 mm, which means >99.99% electromagnetic waves can be absorbed. The effective absorption bandwidth (RC below −10 dB) is 4.2 GHz, covering the whole X-band (8.2−12.4 GHz). The EM wave absorption property is very excellent in comparison to current electromagnetic wave absorbing materials including transition metal-induced nanocrystals-containing ceramics. The carbon-rich Si−C−N ceramic derived from metal-free precursors provides a new strategy for highly efficient EM wave absorbing functional materials with great potential in electronic devices, antenna housings, and radomes in harsh environments.

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