Coupling effect of pressure and flow fields on the crystallization of Poly(vinylidene fluoride)/Poly(methyl methacrylate) miscible blends

Two-Stage Evolution of Gamma-Phase Spherulites of Poly (Vinylidene Fluoride) Induced by Alkylammonium Salt

We investigated the evolution of the γ-phase spherulites of poly(vinylidene fluoride) (PVDF) added to 1 wt% of tetrabutylammonium hydrogen sulfate during the isothermal crystallization at 165 °C through polarized optical microscopy and light scattering measurements. Optically isotropic domains grew, and then optical anisotropy started to increase in the domain to yield spherulite. Double peaks were seen in the time variation of the Vv light scattering intensity caused by the density fluctuation and optical anisotropy, and the Hv light scattering intensity caused by the optical anisotropy started to increase during the second increase in the Vv light scattering intensity. These results suggest the two-stage evolution of the γ-phase spherulites, i.e., the disordered domain grows in the first stage and ordering in the spherulite increases due to the increase in the fraction of the lamellar stacks in the spherulite without a change in the spherulite size in the second stage. Owing to the characteristic crystallization behavior, the birefringence in the γ-phase spherulites of the PVDF/TBAHS was much smaller than that in the α-phase spherulites of the neat PVDF.

Polymer crystallization under external flow

The general aspects of polymer crystallization under external flow, i.e., flow-induced crystallization (FIC) from fundamental theoretical background to multi-scale characterization and modeling results are presented. FIC is crucial for modern polymer processing, such as blowing, casting, and injection modeling, as two-third of daily-used polymers is crystalline, and nearly all of them need to be processed before final applications. For academics, the FIC is intrinsically far from equilibrium, where the polymer crystallization behavior is different from that in quiescent conditions. The continuous investigation of crystallization contributes to a better understanding on the general non-equilibrium ordering in condensed physics. In the current review, the general theories related to polymer nucleation under flow (FIN) were summarized first as a preliminary knowledge. Various theories and models, i.e., coil-stretch transition and entropy reduction model, are briefly presented together with the modified versions. Subsequently, the multi-step ordering process of FIC is discussed in detail, including chain extension, conformational ordering, density fluctuation, and final perfection of the polymer crystalline. These achievements for a thorough understanding of the fundamental basis of FIC benefit from the development of various hyphenated rheometer, i.e., rheo-optical spectroscopy, rheo-IR, and rheo-x-ray scattering. The selected experimental results are introduced to present efforts on elucidating the multi-step and hierarchical structure transition during FIC. Then, the multi-scale modeling methods are summarized, including micro/meso scale simulation and macroscopic continuum modeling. At last, we briefly describe our personal opinions related to the future directions of this field, aiming to ultimately establish the unified theory of FIC and promote building of the more applicable models in the polymer processing.