Flow and Pressure Jointly Induced Ultrahigh Melting Temperature Spherulites with Oriented Thick Lamellae in Isotactic Polypropylene

Morphology of Shear-Induced Polymer Cylindrites Revealed by 3D Optical Imaging

Two-photon confocal laser microscopy was used to obtain three-dimensional (3D) images of the morphology of poly(lactic acid) after shear-induced crystallization. The necessary fluorescence contrast was achieved by doping the polymer with Nile Red. The dye gets partially rejected from the growing crystalline aggregates during their formation, thus creating a renderable high-low fluorescence boundary outlining the shape of the aggregates. Parallel-plate melt-shearing and pulling a glass fiber through the melt were used as the two methods to achieve shear-induced crystallization. This study focuses on the shape of the resulting cylindrites, i.e., large-diameter shish-kebabs. The first 3D images of polymer cylindrites show that, if far from boundaries, they are circular cylinders, highly regular after fiber pull, but less so after parallel-plate shear. In the latter case, the cylindrite reveals the trajectory of the foreign particle that had nucleated its growth. Interestingly, lateral growth of the cylindrites was found to accelerate toward the sample surface when approaching it, giving the cylindrite an elliptical cross section. Furthermore and surprisingly, in the case of fiber pull, a row of spherulites is nucleated at the polymer-substrate interface nearest to the fiber, aligned along the fiber axis and appearing ahead of the rest of the space-filling spherulites. Both the phenomena, elliptical cylindrites and row of spherulites, are attributed to negative pressure buildup peaking at the cylindrite growth front and at the nearby film surface caused by crystallization-induced volume contraction. The pressure and flow distribution in the system is confirmed by numerical simulation. The results illustrate the value of 3D imaging of crystalline morphology in polymer science and polymer processing industry.

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.

Recent advances in post-stretching processing of polymer films with in situ synchrotron radiation X-ray scattering

The stretch-induced structural evolution mechanism is a long-standing scientific question in the post-stretching processing of polymer films. X-ray scattering, especially a combination of small- and wide-angle X-ray scattering (SAXS/WAXS), provides a powerful method to study the hierarchical structure of polymer films. Recent advances in synchrotron radiation (SR) light sources and detection techniques allow one to measure the structural evolution of polymer films during post-stretching processing in real time with ultrahigh time resolution, which benefits the understanding on this topic. This review summarizes some recent investigations on post-stretching processing of polymer films, which combine in situ X-ray scattering techniques with purposely designed tensile apparatus in terms of three aspects: uniaxial stretching, biaxial stretching and stretching with chemical reactions. Concerning the polymer bulk, traditional deformation mechanisms like stretch-induced crystallization (SIC), crystal slipping, phase transition and melting-recrystallization are discussed for the uniaxial and biaxial post-stretching of polymer films. New deformation models have been developed to focus on the structural evolution on the length scale of lamellar stacks, which consider the potential microphase separation of the interlamellar amorphous phase and microbuckling. For solution systems, the coupled effects of the mechanical work from external force and the chemical potential from possible chemical reactions are taken into account for the structural evolution during stretching in solution. Roadmaps of structural and morphological evolution in the processing parameter space (i.e., temperature, stress, strain and the concentration of additive in the bath solution) are eventually constructed for precursor films. The accumulation of a structural evolution database for post-stretching processing of polymer films can be expected to provide a helpful guide for industrial processing for high-performance polymers in the near future.

Flow-Induced Precursor Formation of Poly(l-lactic acid) under Pressure

For the first time, the influences of two inevitable processing fields (pressure and flow fields) on the crystallization of a semirigid molecular chain polymer, that is, poly(l-lactic acid) (PLLA), were explored using a homemade pressuring and shearing device. The results reveal that the shear rate facilitated the generation of precursor because it induced oriented segment formation. It was found that the most sensitive shear temperature for the generation of PLLA precursor under 100 MPa was 180 °C. When the shear temperature was higher (e.g., 190 °C), the relaxation of shear-induced oriented segments was too quick to induce the generation of PLLA precursor. Oppositely, at a lower shear temperature (170 °C), the oriented segments were hard to relax within the whole shear rate range (3.1-31.4 s^-1). Annealing treatment was infaust to the PLLA precursor formation because it promoted the relaxation of oriented segments. Different from the shear and annealing, pressure played a more complicated role in the formation of PLLA precursor. Pressure decreased the free volume between PLLA molecular chains and meantime increased the supercooling of PLLA melt. In addition, PLLA chains tended to form locally oriented segment bundles to adapt to the pressurized state, which facilitated the formation of PLLA precursor and the following crystallization process. These two factors lowered the movability of PLLA chains and suppressed the relaxation of chain, so shear-induced orientation facilitated PLLA precursor formation under pressure. In that case, pressure and shear flow showed a synergetic promoting effect on the generation of PLLA precursor and the following crystallization process. These meaningful results could be helpful for comprehending the relationship between crystallization conditions and the crystallization behavior of PLLA and thus would provide guidance to fabricating the final products through controlling the crystallization process of PLLA.