Polypropylene (PP) Crystallization and Shear
In most polymer processing operations, however, the melt undergoes both elongational and shear flow. Shearing in a molten flow during extrusion or by filling of an injection mold indeed plays a very important role by the creation of the final structure and properties. The relationship between shearing in a molten state processing and the type and level of the crystalline ordering in polymers end products constitutes a very complex matter. Most of the descriptions of the structure ordering are based on the hypothesis that partly frozen chain create an immobilized surface, slowing down the flow of following
These effects are accompanied by an increase of the viscosity, leading to the lowering of the fluid velocity. The adhesion between the die wall and the moving macromolecular chains plays a crucial role by this phenomenon. Several layers in the flow of the molten polymers through the dies and channels may be detected, as it was confirmed by the direct flow measurements, realized byDoppler laser. The number and the thickness of the layers zones is strongly related to the nature of the polymer and to the flow condition, as well to the physical modification of the polymer. Usually, a major role is played by shearing related to the existence of significant flow velocity gradients, in the near die wall zone. Specifically, the alignment of the macromolecules in the flow and the decrease of the intermolecular distances may initiate the crystallization process.
[Nobile et al , Polymer Engineering and Science 45, #2, 153-162 (2005)]
Polymers /Resins
Polypropylene (PP)
Polypropylene (PP) Crystallization
Recent Journal Articles
Negative effect of stretching on the development of β-phase in β-nucleated isotactic polypropylene
(1016–1023)Polymer International 60 #7 (2011)
Zheng et al of Zhengzhou University and Ministry of Education, China, found that for iPP with a low content of β-nucleating agent (β-NA), the crystallinity of β-phase (Xβ) initially increases with increasing shear rate (SR), and then decreases slightly with further increase in SR. However, for iPP containing a higher content of β-NA, with increasing SR, Xβ decreases monotonically, indicating a negative effect of SR on β-phase formation. Small-angle X-ray scattering and polarized optical microscopy experiments reveal that, when SR is less than 30 cm min−1, the increasing amount of row nuclei induced by increasing SR is mainly responsible for the increase of Xβ. In contrast, when SR exceeds 30 cm min−1, the overgrowth of shish structures unexpectedly restrains the development of β-phase, and spatial confinement is considered as a better explanation for the suppression of β-phase. (RDC 6/24/2011)
Formation of β-cylindrites under supercooled extrusion of isotactic polypropylene at low shear stress
(2075-2084) Polymer 52 #9 (2011)
Zhang et al of Zhengzhou University, China, studied the formation of β-cylindrites of isotactic polypropylene under various wall shear stress (σw), supercooled temperature of melt (Te) and crystallization temperature (Tc) by polarized light microscopy (PLM), wide angle X-ray diffraction (WAXD), and differential scanning calorimeter (DSC). The samples were prepared by extruding supercooled melt through capillary die. β-cylindrites can be observed by PLM in the extruded specimen even at a lower σw (0.020 MPa), and the number of β-cylindrites nuclei increases rapidly with the lowering of Te. The nucleation density of β-cylindrites increases with the raising of wall shear stress under a given Te of 160 °C. Furthermore, at lower supercooled temperature of melt (145 °C), the radius of β-cylindrites decreases with the increasing of σw, and the number of β-cylindrites nuclei almost remain invariant. At relatively higher σw (0.090 MPa), a saturation of β-cylindrites nuclei is observed with decreasing Tc. A modified model based on above results has been proposed to explain the effect of the original structure of quiescent supercooled melt on the formation of β-cylindrites under low shear stress. (RDC 6/1/2011)
Shish–kebab-like cylindrulite structures resulted from periodical shear-induced crystallization of isotactic polypropylene
(2970-2978) Polymer 52 #13 (2011)
Zhou et al of Sichuan University, China, prepared isotactic polypropylene (iPP) samples by injection molding (CIM) and pressure vibration injection molding (PVIM), in which a periodical shear field was imposed on the iPP melt during the cooling solidification. Results show that the through-the thickness-morphology of sample prepared by CIM features a typical skin-core structure, as a result of general shear-induced crystallization. This structure can be divided into three layers, including a skin layer in which the shish–kebab structure was found, a transition region with deformed spherulite structure and a core layer with spherulitic structure. However, the morphology of the sample prepared by PVIM, as a result of periodical shear-induced crystallization, features a richer and fascinating supermolecular structure and can not be roughly divided into three layers. A region full of shish–kebab-like cylindrulite structures was found between the transition region and the core layer, which is rare to be seen in conventional injection molding. Based on their various core structures, two kinds of shish–kebab-like cylindrulites were defined: one is multi-fibril-core cylindrulite of which core is an assembly of multiple fibrils, and the other is single-fibril-core cylindrulite of which the core just contains a single fibril. Based on the investigated results, a schematic illustration is proposed to depict the through-the thickness-distribution of supermolecular structure of iPP sample prepared by PVIM. The mechanism of the formation of the two kinds of shish–kebab-like cylindrulite structures is described in terms of periodical shear-induced crystallization. (RDC 6/2/2011)
Graphene Nanosheets and Shear Flow Induced Crystallization in Isotactic Polypropylene Nanocomposites
(2808–2818) Macromolecules 44 #8 (2011)
Xu et al of Sichuan University, China, and Stony Brook University, New York, studied thte combined effects of graphene nanosheets (GNSs) and shear flow on the crystallization behavior of isotactic polypropylene (iPP) by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. For crystallization under quiescent condition (at 145 °C), the half-crystallization time (t1/2) of nanocomposites containing 0.05 and 0.1 wt % GNSs was reduced to at least 50% compared to that of neat iPP, indicating the high nucleation ability of GNSs. The crystallization rate of iPP was directly proportional to the GNS content. Under a relatively weak shear flow (at a rate of 20 s−1 for 5 s duration) and a low degree of supercooling, the neat iPP exhibited an isotropic structure due to the relaxation of row nuclei. However, visible antisotropic crystals appeared in sheared iPP/GNSs nanocomposites, indicating that GNSs induced a network structure hindering the mobility of iPP chains and allowing the survival of oriented row nuclei for a long period of time. The presence of GNSs clearly enhanced the effects of shear-induced nucleation as well as orientation of iPP crystals. Two kinds of nucleating origins coexisted in the sheared nanocomposite melt: heterogeneous nucleating sites initiated by GNSs and homogeneous nucleating sites (row nuclei) induced by shear. The difference of t1/2 of nanocomposites with and without shear was significantly larger than that of neat iPP. The presence of GNSs and shear flow exhibited a synergistic interaction on promoting crystallization kinetics of iPP, although the effect of GNS concentration was not apparent. From WAXD results of isothermal and nonisothermal crystallization of sheared iPP, it was found that the appearance of β-crystals depended on the preservation of row nuclei, where the α-crystals were predominant in the iPP/GNSs nanocomposites, indicating that GNSs could directly induce α-crystals of iPP. (RDC 5/27/2011)