Formation Of Shish-kebab Structures Research Articles

Altering the hierarchical morphology distribution of injection molded polyethylene by the introduction of crosslink network and periodical shear

High density polyethylene (HDPE) with moderate content of crosslink network (CPE) was successfully prepared through chemical method. Specimens for structural characterization have been molded by conventional injection molding (CIM) and pressure vibration injection molding (PVIM). Influence of crosslink network on hierarchical morphology distribution and mechanical properties was systematically studied. Polarized light microscopy (PLM) revealed that both CIM and PVIM PE samples have a typical “skin-core” structure and the thickness of shear layer of CIM PE and PVIM CPE samples obviously increase. Scanning electron microscopy (SEM) showed that shish-kebab structures are clearly observed in shear layer of CIM CPE sample, indicating that the crosslink network can surely improve the formation of shish-kebab structures. Moreover, we suppose that shish-kebab structures emerged in shear and core layer of PVIM CPE sample. Wideangle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) confirmed that more orientation and shish-kebab structures form even in core layer of PVIM CPE sample, which demonstrated that the hierarchical morphology was apparently altered by periodical shear and crosslink network. Finally, the mechanical properties revealed that this oriented structure increase the tensile strength from 31 MPa of CIM PE sample to 46 MPa of PVIM CPE sample. However, the tensile behavior tended to change from ductile fracture to brittle fracture.

Mechanism for Shish Formation under Shear Flow: An Interpretation from an in Situ Morphological Study

Isotactic polypropylene has been systematically studied with in situ optical microscopy to obtain a real space view of the morphology evolution of shear induced crystallization as a function of shear rates and shear time to elucidate the mechanism of shish formation. The critical shear time for inception of shish formation on the shear rate have provided an important understanding of the molecular and entangled network relaxation in relation to the shish formation. Also the observation of a typical shear time dependent comet like shish-kebab structure formation on the interface and the morphological growth of the shish from a fiber inside our sample have led to a new hypothesis that the shish is formed through a multiple discrete steps instead of forming directly to the final most stable state, this means that a transition state may be existed before shish growth. Two steps shear experiments with various time intervals between each step were designed to verify our proposed transition state mechanism, which can be observed much directly and obviously through real-space morphology, especially at low shear rate with long shear time. Another time dependent relaxation time is introduced base on shear rate dependent experiment after the first step shear and has close relationship with the existence of the critical shear time. A general framework for the shish formation has been established which can capture all the observed morphological features well, including the existence of the critical shear time at a given shear rate.

Control of Structural Morphology in Shear-Induced Crystallization of Polymers

Critical examination finds that the longest chains play a catalytic role in the formation of shish kebabs recruiting other chains into the formation of this morphology. The longest chains in an ensemble are stretched by shear flow to form the “shish” upon which the bulk of the material crystallizes as “kebabs”. A universal parameter for the formation of shish kebab structures, the specific mechanical work, and a method by which it may be measured for any given ensemble of polymers is provided. In rotating parallel-plate flow a clear boundary is observed between oriented and unoriented material which is dependent on both the shear rate and the total strain. It has been found that the necessary conditions for the formation of oriented nuclei is that the shear rate should be larger than the inverse Rouse time of the longest chain in the ensemble and that mechanical work above a critical threshold is required. The experimental procedure required to make such measurements, and the precautions necessary to avoi...

Unexpected molecular weight dependence of shish-kebab structure in the oriented linear low density polyethylene/high density polyethylene blends

In this study, highly oriented shish-kebab structure was achieved via imposing oscillatory shear on the melts of linear low density polyethylene (LLDPE)/high density polyethylene (HDPE) blends during the packing stage of injection molding. To investigate the effect of molecular weight of HDPE on the formation of shish-kebab structure, two kinds HDPE with large melt flow index (low molecular weight) and small melt flow index (high molecular weight) were added into LLDPE matrix. The structural characteristics of LLDPE/HDPE blends were systematically elucidated through two-dimensional wide-angle x-ray scattering, scanning electron microscopy, and differential scanning calorimetry. Interestingly, an unexpected molecular weight dependence of shish-kebab structure of the prepared samples was found that the addition of HDPE with low molecular weight resulted in an higher degree of orientation, better regularity of lamellar arrangement, thicker lamellar size, and higher crystal melting temperature than that adding HDPE with high molecular weight. Correspondingly, the blend containing low molecular weight HDPE had better tensile strength. A possible mechanism was suggested to elucidate the role of HDPE molecular weight on the formation of shish-kebab structure in the oriented blends, considering the change of chain mobility and entanglement density with change of molecular weight.

Crystallization of Polyethylene Blends under Shear Flow. Effects of Crystallization Temperature and Ultrahigh Molecular Weight Component

We performed small-angle X-ray scattering (SAXS) measurements on crystallization processes of polyethylene blends of low molecular weight and ultrahigh molecular weight components with various blending ratios in order to clarify effects of ultrahigh molecular component on shish-kebab structure formation. Anisotropic scattering patterns due to kebab structure formation were observed above a certain concentration of ultrahigh molecular weight polyethylene. The critical concentration was about ∼0.1 wt % independent of the crystallization temperature below 125 °C, while it increased with the crystallization temperature above 125 °C. Analyzing the correlation between ultrahigh molecular weight component concentration and crystallization temperature in more detail, these results suggest that both the crystallization rate and the relaxation rate of the ultrahigh molecular weight component have significant effects on the shish-kebab formation process.

The hierarchy structure and orientation of high density polyethylene obtained via dynamic packing injection molding

The evaluation of microstructure and crystal morphology in injected-molded bar becomes much complicated because of the existence of a shear gradient and a temperature gradient from the skin to the core of the samples. To understand the relationship between shear rate-molecular weight and oriented structure of injection molded bar, in this work, the hierarchy structure and the effect of molecular weight on the formation of shish-kebab structure were investigated by examining the lamellar structure of injection molded samples of high density polyethylene (HDPE) with different melt flow index (MFI), layer by layer, along the sample thickness. To enhance the shear effect, so-called dynamic packing injection molding (DPIM), in which the melt is firstly injected into the mold and then forced to move repeatedly in a chamber by two pistons that move reversibly with the same frequency as the solidification progressively occurs from the mold wall to the molding core part, was used to obtain the molded bar. Furthermore, a small amount of ultra-high molecular weight polyethylene (UHMWPE) was added into HDPE to explore the effect of UHMWPE on the crystal morphology and orientation. Our results indicated (1) that the overall orientation in the molded bar increased with decreased MFI, and a small amount of UHMWPE could enhance substantially HDPE orientation; (2) at the skin, there existed intertwined lamellae constituting an interlocked lamellar assembly, a typical shish-kebab structure gradually developed from the subskin-layer to the core, with increased shish content toward the center, but in the core was a spherulite-like superstructure with randomly distributed lamellae; (3) UHMWPE played an important role not only in the formation of shish, but also in the transformation from spherulite to shish-kebab oriented structure for HDPE with a low molecular weight (high MFI).

Effects of high molecular weight component on crystallization of polyethylene under shear flow

Crystallization of polyethylene (PE) blends of low and high molecular weight components under shear flow was studied using time-resolved depolarized light scattering (DPLS), focusing on effects of the high molecular weight component on the shish–kebab structure formation. Anisotropic two-dimensional scattering pattern due to shish-like structure formation was observed above a certain concentration of the high molecular weight PE. The threshold was about 2.5–3 times larger than the chain overlap concentration, suggesting an important role of entanglements of the high molecular weight component. On the basis of these results a gel-spinning-like mechanism for the shish-like structure formation has been proposed. The DPLS results also implied that the shish-like structure was mainly formed from the high molecular weight PE. This was confirmed by small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) measurements on an elongated PE blend of low molecular weight deuterated PE and high molecular weight hydrogenated PE (3wt%).

Mechanism of formation of shish kebab structures

AbstractTo describe the characteristic crystalline structure of polyolefins, Pennings first proposed a model consisting of a combination of an extended chain crystal (a “shish”) and folded chain crystals (“kebabs”). In Pennings' model the “shish” forms first during a crystallization process under stress and then later the “kebabs” overgrow this “shish” structure epitaxially. Because we had some doubts about such a mechanism, we undertook a series of experimental studies on linear polyethylene, particularly in regard to the crystallization process from a solution under shear. Our conclusion is that the crystals grow first by a screw dislocation mechanism, like whiskers, and then later these are deformed by the shear stress to form the shish kebab structures.