Shish-kebab crystal morphology

Study of the Crystalline Regions. According to our morphological studies, the crystalline structure of UHMWPE pseudo-gel was different under different sample preparations. For example, spherulites and lamellar single crystal stacks were observed when the pseudo-gel was prepared under quiescent conditions, shish-kebab crystals under stirring conditions, and a mixture of single and shish-kebab crystals under uncontrolled conditions (10). 

Fig. 10.19 Illustration of crystal morphologies of polymers influenced by the different degree of preload strain rates (percentage in the figure). The arrow indicates the direction of stresses. The right-hand schematic picture for shish-kebab crystals is adopted from Pennings et al. (1970)...

In drawn thin films of PE, shish-kebab crystals have been investigated by Miles and co-workers (191). Using contact mode SFM at minimized forces in liquid, the arrangement of the lamellae and the polymer molecules comprising the lamellae were resolved in molecular detail. Similar morphologies were also observed for poly(l-butene) (192), as well as many processed polymers, because of orientation effects during the production process (193,194). 

Fig. 61 Cartoon presenting the mechanism of wax crystallization on polymer templates. According to the experimental findings such a mechanism yields multiscale structures evolving hierarchically with decrease in temperature a the single coil stage b the formation of polymer rods with modulated density (alternation of crystalline and amorphous sequences along the rod axis) c the shish-kebab-like morphology formed by correlated polymer-wax plates templated by the initial polymer rods d a detail of the polymer-wax common plates, basically the same structure as that shown in Fig. 55. At lower temperatures and high wax content the wax surplus fills the space between the platelets and transforms the shish-kebab into large-scale one-dimensional object...

When polymers are crystallized under flow (stirring, extensional, etc.), the ubiquitous morphology [23] is the shish-kebab structure, consisting of central core (shish) surrounded by lamellae (kebabs) attached along the shish. What is the underlying mechanism behind the formation of shish-kebab stmcture ... 

By variation of the conditions of crystallization (see Sect. 1.3.3.3) polyethylene can be obtained either as folded lamellae, as extended chain crystals (high strength fiber), or as so-called shish kebabs (fibrils with a morphology similar to cellulose). All these variants differ in properties. 

The morphology and microstructure of as-polymerized polytetrafluoro-ethylenes is a study in itself. We observe that fibrils are common in some lots of granular PTFE while other specimens consist of beadlike particles, the surfaces of which bear markings suggesting lamellar crystals. Of special note is the (rare) occurrence of shish-kebab structures in as-polymerized PTFE (Figure 1.3). 

The application of a flow field has pronounced effects on the crystallization dynamics and semi-crystalline morphology of polymeric systems. Under quiescent, no-flow conditions the crystallization dynamics is governed by the temperature, T, and spherulites are formed. In the presence of flow, increasing the flow rate can result in an increase of the nuclei density up to factors of 106, but the morphology remains spherulitic [18]. At even higher flow rates the so-called shish-kebab morphology is observed. This results from the de-... 

Keywords Bands Morphology Nanofibrils Poly(tetrafluorethylene) Single crystals Shish kebabs... 

FIG. 19.1 Morphological models of some polymeric crystalline structures. (A) Model of a single crystal structure with macromolecules within the crystal (Keller, 1957). (B) Model of part of a spherulite (Van Antwerpen, 1971) A, Amorphous regions C, Crystalline regions lamellae of folded chains. (C). Model of high pressure crystallised polyethylene (Ward, 1985). (E) Model of a shish kebab structure (Pennings et al., 1970). (E) Model of paracrystalline structure of extended chains (aramid fibre). (El) lengthwise section (Northolt, 1984). (E2) cross section (Dobb, 1985). 

Information on how orientation during melt crystallization affects the transport properties of polymers is sparse however, increases in the permeability have been attributed to the "shish kebab" morphology (ill). Most of the work involving barrier properties of oriented semicrystalline polymers has dealt with materials drawn at temperatures well below the melting point. The transport properties of cold-drawn polyethylene (34f 42-46), polypropylene (42,42), poly(ethylene terephthalate) (12,42-4 9), and nylon 66 (22) among others have been reported. 

Shish-kebab" morphology (a different highly oriented morphology which develops when an oriented melt is crystallized)... 

---