Shish kebab structure

Figure 8.7. (a) Idealised view of a shish-kebab structure (after Pennings ei cil. 1970, Macklcy and Keller 1975). (b) Shish kebabs generated in a llowing solution of polyethylene in xylene (after... 

Fig. 4.12 SEM and TEM images of PE-MWCNTs shish-kebab structure produced by PE on MWCNTs at 103 °C, (reprinted from [15,16] by permission of Wiley).

T. Tao, L. Zhang, J. Ma, C. Li, Production of flexible and electrically conductive polyethylene-carbon nanotube shish-kebab structures and their assembly into thin films, Ind. Eng. Chem. Res, vol. 51, pp. 5456-5460, 2012. 

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 ... 

Among the numerous challenges faced in understanding the formation and evolution of hierarchical structures in polymer crystallization, we restrict ourselves to explain the essential basic features of folded lamellae. Specihcally, we consider (1) molecular origin of enhanced scattered intensity before any crystallographic features are apparent, (2) spontaneous selection of small lamellar thickness, (3) molecular details of growth front, and (4) formation of shish-kebab structures in the presence of a flow. 

As-polymerized PTFE has exceptionally high crystallinity, a melting endotherm that is prone to superheating, and, in some instances, an unusual fibrous morphology. Melillo and Wunderlich note that some as-polymerized fibers may have a shish-kebab structure. Our electron microscopy confirms that this is indeed the case (Figure 1.3). 

As manufactured, PTFE is of two principal types dispersion polymer, made by suspension polymerization followed by coagulation, and granular PTFE, polymerized and generally comminuted to a desirable particle size. Some details are given by Sperati. We have observed cast films of an aqueous colloidal dispersion and see that it consists of peanut-shaped particles, approximately 0.25 pm in size, which are composed of even finer particles. Electron micrographs of as-polymerized granular particles show three structures bands arranged in parallel, striated humps, and fibrils, some of which have the shish-kebab structure."... 

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). 

As-polymerized PTFE has a number of interesting microstructures including highly developed crystals, fibrils, and occasionally, shish-kebab structures. 

Shish-kebab. In addition to spherulitic crystals, which are formed by plate- and ribbonlike structures, there are also shish-kebab crystals which are formed by circular plates and whiskers. Shish-kebab structures are generated when the melt undergoes a shear deformation during solidification. A typical example of a shish-kebab crystal is shown in Fig. 1.17. 

Another observation by Wunderlich and Melilio [6] of both interest and concern relative to our observations later of individual molecules moving on the substrates above the melting point was the development of fibrous, presumably extended-chain structures by monomer sublimation and repolymerization/crystallization on the walls of an evacuated tube when a PTFE sample in it was heated to 320 °C for 2 h (Fig. 10). Butenuth [11] had earlier described similar observations. On colder surfaces the polymer formed as strings of less than 0.1-micron circular platelets that the authors suggested formed as a shish-kebab structure (Fig. 10, right side). 

Fig. 10 PTFE filaments prepared by heating a PTFE sample in an evacuated tube to 320 °C for 2 h. The filaments collected on the hot wall of the tube. The inset shows the shish-kebab structure formed when the walls were cool. (Reprinted from Ref. [6] with permission from Springer-Verlag)...

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). 

This development started with an observation of Pennings and Kiel (1965) that, when dilute solutions of polyethylene were cooled under conditions of continuous stirring, very fine fibres were precipitated on the stirrer. These fibres had a remarkable morphology a fine central core of extended CH2-chains, with an outer sheath of folded chain material. Electron microscopy revealed a beautiful "shish kebab" structure (see Fig. 19.16). Shish kebabs have also been observed in experiments without any stirring. For example, by washing polyethylene powder with xylene (Jamet and Perret, 1973) and by crystallising nylon 4 from a glycerol/water mixture (Sakaoku et al., 1968). 

Sherwood number, 56, 59 Shielding constant, 368 Shift factor, 443 Shish kebab structure, 705, 731 Shore D hardness, 831 Shore hardness testers, 838 Short-range order, 25 Short shots, 804 line region, 807 Short-term behaviour, 820 Short-time stiffness, 25 Side chain effects, 184 Significance of the shift factor, 450 Silky lustre, 876 Simha/Boyer rule, 100... 

Fig. 12.24. "Shish-kebab" structure showing extended-chain crystals with lamellar overgrowths.

Fig. 24 A series of phase images showing the further growth of the PE shish- kebab structure. The gray scale represents a change in phase angle of 60°. The scan rate was 6.1 lines/sec. The scale bar refers to all the images and represents 300 nm. a Taken at 132 °C. b Taken at 131.5 °C the B indicates a pair of lamellae that have changed direction to avoid joining, c Taken at 131 °C the A indicates a pair of lamellae that have joined, d Taken at 130.5 °C the arrow indicates a point on the extended chain backbone where a new nucleation event has occurred, and the dotted lines show the distorting effect of drift, in which all the lamellae on a series of scan lines are deformed [84]...

Figure 6 (a) Sketch of the chain arrangement in shish-kebab structures. Reproduced from [67] with permission, (b) Polyethylene shish-kebabs crystalled from solution [68]. Reproduced with permission from H. D. Keith, 1996 American Institute of Physics, (c) Mannan-lamellae, epitaxially grown on cellulose fibres [69]. Reproduced from H. Chanzy, reprinted with permission from John Wiley Sons, Inc. 

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