Shish kebab

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... 

Supramolecular structures formed during the crystallization of the melt under a tensile stress have already been described by Keller and Machin25. These authors have proposed a model for the formation of structures of the shish-kebab type according to which crystallization occurs in two stages in the first stage, the application of tensile stress leads to the extension of the molecules and the formation of a nucleus from ECC and the second stage involves epitaxial growth of folded-chain lamellae. 

Fig. 3a-e. Supermolecular structures of polymers crystallized in various force fields a structure of the shish-kebab type, b structure formed during crystallization in a capillary with a conical inlet and c structure of a polymer crystallized at hydrostatic compression at 4 x 108 Pa... 

Heterogeneous nucleation of polymer crystallization resembling a visualized metaphor compare the way meat is prepared in an oriental way shish - kebab. 

The optical and electrochemical properties of porphyrins make these chromophores useful building blocks for the synthesis of electro- and photoactive polymers. Two types of linear polymers have been constructed using the self-assembly approach homo-polymeric assemblies and hetero- or shish kebab polymers. 

Maldotti (96) studied the kinetics of the formation of the pyrazine-bridged Fe(II) porphyrin shish-kebab polymer by means of flash kinetic experiments. Upon irradiation of a deaerated alkaline water/ethanol solution of Fe(III) protoporphyrin IX and pyrazine with a short intense flash of light, the 2 1 Fe(II) porphyrin (pyrazine)2 complex is formed, but it immediately polymerizes with second-order kinetics. This can be monitored in the UV-Vis absorption spectrum, with the disappearance of a band at 550 nm together with the emergence of a new band due to the polymer at 800 nm. The process is accelerated by the addition of LiCl, which augments hydrophobic interactions, and is diminished by the presence of a surfactant. A shish-kebab polymer is also formed upon photoreduction of Fe(III) porphyrins in presence of piperazine or 4,4 -bipyridine ligands (97). 

Hanack has reviewed a number of phthalocyanine and tetrabenzopor-phyrin shish-kebab porphyrin polymers (102). 

Fig. 10 Snapshot of a shish-kebab crystallite induced by a pre-aligned single chain (drawn much thicker than other chains for better visibility) in a solution. The chain length is 32 units and the thickness of crystallites is about 7 units. The bonds are drawn in solid cylinders [58]... 

Fig. 33 Snapshot of a shish-kebab in the Monte Carlo simulation... 

Usually, synthetic polymers crystallize11 j15 from a melt or a solution in form of folded lamellae. Under specific circumstances it is sometimes also possible to obtain extended chain crystals which is the preferred arrangement in the crystallites of many natural polymers (cellulose, silk). Recently it has been found33 31 that in some cases another crystalline modification can be obtained, the so-called shish-kebabs, which are a sort of hybrid between folded lamellae and extended chain crystals. These shish-kebabs are obtained by shear-induced crystallization, a process in which the polymer crystallizes from solution under the influence of an elongated flow. 

By special processes, like polymerization under shear conditions or flash-spinning of polymer solutions, it is furthermore possible to obtain materials which do not only have this new shish-kebab micro-morphology, but also a new macro-morphology, namely the polymer fibrids.35... 

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).

L. Zhang, T. Tao, C. Li, Formation of polymer/carbon nanotubes nano-hybrid shish-kebab via non-isothermal crystallization, Polymer, vol. 50, pp. 3835-3840, 2009. 

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. 

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