Shish structure

There are still hot ongoing debates about the formation mechanism of shish structures in the oriented flow field. Recently, Hashimoto and his coworkers proposed a new scenario for flow-induced phase transitions in polymer solutions to form the hierarchical structures of shish-kebabs (Hashimoto et al. 2010 Murase... 

Finally the relation between the anisotropic structure and the shish structure is discussed. In the transmitted electron microscope (TEM) measurements, shish structures with diameter of about several nm have been reported in some polymers [16], It is clear that such a shish structure is different from the anisotropic oriented structure observed here because the spatial scale is very much different. Judging from the size of the anisotropic oriented structure, it must be a bundle of shish-kebab structures at least at the final stage of crystallization. Why is the 4,m scale structure formed in the initial stage of crystallization under shear flow One speculative answer is that tm scale phase separation occurs in the initial stage of crystallization between the oriented and unoriented regions, and then crystallization of stretched polymer chains occurs in the oriented domain after aggregation of the stretched chains. 

Figure 2.5 Relaxation modes of shish structure in melt with respect to different size (a) faster relaxation of shish structure with smaller size and (h) slow relaxation of shish structure with larger size.

The mechanism by which shish are formed out of pointlike FIPs is stiU not fully understood. Nevertheless, a few experimental observations can guide the modeling of shish formation. Komfield and coworkers observed that the transition to oriented crystallization occurs very abruptly when a critical stress is surpassed [17,18,145] and that long chains lower this critical stress [48]. Kimata et al. [88] concluded from neutron scattering experiments that, although shish formation is accelerated by long chains, the shish structure does not have a higher content of these chains than the rest of the melt. 

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... 

Figure 30 illustrates nine examples of the structures obtained in these simulations. It is clear that the chains group into crystallized kebabs on the shish surface. There are very few areas where the chains are partially or completely stretched under the influence of the shish template. The dominant mode of crystal nucleation on the shish is the growth of folded chains grouped into lamellar nuclei. Also, some of the chains do not join the central structure but drift away from it leaving a large gap on the shish between them. These simulations show clearly that the presence of the ordered template (the shish) influences the nucleation of lamellae and the formation of kebabs. 

Some of the chains, when in contact with the shish, will stretch almost completely. However, these highly stretched chains are not dominant compared to the highly folded bundles that form crystalline kebabs around the shish. Also, none of the highly stretched chains formed a structure with part of it stretched and attached to the shish and part of it in folded crystalline lamella. The formation of kebabs in these simulations is clearly growth of lamellae, nucleated on the shish. 

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. 

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). 

FIGURE 2.17 Crystalline polymer structures formed under applied tension including flow conditions. The center shows the tertiary monofibriUar structure including platelets, the left shows these mono-fibrillar structures bundled together forming a quaternary structure fibril, and the right shows the distorted shish kebab formed with more rapid flow. 

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 ... 

In general, if the chains are oriented by external forces, the melting temperature is increased [28]. Further, it has been known for a long time that complex, row-nucleated structures occur if polymer solutions or melts are crystallized in the presence of flow [29,30]. These so-called shish-kebabs (sketched in Fig. 1.6), consist of a central fiber core, shish, surrounded by lamellar crystalline structures, kebabs, periodically attached along the shish. 

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