Shish formation

Fig. 8.10. Schematic representation of the entanglement network of HMVIT E chains (A) under uniaxial deformation (LMVIT E chains are not shown here). E pon shearing, some chain segments between entanglements (shown as round dots) are stretched along the flow direction, but most segments remained in the coiled state (B). The stretched segments form the precursors for the shish formation, and the coiled segments can grow into kebabs (reprinted with permission from [27])...

Model PE blends consisting of a low molecular weight (LMW) matrix that was kept amorphous by choosing the experimental temperature higher than its melting temperature, that is, acting as a solvent, and a crystallizable HMW minor component were studied by Hsiao and coworkers [85]. Such special blends allow for the examination of the crystallization behavior under fiow of the HMW component only. Shish formation was directly observable from SAXS measurements for sufficiently high HMW concentration and indirectly from WAXD (oriented kebab). 

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. 

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. 

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). 

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. 

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. 

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. 

De Gennes scaling concepts for polymer solutions and melts and the concept of reptation movement of polymer chains in melts Pennings discovery of chain extension and shish-kebab formation in stirred solutions of very high MW polyethylene this eventually led to the ultra-high modulus gel-spinning process of polyethylene... 

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