Fibers shish kebab

Fiber Main fiber is made of secondary fibrils Fiber Shish-Kebab Melt cryst. films Spherulitic Soln. grown Lamellae 500 (fibril width) 100 10 (fibril thickness) Low-magnification electron microscopy 25 High-magnification electron microscopy 8 — 12 ... 

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. 

The fiberlike crystalline structures (shish) are highly stable to the point that they can be superheated [31]. Therefore, it is believed that the core of the shish is formed by crystallization of completely stretched polymer chains. The kebabs are believed to be folded-chain lamellar structures. The direction of growth of the kebabs is normal to the shish. The chain ahgnment in the kebabs is believed to be parallel to the shish. Similar structures are obtained by crystalhzation in polymer melt films exposed to orientational deformation [32,33]. These two-dimensional shish-kebabs also consist of a central fiber, shish, and periodically attached linear kebabs, with growth direction normal to the shish. 

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. 

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). 

No shish are present in Fig. 51a. Indeed there is some question as to whether the oriented rows of bands are shish-kebabs many of the kebab bands fie at an oblique angle to the apparent shear direction. Possibly they were nucleated along fines of smeared particles. On the other hand, Fig. 51b shows unmistakable shish kebabs (350 °C, 30 min, followed by air-quenching). In the shish on the left all of the kebabs would be nucleated at the sides of the apparent fiber, a split being present between the kebabs growing in opposite directions. On the right shish, however, a number of the kebabs extend across the cen-... 

Production. Recognition that the shish-kebab fibers produced by the surface-growth procedure result from the deformation of a gel-like entangled network layer at the rotor surface led to the development of gel-spun polyethylene fibers. The fiber is made by the solution spinning method. The polymer is... 

Two-step processes to generate nanoprotrusions on electrospun fibers have also been studied. Nanofiber "shish kebabs" by electrospinning poly(s-caprolactone) (PCL) followed by solution crystallization of PCL or a copolymer of PCL and PEO was obtained. ... 

Mullin N, Hobbs J (2011) Direct imaging of polyethylene films at single-chain resolution with torsional tapping atomic force microscopy. Phys Rev Lett 107 197801 Murase H, Ohta Y, Hashimoto T (2011) A new scenario of Shish-Kebab formation from homogeneous solutions of entangled polymers visualization of structure evolution along the fiber spinning line. Macromolecules 44 7335-7350... 

The nucleation process of nucleation agents added on purpose to polymer melts, as well as the effects of shear on polymer crystallization, are still not well understood. SFM can provide insight into the imderlying processes as shown by Beekmans and co-workers, who investigated the transcrystalhzation of PCL on PTFE fibers using hot-stage SFM (293), and Hohbs and co-workers who studied the crystallization of sheared PE melts (283,284). In the latter studies, shish-kebab structures, nucleated on oriented backbones, were detected in the melt and could be followed in real time (Fig. 29). 

Mai F, Wang K, Yao M, Deng H, Chen F and Fu Q (2010) Superior reinforcement in melt-spun polyethylene/multiwalled carbon nanotube fiber through formation of a shish-kebab structure,... 

The aspect of self-reinforcement is of great importance for the performance of the single polymer composites. Apart from the stretching of the molecules, which occurs during the fiber and/or tape production, crystalline structures are also induced [11]. So-called shish kebab structures can be specifically developed by means of melt or solid phase deformation [12] and give the stretched thermoplastic tapes or fibers significantly higher mechanical properties than are possible for a comparable compact material that is not self-reinforced. 

Xu H, Xie L, Chen Y-H, Huang H-D, Xu JZ, Zhong G-J, Hsiao BS, Li Z-M (2013) Strong shear flow-driven simultaneous formation of classic shish-kebab, hybrid shish-kebab, and transoys-tallinity in poly(lactic acid)/natural fiber biocomposites. ACS Sustain Chem Eng 1... 

Studies using gold decoration have been performed in several laboratories [282-286]. Thin solution cast films of nylon were studied by Spit [283], while Krueger and Yeh [284] studied stirred PE solutions which formed shish kebabs. Kojima and Magill [287] studied the morphology of spherulites of block copolymers by gold decorating sections. Shimamura [288] applied this method to the study of the internal structure of PE fibers. 

At the same time Pennings (DSM, later of the University of Groningen, The Netherlands) studied the fiber formation from dilute solutions of high molecular weight polyethylene. He started with fibers formed in a Couette device these were stirring-induced fiber crystals, with the famous shish-kebab stmcture [16]. The best properties were obtained when fibers were slowly withdrawn from a gel layer on a rotor. The molecular weight was above 10 , the polymer concentration about 1%, the fiber growth rate below 1 m min, the moduli around 125 GPa [17], and tenacities far above 1 GPa. 

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