Polymer crystallization shish-kebab

The lamellar crystal has a preferential orientation along the fibrillar direction (the c-axis of polymer crystal). Thus, it is assumed that the overgrowth of the folded chain crystal (shish kebab type) takes place in the formation of the lamellar crystal formed from the partial melt upon cooling. 

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. 

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. 

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 ... 

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. 

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. 

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). 

Li et al. (26) reported a polymer crystallization method to modify CNTs and nanofibers with semicrystalline polymers in a periodic manner, leading to a novel nanohybrid shish—kebab superstructure. Zou et al. (27) proposed a "roping and wrapping" method to disperse MWCNTs by non-covalently wrapping with linear polymers. Liang et al. (28) used molding processing technique to modify... 

Jandt, K.D. Buhk, M. Miles, M.J. Petermann, J. Shish-kebab crystals in polyethylene investigated by scanning force microscopy. Polymer 1994, 35, 2458. 

Information on how orientation during melt crystallization affects the transport properties of polymers is sparse however, increases in the permeability have been attributed to the "shish kebab" morphology (ill). Most of the work involving barrier properties of oriented semicrystalline polymers has dealt with materials drawn at temperatures well below the melting point. The transport properties of cold-drawn polyethylene (34f 42-46), polypropylene (42,42), poly(ethylene terephthalate) (12,42-4 9), and nylon 66 (22) among others have been reported. 

We see in these examples that great effect can be created by substitutions on the two ends of the mesogenic cores. The consequence includes not only the formability of liquid crystal phases and transition temperatures, but also the types of the liquid crystalline phase. It is well known that substitution on the end with longer linear alkyls and alkoxyls favor the formation of smectic phases. One example for this remark has been given in Table 3.3. In the design of liquid crystal polymers, especially in the cases of side-group type and molecular shish-kebabs, the concept of end-substitution has been fully utilized. 

Bunn and Alcock and Keller found that polymers show this type of organization [6], Finally, when the polymer solution or the amorphous state is under stress, long chains extend, serving as nuclei (shish), whereas smaller chains crystallize (kebab), usually in the form of a disk. Shish kebab structures were first observed by Mitsuhashi, Pennings, and Binsbergen [7]. 

If crystallisation occurs in an oriented melt, then non-spherulitic microstructures can form, with preferred orientation of the crystals (Section 3.4.10). Fibrous nuclei, believed to contain fully extended polymer chains, can form in an oriented melt. Figure 6.7a shows several fibrous nuclei, in a polyethylene injection moulding, aligned with the flow direction. On either side of these dark nuclei is a bright layer, where lamellar crystals have grown from the nucleus. The c axes of the lamellar crystals are parallel to the fibrous nucleus the microstructure of platelet crystals skewered by a rod-like nucleus has been described as a shish kebab. The rest of the microstructure consists of small spherulites. 

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