Row-nucleated structures

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 phenomenon of strain hardening in polymers is a consequence of orientation of molecular chains in the stretch direction. If the necked material is a semicrystalline polymer, like polyethylene or a crystallizable polyester or nylon, the crystallite structure will change during yielding. Initial spherulitic or row nucleated structures will be disrupted by sliding of crystallites and lamellae, to yield morphologies like that shown in Fig. 11-7. 

Figure 3.41 Row nucleated structure or shish-kebab, obtained from crystallization under strain...

Figure 3.54 shows actual DSC curves for samples A and B melted at 170°C. The fit of the calculated Avrami parameters to the experimental data is also shown. As seen, the sample spun under high stress crystallized much more rapidly than the sample produced under low stress. Moreover, it was observed that the starting spherulitic and row-nucleated structures were retained in the recrystallized samples. 

The crystallisation from strained melt as for instance in a blown film or in the jet during fibre spinning produces a row nucleated structure. " Linear nuclei are formed parallel to the strain direction. They contain more or less extended polymer chains. Secondary epitaxial nucleation on the surface of such linear row nuclei produces folded chain lamellae which are oriented perpendicular to the strain (Fig. 6). In such a case the sample exhibits a high uniaxial orientation of chain axes in the strain direction with random orientation of the a- and b-axes perpendicular to it. If the growing lamellae exhibit a helical twist the chain orientation in the strain direction is very soon replaced by the orientation of the axis of maximum growth rate (b-axis in the case of polyethylene) perpendicular to the strain direction and a more random orientation of the remaining two axes (a- and c-axes in the case of polyethylene) with a maximum in the strain direction. Such a row nucleated structure has parallel cylindrical spherulites (cylindrites) as its basic supercrystalline element. 

The row nucleated structure contains two types of crystals a small fraction of fibrillar crystals (row nuclei) with partially or even fully extended chains and the normal type folded chain lamellae. The existence of two types of crystals is detectable by calorimetry and the resistance to filmic nitric acid attack, high in the row nuclei and low in the surface layers of lamellae. The number of tie molecules between consecutive... 

The surface ripples have been observed previously in polymer solutions subject to shear stress [1-7] and in some cases have been captured in films formed by evaporation of these solutions. These surface bands, termed row nucleated structures by Kiss and Porter [7], are considered to be intimately related to molecular alignment within the film, which, in the case of PBLG, appears to be about 45° to the field direction and thus also to the direction of the striae. 

Fig. 4.22 Electron micrographs showing the effect of applied stress upon the morphologies of crystalline polymers, (a) Row-nucleated structure in melt-crystallized isotactic polystyrene (courtesy of Dr J. Petermann). (b) Shish-kebabs obtained from a stirred solution of polyethylene (courtesy of Dr A. Pennings).

Previous papers focus mainly on the transformed fraction. They often deduce the absolute crystallinity [139,143,144], and calculate the solid layer [139], especially at the end of the filling stage [64, 140, 141,145-147]. They sometimes predict the mean spherulite size [146,147]. Recently, much more detailed predictions of structure development, including the effects of flow on morphology, have been published [68,69]. Two types of morphologies are taken into account spherulites and row-nucleated structures (shish-kebabs). Figure 15.37 shows an example of calculations of the transformed fraction for each type of morphology [69]. 

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