Morphology, cross-hatched

Morphological explanations for the improved ductility of the fl nucleated materials have focussed on the lamellar texture. That crazes in a iPP are more localised and better defined than in /3 iPP may reflect both the influence of the cross-hatched structure on lamellar slip described in the previous section, and the strong correlation between deformation in ft iPP and the local orientation of the lamellae with respect to the tensile axis. Indeed, the trend towards more localised deformation in a spherulites may simply reflect the relatively homogeneous lamellar textures of these latter [24]. 

Two typical images which depict a spherulite and the dense cross-hatched morphology typical of the a-modification of iPP are shown in Fig. 3.30 (see also Fig. 3.21 for a comparison with TM-AFM data). The presence of cross-hatching is often noted in all common CM-AFM modes, namely height, deflection (error signal), and also friction modes. 

Fig. 3.30 CM-AFM deflection images of spherulite left) and cross-hatched morphology right) found at the surface of melt-crystallized iPP films (image sizes, 15 and 1.0 pm, respectively). With kind permission from Springer Science+Business Media from [55]. Copyright (1993). Springer-Verlag...

Fig. 2.5 Morphological effects of the recrystallization of isotactic polypropylene at 145 °C (a) radial growth enclosing a leaf-shaped area of cross-hatched material, (b) a similar area after partial melting (From figure 4 in. Weng J, Olley RH, Bassett DC and Jaaskelainen P, Changes in the Melting Behavior with the Radial Distance in Isotactic Polypropylene Spherulites. J. Polym. Sd. Polym. Phys. 41 2342-2354. Copyright 2003 by John Wiley Sons, hic. Reprinted by permissirai of John Wiley Sons, Inc.)...

This can then be partially melted (following curve 2) and the specimen quickly removed and quenched, so that the material has recrystallized in a much lower melting state (curve 3). The morphological effect can be seen in Fig. 2.5. The lower melting peak is seen to relate to daughter lamellae formed between the dominant ones, and especially to highly cross-hatched regions which melt out entirely. 

Polypropylene can be easily induced to form row structures by extension of the melt. An example of the resultant morphology is shown below. Closely spaced shish-kebab lamellae have grown directly out from the central row nucleus, but as growth proceeds outwards, increasing numbers of cross-hatching lamellae are found. The cross-hatching lamellae are found almost perpendicular to the lamellae growing out from the initial nucleus (Fig. 3.3). 

Auriemma F, De Rosa C (2006) Stretching isotactic polypropylene from cross-p to cross-hatches, from a form to y form. Macromolecules 39(22) 7635-7647 Baird DG (1998) Polymer processing principles and design. Wiley-Interscience, New York Bassett DC, Olley RH (1984) On the lamellar morphology of isotactic polypropylene spherulites. Polymer 25(7) 935-943... 

Some SEM observations (e.because there are two populations of fibrils, the second one being nearly perpendicular to the first one just as in cross-hatched morphology. 

Figure 5.2 TEM bright-field images of ultrathin film samples of [a] pure iPP-g-MA showing cross-hatched morphology and trans-crystalline organization of iPP-g-MAaround [b] MWCNTs and (c) SWCNTs. (Reproduced with permission from ref. 14. Copyright 2008 American Chemical Society)...

Figure 3.9 SEM micrographs of PP surfaces after permanganic etching, showing (a) the typical morphology of a - iPP with the typical cross-hatched arrangement of the lamellae and (b) the typical morphology of f) - iPP with the sheaf-hke lamellar arrangement.

Figure 3.17 compares the morphology of a commercial iPP in the a-form with the so-called cross-hatched arrangement consisting of primary lamellae radiating from a spherulite center and smaller secondary (or daughter ) lamellae perpendicular to the primary ones (at the bottom, compare also with Figs. 3.9 and 3.12). Several preparation techniques, necessary for the different microscopic techniques, are mentioned in Section 3.3. 

Lamellar habit is also known for crystallization from the melt under isothermal and nonisothermal conditions. Typical views of lamellae in isotactic polypropylene or isotactic sequences in elastomeric polypropylene are shown in Figure 6.15, also for comparison with data discussed in Chapter 3. The typical cross-hatched morphology with angles of 80° agrees well with the theoretically predicted and experimentally confirmed value [26]. Independent of the AFM mode (contact mode, intermittent contact mode, pulsed force mode) consistent observations were made. 

Figure 6.15 (a) Contact mode AFM deflection image of cross-hatched morphology found at the surface of melt-crystaUized isotactic PP... 

This difference is reflected in the morphologies of samples. Figure 8.17 shows TEM micrographs of all samples. A typical thick lamellar structure is observed for iPP-IC. For iPP-PMQ and iPP-MQ, thinner lamellae than iPP-IC and a cross-hatched structure are observed. On the other hand, iPP-POW has no lamellar structure, which means the crystalline component is... 

In order to avoid misunderstanding, we emphasize here that the parallel aligned lamellar structure is not the sole morphology of parallel chain polymer epitaxy. As an example, when crystallizing the poly(ethylene adipate) (PEA) on uniaxially oriented PE substrate [97], a cross-hatched edge-on lamellar structure of PEA with its lamellae ca. 66.5° apart from the chain direction of the PE substrate film is observed (see Fig. 12.10), indicating the occurrence of epitaxial crystallization of PEA on oriented PE substrate. Known from the experience of PE/iPP epitaxy, where the 50° intersect of both polymer chain directions leads to the formation of a cross-hatched lamellar structure of PE (see Fig. 12.9a), one may conclude that the PEA and PE molecular chains... 

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