Cross-hatched lamellae

Fig. 3.21 Tapping mode AFM phase image left) and height image right) of cross-hatched lamellae of iPP in elastomeric polypropylene and isotactic PP, respectively. The angles of 80° agree well with the theoretically predicted and experimentally confirmed value [53]. Reproduced with permission from [51]. Copyright 2002. American Chemical Society, and [54]...

The images shown in, Fig. 3.23 display the simultaneously captured height (left) and stiffness images. The (cross-hatched) lamellae are visualized in both modes. 

Keywords entanglement, disentanglement, cross-hatching, lamellae, crystallization, nucleation, reptation, nucleation (crystallization) regimes, nucleation agents, nucleation rate, spherulitic growth rate, Avrami-equation, Ozawa-equation, isothermal crystallization, nonisothermal crystallization, secondary nucleation, supercooling. 

Spherulites consist of crystal lamellae of about 10-20 nm in thickness and a lateral range of some micrometers. In PP, the spherulites show a cross-hatched lamellae structure [4] with radial lamellae as well as tangential lamellae, which grow up on the radial lamellae ( homo-epitacticity ). 

Figure 2 Transmission electron mictt aph of the different deformation behavior of the highly oriented layer (top) with stacked lamellae and the typical cross-hatched lamellae structure of PP (bottom). The lamellae appear bright, the amorphous phase dark and the damaged structures black. The melt flow direction inside of this micrograph is horizontal.

This result indicates a relaxation of the nanostructure while the extensometer stands still. Quantitative analysis shows that during relaxation, the extension of crystalline lamellae is increasing. Lamellae thicknesses are becoming nonuniform. The range of order is shortening. Cross-hatched lamellae are formed. 

Spherulites of aliphatic hydrocarbon polymers such as polyethylene generally have their molecular chains with the highest refractive index oriented perpendicular to the radial growth direction. This is the case with the polypropylene spherulites shown in Fig. 2.9a, which were crystallized at 140 °C and consequently contain very few cross-hatched lamellae, so in this respect they are like polyethylene spherulites. The lower refractive index in this view is radial in direction, giving a yellow colour where the radius is parallel to the slow direction of the tint plate these are therefore termed negative spherulites. 

In Fig. 2.9b, the specimen was crystallized at 120 °C, so that there is a preponderance of cross-hatching lamellae located roughly perpendicular to the dominant radially growing ones. This reverses the birefringence, giving positive spherulites... 

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). 

There are many modifications of lamellar and superlamellar structures in different polymers, for instance, the cross-hatched lamellae in a-iPP (see Fig. 2.2) or the so-called shish kebab structures in oriented PE and PP see Fig. 2.13. Such structures appear when highly oriented melts crystallize (in the form of a bundle of oriented, longer macromolecules, the shish, and smaller perpendicular lamellae of folded macromolecules, the kebabs ). A peculiar shape is formed by spiral lamellae in TPU (also see Fig. 4.95 in Section 4.5.3). 

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]. 

Sanidine is monoclinic (space group C2/m), and there is complete disorder in the occupation of the tetrahedral (T) sites by the A1 and Si atoms. Over geological time, ordering takes place. In low (or maximum) micro-cline, the ordering is complete (all A1 in TiO sites), and the symmetry is reduced to triclinic (CT). There are four main orientational variants in this structure two orientations related by the albite twin law (rotation of 180° about b ) and two orientations related by the pericline twin law (rotation of 180° about b). The composition planes of these two twins are, respectively, (010) and the rhombic section which is parallel to b and approximately normal to (001). Thus, the characteristic cross-hatched pattern observed in (001) sections between crossed-polarizers in the optical microscope has, for many years, been simply interpreted as intersecting sets of albite and pericline twin lamellae formed at the monoclinic-to-triclinic transformation. However, TEM observations indicate that this model is too simple. Because these observations, collectively, also constitute an excellent example of the application of the principal modes of operation of TEM to a specific mineralogical problem, we discuss them in some detail. 

Figure 8.25. Diagram showing the idealized intersection of a set of albite-twin lamellae ABABA... with a set of pericline twin lamellae A B A B A. .. to form the chessboard pattern of cross-hatched twinning. (From McLaren 1978.)...

Every crystallization in polymers is induced by crystallization nuclei. Nuclei give rise to lamellar crystal growth. Lamellae radiate from the central primary nuclei and by sufficiently frequent branching and cross-hatching form crystalline aggregates called spherulites. 

Investigations by transmission electron microscopy (TEM) exhibit the known cross-hatched structure in the central layer but inside the oriented layers there are stacked lamellae. Their orientation is perpendicular to the flow direction of the polymer melt (Figure 2, upper side). The lateral range of these lamellae are shorter than the range of the radial lamellae... 

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