Primary lamellae

As the ageing time was increased, the initial transformation product (identified as primary lamellae with a spacing less than lOnm) was gradually replaced by secondary lamellae with a larger spacing, typically several tens of nanometres (Figure 5.20). 

Figure 5.21 shows the results of an analysis trace made across the primary lamellae/ secondary lamellae interface shown in Figure 5.20 as the line X-X. 

Figure 5.20. TEM micrograph of Co-32 wt%W alloy aged at 875 K for 3 days, showing fine primary lamellae having been replaced by coarser secondary lamellae. (Reproduced with permission by Zieba...

On 0001 surfaces a rosette pattern is sometimes observed by electron microscopy. This could be decoration by lower-MW PTFE or it might be a result of fold surface defects (see Figure 1.6). It is of note that the hexagonal daughter crystal in this figure developed in an orthogonal relationship to the primary lamella. 

Figure 1.6. Fracture surface of sintered PTFE showing a nearly hexagonal crystal viewed with its c-axis perpendicular to the plane of the micrograph. This crystal grew upon and perpendicular to a primary lamella, which is seen side-on and shows the curved edge profile associated with growth by nucleation and thickening. Magnification = 75,200, Scale bar = 0.5 pm.

Dunel-Erb S, Bailly Y, Laurent P. 1982. Neuroepithelial cells in fish gill primary lamellae. J Appl Physiol 53 1342-1353. Duranteau J, Chandel NS, Kulisz A, Shao ZH, Schumacker PT. 

The hght scattering in both the parallel (Vv) and cross polarization (Hv) modes, was measured for PP/partiaUy hydrogenated oligo(styrene-co-indene) blends. The results indicated that the primary lamellae (thick line in Figure 8.13b) sufficiently grew in the... 

In the author s opinion there is ample evidence for the existence of smaller, less perfect lamellae which form in the constrained amorphous phase between primary lamellae. It is perfectly reasonable that this would be so - since secondary crystallisation occurs in a constrained amorphous phase. Accordingly the second interpretation is favoured. However, it is also true that melting and recrystallisation occur during a heating scan and so the small LTMP may represent more of the lamella size distribution than the small size of the peak would suggest. In cold-crystallised samples there may be a lot more small lamellae than the size of the LTMP indicates (because melting is accompanied by a simultaneous crystallisation exotherm) and indeed the clearest evidence for the melting-recrystallisation interpretation comes from cold-crystallised materials. 

The same polymer morphology appears in a somewhat modified version if using different microscopic techniques and preparation methods, but they can reveal different details. As an example, Eig. 1.33 shows the morphology of a commercial a-iPP with the so-called crosshatched arrangement consisting of primary lamellae radiating from a spherulite center and smaller secondary (or daughter )... 

Secondary crystallization in isotactic polypropylene occurs in a peculiar way. On cooling additional crystallites are formed, however, many of them are oriented oblique to the primary lamellae rather than p lrallel as is normally observed. As explained by Lotz and Wittmann [8], this cross-hatching is caused by an epitaxial growth mechanism. If cross-hatching occurs the one-dimensional model used so far in the analysis of SAXS data becomes invalid. There is a simple check which shows the deviations. For a stack of crystallites one expects... 

Fig. 5.19. BA-C8 crystallizing at 22 °C. AFM tapping mode phase images of a nucleus left), one growing primary lamella and the development of branches right). From Chan et al. [46]. Copyright (2002) American Chemical Society...

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