Cooling direction, lamellae

It is clear that the quenched material has lamellae that are both thinner in the chain direction and smaller in lateral extent. Both these parameters increase with slower rates of cooling. So, too, does density. The microstructure of PTFE prepared this way may be likened to a pack of rods (the helical polymer chains), which in turn comprise lamellar crystals. Consider a hexagonal poker chip as a macroscopic analogue of one lamella. The diameter of die poker chip is the lamellar breadth and its thin dimension is the lamellar thickness. The fully extended chain length of a 10-million-MW PTFE macromolecule would be ca. 26 pm while our thickest observed lamellae are ca. 0.5 pm thick. It therefore... 

It should be pointed out that there is no direct physical relation between the phenomenon of fractionated crystallization and the number and the size of spherulites in the pure polymer. Whereas the occurrence of fractionated crystallization is related to the ratio between the number densities of dispersed polymer particles and primary nuclei, the size and the number of spherulites are additionally influenced by the cooling rate and the crystallization temperature. There is, therefore, also no relation between the fractionated crystallization and the type of the arising crystalline entities (complete spherulites, stacks of lamellae,...) both in the pure and in the blended material. There is, finally, no relation between the scale of dispersion which is necessary for the occurrence of fractionated crystallization and the spherulite size in the unblended polymer. 

Figure 1.9. A schematic diagram of the development of the solid, semi-crystalline polymer by growth of crystalline lamellae in the direction r from the melt on cooling. Adapted from Strobl (1996).

An example of the multilayered structures common in polyacetals is shown in the polarized light micrographs (Fig. 5.35). They depict a uniformly nucleated crystalline structure formed due to mold filling and variations in the rate of cooling of the melt. The skin surface in the microtomed section (top in Fig. 5.35A) is birefringent, non-spherulitic and highly oriented. The molecular chains are oriented parallel to the injection direction. The central portion of the bar consists of a core (bottom Fig. 5.35A) with randomly oriented spherulites (Fig. 5.35C). It has no preferred molecular or lamellae orientation. There are usually one or more layers between the skin and core that are transitional shear zones with intermediate... 

The first direct observation of the namre of polymer crystallinity resulted from the growth of single crystals from dilute solution. Either by cooling or by evaporation of solvent, thin pyramidal or plate-like polymer crystals (lamellae) were precipitated from dilute solutions... 

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