Crystal growth lamellae stacks

Figure 31 AFM phase images (1 pm ) recorded in hard tapping during crystallization of PET from the melt at 233 °C elapsed times are 72 (a), 80 (b), 96 (c), and 112 min (d). Thin white stripes correspond to the crystalline lamellae in almost edge-on orientation. The arrows help the reader to identify the lamellar stacks and the same spots on the sample surface where the crystal growth proceeds wathe stack thickening mechanism. With permission from Ivanov, D. A. Amalou, Z. Magonov, S. N. /Macromotecotes2001,34.8944. ...

Multilamellar Crystals, Hedrites, and Spherulites. Upon crystallization from the quiescent melt (or concentrated solution), polymers exhibit higher order supramolecular structures, including stacks of lamellae, hedrites, and finally spherulites (167). Hedrites (177) are entities composed of multiple lamellar crystals and as such correspond to intermediate stages of spherulitic growth. SFM in various modes was used to determine spherulite sizes and details of lamellar arrangement within spherulites for a wide variety of polymers and polymer processing conditions. SFM data acquired at free surfaces, at the surface of microtomed... 

Figure 22 Schematic illustration of the lamellar stack (top) and Its possible evolution during the crystallization process (bottom). The structure in the left bottom panel shows formation of separate stacks of thicker and thinner crystals whereas the structure in the right bottom panel corresponds to the so-called insertion growth mode where the thinner crystal grow In-between the dominant thicker lamellae.

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... 

Figure 2.24 Hierarchy of structures observed when a polymer crystallizes. Molecules adopt an extended conformation, or fold. The crystallized molecules are organized into lamellae. Crystalline layers and amorphous layers form a lamellar stack. Radial growth of lamellae leads to spherulitic supermolecular structures. The spokes of the spherulite result from defects in lamellar organization, such as giant screw dislocations...

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