Single crystal, lamellae

First of all, these materials are ttractive "per se". In particular, poly(styrene-b-caprolac tone) is a semi-crystalline product displaying an amazing resistance to cold-fracture, and also macroscopically biodegradable at least when PCL represents the continuous phase. It can be "organized" into single crystals, wherein lamellae of PSt and PCL alternate with a periodicity of about 80 A. 

Two final points need to be made about secondary nucleation. First, that screw-dislocation defects, described in more detail in Sect. 5.3, prodnce indesttnctible secondary nuclei for growth on top of the fold surfaces of polymer lamellae. This surface would otherwise be inactive for further growth and restrict polymer crystals to single lamellae (see Chap. 5). An example of a series of screw dislocations is shown in Fig. 3.72 on the example of poly(oxyethylene) of 6,000 molar mass grown... 

Fig. 13. A graph showing the variation of growth rate with the inverse of film thickness Ud) for isotactic polystyrene crystals grown at 180 C in nltrathin films. A step in the behavior can be seen at a thickness of 8 nm. At this temperature, this corresponds to the thickness of the single lamella, showing a change in thickness dependence once the film is thinner than the growing crystal. Reprinted from Ref. 98. Copyright (2002), with permission from Marcel Dekker, Inc.

Crystallization from the melt also results in chain-folded lamellar crystals and for numerous polymers single lamellae have been studied by SFM approaches. Since typical lamellar thicknesses, depending on the polymer and the crystallization temperature, are in a range of 10 nm to several tens of nanometers, the finite size of the SFM probe tip leads to convoluted data, if lamellae are viewed edge-on (33). Among the coimtless examples are isotactic PS (157,158), PEO (159,160), PP (161-163), PE (164), polycarbonate (PC) (165), etc. 

These had the same crystallographic form as complexes isolated from melt crystallization and from concentrated solutions. The stereocomplex is a 2 1 molar ratio of syndiotactic isotactic. Since the melting point also varied with crystallization temperature, lamella and single-crystal structure have been suggested for the dilute solution complex. 

Fig. 4.29 Electron micrograph of a nylon 6 single crystal. The lamellae thickness are 50-100 A. (Geil, (1960), reprinted with permission of John WUey and Sons, Inc.)...

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