Spherulitic region

From the images in Fig. 5.17 it is apparent that the PVAc is mostly excluded from the crystalline regions, and is located mainly within the inter-spherulitic regions. 

In the case of acetal, the use of a cold mold results in fast dissipation of heat from the melt into the mold wall. Consequently, the threshold limit for the formation of crystallization nuclei is quickly reached and a skin is formed on the parts that has an amorphous appearance but is actually crystalline, although to a much lower extent than the spherulitic region that is formed below the skin. The thickness to the amorphous zone is dependent on the mold and melt temperatures and other factors. 

FIG. 16 Morphological map for isothermally crystallized molecular mass fractions of PE. Regions marked with b and c denote spherulites region d, thin rods region g, rods and sheet-like crystals region h, randomly oriented lamella. (Reprinted with permission from Ref. 158. Copyright 1984 American Chemical Society.)... 

In the case of the polycrystalline polyester thermoplastic rubbers the simple domain theory does not seem to apply. With these rubbers it would appear that they contain spherulitic structures consisting of 4GT radial lamellae with inter-radial amorphous regions that are mixtures of PTMEG soft segments and noncrystalline hard segments. 

Explain the hierarchy of crystalline regions using the words unit cell, crystallite, lamellae, cylindrites and spherulites. 

We have already mentioned that depending on composition, semicrystalline triblock copolymers can show some conflict between microphase separation and superstructure formation. In fact, one of the controversial aspects is the question whether block copolymers can or cannot exhibit spherulites. This is a relevant question because spherulitic structures greatly affect the ultimate mechanical properties, and the boundaries between adjacent spherulites are often weak points in mechanical performance. Kim et al. [125] studied the competition between crystallization within microphase-separated regions and reorganization into supermolecu-lar spherulites in semicrystalline PS-b-PB-b-PCL triblock copolymers. These authors found that the formation of spherulites is strongly affected by the thickness of the specimen in such a way that thin films crystallize into... 

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