Crystallinity nucleation and growth

Most pure substances have a definite melting temperature below which the change from a random liquid structure to a well ordered, periodic crystalline structure can occur this transformation is called crystallisation the reverse process is called melting. 

Crystallisation is also possible from solutions the reverse process is called dissolving. 

Crystallisation from a solution largely depends on the rate of cooling and on the rate of change in solubility connected with it. Again, the polymers with a regular molecular chain without side groups crystallise fast. 

Since polymers cannot be completely crystalline (i.e. cannot have a perfectly regular crystal lattice) the concept crystallinity has been introduced. The meaning of this concept is still disputed (see Chap. 2). According to the original micellar theory of polymer crystallisation the polymeric material consists of numerous small crystallites (ordered regions) randomly distributed and linked by intervening amorphous areas. The polymeric molecules are part of several crystallites and of amorphous regions. 

TABLE 19.1 Definitions of crystallinity (xc) (after Kavesh and Schultz, 1969) 

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