Nucleation, crystal secondary

The free-energy change for secondary crystal nucleation can be estimated in a way similar to the above primary nucleation. Assuming a rectangular bundle of crystalline stems with the lateral size a and the stem length I,... 

The regime-transition phenomena of polymer crystal growth have been well explained on the basis of the Lauritzen-Hoffman model [14,15]. Nevertheless, the assumptions about the details of secondary crystal nucleation can be replaced by the intramolecular crystal nucleation, without a substantial loss of semi-quantitative predictions about regime transitions. 

Describe the mechanism by which crystallites form, starting with nucleation and ending with secondary crystallization. 

Note Nucleation may be classified as primary or secondary. Primary nucleation can be homogeneous or heterogeneous if heterogeneous nucleation is initiated by entities having the same composition as the crystallizing polymer, it is called self-nucleation. Secondary nucleation is also known as surface nucleation. 

The isotherms obtained in dilatometric measurements of the crystallization rate could be fitted with an Avrami (3) type equation only by assuming the existence of a secondary crystallization process much slower than the rate of spherulitic growth observed microscopically, and by taking into account the experimentally determined form of the nucleation rate. The nucleation rate was found to be a first-order process. Assuming that the secondary crystalliza-... 

The most successful approach to control membrane formation involves segregation of the processes of crystal nucleation and growth [24]. The so-called ex situ or secondary (seeded) growth methods, unlike the direct synthesis procedures just discussed, include a first step in which a closely packed layer of colloidal zeolite crystals, synthesized homogenously, is deposited onto... 

Crystal growth can only start after the crystalline phase has been nucleated, in contrast to a chemical reaction which may proceed on any single encounter of two reaction partners. This difference between the chemical reaction which sets the primary bonds and the crystallization which sets the secondary bonds in crystals of flexible linear high polymers makes the crystal nucleation to the central problem. From the different examples of crystallization during polymerization collected here one can derive five different nucleation processes A. an intermolecular nucleation followed by simultaneous polymerization and crystallization dose to the ceiling temperature, B. an intermolecular nucleation followed by simultaneous polymerization and crystallization far from the ceiling temperature, C. an intermolecular nucleation followed by successive pol3uner-ization and crystallization, D. and E. an intramolecular nudeation which may also show simultaneous or successive polymerization and crystallization. 

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