Nucleation behaviour

Section 3.2 is an introduction to general nucleation behaviour as first used for low molecular weight materials, that is, non-polymeric substances. This topic is frequently not covered in polymer papers but the extensive work done in this area provides the framework for the theory, and indicates the limitations and problems to be expected in predicting the physical behaviour. 

J. S. Parent and P. R. Bishnoi. Investigations into the nucleation behaviour of methane gas hydrates. Chem Eng Commun, 144 51-64, February 1996. 

Parent, J.S., Investigations into the Nucleation Behaviour of the Clathrate Hydrates of Natural Gas Components, M.Sc. Thesis, University of Calgary, Alberta (1993). 

Basicly, the effects are caused by the nucleating activity of different inhomogeneities. They can become more complicated if the second component, in particular their just created crystals, acts as a crystallization nucleating inhomogeneity. Such a - in some cases mutual - nucleating activity is hidden under usual conditions. Finally, some blends, e.g. that of PBTP and PVDF, exhibit a most complicated mutual nucleation behaviour the molten first component acts as nucleating substrate for the second one and becomes then itself nucleated by the newly created crystals of that component. 

Table 3.5 Nucleation behaviour of three palm oleins...

S. Park, and J. Lee, Nucleation behaviour of diamond particles on silicon substrates in a hot-filament chemical vapour deposition, J. Mater. Set, 28(7) 1799-1804... 

Little has yet been achieved in the way of making an explicit identification of these active nucleation sites, though materials like silver iodide are known to show heterogeneous surface effects in water vapour adsorption which may be related to their ice nucleation behaviour (Zettlemoyer et al. 1963). Other recent... 

In fact the only discrepancy between the theoretical prediction of eqn (2) and the experimental data was found to be in the preexponential factor /q. This was over 20 orders of magnitude lower than typical values for other polymers and, although no explanation for the massive discrepancy has been proposed, it must reflect the difficulty in nucleating PHB. It also confirms our earlier observation about the poor nucleation behaviour of PHB that the polymer must be cooled from the melt to a much greater extent than is usual for other materials before nucleation is initiated. 

The product I tm does not contain the quantities k,A, or N, and Scharifker suggests that it is therefore a useful diagnostic criterion for the type of nucleation behaviour. For instantaneous nucleation,... 

The recent work of Day has confirmed that this modified structure is the result of a change in the mode of growth and not the result of a change in nucleation behaviour as was earlier proposed. The coarse silicon structure of slow-cooled alloys is altered if rapid growth rates are used. A similar effect is produced by adding a modifier (sodium) to the slow-cooled alloy. These are illustrated in Fig. 5.10. The sihcon morphology is basically the same in both the growth rate-modified and the additive-modified alloys. 

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