Spherulite initiation and growth general concepts

Carefully conducted experiments have shown that it is possible for spherulites to develop sporadically in both time and space in thin polymer films.(22,23,101,102) Repetitive experiments have indicated that spheruhtes do not necessarily form in identical positions if complete melting of the sample is ensured. However, a study of poly(decamethylene terephthalate) has shown that although spherulites are formed sporadically in time, they appear in identical positions within the sample.(103) Experimental evidence showed that for this sample the spheruUtic centers are initiated from a fixed number of heterogeneities. There is a strong tendency for spherulites to appear in the same position in the field of view after successive crystallizations.(73,74,104-106) In some cases this observation is solely a result of incomplete melting.(33,36,102) In others, it can also be due to the presence of a finite number of nucleation catalysts in the polymer melt. 

It is universally observed that in the vicinity of the melting temperature the rate at which spheruhtes are generated depends very strongly on the crystallization temperature. The rate increases very rapidly as the temperature is lowered. As was pointed out earlier, the rate at which spherulite centers are generated in poly(decamethylene adipate) decreases by a factor of 10 as the crystallization temperature is raised from 67 to 72°C.(22,107) The effect is quite general and is illustrated in Fig. 9.26 for the crystallizahon of poly(hexamethylene adipamide).(108) Here the number of spheruhtes that are formed per unit volume is plotted against the rime for a series of 

With this introduction, the discussion of the temperature coefficient begins with a detailed analysis of the crystallization kinetics in the vicinity of T. It will become 

A simple, but instructive example is found in the formation of a spherical nucleus. The free energy of homogeneously forming such a nucleus from the melt is 

Here AG is the bulk free energy change per unit volume, a is the surface free energy per unit area and r is the radius of the sphere. This function is illustrated in Fig. 9.27. As the radius increases, AG initially increases until a maximum is reached at r = r. The free energy then decreases precipitously and becomes 

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