Nucleation, athermal thermal

In immiscible polymer blends with a high degree of immiscibility such as PP/PS, it has been shown that nucleation at the interface affects the crystallization behavior. Wenig et al. [1990] showed that, with increasing the amount of PS in a blend with PP, the nucleation shifted from preferentially thermal (related to the degree of undercooling) to more athermal. This was... 

The number of nuclei is assumed to be either constant (athermal nucleation) or varying with time (thermal nucleation). 

The average values n are indicative of thermal and/or athermal nucleation followed by a three-dimensional crystal growth. Indeed, for spherulitic growth and athermal nucleation, n is expected to be 3. In the case of thermal nucleation, it is expected to be 4 [2], However, complications in the Avrami analysis often arise because several assumptions, not completely applicable to polymer crystallization, are involved in the derivation. A comparison of some crystallization kinetics parameters is summarized in Table 3.5 [70-80]. 

There is also another type of primary nucleation called self-nucleation investigated first by Blundell et al. (1966). The foreign surfaces for self-nucleation are provided by crystals of the same species which survived during thermal history. Since there is no extra surface free energy change during self-nucleation, it is also called athermal nucleation. This type of nucleation is an important source of memory effects for polymer crystallization. 

As can be seen from Table 2.3, in the case of athermal nucleation the Avrami exponent equals the dimension of the geometry of the growing crystal entities 1 for fibrillar crystals, 2 for lamellar crystals, and 3 for spherulites. In the case of thermal nucleation, the Avrami exponent equals the geometry of the growing entities plus 1 (however, this is not true for diffusion-controlled processes). 

The nucleation is seldom either simple athermal or simple thermal. A mixture of the two is common. 

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