Nucleation athermal

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... 

Experimental time-temperature-transformation (TXT) diagram for Ti-Mo. Xhe start and finish times of the isothermal precipitation reaction vary with temperature as a result of the temperature dependence of the nucleation and growth processes. Precipitation is complete, at any temperature, when the equilibrium fraction of a is established in accordance with the lever rule. Xhe solid horizontal line represents the athermal (or nonthermally activated) martensitic transformation that occurs when the p phase is quenched. 

From the slope on the right graph of Fig. 99 an Avrami exponent of 3.2 results, close to the value expected for athermal nucleation followed by sphemlitic growth, but because of the many assumptions that went into the derivation of the Avrami equation still not proven without a detailed structural analysis. 

For athermal nucleation (self-seeding nucleatiOTi as a special case of heterogeneous nucleation), the number of nuclei is fixed. Thus the number of spherulites is fixed, with a density mlV, = Anr 3, r = vt, and then... 

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]. 

Promotion of a number of sub critical clusters to critical nuclei without growth, caused by reduction of critical cluster volume in time, is athermal nucleation. The concept of athermal nucleation, introduced by Fisher and Turnbull [43], consists in changing thermodynamic criterion of cluster stability. General expression of athermal nucleation in the systems with time-dependent thermodynamic parameters was derived in [21,45]. Angular distribution of athermal nucleation in the transient system is proportional to the distribution of critical clusters and the time derivative of the critical cluster volume... 

The flux of athermal nucleation is positive when associated with a reduction of the critical cluster volume in time due to transient elastic potential of chain deformation and orientation of the chain segments. Fast changes in the critical cluster size may result in athermal nucleation. 

Contribution of the athermal nucleation in the transient system can be characterised by the ratio of the athermal flux to the growth flux. With transient effects in the elastic free energy scaled with the chain relaxation time, contribution of the athermal nucleation is of the order of of the growth flux... 

Athermal nucleation is induced by time-dependent chain deformation during chain relaxation, tending to an equiUbrium steady-state conformations under the flow and depends on the orientation angle. 

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