Heterogeneous particle nucleation

The average volume of dispersed PBTP particles in the PVDF/PBTP = 85/15 blend after the first and the fourth extrusion cycles amounts to about 4pm and 5x10 pm, respectively. By the same arguments as above one has to conclude from the fractionation of the crystallization of the dispersed PBTP droplets even in the one time extruded blend that the number density of the heterogeneities which nucleate PBTP above 148°C is below about 0.2pm . ... 

Klemmer and Jungnickel [1984] have reported on the fractionated crystallization of POM in an HOPE matrix. They found an additional crystallization peak of POM to occur 14°C lower than the bulk crystallization peak. This was attributed to the fractionated crystalhzation of POM, caused by an interface-induced additional inhomogeneous nucleation and crystallization. It was shown that this phenomenon only occurs in those blends where the number of the dispersed particles was higher than the number of available heterogeneous particles. Moreover, the preparation method clearly influenced the fractionation due to the change of the particle sizes - fractionated crystallization has been observed only in melt-mixed blends. 

During nucleation, monomer droplets, monomer swollen micelles and monomer swollen polymer particles coexist in the batch reactor. Polymer particles efficiently compete for radicals and as their number increases, they become the main polymerization lod. The monomer that is consumed by free-radical polymerization in the polymer particles is replaced by monomer that diffuses from the monomer droplets through the aqueous phase. Therefore, the size of the particles increases and that of the monomer droplets decreases. The number of micelles decreases because they become polymer particles upon entry of a radical, and also because they are destroyed to provide surfactant to stabilize both the polymer chains that precipitate in the aqueous phase and the increasing surface area of the growing polymer particles. After some time, all micelles disappear. This is considered to be the end of the nucleation and only limited formation of new particles may occur after this point because heterogeneous nucleation is not possible and there is no free surfactant available in the system to stabilize the particles formed by homogeneous nucleation. The stage of the batch emulsion polymerization in which particle nucleation occurs is called Interval I [24,29]. At the end of Interval I, which typically occurs at a monomer conversion... 

Particle nucleation may occur through both heterogeneous nucleation and homogeneous nucleation (Figure 6.2). The rate of particle formation depends on the nucleation mechanism. 

Surface active substances deactivate heterogeneous particles and thus increase the width of the metastable region (165,183]. The extent of this action Is given by the amount and catalytic activity of foreign partKdes. An opposite Influence [203.204] can be explained by the fact that surface active substances decrease the surface energy so that the nucleation rate can Increase. The shape of the curve of nucleation rate vs. the admixture concentration resembles the adsorption Isotherms of surface active substances on solid surfaces so that there may be expected a direct link of the nucleation rate rise with the adsorption of the admixture on the surface. 

---