Kinetics factors influencing crystallization

Since interactions at the molecular level between polymer components in the blends occur only in the amorphous phase, it is reasonable to assume that these effects are due to kinetic factors and, in particular, to the influence of a polymer component on the nucleation or crystallization kinetics of the other one. 

Although the basic principle and procedure of diastereomeric resolution are not difficult to understand, the chiral discrimination mechanism involved in the selective crystallization of one diastereomer from the mixture is very complicated. The chiral discrimination mechanism for diastereomeric resolution changes in accord with the resolving system, since not only the properties of diastereomeric crystals but also the conditions for crystallization strongly influence the chiral discrimination mechanism. In particular, the polymorphism of crystal, the severe solvent effect on solubility, and the kinetic factor for crystal growth are still not perfectly understood regarding this chiral discrimination phenomenon. The study is therefore limited in its investigation of the chiral discrimination mechanism for the diastereomeric resolution, as the mechanism involves both the crystal and solution properties of diastereomers.7... 

To truly control crystallization to give the desired crystalline microstructure requires an advanced knowledge of both the equilibrium phase behavior and the kinetics of nucleation and growth. The phase behavior of the particular mixture of TAG in a lipid system controls both the driving force for crystallization and the ultimate phase volume (solid fat content) of the solidified fat. The crystallization kinetics determines the number, size, polymorph, and shape of crystals that are formed as well as the network interactions among the various crystalline elements. There are numerous factors that influence both the phase behavior and the crystallization kinetics, and the effects of these parameters must be understood to control lipid crystallization. 

It is essential to clarify the crystallization kinetics of magnesium hydroxide and to investigate the influence of various factors on the reactive crystallization characteristics of magnesium hydroxide for better understanding of its crystallization process. The aims of this study are to discuss the followings ... 

The initiation of the polymerization with y-radiation from a Co source was studied by Usmanov et al. [460,484,485]. Polymerization was carried out by irradiation of the monomer, in both liquid and gaseous phase, with the use of y-rays at 38 °C. The dose rates were lOrad/s and 0.5Mrad/s. Impurities such as acetylene greatly inhibited polymerization. Oxygen influenced the kinetics, a factor that confirms a free-radical mechanism. Liquids such as difluoroethane, benzene, and carbon tetrachloride reduced the polymerization rate and caused low-molar-mass polymer. On studying the thermal behavior of PVF it was found that the polymers obtained by y-ray initiation in bulk were the most crystallized and had the lowest degree of irregularity in the polymer chain. Nearly no branches were found in contrast to the chemically initiated polymers produced in suspension [482,484]. 

In this chapter, we will briefly describe experimental measurements and computer modeling of the overall crystallization kinetics. Furthermore, we will review the overall crystallization theories including the recent developments. Factors influencing the overall crystallization kinetic and reasons for discrepancies between the theories and experimental results will be also addressed. 

A key factor in governing the extent of co-crystallization in binary polyethylene blends is the closeness of the crystallization rates of each of the components.(50-52) The difference in rate diminishes with increasing concentration of the linear component in the blend. The amount of co-crystallization is favored by lower isothermal crystallization and is maximized by rapid, quenched crystallization conditions. Blend composition and molecular structure of the components also have an influence on co-crystallization. The different results that can be obtained for different modes of crystallization have important implications for the morphology and properties in the solid state. Therefore, to gain further insight it should be fruitful to examine and analyze the crystallization kinetics of such blends in more detail. ... 

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