Interface-induced nucleation

Nucleation of oil in individual emulsion droplets can be either homogeneous or heterogeneous, with the catalytic impurity distributed throughout the bulk (bulk heterogeneous nucleation) or at the oil/water interface [77-79] (surface heterogeneous nucleation). A fourth mechanism, interdroplet heterogeneous nucleation, has also been proposed [80,81]. It is the third mechanism, interface-induced nucleation, that concerns us most in this volume. 

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. 

The mini-contact test is based on homogeneous induced nucleation, which is faster than primary nucleation. However, this test does not take into account the particle size of the seed tartrate, although the importance of its effect on the crystallization rate is well known. The operative factor in this test is the surface area of the liquid/solid contact interface. Furthermore, this test defines the stability of the wine at 0°C and in its colloidal state at the time of testing. In other words, it makes no allowance for colloidal reorganization in wine, especially red wine, during aging. 

A special case of interest is reinforced polypropylene with various fibers. Often transcrystallinity in polypropylene occurs which is due to dense heterogeneous nucleation by a substrate. The occurrence of transcrystallinity depends on the type of fiber and the temperature. In contrast to transcrystallinity in quiescent crystallization, the application of stress at the interface between a fiber and a PP melt results in the crystallization of polypropylene on a row-nuclei around a fiber. This effect is caused by strain-induced nucleation via some self-nucleation mechanism and is independent of the type of fiber and less dependent on the temperature of crystallization [5,6]. Axial stress arises also during cooling of two materials with a large difference in thermal expansion coefficients. As such, the stress-induced nucleation in reinforced PP depends also on the cooling rate, fiber length, position along the fiber and viscoelastic properties of the PP melt [5]. 

Preexisting surfaces (e.g. aerosol or dust particles), ions or large polymer molecules greatly accelerate the rate of nucleation by lowering W, defined by Eq. (2). Such surfaces accomplish this by reducing the amount of work required to provide the interface in nucleation (since a surface already exists) while ions accomplish this (especially with polar molecules) by dielectric polarization so that the barrier W can be lowered to the point where a single ion can induce the formation of a macroscopic liquid drop. This represents almost the ultimate in amplification and detection. 

A difiiculty with this mechanism is the small nucleation rate predicted (1). Surfaces of a crystal with low vapor pressure have very few clusters and two-dimensional nucleation is almost impossible. Indeed, dislocation-free crystals can often remain in a metastable equilibrium with a supersaturated vapor for long periods of time. Nucleation can be induced by resorting to a vapor with a very large supersaturation, but this often has undesirable side effects. Instabilities in the interface shape result in a degradation of the quality and uniformity of crystalline material. 

Molecular recognition of crystal interfaces makes possible the control of crystal growth processes in that suitably designed auxiliary molecules aci as promoters or inhibitors of crystal nucleation inducing, for instance, the resolution of enantiomers or Ihe crystallization of desired polymorphs and crystal habits. 

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