Nucleating ability

Nucleation in a cloud chamber is an important experimental tool to understand nucleation processes. Such nucleation by ions can arise in atmospheric physics theoretical analysis has been made [62, 63] and there are interesting differences in the nucleating ability of positive and negative ions [64]. In water vapor, it appears that the full heat of solvation of an ion is approached after only 5-10 water molecules have associated with... 

A comparable folding mechanism was found in silks. A seminal study by Li et al. (2001) found that in vitro formation of silk fibrils is conformation dependent and occurred via a nucleation mechanism. Although now established as amyloidogenic (Kenney et al., 2002), the nature of the silk fibril assembly remains unclear. Noteworthy is the evidence for a cross-nucleation ability of silk proteins, supporting the amyloidogenicity of silk (Lundmark et al., 2005). 

It is not only sodium salts of monocarboxylic acids that are effective nucleating agents for PET lithium, calcium and barium salts of monocarboxylic acids have also been found to impart nucleation ability [33],... 

According to Cimmino et al. [25], OM measurements show that in the melt (340 °C) demixing of PVME occurs at all compositions, and during crystallization segregated PVME particles remain occluded in the interspherulitic region of sPS crystals. At the end of the crystallization process, an increase in the spherulite dimensions is observed, indicating that PVME addition reduces the nucleation ability of sPS. 

With some types of glass, crystallization takes place more readily at the surface than within the glass. A crystalline layer is then formed at the surface while the internal zones of glass remain free from crystals as a result of a poor nucleating ability. This preferential surface crystallization may result from a lower content of volatile components in the glass surface or on the contrary from enrichment of the surface with components reducing surface tension, from surface contamination, etc. 

Figure 10.6 gives a summary of data on different fillers in UHMWPE. The total degree of crystallinity, as determined by the enthalpy of crystallization, increases with filler concentrations up to 40-50% and then gradually decreases. This decrease is caused by filler aggregation which decreases its nucleation ability. 

A small but important class of atmospheric aerosol particles are the ice nuclei. These nuclei promote the freezing of water drops in clouds (see Fletcher, 1962). In this way they play a definite role in the formation of precipitation in mixed clouds containing both water drops and ice crystals. This kind of precipitation formation is due to the fact that the saturation vapour pressure over ice is smaller than over liquid water. In this way ice crystals grow by condensation while drops tend to evaporate. Thus, if human activity emits ice nuclei to the atmosphere the precipitation distribution can be modified. Results of measurements show that in the vicinity of steel works and aluminum foundries the concentration of ice nuclei active at a temperature of — 20 °C is unusually high. It is believed that this is caused by the presence of some metal oxides in the air (Pruppacher, 1973). More recent studies on ice nuclei also showed that lead compounds (e.g. Pbl 2) in exhaust gases of vehicles also have ice nucleating ability. It is believed, however, that anthropogenic ice nuclei cannot play an important role, except in local scale processes (see Pruppacher, 1973). 

The environmental implications of biological ice nucleation are still a subject of intensive research. For instance, ice-nucleating ability has been observed in spider web fibroin, but it is uncertain whether this process has any ecological consequences. Unfortunately, no agreement has yet been reached on nomenclature. Care is required to distinguish between terms such as nucleation temperature , kinetic freezing point , supercooling point , and their multifarious interpretations. ... 

Formation of ice particles in the absence of IN is possible only at very low temperatures, below —40°C (Hobbs, 1995). The presence of IN allows ice formation at higher temperatures. Aerosols that can serve as IN are rather different from those that serve as CCN. Ice-forming nuclei are usually insoluble in water and have chemical bonding and crystallographic structures similar to ice. Larger particles are more efficient than smaller ones. While our understanding of the ice nucleating abilities of aerosols remains incomplete, particles that are known to serve as IN include dust particles (especially clay particles such as... 

Schaefer (1966) reported the activation of large numbers of ice nuclei on the addition of trace levels of iodine vapor to car exhaust (containing lead oxide nanoparticles) at temperatures from —3 to —20°C in the laboratory. The formation of lead iodide was concluded to have a seeding effect similar to that of silver iodide particles (Vonnegut, 1947), which had been used in an attempt to artificially modify cloud properties and enhance precipitation. Consequendy this method was proposed as a means to remove harmful aerosol formed in polluted urban areas, and also in artificial weather modification. However, the development of unleaded fuels, for which no similar ice nucleating ability was shown to occur in the presence of iodine (Hogan, 1967), provided a better long-term solution to this problem. 

Organic pigments commonly used for the coloration of polypropylene fibres exhibit nucleating ability toward polypropylene crystallisation. The rough surface of pigments ... 

By further increment of the take-up velocity the nucleation ability of pigments does not influence the polypropylene crystallisation. Inside fibres under high molecular orientation numerous row nuclei are formed. The crystallisation proceeds on row nuclei without... 

Broda, J. (2003d). Structure of polypropylene fibres coloured with a mixture of pigments with different nucleating ability. Polymer Vol.44, Issue 22, pp 6943-6949. 

Kawamoto, N., Sakai, A., Horikoshi, T. et al. (2007) Nucleating agent for poly(L-lactic acid)-An optimization of chemical structure of hydrazide compound for advanced nucleation ability. Journal of Applied Polymer Science, 103, 198-203. 

Assouline, E. et al. Nucleation ability of multiwall carbon nanombes in polypropylene composites. J. Polym. Sci. Part B Polym. Phys. 41, 520-527 (2003). 

Further, this chapter provides a survey about the formation of a transciystalline layer in the composite system. The occurrence of transcrystallization was found to strongly depend on the type of chemical treatment of the fiber surface. Predominant nucleation ability was found for unmodified fibers. However, chemical modification of fiber surface slightly depressed the nucleation of polypropylene matrixes. 

In thermoplastic/fiber systems, the fiber may act as a series of nucleating sites for the polymer, resulting in a transcrystaUine region around the fiber. In this chapter, a large number of factors have been demonstrated to contribute to the interactions across the interface. The effect of various conditions such as the chemical modification of natural fibers and nucleation ability of semicrystalUne matrix on the formation of transcrystallinity was investigated. 

Results of many researches confirm the presence of TCL as the effect of nucleation ability of different substances. Among the factors inducing transcrystallization in polyolefin matrix, the following fillers can be listed ... 

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