Homogeneous nucleation mechanism

The particle generation rate was calculated by a step mechanism, namely formation of primary precursor particles by homogeneous nucleation (JLQ.) followed by coagulation to latex particles (8-9). This homogeneous nucleation mechanism is often referred to as the HUFT mechanism for its originators Hansen, Ugelstad, Fitch, and Tsai. 

Emulsifier is not a necessary component for emulsion polymerization if ihe following conditions are satisfied The particles are formed by homogeneous nucleation mechanism, and the particles are stabilized by factor(s) olher than emulsifier. As to the latter, the sulfate end group that is the residue of persulfate initiator serves for stabilization of dispersion via interparticle electrorepulsive force (20). When the stabilization mechanism works well, a small number of particles grow during polymerization without aggregation, keeping the size distribution narrow. Finally stable, monodisperse, anionic particles are obtained. 

Thus in the emulsifier-free emulsion copolymerization the emulsifier (graft copolymer, etc.) is formed by copolymerization of hydrophobic with hydrophilic monomers in the aqueous phase. The ffee-emulsifier emulsion polymerization and copolymerization of hydrophilic (amphiphilic) macromonomer and hydro-phobic comonomer (such as styrene) proceeds by the homogeneous nucleation mechanism (see Scheme 1). Here the primary particles are formed by precipitation of oligomer radicals above a certain critical chain length. Such primary particles are colloidally unstable, undergoing coagulation with other primary polymer particles or, later, with premature polymer particles and polymerize very slowly. 

This particular model allows for particle nucleation to occur by either micellar or homogeneous nucleation mechanisms. The details of the mathematical development are available in the paper by Kiparissides (9). Solution of the set of differential equations (2)-(8) requires the additional iteration over the number of reactors in the train. 

Although the fly ash particle size distribution in the submicron regime is explained qualitatively by a vaporization/homogeneous nucleation mechanism, almost all of the available data indicate particles fewer in number and larger in size than predicted theoretically. Also, data on elemental size distributions in the submicron size mode are not consistent with the vapor-ization/condensation model. More nonvolatile refractory matrix elements such as A1 and Si are found in the submicron ash mode than predicted from a homogeneous nucleation mechanism. Additional research is needed to elucidate coal combustion aerosol formation mechanisms. 

Unzueta et al. [18] derived a kinetic model for the emulsion copolymerization of methyl methacrylate (MMA) and butyl acrylate (BA) employing both the micellar and homogeneous nucleation mechanisms and introducing the radical absorption efficiency factor for micelles, F, and that for particles, Fp. They compared experimental results with model predictions, where they employed the values of Fp=10 and Fn,=10", respectively, as adjustable parameters. However, they did not explain the reason why the value of Fp, is an order of magnitude smaller than the value of Fp. Sayer et al. [19] proposed a kinetic model for continuous vinyl acetate (VAc) emulsion polymerization in a pulsed... 

When a fat is emulsified, nucleation is substantially altered compared with the same fat in bulk liquid form. This is primarily because of the distribution of heterogeneous nucleation sites among the emulsion droplets. If there are more droplets than heterogeneous nucleation sites, then some of the droplets will nucleate by a homogeneous nucleation mechanism. That is, as a finely dispersed emulsified system is cooled, one population of droplets nucleates at relatively higher temperatures because of heterogeneous nucleation, whereas another population nucleates at substantially lower temperature because of homogeneous nucleation. 

It seems therefore that little or no stability is to be expected for the point defect aggregates which provide the necessary shear-plane precursors in the homogeneous shear-plane formation mechanisms. These homogeneous nucleation mechanisms are therefore unlikely to operate, and we turn our attention now to a heterogeneous mechanism, in which point defects aggregate at pre-existing planar-defect sites. 

In spite of these studies and results, the relative importance of the gas-phase nucleation compared to the surface nucleation is unclear as yet. In fact, the number of diamond particles collected from the gas phase is very small compared to the typical surface nucleation densities, thus the homogeneous nucleation mechanism cannot account for the large variety of nucleation densities observed on different substrate materials and from different surface pretreatments. It is speculated and also supported by a recent experimentl l that the nuclei formed in the gas phase may reach the growing surface and increase the surface nucleation density. However, how the diamond particles formed in the gas phase could serve as seeds on the substrate surface for the subsequent growth of a diamond film remains unknown. 

Figure 6,3 Empirical dependence of induction period of Ni(NH4)2(S04)2 6H2O at 25 °C on relative supersaturation in logarithmic coordinates (after Sohnel and Mullin 1979) Region 1—predominant heterogeneous nucleation mechanism Region III—predominant homogeneous nucleation mechanism, and Region II—possible coexistence of both nucleation mechanisms.

Figure 7 Mass of water (g) likely to contain one critical nucleus, as a function of temperature, assuming a homogeneous nucleation mechanism. Reproduced from Franks, with permission from Cambridge University Press...

The kinetics of inverse emulsion polymerization can be classified more or less arbitrarily into two subclasses according to the solubility of the initiator. Note that the solubility in water of oil-soluble initiators was found to be oihanced (up to a factor of 3) by the presence of monomo [23,28,29], making possible initiation by radical pairs formed in the aqueous dispersed phase. On the other hand, homogeneous nucleation mechanisms generating oligoradicals in the continuous phase can also be operative in inverse emulsions due to the maiginal solubility of acrylamide in organic media (1.6 wt% in isoparaffinic solvents and 2 wt% in toluene) [3]. 

Summarizing, the biosynthesis of PHB can be understood in terms of the kinetic processes of initiation, propagation and chain transfer and the affect of latex particle size on these. Rirther, the colloidal aspects of the formation of PHB latex particles can be explained by the homogeneous nucleation mechanism, well known in conventional emulsion polymerization processes. 

The template behavior of the nanodispersed structure in the course of the foaming process was confirmed by Nemoto et al. [89] in the case of nanostructured polymer blends with extremely high differences in CO2 solubility and viscoelastic properties of both phases (PP and rubber). In these blends the homogeneous nucleation mechanism inside the PP matrix can be considered completely hindered. This kind of behavior can be applied to amorphons polymers (eg, PMMA, PS) combined with ideal nucleant nanostructures. 

Figure 9.30 (a) Evidences about the homogeneous nucleation mechanism in neat poly(methyl methacrylate) (PMMA) and (b) evidences about the heterogeneous nucleation mechanism in 90/10 PMMA/MAM blends. SEM, scanning electron microscopy RT, room temperature. 

The homogeneous nucleation mechanism described above resembles in every way La Mer s self-nucleation process. Hence, we can consider the monomer as a precursor, playing the same role as the sodium thiosulfate, whilst the growing chain in the aqueous phase corresponds to the nucleating species (in that case, sulfur). Indeed, R. Fitch referred to La Mer s work when he proposed the mechanism in 1965, to describe emulsion polymerisations carried out with no micelle. 

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