Homogeneous particle nucleation

The debate as to which mechanism controls particle nucleation continues. There is strong evidence the HUFT and coagulation theories hold tme for the more water-soluble monomers. What remains at issue are the relative rates of micellar entry, homogeneous particle nucleation, and coagulative nucleation when surfactant is present at concentrations above its CMC. It is reasonable to assume each mechanism plays a role, depending on the nature and conditions of the polymerization (26). 

The basic principle of the Fitch theory is that the formation of primary particles will take place up to a point where the rate of formation of radicals in the aqueous phase is equal to the rate of disappearance of radicals by capture of radicals by particles already formed. According to the Fitch theory of homogeneous particle nucleation, the addition of emulsifier does not lead to any... 

Deriving an expression for f(t) a considerable simplification occurs if one takes all polymer particles to be nucleated at the same size dp(t,t). The generation of new polymer particles in an emulsion system is basically due to two mechanisms micellar and homogeneous particle production. Then, the rate of particle nucleation, fit), can be expressed as (12) ... 

Expressing the specific homogeneous nucleation rate constant, kh, according to (2) and substituting equation (1-4) into equation (II-6), (II-6) can finally yield a general expression for the particle nucleation rate, fit), (12,58, 9). 

The depth profiling studies suggest that two different processes govern the formation of automotive exhaust particles. The elemental surface predominance on large particles is attributed to the deposition of volatile Pb and S species (e.g. PbBrCl, SO2) onto the surfaces of refractory iron-containing particles in the automotive exhaust system (11,12). The iron-rich particles are probably derived from corrosion and ablation of the exhaust system. The smaller, more homogeneous particles may form by a nucleation process in which PbBrCl forms rather pure molten droplets when the exhaust system temperature falls below the saturation point (12). 

The size of the monomer droplets plays the key role in determining the locus of particle nucleation in emulsion and miniemulsion polymerizations. The competitive position of monomer droplets for capture of free radicals during miniemulsion polymerization is enhanced by both the increase in total droplet surface area and the decrease in the available surfactant for micelle formation or stabilization of precursors in homogeneous nucleation. 

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. 

In Stage II (referred to as Stage II to differentiate it from the classical Interval II), the rate of polymerization and the number of polymer particles continue to increase but at a slower rate. Polymer particles are formed by homogeneous nucleation as long as monomer droplets and enough emulsifier (>0.05 mM) are present in the system. The end of this stage is marked by the disappearance of monomer droplets, but particle nucleation may or may not end at this time. 

As pointed out above, particle nucleation includes all three mechanisms -micellar, homogeneous, and droplet, since these mechanisms may compete and coexist in the same system. Often one will dominate. Therefore, any general model of emulsion polymerization should include all three mechanisms. Hansen and Ugelstad [31] and Song [10] have presented probabilities for each of these mechanisms in the presence of all three. 

Claverie et al. [325] have polymerized norbornene via ROMP using a conventional emulsion polymerization route. In this case the catalyst was water-soluble. Particle nucleation was found to be primarily via homogenous nuclea-tion, and each particle in the final latex was made up of an agglomeration of smaller particles. This is probably due to the fact that, unlike in free radical polymerization with water-soluble initiators, the catalyst never entered the polymer particle. Homogeneous nucleation can lead to a less controllable process than droplet nucleation (miniemulsion polymerization). This system would not work for less strained monomers, and so, in order to use a more active (and strongly hydrophobic) catalyst, Claverie employed a modified miniemulsion process. The hydrophobic catalyst was dissolved in toluene, and subsequently, a miniemulsion was created. Monomer was added to swell the toluene droplets. Reaction rates and monomer conversion were low, presumably because of the proximity of the catalyst to the aqueous phase due to the small droplet size. 

Usually, monomer droplets are bdieved not to play any role in emulsion polymerization other than as a source of monomer. Ugelstad and associates have shown, however, that in cases with very small monomer droplets, these may become an important, or even the sole, loci for particle nucleation. The system may then be regarded as a microsuspension polymerization with water-soluble initiators. It has therefore been pointed out (Hansen and Ugelstad, 1979c) that particle nucleation mndels should include all three initiation mechanisms— micellar, homogenous, and droplet—since all these mechanisms may compete and coexist in the same system, even if one of them usually dominates. 

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