Formation of Particle Nuclei in the Continuous Aqueous Phase

1 Formation of Particle Nuclei in the Continuous Aqueous Phase 

Roe [26] pointed out that the derivation of Eqs. (3.3) and (3.5) did not require the assumption of particle nucleation in monomer-swollen micelles. With the assumption that particle nuclei formed outside the micelles and particle nude- 

The above ideas were then incorporated into the following kinetic model developed by Fitch and Tsai [28-30]  

The principle behind the homogeneous nucleation mechanism [see Eq. (3.16)] is that the rate of generation of particle embryos is primarily governed 

The collision theory [15] and the concept of the limited flocculation process were originally used to calculate the values of Rc and Rf, respectively. Later, the diffusion theory [17,18] was adopted by Fitch to calculate the value of Rc [31]. It was pointed out that the concentration of oligomeric radicals with chain length of j in the continuous aqueous phase, which was required to carry out the calculation, was very difficult to be determined. 

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Other than micellar nucleation, many mechanisms have been proposed to explain the particle nucleation stage. The best-known alternative theory for particle nucleation is that of "homogeneous nucleation" which includes the formation of particle nuclei in the continuous aqueous phase. This theory is proposed by Priest, Roe and Fitch and Tsai, and extended by Hansen and Ugelstad (HUFT) describes the emulsion polymerization of water-solubble monomers such as vinyl acetate and acrylonitrile, their water solubility though low (< 3%) is much in excess of the amount of monomer which may be solubilized by the emulsifier [43-48]. It is also the only mechanism which can apply to monomers of low water-solubility, such as styrene, in emulsifier-free reaction system, and also in reaction system which contain a micellizing emulsifier but at such a concentration that is below the CMC. When the monomers are somewhat soluble in the continuous phase, emulsifier micelles have little influence on particle formation. Emulsifier may be required, however, to ensure colloidal stability of the product as it is formed and subsequently "on the shell". 

Increasing the persulfate initiator concentration in miniemulsion polymerization promotes the formation of particle nuclei in the continuous aqueous phase (homogeneous nucleation) [36,43]. Both the number of latex particles originating from monomer droplet nucleation and the number of water-borne particles increase with increasing the surfactant concentration. It is noteworthy that particle nuclei formed via the micellar nucleation mechanism as expected may become important if the surfactant concentration is well above its critical micelle concentration [44,45]. 

The influence of the type of initiators (sodium persulfate versus 2,2 -azobisisobutyronitrUe) on the particle nucleation mechanisms and kinetics involved in the styrene miniemulsion polymerizations has also been studied [39]. As expected, the oil-soluble 2,2 -azobisisobutyronitrile promotes nucleation in the homogenized monomer droplets. On the other hand, formation of particle nuclei in the continuous aqueous phase becomes more important when water-soluble sodium persulfate is used. This result is consistent with the vinyl chloride work of Saethre et al. [44]. The number of polyvinyl chloride latex particles generated by mechanisms other than monomer droplet... 

Based on the above reaction scheme and the assumptions that (a) a monomer-swollen micelle can be successfully converted into a particle nucleus via the capture of a free radical in the continuous aqueous phase, (b) the volumetric growth rate for particle nuclei (p = dvpidt, where Vp is the volume of a particle nucleus) is constant, (c) desorption of free radicals out of the particle does not occur, and (d) the amount of surfactant molecules dissolved in the continuous aqueous phase and adsorbed on the monomer droplet surfaces is insignificant, the rate of formation of particle nuclei is then equal to the rate of generation of free radicals in the continuous aqueous phase. 

Figure 3.5. Paths for the formation of particle nuclei starting from persulfate initiator radicals generated in the continuous aqueous phase. The symbols M and S represent monomer and surfactant species, respectively.

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