Micellar particle nucleation

Hansen FK, Ugelstad J. The effect of desorption in micellar particle nucleation in emulsion polymerization. Makromol Chem 1979 180 2423-2434. 

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). 

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) ... 

The rate of polymerization was found to be independent of emulsifier concentration around CMC (1.8x10 4mol dm 3) and up to ca. 10 3 mol dm 3 and then strongly increased with increasing emulsifier concentration (Fig. 5). It can be seen that, for this system, the break in the dependence of the rate on surfactant concentration does not coincide with the CMC of either the surfactant or the surfactant/PEO-MA macromonomer. In fact, these two values are identical at room temperature at 50 °C the CMC of the surfactant is lower than at 20 °C. The kinetics of particle nucleation for the present nonionic polymerization of BA may not follow a micellar mechanism. 

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. 

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. 

Their recipe of polymerization is reproduced in Table I, and Figure 1 shows the rate of polymerization, Rp, plotted against SLS concentration in the mixed surfactants. In that plot, the concentration of SLS was expressed in parts of SLS/5-parts BC-840/100 parts styrene. Based on this study, Kamath ( 8) proposed that particle-nucleation in his system was controlled by micellar nucleation mechanism following Harkins and Smith-Ewart s theory. 

Experimental results and interpretations so far presented lead to the justification of proposing a hypothesis concerning the micellar size effect on particle nucleation in mixed surfactant systems of emulsion polymerization. Essentials of this hypothesis are as follows ... 

The present study thus supports a theory of particle nucleation where micelles do play a vital part, it does not necessarily support or negate micellar initiation. 

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. 

In mini-emulsion polymerization, the particle nucleation mechanism may be evaluated by the ratio of the final number of polymer particles to the initial number of monomer droplets (Np f/Nm i). If the particle nucleation process is primarily governed by entry of radicals into the droplets, then the value of Np>f/Nm>i should be around 1. A lower value of Np f/Nm i may imply incomplete droplet nucleation or coalescence. On the other hand, a higher value of Npf/Nm>i may indicate that the influence of micellar or homogeneous nucleation comes into play in the particle formation process, since one droplet feeds monomer to more than one micelle in the classical emulsion polymerization. For pure micel-... 

One of the key features of this reaction mechanism is the particle nucleation beyond the first maximal Rp. Furthermore, the disappearance of monomer droplets in the conventional emulsion polymerization at ca. 30-50% conversion results from the transfer of monomer from the monomer droplets to the locus of polymerization (polymer particles). In mini-emulsion polymerization, the concentration of monomer decreases and monomer droplets may exist throughout the polymerization. In the context of the micellar nucleation model,both Np and the concentration of monomer in the particles in the constant reaction rate region contribute to Rp. Therefore, the first maximal Rp observed in the course of mini-emulsion polymerization does not necessarily correspond to the end of particle nucleation. This is because Np may increase in the course of polymerization, but the contribution of the increased Np to Rp can be outweighed by the decreased monomer concentration in the reaction loci. 

The ultrasonification process is connected with the rapidly increased oil-water interfacial area as well as the significant re-organization of the droplet clusters or droplet surface layer. This may lead to the formation of additional water-oil interface (inverse micelles) and, thereby, decrease the amount of free emulsifier in the reaction medium. This is supposed to be more pronounced in the systems with non-ionic emulsifier. Furthermore, the high-oil solubility of non-ionic emulsifier and the continuous release of non-micellar emulsifier during polymerization influence the particle nucleation and polymerization kinetics by a complex way. For example, the hairy particles stabilized by non-ionic emulsifier (electrosteric or steric stabilization) enhance the barrier for entering radicals and differ from the polymer particles stabilized by ionic emulsifier. The hydro-phobic non-ionic emulsifier (at high temperature) can act as hydrophobe. 

Formation of latex particles can proceed via the micellar nucleation, homogeneous nucleation and monomer droplet nucleation. The contribution of each particle nucleation mechanism to the whole particle formation process is a complex function of the reaction conditions and the type of reactants. There are various direct and indirect approaches to determine the particle nucleation mechanism involved. These include the variations of the kinetic, colloidal and molecular weight parameters with the concentration and type of initiator and emulsifier. There are some other approaches, such as the dye method where the latex particles generated via homogeneous nucleation do not contribute to the amount of dye detected in the latex particles since diffusion of the extremely hydrophobic dye molecules from the monomer droplets to the latex particles generated in water is prohibited. On the contrary, nucleation of the dye containing monomer droplets leads to the direct incorporation of dye into the polymer product. However, the dye also act as a hydrophobe and enhances the stability of monomer droplets as well as the monomer droplet nucleation. 

Hansen and Ugelstad [37,38] have discussed the effect of desorption and reabsoiptitm of radicals on particle number for both homogenous and micellar nucleation, applying a steady-state treatment to describe the fate of the radicals formed from the initiator and those formed by chain transfer to monomer. The ect of desorption of monomer radicals on micellar particle nircleation was demonstrated in a simplified treatment. The rate of micellar particle formation was expressed by... 

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