Homogeneous nucleation, droplet formation

The number of polymer particles is the prime determinant of the rate and degree of polymerization since it appears as the first power in both Eqs. 4-5 and 4-7. The formation (and stabilization) of polymer particles by both micellar nucleation and homogeneous nucleation involves the adsorption of surfactant from the micelles, solution, and monomer droplets. The number of polymer particles that can be stabilized is dependent on the total surface area of surfactant present in the system asS, where as is the interfacial surface area occupied by a surfactant molecule and S is the total concentration of surfactant in the system (micelles, solution, monomer droplets). However, N is also directly dependent on the rate of radical generation. The quantitative dependence of N on asS and R,- has been derived as... 

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. 

The competition for oligomeric radicals also includes particles that have been created. In miniemulsion polymerizations, the nucleation of one droplet results in the formation of one particle of equal surface area. Therefore, nucleation therein has little effect on competition for radicals. This is not so with macroemulsions, since both micellar and homogeneous nucleation result in a large shift in the surface area from micelles to particles as the particles are created and grow. 

Nucleation is the science investigating the kinetics and thermodynamics of the formation of a new phase of a material at a size just sufficient to be stable. In addition to their role in new particle formation, nucleation processes are also critical to an accurate understanding of a number of other atmospheric events, including cloud droplet activation on CCN, ice formation, and the deliquescence/efflorescence of particles. In this section we focus on the nucleation of new particles through homogeneous nucleation, i.e., from gaseous precursors. The theoretical treatment of new particle nucleation, as well as field and laboratory measurements of nanoparticle formation, are addressed. 

Mixed-phase cloud processes A variety of mixed-phase and ice cloud models exist, describing the homogeneous and heterogeneous formation of water droplets and ice crystals. The implications of aerosol particles on mixed-phase clouds may be evaluated if their ice nucleating properties are known. 

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-... 

The influence of the emulsifier (SHS) concentration on Np is more pronounced in the conventional emulsion polymerization system (Rp°c[SHS]y, y= 0.68) than in mini-emulsion polymerization (y=0.25). This result is caused by the different particle formation mechanism. While homogeneous nucleation is predominant in the conventional emulsion polymerization, monomer droplets become the main locus of particle nucleation in mini-emulsion polymerization. In the latter polymerization system, most of the emulsifier molecules are adsorbed on the monomer droplet surface and, consequently, a dense droplet surface structure forms. The probability of absorption of oligomeric radicals generated in the continuous phase by the emulsifier-saturated surface of minidroplets is low as is also the particle formation rate. 

The presence of a small amount of polymer (PSt or PMMA) inside the homogenized monomer droplets reduces the sensitivity of Np to changes in [initiator]51 and the decrease of the dependency is more pronounced for the miniemulsion polymerization with PMMA (Table 2) [19, 79, 93]. Under these circumstances, the added polymer increases the lifetime of monomer droplets and the probability of monomer droplet nucleation. The ratio Np>f/Nm>i was found to be very close to 1 for the mini-emulsion polymerization with PMMA, but it is much above 1 for the run with PSt. The interaction between the polymer particle surface and emulsifier increases with increasing hydrophobicity of polymer and, thence, PSt should promote the formation of more stable monomer droplets in the preparation of the mini-emulsion. However, the reverse seems to be true for the highly diluted polymer particles with the predissolved PSt. PSt mainly locates in the monomer droplet core, whereas the more hydrophilic PMMA tends to diffuse closer to the droplet surface layer and interact with emulsifier therein. Thus, the stronger interaction of PMMA with the droplet surface in comparison with PSt makes PMMA a more efficient hydrophobe. 

The reaction order 0.56 obtained from Rp°c [SDSJ0 56 indicates the emulsifier-flooded condition (the monomer droplet surface not saturated with SDS). However, the increased coverage of the droplet surface by emulsifier is accompanied by the enhanced homogeneous nucleation, i.e., Nw increases significantly with increasing [SDSJ. Monomer droplet nucleation predominates in the particle formation process for the run with the lowest [SDSJ (4 mM). By contrast, mixed modes of particle nucleation are operative in the polymerizations with... 

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. 

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