Monomer droplet nucleation

However, in the case of mini- and microemulsions, processing methods reduce the size of the monomer droplets close to the size of the micelle, leading to significant particle nucleation in the monomer droplets (17). Intense agitation, cosurfactant, and dilution are used to reduce monomer droplet size. Additives like cetyl alcohol are used to retard the diffusion of monomer from the droplets to the micelles, in order to further promote monomer droplet nucleation (18). The benefits of miniemulsions include faster reaction rates (19), improved shear stabiHty, and the control of particle size distributions to produce high soHds latices (20). 

Mouran et al. [105] polymerized miniemulsions of methyl methacrylate with sodium lauryl sulfate as the surfactant and dodecyl mercaptan (DDM) as the costabilizer. The emulsions were of a droplet size range common to miniemulsions and exhibited long-term stability (of greater than three months). Results indicate that DDM retards Ostwald ripening and allows the production of stable miniemulsions. When these emulsions were initiated, particle formation occurred predominantly via monomer droplet nucleation. The rate of polymerization, monomer droplet size, polymer particle size, molecular weight of the polymer, and the effect of initiator concentration on the number of particles all varied systematically in ways that indicated predominant droplet nucleation. 

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. 

These data indicate that the increase in Np f with [SPS] is much more pronounced by homogeneous nucleation in comparison with monomer droplet nucleation. The dependencies of Rp and Np f on [SPS] indicate that the increased Rp... 

Monomer droplet nucleation plays an important role in both the DMA (1) and SM A (2) containing mini-emulsion polymerization of St initiated by AIBN [116]. On the other hand, increasing [SPS] promotes the homogeneous nucleation in the St mini-emulsion polymerization system (see above). For example, for the SMA series, the degree of homogeneous nucleation decreases in the series ... 

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... 

A common feature of the emulsion polymerization of allyl methacrylate (AMA) is the slow rise to a maximal Rp and two rate regions [119]. The increase of Rp is ascribed to the increased number of latex particles, while the decrease of Rp is attributed to the decreased monomer concentration at the reaction loci. The slow accumulation of polymer in the monomer droplets (due to the very low reactivity of AMA) retards the monomer droplet degradation and the transfer of monomer from monomer droplets to the reaction loci but enhances the monomer droplet nucleation. Rp/Np increases with increasing particle size. The slight increase in Rp with increasing [SDS] is due to the high coverage of the droplet surface by SDS. 

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 [37-39] suggested that all the micellar nucleation, homogeneous nucleation and monomer droplet nucleation were operative in emulsion polymerization with a concentration of surfactant greater than its CMC. This indicates that monomer-swollen micelles and particle nuclei and emulsified monomer droplets compete with one another for the incoming ohgomeric radicals from the continuous aqueous phase. Thus, the total rate of particle nucleation is given by... 

Poehlein [40] summarized previous work and proposed a comprehensive particle nucleation mechanism involved in a persulfate initiated emulsion polymerization system, as shown schematically in Figure 3.5. Song and Poehlein [41, 42] developed a general kinetic model taking into account micellar nucleation, homogeneous nucleation, and monomer droplet nucleation in emulsion polymerization. The chain transfer and termination reactions occurring in the continuous aqueous phase, capture of oligomeric radicals by particle nuclei, and flocculation of particle nuclei were also incorporated into the model development. The resultant expressions for calculation of the rate of particle nucleation can be written as... 

The most important characteristic of miniemulsion polymerization is the transformation of the homogenized monomer droplets into latex particles via the capture of free radicals when a water-soluble initiator such as the persulfate initiator is used to initiate the free radical polymer reactions. However, this feature does not necessarily guarantee that the particle nucleation mechanisms other than monomer droplet nucleation can be ruled out. As will be shown later, previous studies dealing with nucleation of particle nuclei in miniemulsion polymerization often resulted in controversial conclusions. This subject is still open to discussion, and it represents a great challenge to polymer scientists. 

---