Emulsion polymerization. See

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

The reaction described in this example is carried out in miniemulsion.Miniemulsions are dispersions of critically stabilized oil droplets with a size between 50 and 500 nm prepared by shearing a system containing oil, water,a surfactant and a hydrophobe. In contrast to the classical emulsion polymerization (see 5ect. 2.2.4.2), here the polymerization starts and proceeds directly within the preformed micellar "nanoreactors" (= monomer droplets).This means that the droplets have to become the primary locus of the nucleation of the polymer reaction. With the concept of "nanoreactors" one can take advantage of a potential thermodynamic control for the design of nanoparticles. Polymerizations in such miniemulsions, when carefully prepared, result in latex particles which have about the same size as the initial droplets.The polymerization of miniemulsions extends the possibilities of the widely applied emulsion polymerization and provides advantages with respect to copolymerization reactions of monomers with different polarity, incorporation of hydrophobic materials, or with respect to the stability of the formed latexes. 

These superabsorbents are synthesized via free radical polymerization of acrylic acid or its salts in presence of a crosslinker (crosslinking copolymerization). Initiators are commonly used, water-soluble compounds (e.g., peroxodi-sulfates, redox systems). As crosslinking comonomers bis-methacrylates or N,hT-methylenebis-(acrylamide) are mostly applied. The copolymerization can be carried out in aqueous solution (see Example 5-11 or as dispersion of aqueous drops in a hydrocarbon (inverse emulsion polymerization, see Sect. 2.2.4.2). 

Large volumes of soap are used in industrial applications as gelling agents lor kerosene, paint driers, and as surfactants in emulsion polymerization. See also Soaps. Concern over water eutrophication resulted in a ban of phosphorus in laundry detergents. Phosphates have been effectively replaced by combinations of zeolite, citrate, and polymers, coupled with rebalanced synthetic active systems. Soap itself is generally present only as a minor component of surfactants. 

American scientists had learned a little about Buna and Buna-S that the Germans had developed. Building on this rudimentary knowledge, a procedure was worked out to produce GR-S by emulsion polymerization (see Chapter 5). 

Somewhat surprisingly. Increasing the concentration of AIBN did not decrease the polymer molecular weight, as Is observed In conventional emulsion polymerizations (see Figure 9). This type of behavior has been observed In mlcroemulslon polymerizations (M) f and has been attributed to the difficulty of the initiator to penetrate the Interface because of the larger amount of surfactant In mlcroemulslon systems than In conventional emulsion polymerizations. 

The earliest mathematical model of emulsion polymerization was that of Smith and Ewart [1-3], and was based on the Harkins [4-6] mechanistic understanding of emulsion polymerization (see Sections 4.3 and 4.4). This model was applicable only to a batch polymerization in which all formulation components were added at the start of the reaction. Modifications to this basic understanding were made by Gardon [7] in his model. A good review of emulsion polymerization modelling is provided by Penlidis et al. [8]. 

Using KPS, the rate of polymerization and the nuniber of particles were found to increase with the 0.33 and 037 powers respectively, of the initial inidator concentration. These values are not far fiom the 0.4 power predicted by Smith and Ewart for conventional emulsion polymerization (see Section 4.9) and are within the range reported experimentally. Similar values were more recently reported by Miller et al. (namely, oc [I]° , Ap oc [1]° ) [14]. However, significant differences exist between these two efforts. The resulting particle... 

If monomer, initiator, surfactant and water are the only reactants (so-called ab initio emulsion polymerization), it is possible for particle nucleation and particle growth to proceed simultaneously for a significant period of the reaction, especially if the initiator concentration is low and the surfactant concentration is high. However, in most instances, the particle nucleation period in an nb initio batch process is short, thereby giving rise to distinct nucleation and growth periods. In such situations the conversion-time curves take on the classic shape showing the three intervals of emulsion polymerization (see Section 2.4. and Section 4.3 and Figure 4.3). 

The styrene is transferred from the storage tank into a water-cooled reaction vessel. Following the addition of water and additives, it is polymerized to make PS while being mixed constantly (emulsion polymerization, see Table 2). The reaction mixture is then drained into a cooling vat, from where it enters a centrifuge in which water, residual monomer and additives are separated out. The pure PS, obtained in powder form, then requires only drying. 

In liquid-liquid two-phase systems, the solubility characteristics of the initiator are important. The distribution of initiator between oil and water phases should be such that it is predominantly present in the phase where the polymerizable function is located. The polymerization depicted in Figure 17.24 is best served with a water-soluble initiator, whereas that of Figure 17.25 proceeds best with an oil-soluble one. In the most important application of polymerizable surfactants, that of emulsion polymerization (see below), water soluble initiators, such as potassium persulfate and hydrophilic azo compounds, are used. 

Latices can be prepared by emulsion polymerization (see also Section 20.6.5) or by the subsequent dispersing of polymer solutions or melts in water. These latices have very high solids contents up to 74% in the case of perfect spheres and up to 80% with imperfect spheres. In addition, aqueous dispersions can be economically produced. Water is a nontoxic and non-inflammable solvent. These advantages face the disadvantage that water can only be removed slowly and in a relatively uncontrolled manner and the retained water unfavorably influences the polymer properties. 

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