Monomer continued swollen micelles

Emulsion polymerization typically refers to the polymerization of a nonaqueous material in water. The polymerization of a water-soluble material in a nonaqueous continuum has been called inverse emulsion polymerization. The inverse emulsion polymerization technique is used to synthesize a wide range of polymers for a variety of applications such as wall paper adhesive, waste water fiocculant, additives for oil recovery fluids, and retention aids. The emulsion polymerization technique involves water-soluble polymer, usually in aqueous solution, emulsified in continuous oil phase using water in oil emulsifier. The inverse emulsion is polymerized using an oil- or water-soluble initiator. The product is a colloidal dispersion of sub-microscopic particles with particle size ranging from 0.05 to 0.3 pm. The typical water-soluble monomers used are sodium p-vinyl benzene sulfonate, sodium vinyl sulfonate, 2-sulfo ethyl acrylate, acrylic acid, and acrylamide. The preferred emulsifiers are Sorbitan monostearate and the oil phase is xylene. The proposed kinetics involve initiation in polymer swollen micelles, which results in the production of high molecular weight colloidal dispersion of water-swollen polymer particles in oil. 

Harkins calculated from the solubility of styrene in water (0.00368 mol dm at 50 °C [50]) that there are 4 x 10 molecules dm . In a 3% solution of potassium dodecanoate there are about 1 x 10 micelles dm , but with 61 molecules per micelle with an unswollen radius of 2.1 nm the cross-sectional area of the monomer-swollen micelles exceeds that of the styrene molecules by a factor of at least 12. Hence the micelles are more likely to capture initiator radicals produced in the aqueous phase. Polymerization within the micelles must be much faster than in the water because the concentration of styrene will be much the same as in bulk (8.5 mol dm ). The molar mass of the polystyrene produced is much larger than the molar mass of all the styrene molecules solubilized in a micelle thus, the monomer must be able to diffuse through the aqueous phase from other micelles and monomer droplets to allow the polymer radical to continue to grow until it is finally terminated by the entry of another initiator radical from the aqueous phase. Under the standard conditions of the mutual recipe (Table 4.1) there is 180 g water to 100 g styrene taking the emulsion droplets to have a radius of 1 pm, the ratio of the total cross-sectional areas of droplets to micelles to monomer molecules is about 1 30 2.5. The ratio of total surface areas would be even more heavily biased in favour of micelles. Hence it is probable that many more radicals will be captured from the aqueous phase by the micelles than by the emulsion droplets or than react with the monomer molecules in aqueous solution. 

The second difference lies in the structure of the initial systems. In an emulsion the monomer is located in large monomer droplets ( f l-10 / m) and in small micelles 5-10 nm) and is partially solubilized in the continuous phase. In a globular microemulsion, it is solubilized within swollen micelles of the same size = 5-10 nm). These features coupled with the dynamic character of microemulsions are the origin of the difference in the mechanisms observed in the two processes. 

Micro-emulsion polymerization In micro-emulsion polymerization, the initial system is microemulsion which consist of monomer droplets (varying from 10 to 100 nm) dispersed in water with the aid of a classical emulsifier (e.g. sodium dodecyl sulfate, SLS) and a "cosurfactant" such a low molar mass alcohol (pentanol or hexanol). Micro-emulsions are thermod3mamically stable and optically one-phase solution. There is an excessive amount of emulsifier in these emulsions. Therefore, they are concentrated systems of micelles and the micelles exist throughout the reaction. One of the most interesting aspects of these micelles is their ability to accommodate monomer molecules. Furthermore, their high total surface area relative to nucleated particles implies the monomer-swollen micelles preferentially capture primary radicals generated in the continuous aqueous phase. Then the probability... 

Monomer solubility in the continuous phase plays an important role in all of this. The rate at which oligomeric radicals grow depends on the concentration of monomer, while at the same time the solubility of the oligoradicals (which is a direct function of monomer solubility) affects how readily they self-nucleate or are captured. The partition of monomer between the disperse and continuous phases, either monomer-swollen micelles or particles, also depends on the solubility. A so-called "water-insoluble monomer will have a low concentration in the continuous phase and a high concentration in the micelles, with the result that pol3nnerlzation is slow in early stages unless micelles are present. For water-soluble monomers, micelles are unnecessary for rapid particle formation because polymerization can progress readily in the aqueous phase. 

Polymerization of 0/W microemulsions is referred to as microemulsion polymerization and that of W/0 microemulsions as inverse microemulsion polymerization. Both of them proceed in a similar manner. The radicals are produced in the continuous phase and react with the monomer dissolved in the continuous medium. Once they become surface active, they are able to enter into the monomer-swollen micelles, which become polymer particles [102,103]. 

The continuous nucleation of polymer particles during polymerization results from the very high number of monomer-swollen micelles or microdroplets. The ratio of monomer to emulsifier or the monomer concentration at the reaction loci decreases with increasing conversion. The result of these two opposing effects is the appearance of maximal rate at ca. 10 - 20% conversion. The light scattering measurements prove the presence of both the microdroplets and mixed micelles (monomer - starved microdroplets). The ratio of microdroplets to mixed micelles... 

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