Micelles during dispersion polymerization

The surfactant is initially distributed through three different locations dissolved as individual molecules or ions in the aqueous phase, at the surface of the monomer drops, and as micelles. The latter category holds most of the surfactant. Likewise, the monomer is located in three places. Some monomer is present as individual molecules dissolved in the water. Some monomer diffuses into the oily interior of the micelle, where its concentration is much greater than in the aqueous phase. This process is called solubilization. The third site of monomer is in the dispersed droplets themselves. Most of the monomer is located in the latter, since these drops are much larger, although far less abundant, than the micelles. Figure 6.10 is a schematic illustration of this state of affairs during emulsion polymerization. 

One of the fascinating application of macromonomers is in the field of dispersion polymerization. The dispersion polymerization in the presence of suitable stabilizers affords mostly monodisperse submicron- and micron-sized microspheres (particles). The macromonomers are graft-copolymerizaed during copolymerization in the continuous phase and so accumulate on the particle surface, so that the resulting particles are effectively sterically stabilized against flocculation. Amphiphilic copolymers s mthesized by copolymerization of a hydrophobic conventional monomer with a hydrophilic macromonomer and vice verse present all the typical properties of conventional surfactants. They aggregate between themselves and form a micelle in the aqueous or non-aqueous media. The conformation of a micelle formed by PEO-g-PSt polymer in the aqueous medium consists of a hydrophobic PSt core and a hydrophilic PEO shell (Fig. 10). 

The rapid development of biotechnology during the 1980s provided new opportunities for the application of reaction engineering principles. In biochemical systems, reactions are catalyzed by enzymes. These biocatalysts may be dispersed in an aqueous phase or in a reverse micelle, supported on a polymeric carrier, or contained within whole cells. The reactors used are most often stirred tanks, bubble columns, or hollow fibers. If the kinetics for the enzymatic process is known, then the effects of reaction conditions and mass transfer phenomena can be analyzed quite successfully using classical reactor models. Where living cells are present, the growth of the cell mass as well as the kinetics of the desired reaction must be modeled [16, 17]. 

Classical emulsion polymerization is divided into three kinetic stages. At the start of the process, the unsaturated monomers are dispersed into small droplets, stabilized with surfactants. Additional surfactant aggregates into micelles. These micelles are very small ( 10nm) relative to monomer droplets ( 1-10 pm). During stage 1 the initial formation of polymer... 

Mini emulsion polymerization processes have multiple advantages apparent in reactions conducted in a heterogeneous medium, but they also maintain distinct characteristics observed in classic polymerization processes, such as perceived in emulsion and suspension. The main difference is evident during the nucleation particle process, which does not require the presence of micelles in the medium. This is a result of the nucleation process, which starts directly in the reactive species (monomer droplets) and is dispersed throughout the continuous phase. Hence, after the polymerization reaction, this process yields a final product comprising very stable polymer particles with sizes that range from 50 to 500 nm [25-27],... 

The monomer droplets and the micelles swollen with monomer compete for the free radicals generated in the aqueous phase, but since there are many more micelles than droplets in the system most of the free radicals enter micelles. Polymerization is initiated within individual micelles. The monomer consumed during the resulting polymerization is replenished by diffusion of new monomer molecules from the aqueous phase, which in turn, is kept saturated with monomer from the droplets of monomer. Polymerization continues within a given micelle until a second free radical enters the micelle, in which case termination quickly occurs because of the small volume of the reaction locus. The micelle then remains inactive until a third free radical enters, and so on. As reaction proceeds the micelles become larger and are disrupted to form particles of polymer swollen with monomer which are stabilized by soap molecules around the periphery. Monomer continues to diffuse into these particles and polymerization is maintained therein until the monomer supply is exhausted. The final product is a stable dispersion (latex)... 

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