Micelles in emulsion polymerization

Why does polymerization not take place in the droplets instead of in the micelles in emulsion polymerization ... 

Monomer-Swollen Emulsifier Micelles. The role of micelles in emulsion polymerization has been extensively discussed. The original work of Harkins ( ) and Smith and Ewart ( ) treated micelles as sites for particle nucleation. They proposed that particle nucleation occurs when a free radical enters a monomer-swollen micelle and begins polymerization. As the particles grow the micelles disband to provide emulsifier for the new organic surface. 

Figure C2.3.11 Key surfactant stmctures (not to scale) in emulsion polymerization micelles containing monomer and oligomer, growing polymer particle stabilized by surfactant and an emulsion droplet of monomer (reservoir) also coated with surfactant. Adapted from figure 4-1 in [67],...

Suspension polymerization produces beads of plastic for styrene, methyl methacrviaie. viny l chloride, and vinyl acetate production. The monomer, in which the catalyst must be soluble, is maintained in droplet fonn suspended in water by agitation in the presence of a stabilizer such as gelatin each droplet of monomer undergoes bulk polymerization. In emulsion polymerization, ihe monomer is dispersed in water by means of a surfactant to form tiny particles held in suspension I micellcsK The monomer enters the hydrocarbon part of the micelles for polymerization by a... 

In emulsion polymerization, a solution of monomer in one solvent forms droplets, suspended in a second, immiscible solvent. We often employ surfactants to stabilize the droplets through the formation of micelles containing pure monomer or a monomer in solution. Micelles assemble when amphiphilic surfactant molecules (containing both a hydrophobic and hydrophilic end) organize at a phase boundary so that their hydrophilic portion interacts with the hydrophilic component of the emulsion, while their hydrophobic part interacts with the hydrophobic portion of the emulsion. Figure 2.14 illustrates a micellized emulsion structure. To start the polymerization reaction, a phase-specific initiator or catalyst diffuses into the core of the droplets, starting the polymerization. 

According to the first method, each micelle in an emulsion behaves like a separate micro-continuous reactor which contains all the components, i.e. monomers and radicals from the aqueous phase. Thus, analogous to the latex particles in emulsion polymerization, microgels formed by emulsion polymerization are distributed in the whole available volume. 

The initiator is present in the water phase, and this is where the initiating radicals are produced. The rate of radical production if, is typically of the order of 1013 radicals L-1 s-1. (The symbol p is often used instead of Rj in emulsion polymerization terminology.) The locus of polymerization is now of prime concern. The site of polymerization is not the monomer droplets since the initiators employed are insoluble in the organic monomer. Such initiators are referred to as oil-insoluble initiators. This situation distinguishes emulsion polymerization from suspension polymerization. Oil-soluble initiators are used in suspension polymerization and reaction occurs in the monomer droplets. The absence of polymerization in the monomer droplets in emulsion polymerization has been experimentally verified. If one halts an emulsion polymerization at an appropriate point before complete conversion is achieved, the monomer droplets can be separated and analyzed. An insignificant amount (approximately <0.1%) of polymer is found in the monomer droplets in such experiments. Polymerization takes place almost exclusively in the micelles. Monomer droplets do not compete effectively with micelles in capturing radicals produced in solution because of the much smaller total surface area of the droplets. 

The size of the micelles is significantly increased by the addition of monomer up to a diameter of 4.5-5 nm. However, the size of the monomer droplets is stilt very much larger than that of the micelles (diameters up to 1 pm). In emulsion polymerization, one generally uses 0.5-5 wt% of emulsifier relative to monomer. With the usual oil-in-water emulsions, the water content varies from half to four times the amount of monomer. 

Figure 3.26 Schematic representation of micelle formation in emulsion polymerization.

Surfactants play a major role in the preparation of suspensions of polymer particles by heterogeneous nucleation. In emulsion polymerization, the monomer is emulsified in a nonsolvent (usually water) using a surfactant, whereas the initiator is dissolved in the continuous phase. The role of surfactants in this process is obvious since nucleation may occur in the swollen surfactant micelle. Indeed, the number of particles formed and their size depend on the nature of surfactant and its concentration (which determines the number of micelles formed). 

Superficially, emulsion polymerization resembles suspension polymerization, but there are a number of important differences. Water is used as the continuous phase and heat transfer is very good for both suspension and emulsion polymerization. In contrast to suspension polymerization, the polymer particles produced in emulsion polymerization are on the order of 0.1 xm in diameter.33 Another important difference is the presence of an emulsifying agent or soap. At the beginning of polymerization the soap molecules aggregate together in a group of about 50-100 molecules to form what is called a micelle. Some of the... 

As we discussed in Sect. 3.1.1, Hansen et al. [15] made significant improvements to the concept of the radical capture efficiency proposed by Nomura et al. [ 14]. Taking this concept into consideration, they examined the effect of radical desorption on micellar particle formation in emulsion polymerization [ 65 ]. Assuming that radical entry is proportional to the x power of the micelle radius and the polymer particle radius, they proposed the following general expression for the rate of particle formation ... 

Strictly speaking, if the size of the micelle, is to play a role in emulsion polymerization, it should be the size of the monomer-swollen surfactant micelle, and not the monomer free one. However, these two sets of "sizes" should be proportional in the present case... 

The effects of emulsifiers in emulsion polymerization systems may be enumerated as follows (l stabilization of the monomer in emulsion, (2) solubilization of monomer in micelles, (3) stabilization of polymer latex particles, (4) solubilization of polymer, (5) catalysis of the initiation reaction, and (6) action as transfer agents or retarders which leads to diemical binding of emulsifier residues in the polymer obtained. 

From the discussion above, it is clear that there is no evidence for catalysis of persulfate initiation in emulsion polymerization systems. However, many ionic reactions have been shown to be subject to large catalytic effects in the presence of emulsifier micelles (Fendler and Fendler, 1975) so that the question arises as to whether there are any radical reactions that are subject to micellar catalysis and whether this phenomenon plays any part in any emulsion polymerization systems, Prima fade evidence that uiicellar catalysis may be important when emulsified monomer is allowed to polymerize thermally is provided by the work of Asahara et al. (1970, 1973) who find that several emulsifiers decrease the energy of activation for thermal initiation of alkyl methacrylate and styrene, [n particular, the energy of activation for thermal initiation of styrene emulsified with sodium tetrapropylene benzene solfonate was reported as S3 kl mol. much lower than any value determined in bulk. Hui and Hamielec s value of ] IS kj tnol (1972) seems to be representative of the data available on thermal initiation in bulk. The ctmclusions of Asahara et al. are based on observations of the temperature dependence of the degree of polymerization and are open to several objections. 

When an emulsifier or soap is dissolved in water, the solute molecules associate to form small clusters called micelles. The hydrocarbon parts of the emulsifier molecules constitute the interior of the micelles, the surface of which is formed by the ionic groups of the emulsifier. A small fraction of the soap is molecularly dissolved in the water and there is a dynamic equilibrium between the micelles and these single molecules in the aqueous phase. Micelles are of colloidal size, consisting of a relatively small number of soap molecules of the order of 100 molecules. This corresponds to a diameter of about 50 A., if one assumes the cluster to be spherical. At the concentrations usually employed in emulsion polymerization, there are some 10 micelles per milliliter of water. 

The emulsifier in emulsion polymerization has three key functions, namely stabilizing the monomer droplets during the first stage of the emulsion polymerization, supplying surfactant micelles as the site of the polymerization reaction (literally the micelles can be regarded as some kind of micro-reactors) and stabilizing the latex particles at the end of the emulsion polymerization process pending transportation,... 

Rate of polymerization. Under general conditions employed in emulsion polymerization, a typical value of the rate of generation of free radicals (Rr) in the aqueous phase is 10 per second per milliliter and a typical value of the number of polymer particles is 10 per milliliter. If all the radicals generated eventually enter M/P particles, since the micelles will have already disappeared, the rate of radical entry in a particle will average out to about one every 10 seconds, which means that the free radicals will generally enter the particles singly. 

---