Emulsion polymerization qualitative picture

The physical picture of emulsion polymerization is based on the original qualitative picture of Harkins [1947] and the quantitative treatment of Smith and Ewart [1948] with subsequent contributions by other workers [Blackley, 1975 Casey et al., 1990 Gao and Penlidis, 2002 Gardon, 1977 Gilbert, 1995, 2003 Hawkett et al., 1977 Piirma, 1982 Poehlein, 1986 Ugelstad and Hansen, 1976]. Table 4-1 shows a typical recipe for an emulsion polymerization [Vandenberg and Hulse, 1948]. This formulation, one of the early ones employed for the production of styrene-1,3-butadiene rubber (trade name GR-S), is typical of all emulsion polymerization systems. The main components are the monomer(s), dispersing medium, emulsifier, and water-soluble initiator. The dispersing medium is the liquid, usually water,... 

Before describing a qualitative picture of emulsion polymerization a note on monomer solubility and type of surface active agents is in order. Monomers for emulsion polymerization should be nearly insoluble in the dispersing medium but not completely insoluble. The solubility must be less than about 0.004 mol/L, as otherwise the aqueous phase will become a major locus of polymerization and the system will then not be typical emulsion polymerization. At the same time the monomer must be slightly soluble as this will allow the transport of monomer from the emulsified monomer reservoirs to the reaction loci (see later). 

The physical picture of emulsion polymerization is based originally on the qualitative picture of Harkins [18] and the quantitative treatment of Smith and Ewart [19], followed by other contributions. Gilbert shaped the qualitative and quantitative picture of the emulsion polymerization process as it is now generally accepted [16]. The main components of an emulsion polymerization recipe are the monomer(s), dispersing medium (usually water), surfactant and initiator. 

Give a qualitative picture of emulsion polymerization as described by Harkins. 

The preparation of waterborne latices by emulsion polymerization usually employs polymerization in aqueous micellar surfactant solutions. The simplest manifestation of the process involves the presence in an aqueous medium of emulsified monomer drops, micellar surfactant, and a water-soluble polymerization (free-radical) initiator. A combination of monomers is often used, exemplified by combinations of compounds such as methyl methacrylate, butyl acrylate, and styrene. Typically monomers have only slight water solubility. The classic qualitative picture of this process was described by Harkins [2] more than 60 years ago. A schematic of that process is shown in Figure 9.1. The key step is solubilization of the monomers in the micelles where polymerization is initiated. Both the low solubility of the monomer and the relatively low surface area of the monomer emulsion drops means that initiation of polymerization is essentially confined to the micelles. Polymerization now proceeds as more monomer is transported to the swelling micelles from the... 

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