Particle nucleation in emulsion polymerization

Particle morphology, of fillers, 11 303-304 Particle nucleation, in emulsion polymerization, 14 713-714... 

Besides giving latices of narrow particle size distribution, mixed surfactant systems have shown several other interesting characteristics which lighten some aspects concerning the mechanism of particle nucleation in emulsion polymerization process. 

In persulfate-initiated emulsion polymerizations carried out in the presence of conventional surfactants, firee radicals are generated in the aqueous phase as a result of dissociation of the persulfate- The complexity of particle nucleation in emulsion polymerization stems from the various physical processes and chemical reactions which follow the formation of these ionic radicals and determine their destiny. Figure 2.2 illustrates the possible fates of these aqueous i iase radicals and divides them into chemical and physical processes. They include ... 

Particle Nucleation and the Role of the Surfactant.—A recent paper by Hansen and Ugelstad is the first of a series which will deal comprehensively with the theory of particle nucleation in emulsion polymerization. The aim of the series is to discuss in detail the different parameters which affect particle formation, including the nature of the monomer, the capture efficiency of oligomers by particles of different size and nature, the effect of surfactant, and the electrostatic repulsion caused by charges on the surface of the particles. This first paper is concerned with the behaviour of monomers having relatively low water solubility, and with the case where either the system is surfactant-free, or, if... 

Hansen FK, Ugelstad J. The effect of desorption in micellar particle nucleation in emulsion polymerization. Makromol Chem 1979 180 2423-2434. 

Hansen, F. K., and J. Ugelstad, Particle Nucleation in Emulsion Polymerization la. Theory of Homogeneous Nucleation, J. Polym. Sci. Polym. Chem., 16,1953,1978 II. Nucleation in Emulsifier-Free Systems Investigated by Seed Polymerization III. Nucleation in Systems with Anionic Emulsifier Investigated by Seeded and Unseeded Polymerization IV Nucleation in Monomer Droplets, J. Polym. Sci. Polym. Chem., 17, 3033-3046, 3047-3068, 3069, 1979. 

The size of the monomer droplets plays the key role in determining the locus of particle nucleation in emulsion and miniemulsion polymerizations. The competitive position of monomer droplets for capture of free radicals during miniemulsion polymerization is enhanced by both the increase in total droplet surface area and the decrease in the available surfactant for micelle formation or stabilization of precursors in homogeneous nucleation. 

In 1952 W. J. Priest, in an important paper, laid out all of the basic qualitative features of the theory of homogeneous nucleation in emulsion polymerization as it is known today (12). This was based upon his studies of particle size distributions in vinyl acetate polymerization initiated by potassium persulfate (K2S20g) in the presence of varying amounts of different stabilizers and inhibitors at several temperatures. Priest proposed that (1) "polymerization in solution is the initial process" ... 

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). 

Microemulsion polymerization is an emulsion polymerization with very much smaller monomer droplets, about 10-100 nm compared to 1-100 pm. Micelles are present because the surfactant concentration is above CMC. The final polymer particles generally have diameters of 10-50 nm. Although many of the characteristics of microemulsion polymerization parallel those of emulsion polymerization, the details are not exactly the same [Co et al., 2001 de Vries et al., 2001 Lopez et al., 2000 Medizabial et al., 2000]. Water-soluble initiators are commonly used, but there are many reports of microemulsion polymerization with oil-soluble initiators. Nucleation in emulsion polymerization occurs almost exclusively in the early portion of the process (interval I). Nucleation occurs over a larger portion of the process in microemulsion polymerization because of the large amount of surfactant present. Nucleation... 

Three major mechanisms have been proposed for particle formation in emulsion polymerization, namely, micellar, homogeneous and droplet nucleation. These are described in the following sections. 

One of the outstanding issues in this area is the need for reliable experimental data to quantify the contribution of each one of the nucleation mechanisms to particle formation in emulsion polymerization. Also outstanding is the development of a complete mathematical model which combines the contributions of all particle nucleation mechanisms. 

The five different mechanisms currently in use to discuss particle nucleation during emulsion polymerization are summarized in Figure 25.5. 

Tauer, K. and Ktihn, I. (1995) Modeling particle formation in emulsion polymerization - an approach by means of the classical nucleation theory. Macro-molecules, 28, 2236. 

Nucleation in emulsion polymerization has been a matter of controversial discussion because for more than five decades no direct experimental data on the nucleation process were available. Discussions were mainly centered on the question of the role of emulsifying agents, in particular on the role of micelles as precursors of polymer particles. Micellar nucleation theory, as it was expressed by Smith-Ewart (77) on the baseis of ideas developed by William Harkins (78-80), states ... 

Figure 5 Effect of level of sodium dodecyl sulfate upon number of particles nucleated during emulsion polymerization of styrene in presence of fixed amoiint of carboxylic-acid monomers (42). Monomers comprised styrene, acrylic acid and fumaric acid in ratio 90.9/5/ 2.5. Degree of neutralization of acids was 0,85 Polymerization temperature 70 C.

Gonzalez-Ortiz LJ, Asua JM. Development of particle morphology in emulsion polymerization. 3. Cluster nucleation and dynamics in polymerizing systems. Macromolecules 1996 29 4520-4527. 

A number of mechanisms and models have been proposed for latex particle formation in emulsion polymerization systems [18-20]. These include particle formation by entry of a free radical into a micelle [6,7] or by homogeneous nucleation of oligomeric free radicals in the aqueous phase [18,21-23] or within microdroplets of the monomeric emulsion [24]. After their formation, these primary particles may simply grow by conversion of monomer into polymer within these particles, or undergo coagulation [22,25]. 

Poehlein [40] summarized previous work and proposed a comprehensive particle nucleation mechanism involved in a persulfate initiated emulsion polymerization system, as shown schematically in Figure 3.5. Song and Poehlein [41, 42] developed a general kinetic model taking into account micellar nucleation, homogeneous nucleation, and monomer droplet nucleation in emulsion polymerization. The chain transfer and termination reactions occurring in the continuous aqueous phase, capture of oligomeric radicals by particle nuclei, and flocculation of particle nuclei were also incorporated into the model development. The resultant expressions for calculation of the rate of particle nucleation can be written as... 

Sajjadi [85] investigated the diffusion-controlled nucleation and growth of particle nuclei in the emulsion homopolymerizations of styrene and methyl methacrylate. The polymerization starts with two stratified layers of monomer and water containing surfactant and initiator, with the water layer being stirred gently. In this manner, the rate of transport of monomer becomes diffusion-limited. As a result, the rate of growth of particle nuclei is reduced significantly, and more latex particles can be nucleated in emulsion polymerization. 

The emulsifier provides sites for the particle nucleation and stabilizes growing or the final polymer particles. Even though conventional emulsifiers (anionic, cationic, and nonionic) are commonly used in emulsion polymerization, other non-conventional ones are also used they include reactive emulsifiers and amphiphilic macromonomers. Reactive emulsifiers and macromonomers, which are surface active emulsifiers with an unsaturated group, are chemically bound to the surface of polymer particles. This strongly reduces the critical amount of emulsifier needed for stabilization of polymer particles, desorption of emulsifier from particles, formation of distinct emulsifier domains during film formation, and water sensitivity of the latex film. 

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