Classical emulsions

Kinetics and Mechanisms. Early researchers misunderstood the fast reaction rates and high molecular weights of emulsion polymerization (11). In 1945 the first recognized quaHtative theory of emulsion polymerization was presented (12). This mechanism for classic emulsion preparation was quantified (13) and the polymerization separated into three stages. 

Emulsion Polymerization. When the U.S. supply of natural mbber from the Far East was cut off in World War II, the emulsion polymerization process was developed to produce synthetic mbber. In this complex process, the organic monomer is emulsified with soap in an aqueous continuous phase. Because of the much smaller (<0.1 jira) dispersed particles than in suspension polymerization and the stabilizing action of the soap, a proper emulsion is stable, so agitation is not as critical. In classical emulsion polymerization, a water-soluble initiator is used. This, together with the small particle size, gives rise to very different kinetics (6,21—23). 

The reaction described in this example is carried out in miniemulsion.Miniemulsions are dispersions of critically stabilized oil droplets with a size between 50 and 500 nm prepared by shearing a system containing oil, water,a surfactant and a hydrophobe. In contrast to the classical emulsion polymerization (see 5ect. 2.2.4.2), here the polymerization starts and proceeds directly within the preformed micellar "nanoreactors" (= monomer droplets).This means that the droplets have to become the primary locus of the nucleation of the polymer reaction. With the concept of "nanoreactors" one can take advantage of a potential thermodynamic control for the design of nanoparticles. Polymerizations in such miniemulsions, when carefully prepared, result in latex particles which have about the same size as the initial droplets.The polymerization of miniemulsions extends the possibilities of the widely applied emulsion polymerization and provides advantages with respect to copolymerization reactions of monomers with different polarity, incorporation of hydrophobic materials, or with respect to the stability of the formed latexes. 

For nonionic surfactants, an optimization of the process was achieved by using a similar approach to the so-called Cohesive Energy Ratio (CER) concept developed by Beerbower and Hill for the stability of classical emulsions (H). Its basic assumption is that the partial solubility parameters of oil and emulsifier lipophilic tail and of water and hydrophilic head are perfectly matched. Thus, the Vinsor cohesive energy ratio Ro, which determines the nature and the stability of an emulsion, is directly related to the emulsifier HliB (hydrophile-lipophile balance) by... 

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... 

In mini-emulsion polymerization, the particle nucleation mechanism may be evaluated by the ratio of the final number of polymer particles to the initial number of monomer droplets (Np f/Nm i). If the particle nucleation process is primarily governed by entry of radicals into the droplets, then the value of Np>f/Nm>i should be around 1. A lower value of Np f/Nm i may imply incomplete droplet nucleation or coalescence. On the other hand, a higher value of Npf/Nm>i may indicate that the influence of micellar or homogeneous nucleation comes into play in the particle formation process, since one droplet feeds monomer to more than one micelle in the classical emulsion polymerization. For pure micel-... 

Monodispersity, spatial ordering and absence of coalescence from phase separations in liquid crystals provide new and potentially helpful tools for the design of ordered composites and functional materials. The unusual behaviors of inclusions in the presence of an electrical field could also be useful for the development of a new class of field-responsive fluids. To practically estimate the potential of these systems for future appHcations, it is now time to start exploring the physical properties of these materials. Little is still known about their optical or rheological properties. They will presumably differ from that of classical emulsions opening thereby the possibiHty for the development of novel emulsion appHcations. 

In the initial stages of the reaction a suspension polymerization is a liquid-in-liquid system and from a colloidal point of view is a classical emulsion, i.e. macroemulsion. However, the system undergoes a transition during polymerization and in the final stages becomes a solid-in-liquid suspension , in analogy with the sol of gold [9]. 

A classical emulsion experiment involves the polymerization of styrene dispersed in water with a high HLB emulsifier such as sodium dodecyl sulfate. A... 

Effective compatibilization of binary polymer blends by addition of a copolymer reduces the dispersed particles size and Vj [Anastasiadis et al, 1987 Wu, 1987 Patterson et ai, 1971]. An illustration is shown on Figure 4.15. The effect of compatibilizer addition is similar to the emulsification of the classical emulsions. In the former systems, the compatibilizer effect on the drop size and Vj follows the same behavior as the emulsion drop size reduction upon addition of a surfactant. The latter behavior is usually described as the titration curve that characterizes the surfactant efficiency. The shape of the titration curve depends on the type of emulsifier and the emulsification process, e.g., mixing time and equipment. However, the amount of emulsifier to saturate the interface also depends on the affinity of emulsifier to the dispersed phase, the size of the dispersion, the orientation of the emulsifier at the interface and its ability to prevent flocculation and coalescence [Djakovic et al., 1987]. A similar behavior is to be expected for polymer blends upon addition of a compatibilizer. 

Effective compatibilization of binary polymer blends by addition of a copolymer should reduce the interfacial tension coefficient. Often, it also alters the molecular structure of the interface (as measured by the scattering methods). The process is similar to the emulsification in the classical emulsions. The emulsifier effect on the droplet size follows generally the same behavior as the interfacial tension. This behavior is described by the emulsification curves (evolution of the particle s size with the emulsifier content) and characterizes the additives efficiency. The shape... 

Some microemulsions are thermodynamically stable, /. e., they form immediately, and the oil and water phases cannot be separated without an energy input. However, most systems are not thermodynamically stable, althou they always present a long term kinetic stability. If the water and oil phase separation does occur in a microemulsion system, the process will take several orders of magnitude longer (months to years) compared to the phase separation time interval of a classical emulsion [33,34],... 

Classical emulsion polymerization processes are conducted in a heterogeneous medium, but they have some characteristics that differentiate them from other processes. As a result, their unique characteristics may be exploited for the production of specific materials. There are specific requirements that must be met for this process to occur in a heterophase reaction system. First, the reactive organic phase (monomer) must be almost completely insoluble in the continuous phase (water). Subsequently,... 

Figure 8.2 A representation of a classical emulsion polymerization process and depiction of the radical initiators and their entry mechanism. The diffusion of monomers into the micelles and the micellar structure are shown as a process formed via self-organization, containing monomer droplets (head as the hydrophilic part, and the tail as the hydrophobic part) with a surfactant concentration above the critical micelle concentration are also illustrated.

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