Monomer droplet-water interface

On the basis of the above dimer formation model, a time course of the absorbance of Y-Dye (/4(f)) produced in an individual droplet has been analyzed [104]. The coupling reaction between QDI and Y-Cp is assumed to proceed at the oil-droplet/water interface (rate constant, kt) and the association equilibrium between the Y-Dye monomer and dimer is attained immediately upon the dye formation. In the actual experiments, QDI is oxidized partly by 02 dissolved in the water phase, so that Y-Dye is produced in a DBP droplet even before electrolysis of QDI. To correct this contribution, [Y-Dye] is defined as [Y-Cp]0 — [Y-Cp]j exp( — fc,[QDI]wf), where [Y-Cp]0 and [Y-Cp], are the Y-Cp concentrations at the emulsion preparation and t = 0 (before electrolysis), respectively. According to the... 

However, there is no doubt that the monomer miniemulsion itself has a strong effect on the polymerization. Due to the large droplet interface, all exchange processes are enhanced according to equation (8.33) where Cm,i is the monomer concentration at the droplet-water interface. (Note that this a similar expression to the relationship given by equation (8.24) for the characteristic exchange time of a droplet.)... 

The monomer has only a very limited (but finite) solubility in the solvent (e.g. styrene in water). Most of it is present initially in dispersed droplets (hence the term emulsion polymerisation) one role of the (anionic) surfactant is to help stabilise these droplets, by adsorbing at the droplet/water interface. However, some of the monomer is present in the water phase. 

Two types of suspending agent are used. A protective colloid is a water-soluble polymer whose function is to increase the viscosity of the continuous water phase. This hydrody-namically hinders coalescence of the monomer droplets, but is inert with regard to the polymerization. A finely divided insoluble inorganic salt such as MgCOs may also be used. It collects at the droplet-water interface by surface tension and prevents coalescence of the... 

Suspension Polymerization. Suspension polymerization is carried out in small droplets of monomer suspended in water. The monomer is first finely dispersed in water by vigorous agitation. Suspension stabiUzers act to minimize coalescence of droplets by forming a coating at the monomer—water interface. The hydrophobic—hydrophilic properties of the suspension stabiLizers ate key to resin properties and grain agglomeration (89). 

Suspension polymerization may be the most important particle-forming polymerization from an industrial viewpoint. The system is very simple, composed of monomer, initiator, stabilizer, and medium (water in most cases). The monomer droplets with dissolving initiator are dispersed in water and the stabilizer exists at the interface. But suspension polymerization is regarded as a kind of homogeneous polymerization because the polymerization occurs only in monomer droplets and water does not affect the polymerization. Water contributes only to dividing the polymerization locus into small droplets and absorbing the heat evolved by polymerization. On the contrary, in emulsion polymerization, which is another type of polymerization performed in water and as practically important as suspension polymerization, water affects the polymerization significantly. In this section, emulsion polymerization is first discussed, and then some modified emulsion polymerizations such as soap-free emulsion polymerization and micro and mini emulsion polymerizations are described. 

Free-Radical Polymerization in Emulsion. In suspension polymerization, the particle size is fixed by the size of the monomer droplet which contains the initiator. Emulsion polymerization differs from suspension polymerization in that the initiator is dissolved in the aqueous phase and the polymer particle grows during polymerization. Free radicals are generated in the water and diffuse to the monomer-water interface. The length of the polymer chain formed, or equivalently the molecular weight, depends on the rate of free radical arrival and termination. S. Katz,... 

With cetyl alcohol, there is the complication that the polarity of the molecule may cause it to reside at the surface of the droplet, imparting additional colloidal stability. Here, the surfactant and costabilizer form an ordered structure at the monomer-water interface, which acts as a barrier to coalescence and mass transfer. Support for this theory lies in the method of preparation of the emulsion as well as experimental interfacial tension measurements [79]. It is well known that preparation of a stable emulsion with fatty alcohol costabilizers requires pre-emulsification of the surfactants within the aqueous phase prior to monomer addition. By mixing the fatty alcohol costabilizer in the water prior to monomer addition, it is believed that an ordered structure forms from the two surfactants. Upon addition of the monomer (oil) phase, the monomer diffuses through the aqueous phase to swell these ordered structures. For long chain alkanes that are strictly oil-soluble, homogenization of the oil phase is required to produce a stable emulsion. Although both costabilizers produce re-... 

The llux of monomers diffusing from the droplets to the particles will decrease as the former reservoirs are depleted, but reasonable calculations lead to the expectation that the arrival rate at the particle-water interface will exceed the usage rate even when more than 90% of the monomer has already left the droplets. 

At the beginning of a batch reaction the continuous aqueous phase contains the water-soluble initiator, emulsifiers, and buffers. Common ionic emulsifiers will be present as molecularly dissolved electrolytes, as surface active agents at the various interfaces, and as molecular clusters called micelles. The monomer will be in three different locations. A small amount will be dissolved in the water phase. Some will be solublized within the emulsifier micelles. The bulk of the monomer, however, will exist in the relatively large (ca. 5 um) monomer droplets. Any oil-soluble components such as chain transfer agents will be distributed with the monomer if the water solubility is sufficient to permit transport from the droplets. 

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