Water-monomer interface

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). 

The surface active agents (surfactants) may be cationic, anionic or non-ionic. Surfactants commonly used are cetyltrimethyl ammonium bromide (CTABr), sodium lauryl sulphate (NaLS) and triton-X, etc. The surfactants help to lower the surface tension at the monomer-water interface and also facilitate emulsification of the monomer in water. Because of their low solubility surfactants get fully dissolved or molecularly dispersed only at low concentrations and at higher concentrations micelles are formed. The highest concentration where in all the molecules are in dispersed state is known as critical micelle concentration (CMC). The CMC values of some surfactants are listed in table below. 

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-... 

Figure 5. Variation of the surface occupied by a SLS molecule in the saturated layer as a function of surface tension at the monomer-water interface...

The advantage of the emulsion method of polymerization lies in the wide choice of catalysts that may be used. In mass reactions such agents as oil-soluble peroxides, diazoamine derivatives, and diazothioethers must be used. Emulsification permits the use of water-soluble catalysts, as well as mixtures of these with oil-soluble materials. The reaction then occurs at the monomer-water interface, with a high probability that the free radicals formed will immediately react with the adjacent monomer. Use of quarternary emulsifiers such as CTAB or Emulsol 607 in such... 

One of the most important problems of emulsion polymerization is the locus in which the elementary processes (initiation propagation, etc.) take place during polymerization, i.e. monomer droplets, monomer-water interface, monomer-saturated emulsifier micelles, or the monomer aqueous solution. Depending on the existing conditions (emulsifier, monomer and initiator) the polymerization reaction will preponderantly develop in one of the possibilities. 

These growing radical ions tend to diffuse into the monomer-water interfaces. The probability that the diffusion takes place into monomer swollen micelles rather than into monomer droplets is backed by the considerations of the relative surface areas of the two. There are, on the average, 10 micelles in each milliliter of water. These are approximately 75 A in diameter and each swollen micelle contains on the average 30 molecules of the monomer. ) th the monomer droplet diameter of 1 // and with approximately 10 such droplets per milliliter of water, the micelles offer 60 times more surface for penetration than do the droplets. 

Commercially, emulsion polymerization is the preferred method. The process is quite complex since polymerization occurs in the water phase and at the monomer-water interface in addition to the usual reaction within the soap micelles. Anionic surfactants are usually used but cationic soaps such as cetyl pyridinium bromide may be employed. 

The initiation-in-micelles mechanism (4) postulated that the monomer droplets serve as reservoirs, feeding monomer to the polymerizing monomer-swollen micelles and -polymer particles by diffusion through the aqueous phase. The failure of the 1-lOym diameter monomer droplets to capture radicals was attributed to their negligible surface area relative to that of the monomer droplets. Indeed, hypothetical calculations of the relative surface areas of monomer-swollen micelles and monomer droplets show that it is unlikely that radicals diffusing to the monomer-water interface would enter monomer droplets and initiate polymerization. This conclusion was supported for isoprene emulsion polymerization by the low concentration of monomer found in monomer droplets separated by centrifugation and the retention of a spherical form by monomer drops in a polymerizing medium (4). 

Figure 10.3b). In this case, one can observe that although the clay is located at the monomer/water interface, it is preferentially placed within the organic phase, but rather close to the interface, avoiding the unfavorable interaction with water.

Figure 10.4 Equilibrium morphology map of a clay-monomer hybrid miniemulsion droplet. The clay is simulated by 15 rigid chains composed of 10 beads (simulating clay smaller than montmorillonite). The density distribution, P/(x,y), is perpendicular to the xy plane. The water is depicted in light gray, monomer in dark gray and clay platelets in black. The monomer/ water interface is the darker gray region.

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