Monomer droplets, free

Furthermore, should free radicals be present, the vinyl groups would much more rapidly polymerise depleting the emulsion droplets of monomer, providing the control required for a particular particle size. The composition of the solution thus determines not only the phase behaviour, but the rate of polymerisation and the particle size. If, the organism has in its genetic code, the abihty to synthesise the monomer, it presumably has... 

An intermediate polymerization technique between emulsion polymerization and suspension polymerization has been described. Here, the monomers are first dispersed in water containing a small amount of surfactant and a high molecular weight alcohol to form very small droplets of monomer. The polymerization is effected with a water-soluble free radical initiator, such as potassium per-oxydisulfate (4). 

It is usual to consider the course of emulsion polymerization to proceed through three intervals [16,17]. The particle number increases with time in Interval I, where latex particles are being formed, and then remains constant during Intervals II and II. The monomer concentration in particles is in equilibrium with a monomer saturated aqueous solution. Swelling is limited only by the opposite force of the particle surface/water tension. Hence, the concentration of monomer in the particles is usually taken as constant up to the point where free monomer droplets disappear. In Intervals I and II, the monomer concentration... 

The mechanism of emulsion polymerisation is complex. The basic theory is that originally proposed by Harkins21. Monomer is distributed throughout the emulsion system (a) as stabilised emulsion droplets, (b) dissolved to a small extent in the aqueous phase and (c) solubilised in soap micelles (see page 89). The micellar environment appears to be the most favourable for the initiation of polymerisation. The emulsion droplets of monomer appear to act mainly as reservoirs to supply material to the polymerisation sites by diffusion through the aqueous phase. As the micelles grow, they adsorb free emulsifier from solution, and eventually from the surface of the emulsion droplets. The emulsifier thus serves to stabilise the polymer particles. This theory accounts for the observation that the rate of polymerisation and the number of polymer particles finally produced depend largely on the emulsifier concentration, and that the number of polymer particles may far exceed the number of monomer droplets initially present. 

Number of Monomer Droplets, Free Emulsifier Concentrations, and Probabilities for Droplet and Homogeneous Initiation ... 

Period of Particle Formation. An important factor which influences the duration of the period of particle formation and hence the number of particles formed is the rate of growth of the particles. This rate in turn is strongly influenced by the solubility of the monomer in its polymer or rather the partition of monomor between polymer and aqueous phase. Table III lists approximate values of C, the conversion at which free monomer droplets disappear... 

In bulk- and solution-phase free-radical polymerization, there is a tradeoff between molecular weight and polymerization rate. This is especially true for controlled/living radical polymerization. In emulsion polymerization, however, high molecular weight polymers can be made at fast polymerization rates. Emulsion polymerization is a type of radical polymerization that is frequently used for making polymers of high molecular weight. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified with surfactants in a continuous phase of water. 

Miniemulsions are typically formed by subjecting the oil/water/surfactant/ cosurfactant system to a high shear field created by devices such as an ultra-sonifier, the Manton Gaulin homogenizer and the Microfluidizer. These rely tn mechanical shear and/or cavitation to break the oil phase into submicron size droplets. When monomer is used as the oil phase, free radical polymerizatim can subsequently be carried out by addition of an initiator (e.g. potassium persulfate, KPS). 

During the second stage of the emulsion polymerization, therefore, the loci for available monomer consist of the dissolved monomer, the free monomer droplets, and the monomer imbibed by the numerous polymer particles. As before, the first two of these loci make a minor contribution, whereas the polymer-monomer particles provide a major locus for reaction with the initiator radicals diffusing from the aqueous phase. The major portion of the... 

The monomer droplets and the micelles swollen with monomer compete for the free radicals generated in the aqueous phase, but since there are many more micelles than droplets in the system most of the free radicals enter micelles. Polymerization is initiated within individual micelles. The monomer consumed during the resulting polymerization is replenished by diffusion of new monomer molecules from the aqueous phase, which in turn, is kept saturated with monomer from the droplets of monomer. Polymerization continues within a given micelle until a second free radical enters the micelle, in which case termination quickly occurs because of the small volume of the reaction locus. The micelle then remains inactive until a third free radical enters, and so on. As reaction proceeds the micelles become larger and are disrupted to form particles of polymer swollen with monomer which are stabilized by soap molecules... 

In emulsion polymerization, the polymerization process (typically radical initiated) takes place in micellar reactors composed of monomer droplets stabilized by surfactant molecules and dispersed in water (Figure 9.1). A colloidally stable polymer dispersion or latex is formed in this reaction by a complex mechanism consisting of three distinct intervals termed Smith-Ewert intervals [ 3-5]. On addition of a dispersed phase soluble monomer to the surfactant/solvent system, the system contains monomer-swollen small surfactant micelles ( 10 nm in diameter) and large emulsion droplets of monomer. On the subsequent addition of a continuous phase soluble initiator, free radical spedes form which diffuse into the micelles. The monomer quickly polymerizes in the micelle and, as diffusion of monomer from the emulsion droplet to the micelle is rapid on the timescale of polymerization, the micelles contain both monomer and polymer. As the concentration of free monomer reduces to zero, the polymerization of the remaining monomer in the latex particles takes place, ending the reaction. Monodispersity is retained throughout the reaction to the final product as all polymerization takes place within the surfactant micelles [6, 7j. 

Semicontinuous and Continuous Emulsion Polymerization In senticontinuous reactors, monomers, surfactant, initiator, and water are continuously fed into the reactor. Monomer droplets form if the rate at which the monomer is fed into the reactor exceeds the polymerization rate. This is not a desirable situation because the presence of free monomer in the system lowers the capability for controlling the polymer characteristics [8]. 

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

The completion stage is identified by the fact that all the monomer has diffused into the growing polymer particles (disappearance of the monomer droplet) and reaction rate drops off precipitously. Because the free radicals that now initiate polymerization in the monomer-swollen latex particle can more readily attack unsaturation of polymer chains, the onset of gel is also characteristic of this third stage. To maintain desirable physical properties of the polymer formed, emulsion SBR is usually terminated just before or at the onset of this stage. 

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