Homogenization, monomer emulsions after

Figure 13. Electron micrograph of monomer emulsion of Figure 12 with OPB + hexadecanol (HD) immediately after homogenization...

Hart and de Pauw 98) used this emulsion technique on the system vinyl acetate-acrylic acid. It is well known that the copolymerization parameters rx and r2 are unfavorable in this system therefore the relative solubility of the two monomers exerces only a small influence on the composition of both sequences. The degree of homogeneity of the sequences has been evaluated, after alkaline hydrolysis, by measuring the tendency to lactonization in acid medium. While 72% of the acetate groups could be lactonized in the case of a random copolymer containing 37% vinyl acetate, only 14% are transformed in a block copolymer with the same initial composition. 

During nucleation, monomer droplets, monomer swollen micelles and monomer swollen polymer particles coexist in the batch reactor. Polymer particles efficiently compete for radicals and as their number increases, they become the main polymerization lod. The monomer that is consumed by free-radical polymerization in the polymer particles is replaced by monomer that diffuses from the monomer droplets through the aqueous phase. Therefore, the size of the particles increases and that of the monomer droplets decreases. The number of micelles decreases because they become polymer particles upon entry of a radical, and also because they are destroyed to provide surfactant to stabilize both the polymer chains that precipitate in the aqueous phase and the increasing surface area of the growing polymer particles. After some time, all micelles disappear. This is considered to be the end of the nucleation and only limited formation of new particles may occur after this point because heterogeneous nucleation is not possible and there is no free surfactant available in the system to stabilize the particles formed by homogeneous nucleation. The stage of the batch emulsion polymerization in which particle nucleation occurs is called Interval I [24,29]. At the end of Interval I, which typically occurs at a monomer conversion... 

When cholesteric liquid crystals are encapsulated in droplet form, the bistability can be preserved when droplet size is much larger than the pitch [64]. There arc two methods which are used to encapsulate Ch liquid crystals phase separation and emulsification. In phase separation [69], the Ch liquid crystal is mixed with monomers or oligomers to make a homogeneous mixture. The mixture is coated on plastic substrates and then another substrate is laminated on. The monomers or oligomers are then polymerized to induce phase separation. The liquid crystal phase separates from the polymer to form droplets. In the emulsification method [70-73], the Ch liquid crystal, water, and a water dissolvable polymer are placed in a container. Water dissolves the polymer to form a viscous solution, which does not dissolve the liquid crystal. When this system is stirred by a propeller blade at a sufficiently high speed, micron-size liquid crystal droplets are formed. The emulsion is then coated on a substrate and the water is allowed to evaporate. After the water evaporates, a second substrate is laminated to form the Ch display. 

A special case of phase separation is met with in the case that the polymer is insoluble in its own monomer as is the case with vinyl chloride. Emulsions of vinyl chloride in water may be formed by the diffusion method described above. A small amount of a compound Y is homogenized in water containing emulsifier. After further dilution with water, vinyl chloride is added and a stable emulsion of vinyl chloride is formed. By the subsequent polymerization,PVC is precipitated inside the droplets. Each droplet maintains its identity but is made up of two phases, one consisting of polymer swollen with monomer and compound Y (this phase may be subdivided into several particles inside the droplet) and an outer phase being composed of monomer and compound Y. The situation is different from the case of aqueous dispersions discussed so far in that the compound Y is present in what may be denoted the continuous phase inside the particles. Thus, in this case Y acts as a compound Y in relation to water and functions as a compound L inside the droplets. The continuous phase inside the droplet will contain only traces of polymer and the situation may therefore be described by the following equation ... 

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]. 

For instance, this procedure is followed in manypapers dealing with the semi-continuous emulsion copolymerisation of vinyl acetate and butyl acrylate (e.g. El-Aasser et al, 1983). Two main situations can be distinguished with respect to the monomer addition rate, (a) Flooded conditions the addition rate is higher than the polymerisation rate, (b) Starved conditions the monomers are added at a rate lower than the maximum attainable polymerisation rate (if more monomers were to be present). The latter process (starved conditions) is often applied in the preparation of homogeneous copolymers/latex particles. In this case after some time during the reaction, because of the low addition rates, a steady state is attained in which the polymerisation rate of each monomer is equal to its addition rate and a copolymer is made with a chemical composition identical to that of the monomer... 

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