Growth of Latex Particles

After the completion of particle nucleation, the number of latex particles remains relatively constant with the progress of polymerization, provided that secondary nucleation of particle nuclei and flocculation of particles are absent from the reaction system (Smith-Ewart Interval II) [1, 72-76], Immediately after the end of particle nucleation, monomer conversion is relatively low and a major proportion of monomer is present in the emulsified monomer droplets ( 10°pm in diameter, 10 M0 dm in number). These monomer droplets serve as a reservoir to supply the growing particle nuclei with monomer for free radical polymerization taking place therein. 

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In addition to their obvious commercial potential, continuous reactors can be excellent tools for the study of fundamental kinetic mechanisms and reaction rates. This later factor is especially important for the study of mechanisms controlling the competitive growth of latex particles. 

Vanderhoff and Co-workers [98, 99] proposed the following empirical equation for the competitive growth of latex particles ... 

Figure 6.4. A schematic model for the nucleation and growth of latex particles in the acrylamide microemulsion polymerization stabilized by sodium bis(2-ethylhexyl)sulfosuccinate. (I) The initial condition of the polymerization system consists of a very large population of the acrylamide/ water-swollen micelles ( 6nm in diameter) dispersed in the continuous oily phase. Nucleation of particle nuclei occurs when free radicals are absorbed by the microemulsion droplets. (II) Growth of latex particles are achieved by (a) collision and then coalescence between two particles and (b) diffusion of monomer molecules from the microemulsion droplets through the continuous oily phase and then into the particles, (c) The polymerization system comprises water-swollen polyacrylamide particles ( 40nm in diameter) and acrylamide/water-swollen micelles ( 3nm in diameter) dispersed in the continuous oily phase at the end of polymerization [81].

The above particle nucleation and growth mechanisms were verified experimentally [61]. It was shown that the number density of latex particles increases linearly with increasing monomer conversion. On the other hand, the latex particle size remains relatively constant with the progress of polymerization. Such reaction mechanisms with some minor modifications may also be adequate for the qualitative description of the nucleation and growth of latex particles in the OAV microemulsion polymerization. 

It is important to note that in addition to microporous solids, other chemical systems have been used to template the growth of nanomaterials. For example, emulsions have been used to pattern both the pores in titania [14] and the packing of latex particles [46]. Reversed micelles have also been used as patterning agents. Examples include the syntheses of super-paramagnetic ferrite nanoparticles [15] and BaC03 nanowires [47]. Finally, carbon nanotubules have also been used as templates [16,48,49]. A variety of nanomaterials including metal oxides [16,48,49] and GaN have been synthesized inside such tubules [50]. 

Fig. 14 A Schematic diagram of motion of latex particles toward the base of myelinic figures, confirming that their growth is due to swelling, not to diffusion as in (B)...

The results of these experiments are depicted in Figure 3. They show that the number of particles formed in the initial stages of the latex preparation depends on the amount of anionic emulsifier present. When this amount was small, few particles were formed, and their number remained constant during further latex preparation. Thus, after initial particle formation, only a growth of existing particles took place. 

In polymerizations where desorption is very high, all micelles grow simultaneously. This is because radical absorption is followed almost iimiediately by desorption with only a brief micellar growth period. As a result, all micelles experience an equal but intermittant growth. The number of latex particles is determined solely by the number of micelles initially present. The number of micelles, m, is given by the expression... 

Interval III is characterized by polymerization in a constant number of latex particles in the absence of monomer droplets. The concentration of monomer in the latex particles thus declines as polymerization progresses. The principal modification required for the application ofEq. (S) in modeling Interval III is its coupling with a monomer balance equation and the modification of the growth factor K to incorporate the declining monomer concentration. A full monomer balance equation for a batch reactor demands the consideration of the monomer consumed in both the aqueous phase and the particles iMin and Ray, 1974). Frequently, however, the aqueous phase consumption is relatively small so that only consumption in the latex phase is significant. The latter is given by... 

S6). It depended on the variation of the number of latex particles formed iV with temperature. Unfortunately, they have overlooked the fact that the particle growth rate fi which appears to the power —f in the Smith-Ewart expression for the number of latex particles formed coitains the propa gation rate constant which is temperature dependent. It has also recently been realized that another factor on which JV depends, the area occupied by a surfactant molecule at the polymer-water interface Og, is also temperature dependent- Dunn et al. (1981) observed that the temperature dependence of N in the thermal polymerization of styrene differed from different emulsifiers. It seems unlikely that the differences ran be wholly explained by differing enthalpies of adsorption of the emulsifiers and, if not, this observation implies that the energy of activation for thermal initiation of styrene in emulsion depends on the emulsifier used. Participation of emulsifiers in thermal initiation (and probsbly also in initiation by oil-soluble initiators) is most probably attributable to transfer to emulsifier and desorption of the emulsifier radical frcan the micelle x>r latex particle into the aqueous phase the rates of these processes are likely to differ with the emulsifier. 

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