Micellar nucleation

Nevertheless micelles are normally present during Interval I of an emulsion polymerization in which latex particles are nucleated. Micellar nucleation of latex particles is dominant for monomers which have only a low solubility in water (e.g. styrene). For such a monomer any effect of micellar catalysis is likely to be revealed by an increase in the number of latex particles formed which would also result in an increased rate of polymerization. The thermal emulsion polymerization cited above seem to be a prima facie case of micellar catalysis. The thermal emulsion polymerization of styrene is investigated further here. 

The initiator-derived radicals can also undergo bimolec-ular termination in the aqueous phase or enter into a preexisting polymer particle, preventing the formation of a new particle. In other words, homogenous nucleation, micellar nucleation, termination of radicals in the aqueous phase, and free-radical capture in particles are competing processes. 

Depending on operating conditions such as the level of surfactant, the solubility of the monomers, the rate of radical generation, and so on, one or more of the three mechanisms (homogeneous nucleation, micellar nucleation, and nucleation in droplets) will predominate. However, the first two processes tend to be the most common. Indeed, the nuclei, produced by homogeneous nucleation, and also the micelles, have sizes of several nanometres and are relatively numerous, whereas the monomer droplets have diameters of several tens of microns, and are so scarce that the capture area they present to growing radicals is negligible compared with that of the tiny colloids. It is only when rather... 

The debate as to which mechanism controls particle nucleation continues. There is strong evidence the HUFT and coagulation theories hold tme for the more water-soluble monomers. What remains at issue are the relative rates of micellar entry, homogeneous particle nucleation, and coagulative nucleation when surfactant is present at concentrations above its CMC. It is reasonable to assume each mechanism plays a role, depending on the nature and conditions of the polymerization (26). 

Various kinetic models on particle formation were proposed by different researchers. These may be classified as follows (1) radical absorption mechanisms by Gardon [28-34] and Fisch and Tsai [13], (2) micellar nucleation newer models by Nomura et al. [35,36] and by Hansen and Ugelstad [37], (3) homogeneous nucleation by Fistch and coworkers [13,38,39]. 

Prindle and Ray (ZB.) have recently analyzed the same styrene data using a hybrid model consisting of the micellar nucleation mechanism above the CMC and of the homogeneous nucleation and coagulation mechanism below the CMC. Their simulations show a much steeper rise in the particle number concentration precisely at the CMC than predicted by EPM. Their hybrid model does not appear to predict that the particle concentration levels off at high surfactant concentrations. 

T. Nucleation Rates. Due to the large amount of emulsifier used in all the considered cases, only the micellar nucleation... 

Deriving an expression for f(t) a considerable simplification occurs if one takes all polymer particles to be nucleated at the same size dp(t,t). The generation of new polymer particles in an emulsion system is basically due to two mechanisms micellar and homogeneous particle production. Then, the rate of particle nucleation, fit), can be expressed as (12) ... 

The number of polymer particles is the prime determinant of the rate and degree of polymerization since it appears as the first power in both Eqs. 4-5 and 4-7. The formation (and stabilization) of polymer particles by both micellar nucleation and homogeneous nucleation involves the adsorption of surfactant from the micelles, solution, and monomer droplets. The number of polymer particles that can be stabilized is dependent on the total surface area of surfactant present in the system asS, where as is the interfacial surface area occupied by a surfactant molecule and S is the total concentration of surfactant in the system (micelles, solution, monomer droplets). However, N is also directly dependent on the rate of radical generation. The quantitative dependence of N on asS and R,- has been derived as... 

The predicted dependence of N on S and R,- for the formation of polymer particles by micellar and homogeneous nucleation followed by coagulative nucleation is given by Eq. 4-11 [Feeney et al., 1984] ... 

The reaction described in this example is carried out in miniemulsion.Miniemulsions are dispersions of critically stabilized oil droplets with a size between 50 and 500 nm prepared by shearing a system containing oil, water,a surfactant and a hydrophobe. In contrast to the classical emulsion polymerization (see 5ect. 2.2.4.2), here the polymerization starts and proceeds directly within the preformed micellar "nanoreactors" (= monomer droplets).This means that the droplets have to become the primary locus of the nucleation of the polymer reaction. With the concept of "nanoreactors" one can take advantage of a potential thermodynamic control for the design of nanoparticles. Polymerizations in such miniemulsions, when carefully prepared, result in latex particles which have about the same size as the initial droplets.The polymerization of miniemulsions extends the possibilities of the widely applied emulsion polymerization and provides advantages with respect to copolymerization reactions of monomers with different polarity, incorporation of hydrophobic materials, or with respect to the stability of the formed latexes. 

The influence of the addition of cetyl trimethyl ammonium chloride, CTAC, to the reverse micellar solution affects the droplet size and micellar interactions, as demonstrated by the DQLS experiment (64). Addition of CTAC to micellar system at a given water content leaves the droplet size unchanged, whereas a decrease in the intermicellar attraction has been observed. This decrease is more important for high CTAC concentrations. This has been interpreted to steric repulsion induced by the long hydrocarbon tail of CTAC (C ft). Thus, the addition of this compound to CdS synthesis could modify the nucleation and/or growth process. The experiments were performed by solulization of CTAC in the micellar solution containing either sodium sulfide or Cd(AOT)2. 

When CTAC is solubilized in micellar solution with sulfide S2- ions, at low water contents (w < 10), the presence of CTAC induces a strong decrease in CdS nanocrystallite size. For a given water content, the absorption spectra are blue shifted when the syntheses are performed in the presence of CTAC compared to that obtained in its absence. The temporal evolution of absorption at 250 nm is approximated to nucleation rate of CdS. It slows down in the presence of CTAC. This blue shift is more pronounced at low water content and high CTAC concentration. Hence it is observed a decrease in the particle size by increasing CTAC concentration. This can be related to the decrease in the intermicellar potential in the presence of CTAC (64). 

The nucleation rate is slowed down with increasing CTAC concentration, notably at a water content w equal to 3. However, this phenomenon is less important compared to what is obtained previously by solubilizing CTAC in micellar solution with sulfide S2 ions. 

The dependence of final particle number on the initiator or emulsifier concentrations according to the micellar and homogeneous nucleation theories is given by the following equation ... 

Exponents 0.6 and 0.8 deviate from both the homogeneous nucleation and micellar models. Thus the bimolecular termination between the growing radicals is suppressed. This behavior may also result from the surface activity of the graft copolymer formed. The higher the surface activity of graft copolymer the higher the particle number. This behavior would indicates that the graft copolymer formed within the particles (with DBP) is more efficient. 

The rate of polymerization was found to be independent of emulsifier concentration around CMC (1.8x10 4mol dm 3) and up to ca. 10 3 mol dm 3 and then strongly increased with increasing emulsifier concentration (Fig. 5). It can be seen that, for this system, the break in the dependence of the rate on surfactant concentration does not coincide with the CMC of either the surfactant or the surfactant/PEO-MA macromonomer. In fact, these two values are identical at room temperature at 50 °C the CMC of the surfactant is lower than at 20 °C. The kinetics of particle nucleation for the present nonionic polymerization of BA may not follow a micellar mechanism. 

In the emulsifier free-emulsion polymerization the reaction loci are formed by nucleation of amphiphilic macromomer micelles (micellar mechanism) or by... 

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