Competitive particle growth

Oil-soluble initiators such as benzoyl peroxide, 2,2-azobis(isobutyronitrile) [67] are sometimes used [43]. They partition preferentially into the monomer-swollen particle and generate the free radical in that confined space. The expectation that free radicals generated within the particles would quickly annihilate each other was not experimentally observed when styrene was polymerized with 0.2% benzoyl peroxide. The competitive particle growth rates were the same as in the presence of water-soluble initiators [68]. 

On the other hand Kao et al. [86] concluded on the basis of simulations that discrimination would not be possible from the results of batch experiments but that experimental results on the semi-continuous emulsion polymerization of styrene (Figure 4.6) fitted their model based on collisional entry. Moreover this hypothesis led to Rp a as found by Vanderhoff et al. [87] in competitive particle growth experiments published 28 years earlier. However, their value of... 

At the suggestion of the author. Flicker and Ross [11] allowed a uniform Phl2 sol to age to see if a symmetry breaking instability would occur - see also [12,13] The system did indeed pattern, displaying phenomenon (vi). To explain this phenomenon a new mechanism of precipitate pattern formation, the "competitive particle growth" (CPG) model was introduced [4,5,12, 14] and found strong experimental support [4,5,12-15] ... 

Feeney, R., Strickholm, P. and Ortoleva, P., Quantitative Comparison of Precipitation Banding Experiments and Competitive Particle Growth Theory (in preparation)... 

Feeney, R , Schmidt, S., Strickholm, P., Chadam, J. and Orto-leva, P. 1983, Periodic precipitation and coarsening waves applications of the competitive particle growth model. Jour. Chem Phys, in press. 

As mentioned above, the two most popular reaction mechanisms involved in the absorption of free radicals by the monomer-swoUen micelles and polymer particles are the diffusion- and propagation-controlled models. Nevertheless, liotta et al. [39] were inclined to support the colUsion-controlled model. A dynamic competitive particle growth model was developed to study the emulsion polymerization of styrene in the presence of two distinct populations of latex particles (i.e., bimodal particle size distribution). Comparing the on-line density and particle size data with model predictions suggests that absorption of free radicals by the latex particles follows the collision-controlled mechanism. 

Two major entry models - the diffusion-controlled and propagation-controlled models - are widely used at present. However, Liotta et al. [28] claim that the collision entry is more probable. They developed a dynamic competitive growth model to understand the particle growth process and used it to simulate the growth of two monodisperse polystyrene populations (bidisperse system) at 50 °C. Validation of the model with on-line density and on-line particle diameter measurements demonstrated that radical entry into polymer particles is more likely to occur by a collision mechanism than by either a propagation or diffusion mechanism. 

Competition occurs between the processes of particle growth, homogenous nucleation, and micellar nucleation. 

AgBr nanoclusters have been also synthesized by y-radiolysis under various conditions of irradiation dose and dose rate. The final cluster size is governed by the competition between two mechanisms of particle growth, one by the coalescence (controlled by second order kinetics and dose rate sensitive) ... 

Seeded batch processes have proven to be particularly useful in studies of emulsion polymaizadon because they facilitate cortirol of the initial particle number craicentration and particle size [e.g. consult 34]. For example, by using preformed latex particles as one reactant. Interval I can be eliminated, thereby enabling more accurate studies of Intmrals II and IQ to be made, including effects of particle size. Additionally, by appropriate selection of the initial particle number concentration, competition between particle nucleation and particle growth can be studied. 

Figure 2. Schematic of the alkanethiol stabilized cluster synthesis After reduction of the gold ions, the competitive processes of gold particle growth d alkanethiol surfece complexation determine the size of the gold nanorfiistA-(See page 3 of color insert.)...

Equations 6.57 and 6.58 show that PSD is the result of the competition between nuc-leation, particle growth and coagulation. Particle growth broadens the PSD because of the higher number of radicals in large particles. This is an intrinsic feature of the system and there is little that the operator can do to modify it. Particle nucleation depends mainly on the availability of the emulsifier, but it is also affected by the radical generation rate, the water-solubility of the monomers and the number of particles already... 

The disadvantage of the broad PSD in the effluent from a CSTR can actually prove to be an asset if one wishes to study competitive growth and transport rates among the particles. Kinetic model components for particle growth, free radical adsorption and free radical desorption must be of the proper form if one is to be able to predict the PSD of the effluent latex. Batch data will not provide an analysis of the same sensitivity. Hence, measurement of feed and effluent PSD s from a CSTR can be an effective way of measuring fundamental kinetic constants. 

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