Nucleation locus

Radicals generated from water-soluble initiator might not enter a micelle (14) because of differences in surface-charge density. It is postulated that radical entry is preceded by some polymerization of the monomer in the aqueous phase. The very short oligomer chains are less soluble in the aqueous phase and readily enter the micelles. Other theories exist to explain how water-soluble radicals enter micelles (15). The micelles are presumed to be the principal locus of particle nucleation (16) because of the large surface area of micelles relative to the monomer droplets. 

Four types of fundamental subjects are involved in the process represented by Eq. (1.1) (1) metal-solution interface as the locus of the deposition process, (2) kinetics and mechanism of the deposition process, (3) nucleation and growth processes of the metal lattice (Mi ttice), and (4) structure and properties of the deposits. The material in this book is arranged according to these four fundamental issues. We start by considering in the first three chapters the basic components of an electrochemical cell for deposition. Chapter 2 treats water and ionic solutions Chapter 3, metal and metal surfaces and Chapter 4, the metal-solution interface. In Chapter 5 we discuss the potential difference across an interface, and in Chapter 6,... 

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. 

Microemulsion polymerizations follow a different mechanism from the conventional emulsion polymerizations. The most probable locus of particle nucle-ation was suggested to be the microemulsion monomer droplets [27], although homogeneous nucleation was not completely ruled out. The particle generation rate in microemulsion polymerization is given by an expression similar to Eq. (21), which was used for the miniemulsion polymerization of styrene [28] ... 

The size of the monomer droplets plays the key role in determining the locus of particle nucleation in emulsion and miniemulsion polymerizations. The competitive position of monomer droplets for capture of free radicals during miniemulsion polymerization is enhanced by both the increase in total droplet surface area and the decrease in the available surfactant for micelle formation or stabilization of precursors in homogeneous nucleation. 

The preparation of a latex by emulsion polymerization comprises two stages (i) particle nucleation (ii) particle growth. For the latex to be monodisperse, the particle nucleation stage must be short relative to the particle growth stage. Despite many investigations, there is disagreement as to the locus of particle nucleation (i) monomer-swollen emulsifier micelles (ii) ad-... 

The reaction model assumed is one in which free-radical polymerisation is compartmentalised within a fixed number of reaction loci, all of which have similar volumes. As has been pointed out above, new radicals are generated in the external phase only. No nucleation of new reaction loci occurs as polymerisation proceeds, and the number of loci is not reduced by processes such as particle agglomeration. Radicals enter reaction loci from the external phase at a constant rate (which in certain cases may be zero), and thus the rate of acquisition of radicals by a single locus is kinetic-ally of zero order with respect to the concentration of radicals within the locus. Once a radical enters a reaction locus, it initiates a chain polymerisation reaction which continues until the activity of the radical within the locus is lost. Polymerisation is assumed to occur almost exclusively within the reaction loci, because the solubility of the monomer in the external phase is assumed to be low. The volumes of the reaction loci are presumed not to increase greatly as a consequence of polymerisation. Two classes of mechanism are in general available whereby the activity of radicals can be lost from reaction loci ... 

The effect of solution concentration on nucleation rate is shown qualitatively in Fig. 9. At low levels of supersaturation, the rate is essentially zero but, as concentration is increased, a fairly well defined critical supersaturation is reached (point 1), beyond which nucleation rate rises steeply (curve 1-2). Point 1 may be regarded as the threshold of the labile region. Data from a series of such curves at different temperatures establish the locus of points at which nucleation starts, i.e., the Miers supersolubility curve discussed in Section II. 

Polymerization rate represents the instantaneous status of reaction locus, but the whole history of polymerization is engraved within the molecular weight distribution (MWD). Recently, a new simulation tool that uses the Monte Carlo (MC) method to estimate the whole reaction history, for both hnear [263-265] and nonlinear polymerization [266-273], has been proposed. So far, this technique has been applied to investigate the kinetic behavior after the nucleation period, where the overall picture of the kinetics is well imderstood. However, the versatility of the MC method could be used to solve the complex problems of nucleation kinetics. 

One of the most unique properties of miniemulsion polymerization is the lack of monomer transport. Recall from Fig. 1 that with macroemulsion polymerization, the monomer must diffuse from the monomer droplets, across the aqueous phase, and into the growing polymer particles. In contrast, in an ideal miniemulsion (nucleation of 100% of the droplets), there is no monomer transport, since the monomer is polymerized within the nucleated droplets. This lack of monomer transport leads to some of the most interesting properties of miniemulsions. For most monomers, macroemulsion polymerization is considered to be reaction, rather than diffusion limited. However, for extremely water insoluble monomers, this might not be the case. In this instance, polymerization in a miniemulsion might be substantially faster than polymerization in an equivalent macroemulsion. For copolymerization in a macroemulsion, where one of the comonomers is highly water insoluble, the comonomer composition at the locus of polymerization might be quite different from the overall comonomer composition, resulting in copolymer compositions other than those predicted by the reactivity ratios. 

The number of reaction loci is assumed not to vary with time. No nucleation of new reaction loci occurs as polymerization proceeds, and the number of loci is not reduced by processes such as particle agglomeration. The monomer is assumed to he only sparingly soluble in the external phase (a typical examide is styrene as monomer and water as the external phase), and thus polymerization is assumed to occur exclusively within the reaction loci and not within the external phase. The monomer is assumed to be present in sufficient quantity throughout the reaction to ensure that monomer droplets are present as a separate phase, and the rate of transfer of monomer to the reaction loci from the droplets is assumed to be rapid relative to the rate of consumption of moncuner in the loci by polymerization. The monomer concentration within the reaction loci is then taken to be constant throughout the reaction. This assumption is important if an attempt is made to relate the overall rate of polymerization to the average number of propagating radicals per reaction locus. The assumption will therefore be examined in further detail below. 

An early contribution to the cognate matter of the prediction of the eventual distribution of locus sizes was made by Ewart and Carr (1954). The two principal faertors that affect the nature of this distribution were considered to be (i) the distribution of locus sizes present when locus nucleation ceases, and (ii) the manner in which the loci grow subsequently as a consequence of polymerization. A third factor that may influence the growth pattern is the tendency for large loci to imbibe more monomer than do small loci. 

The presence and width of a metastable zone, in which nucleation is not spontaneous, have been discussed in Chapter 2. The thermodynamic limit of the metastable zone is a locus of points known as the spinodal curve, where spinodal decomposition replaces nucleation and crystal growth as the phase separation. In typical industrial crystallization, nucleation (and release of supersaturation) occurs at much lower supersaturations than the spinodal curve. 

As mentioned earlier, a supersaturated solution is not in the equilibrium condition. Crystallization moves the solution toward equilibrium by relieving its supersaturation. A supersaturated solution is thus not stable. There is a maximum degree of supersaturation for a solution before it becomes unstable. The region between this unstable boundary and the equilibrium (binodal) curve is termed the metastable zone, and it is here that the crystallization process occurs. The absolute limit of the metastable zone, known as the spinodal curve (8), is given by the locus of the maximum limit of supersaturation at which nucleation occurs spontaneously. Thermodynamically, the spinodal curve within the two-phase region is defined by the criterion... 

The influence of the emulsifier (SHS) concentration on Np is more pronounced in the conventional emulsion polymerization system (Rp°c[SHS]y, y= 0.68) than in mini-emulsion polymerization (y=0.25). This result is caused by the different particle formation mechanism. While homogeneous nucleation is predominant in the conventional emulsion polymerization, monomer droplets become the main locus of particle nucleation in mini-emulsion polymerization. In the latter polymerization system, most of the emulsifier molecules are adsorbed on the monomer droplet surface and, consequently, a dense droplet surface structure forms. The probability of absorption of oligomeric radicals generated in the continuous phase by the emulsifier-saturated surface of minidroplets is low as is also the particle formation rate. 

One of the key features of this reaction mechanism is the particle nucleation beyond the first maximal Rp. Furthermore, the disappearance of monomer droplets in the conventional emulsion polymerization at ca. 30-50% conversion results from the transfer of monomer from the monomer droplets to the locus of polymerization (polymer particles). In mini-emulsion polymerization, the concentration of monomer decreases and monomer droplets may exist throughout the polymerization. In the context of the micellar nucleation model,both Np and the concentration of monomer in the particles in the constant reaction rate region contribute to Rp. Therefore, the first maximal Rp observed in the course of mini-emulsion polymerization does not necessarily correspond to the end of particle nucleation. This is because Np may increase in the course of polymerization, but the contribution of the increased Np to Rp can be outweighed by the decreased monomer concentration in the reaction loci. 

Upon initiation of the polymer reaction, these monomer droplets become the main locus of particle generation. At this stage of polymerization, the function of hydrophobe is to retain the monomer originally present in the nucleated droplets. 

Table 1 compares the Suspension, Emulsion and Microemulsion regimes. The level of surfactant with respect to the CMC and the stability threshold, the locus of nucleation, and the particle size clearly distinguish these three regimes. This table also illustrates the analogies between direct oil-in-water and inverse water-in-oil polymerization processes with respect to the aforementioned attributes. To summarize As the concentration of emulsifier increases, at a fixed temperature, monomer level, and aqueous and organic phase ratio, a transition between the suspension, emulsion and microemulsion domains ensues. This occurs with a corresponding decrease in particle size. Within the... 

Nomenclature Thermo- dynamic stability Surfactant concentration relative to erne Existence of micelles/ inverse-micelles Primary Locus of Nucleation ... 

As predicted by Helfand and Tagami, the interface is the locus of low molecular weight impurities that have been shown to nucleate crystallization, as for example in PS/PP blends [Wening et al, 1990]. Compatibilization by addition of a third component may either reduce or enhance the tendency for crystallization. On the one hand, addition of compatibilizer increases the... 

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