Particle Nucleation and Growth Mechanisms

The pioneering work of Gershberg and Longfield [52] deals with the steady-state number of polystyrene particles per unit volume of water nucleated in a stirred tank reactor or a series of stirred tank reactors based on the conventional Smith-Ewart theory (see Chapter 3, Section 3.1.1), as shown by the following balance equation. 

The residence time distribution function E i) of the growing latex particles in a perfectly mixed stirred tank reactor is defined as follows  

For constant surfactant and initiator concentrations and particle growth rate, the number of latex particles per unit volume of water is linearly proportional to mean residence time when 0 approaches zero and the second term in the denominator can be neglected. In other words, at small 0 values, Np increases linearly with increasing 0. On the other hand, the relationship Np 0 holds 

It is also interesting to note that the maximal number of latex particles per unit volume of water (Np ) that can be achieved in a continuous emulsion polymerization system is 58% of that (Npj) nucleated in the batch counterpart with exactly the same recipe and temperature [cf. Eq. (7.9) with Eq. (3.5)]. In addition, the optimal mean residence time (0max) is 83% of the time (h) at which particle nucleation stops as a result of complete depletion of monomer-swollen micelles in a batch emulsion polymerization system (see Chapter 1, Section 1.1.1). 

The rate of polymerization in a continuous stirred tank reactor can be written as 

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Chem C-S, Chang H-T (2002) Particle nucleation and growth mechanisms in miniemulsion polymerization of styrene. Polym Int 51 1428-1438... 

Moreover the miniemulsion, microemulsion and conventional emulsion polymerizations techniques show quite different particle nucleation and growth mechanisms and kinetics [271]... 

The product obtained from (conventional) emulsion polymerization is a colloidal dispersion comprising a very large population of submicron hydrophobic polymer particles dispersed in the continuous aqueous phase. This colloidal system is not thermodynamically stable because of the incompatibility between polymer and water (i.e., the very low solubility of polymer in water) in nature. As a matter of fact, the fate of most common latex products is the coagulation of polymer particles in order to minimize the particle-water interfacial area. Moreover, the monomer-swollen particles may even lose their colloidal stability and flocculate with one another in the course of emulsion polymerization. This will inevitably make the particle nucleation and growth mechanisms more complicated. 

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. 

Introduction. After we have discussed examples of uncorrelated but polydisperse particle systems we now turn to materials in which there is more structure - discrete scattering indicates correlation among the domains. In order to establish such correlation, various structure evolution mechanisms are possible. They range from a stochastic volume-filling mechanism over spinodal decomposition, nucleation-and-growth mechanisms to more complex interplays that may become palpable as experimental and evaluation technique is advancing. 

The formation of highly dispersed particles or crystallites in the synthesis process of, for example, a supported metal catalyst, is governed by nucleation and growth mechanisms (vide supra) that have been described in the literature [15, 16, 21-23]. For sintering or redispersion (spreading and film formation) to occur, particles or atoms, molecules or clusters of the active... 

The entire region enclosed by the outer dome represents immiscibility. The inner dome is known as the spinodal. The outer dome is known as the binodal. In the region of composition between the binodal and spinodal lines, phase separation occurs by the nucleation and growth mechanism and leads to the formation of dispersed micro-spherical glass particles in the matrix (see also Shelby, 1997). Spinodal decomposition which takes place inside the dotted region is a special type of phase separation. In order to understand this, consider two materials A and B, melts of which... 

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