Particle nucleation mechanisms polymerization

In summary, formation of particle nuclei from emulsified monomer droplets is almost certain to occur in any emulsion polymerization system in which these droplets are present. As mentioned earlier, however, monomer droplets containing polymer will primarily serve as reservoirs to provide monomer to the much more numerous and smaller latex particles formed by other particle nucleation mechanisms. Polymerization in monomer droplets can be eliminated or at least minimized by using seed polymer particles and slowly adding monomer (neat or as an emulsion) to supply the growing seed particles (i.e., seeded semibatch emulsion polymerization under the monomer-starved condition). 

In mini-emulsion polymerization, the particle nucleation mechanism may be evaluated by the ratio of the final number of polymer particles to the initial number of monomer droplets (Np f/Nm i). If the particle nucleation process is primarily governed by entry of radicals into the droplets, then the value of Np>f/Nm>i should be around 1. A lower value of Np f/Nm i may imply incomplete droplet nucleation or coalescence. On the other hand, a higher value of Npf/Nm>i may indicate that the influence of micellar or homogeneous nucleation comes into play in the particle formation process, since one droplet feeds monomer to more than one micelle in the classical emulsion polymerization. For pure micel-... 

It was found that the SDS/CA containing polymerization system shows a bi-modal PSD of latex particles, and the SDS/DMA containing system is characterized by a quite broad PSD. The fraction of small latex particles results from the initially generated highly monomer-swollen polymer particles, which serve as the monomer reservoir in the later stage of polymerization. On the contrary, the latex products obtained from both the SDS/SMA and SDS/HD containing systems show a relatively narrow PSD. These data further support the proposed competitive particle nucleation mechanism. 

Nucleation mechanism polymerization of particles initial hydrolysis of water glass intermediate to final polymerization of particle... 

A polymerization with n very large (102-6) indicating suspension-like (bulk polymerization) kinetic behavior, and a particle nucleation mechanism residing outside the monomer droplets, which delineates an emulsion process. Smith-Ewart case III systems are examples of this type of behavior provided they have evolved from case I and/or case II polymerization at low conversions, which is common. 

Emulsions and suspensions can be distinguished by the particle nucleation mechanism and the kinetics. A polymerization is considered an emulsion if... 

As explained before, when surfactant, water, and monomer(s) are mixed, the colloidal system obtained consists of monomer-swollen micelles (if the surfactant concentration exceeds its CMC) and monomer droplets dispersed in an aqueous phase that contains dissolved molecules of surfactant and a small amount of the sparingly water-soluble monomer(s). When free radicals are generated in the aqueous phase by action of an initiator system, then the emulsion polymerization takes place. Its evolution is such that the colloidal entities initially present tend to disappear and new colloidal entities (polymer latex particles) are bom by a process called nucleation. For convenience, we first focus on the particle nucleation mechanisms, a very important aspect of emulsion polymerization. 

One of the outstanding issues in this area is the need for reliable experimental data to quantify the contribution of each one of the nucleation mechanisms to particle formation in emulsion polymerization. Also outstanding is the development of a complete mathematical model which combines the contributions of all particle nucleation mechanisms. 

The mechanism of polymerization in ternary and quaternary oil-in-water microemulsions has become understood only in recent years. The onset of turbidity upon polymerization and the lack of stability with time observed by most authors, particularly for MM A monomer, is likely the reason for the slow progress in the comprehension of the mechanism of O/W systems. Only slight changes in the formulation are sufficient to significantly affect the polymerization process and to induce particle coagulation at any stage of the reaction. This may explain the disparity in the kinetic data reported by some authors for very similar systems. With this remark in mind, one can, however, conclude that the scheme that is now well accepted is that of a continuous particle nucleation mechanism as in the case of inverse systems. This view is supported by several features. 

Oil-in-water emulsion polymerization systems are typically classified as possessing the characteristics of one of three types of emulsions macro-emulsions, mini-emulsions or microemulsions. These emulsions are the initial systems for emulsion polymerization. There are quite differences between these systems in some aspects such as the size of the droplets (i.e. the discontinuous or dispersed phase), the interfacial area of the droplets, the particle nucleation mechanism and the stability of the emulsion. 

The technique of pyrene fluorescence intensity measurements was proposed to study the particle nucleation mechanisms involved in the 0/W microemulsion polymerization (31). The experimental data showed that microemulsion droplets are the major particle nucleation loci for the polymerization system with the more hydrophobic ST as the monomer. This is followed by the flocculation of latex particles with the remaining droplets. In contrast, the polymer reactions taking place initially in the continuous aqueous phase (homogeneous nucleation) plays an important role in the MMA microemulsion polymerization. 

For conciseness, this chapter primarily deals with three well-established particle nucleation mechanisms (i.e., micellar nucleation, homogeneous nucleation and coagulative nucleation). This is followed by the discussion of emulsion polymerization kinetics in Chapter 4. 

Poehlein [40] summarized previous work and proposed a comprehensive particle nucleation mechanism involved in a persulfate initiated emulsion polymerization system, as shown schematically in Figure 3.5. Song and Poehlein [41, 42] developed a general kinetic model taking into account micellar nucleation, homogeneous nucleation, and monomer droplet nucleation in emulsion polymerization. The chain transfer and termination reactions occurring in the continuous aqueous phase, capture of oligomeric radicals by particle nuclei, and flocculation of particle nuclei were also incorporated into the model development. The resultant expressions for calculation of the rate of particle nucleation can be written as... 

Sutterlin [46] studied the effect of the polarity of various monomers (styrene, acrylate ester monomers, and methacrylate ester monomers see Table 3.1) on the particle nucleation mechanisms involved in emulsion polymerization. When the surfactant concentration is above its CMC, the emulsion polymerization of styrene follows the Smith-Ewart theory (Npj 5o ) except those experiments with relatively low levels of surfactant. The exponent x in the relationship Npj So decreases with increasing monomer polarity when the surfactant concentration is above its CMC. This trend is attributed to the increased tendency of agglomeration of particle nuclei with monomer polarity. The emulsion polymerizations of less polar monomers deviate significantly from the Smith-Ewart theory (x 0.6) if the surfactant concentration is reduced to a level just below its CMC. This implies that some mechanisms other than micellar nucleation (homogeneous nucleation or coagulative nucleation) must operate in these emulsion polymerization systems. 

Nomura et al. [74,75] proposed an experimental method to study the competitive particle nucleation mechanisms (micellar nucleation versus homogeneous nucleation) in a given emulsion polymerization system. This approach involves the emulsion copolymerization of relatively hydrophobic styrene with relatively hydrophilic monomers such as methyl methacrylate or methyl acrylate. The composition of copolymer produced during the very early stage of polymerization (far lower than 1% monomer conversion), which reflects the characteristic of copolymer at the locus of particle nucleation, is then determined. Emulsion copolymerization of styrene with methyl methacrylate (or methyl acrylate) was carried out, where sodium dodecyl sulfate was used to stabilize the emulsion polymerization system and where the weight ratio of styrene to methyl methacrylate (or methyl acrylate) was kept constant at 1 1. The experimental results show that the compositions of copolymers obtained from emulsion polymerizations in the presence and absence of monomer-swollen micelles are quite different. This provides supporting evidence of the generally accepted Smith-Ewart theory that micellar nucleation controls the particle nucleation process in the emulsion copolymerization of styrene with... 

Regardless of which particle nucleation mechanisms predominate in the particle formation process, the amount of surfactant available for stabilizing particle nuclei is perhaps the most important parameter that controls the size of the population of latex particles produced during emulsion polymerization. Figure 3.10 shows a schematic representation of the number of latex particles per unit volume of water as a function of the concentration of surfactant initially present in the polymerization system. For emulsion polymerizations carried out at surfactant concentrations lower than the CMC, the number of latex particles per unit volume of water first remains relatively constant and then increases with increasing surfactant concentration. This is followed by the rapidly increased number of latex particles per unit volume of water with surfactant concentration when the surfactant concentration is in the vicinity of the CMC. The number of latex particles per unit volume of water then levels off when the surfactant concentration is increased to a level well above the CMC. 

Unzueta and Forcada [93] developed a mechanistic model for the emulsion copolymerization of methyl methacrylate and n-butyl acrylate stabilized by mixed anionic and nonionic surfactants, which was verified by the experimental data. This model is based on the mass and population balances of precursor particles and the moments of particle size distribution. It is sensitive to such parameters as the composition of mixed surfactants and the total surfactant concentration. A competitive particle nucleation mechanism is incorporated into the model to successfully simulate the evolution of particle nuclei during polymerization. 

It should be noted that the rate of absorption of free radicals by the latex particles from the continuous aqueous phase (p or a ) is not equal to the rate of generation of free radicals in the continuous aqueous phase (p, or a) when desorption of free radicals out of the latex particles (m) and/or the bimolecular termination of free radicals in the continuous aqueous phase (Y) cannot be neglected in the emulsion polymerization system. In addition to the particle nucleation mechanisms discussed in Chapter 3, to gain a fundamental understanding of transport of free radicals in the heterogeneous reaction system (e.g., absorption of free radicals by the latex particles, desorption of free radicals out of the latex particles and reabsorption of the desorbed free radicals by the latex particles) is thus required to predict the emulsion polymerization... 

The most important characteristic of miniemulsion polymerization is the transformation of the homogenized monomer droplets into latex particles via the capture of free radicals when a water-soluble initiator such as the persulfate initiator is used to initiate the free radical polymer reactions. However, this feature does not necessarily guarantee that the particle nucleation mechanisms other than monomer droplet nucleation can be ruled out. As will be shown later, previous studies dealing with nucleation of particle nuclei in miniemulsion polymerization often resulted in controversial conclusions. This subject is still open to discussion, and it represents a great challenge to polymer scientists. 

The influence of the type of initiators (sodium persulfate versus 2,2 -azobisisobutyronitrUe) on the particle nucleation mechanisms and kinetics involved in the styrene miniemulsion polymerizations has also been studied [39]. As expected, the oil-soluble 2,2 -azobisisobutyronitrile promotes nucleation in the homogenized monomer droplets. On the other hand, formation of particle nuclei in the continuous aqueous phase becomes more important when water-soluble sodium persulfate is used. This result is consistent with the vinyl chloride work of Saethre et al. [44]. The number of polyvinyl chloride latex particles generated by mechanisms other than monomer droplet... 

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