Nucleation in monomer droplets

The first mathematical model for nucleation in monomer droplets was proposed by Chamberlain et al. [25]. In this model, polymer particles were considered to be formed only upon the entry of the radicals into the monomer droplets. The rate of particle formation was expressed as a first-order entry process into monomer droplets ... 

The basic CRP techniques and mechanisms are discussed in Chapter 4 here only those issues associated with the presence of water in the system are dealt with. The subject has been reviewed by several authors [206, 266, 267]. Perhaps the most important challenge in this field is the development of a robust and general ab initio emulsion process (without using a seed). An essential problem in this endeavor is to avoid the nucleation in monomer droplets, which causes colloidal instability. 

RAFT chemistry is probably the most versatile and robust one for polymerization in aqueous dispersions amongst the different CRP techniques. The RAFT agent is bonded to the polymeric chains and therefore does not tend to partition back to the aqueous phase. However, this type of systems also exhibits problems of colloidal stability. In miniemulsion systems, these are attributed to nucleation in monomer droplets and superswelling and can be... 

Although it may seem obvious, the definition of miniemulsion polymerizatim can vary depending on the objectives of the specific research programme. In its narrowest sense, miniemulsion polymerization could be defined as the polymerization of all the monomer droplets present in the initial emulsion, where the final particle size distribution is reflected in the initial droplet size distribution (i.e. there is a one-to-one correspondence between the droplets and particles). It quickly became evident that this definition was too narrow to accommodate most (if not all) of the reactions termed miniemulsion polymerization because there was no such correspondence. Typically, fewer particles were found than the original number of monomer droplets. The definition, therefore, could be taken as the polymerization in miniemulsion droplets where not all the droplets succeed in becoming polymer particles. This definition only allows for nucleation in monomer droplets. 

Hansen, F. K., and J. Ugelstad, Particle Nucleation in Emulsion Polymerization la. Theory of Homogeneous Nucleation, J. Polym. Sci. Polym. Chem., 16,1953,1978 II. Nucleation in Emulsifier-Free Systems Investigated by Seed Polymerization III. Nucleation in Systems with Anionic Emulsifier Investigated by Seeded and Unseeded Polymerization IV Nucleation in Monomer Droplets, J. Polym. Sci. Polym. Chem., 17, 3033-3046, 3047-3068, 3069, 1979. 

In EP of bifunctional vinyl monomers, the reaction rate increases with the emulsifier concentration because the number of particles increases. However, in the crosslinking EP of divinyl monomers, the reaction rate is inversely proportional to the emulsifier concentration. This unusual behavior is due to nucleation taking place in both micelles and monomer droplets. In monomer droplets, the kinetics resembles that of bulk polymerization and therefore the reaction rates... 

Formation of latex particles can proceed via the micellar nucleation, homogeneous nucleation and monomer droplet nucleation. The contribution of each particle nucleation mechanism to the whole particle formation process is a complex function of the reaction conditions and the type of reactants. There are various direct and indirect approaches to determine the particle nucleation mechanism involved. These include the variations of the kinetic, colloidal and molecular weight parameters with the concentration and type of initiator and emulsifier. There are some other approaches, such as the dye method where the latex particles generated via homogeneous nucleation do not contribute to the amount of dye detected in the latex particles since diffusion of the extremely hydrophobic dye molecules from the monomer droplets to the latex particles generated in water is prohibited. On the contrary, nucleation of the dye containing monomer droplets leads to the direct incorporation of dye into the polymer product. However, the dye also act as a hydrophobe and enhances the stability of monomer droplets as well as the monomer droplet nucleation. 

Choi et al. [S] used dilatometry to monitor the kinetics of styrene miniemulsion polymerizations employing not only varying KPS concentrations but, in addition, the oil-soluble initiator 2,2 -azobis(2-methyl butyronitrile) (AMBN). The latter was initially thou t to provide an increased probability of nucleating all monomer droplets considering that the main locus of initiator decomposition and subsequent ch growth would be in the monomer droplets. This, however, did not prove to be the case. 

The relatively large monomer droplets (generally 2-5ym in diameter) have too small a surface area to capture radicals from the aqueous phase and therefore serve as reservoirs for the diffusion of monomer through the aqueous phase to the pol3onerizing oligomeric radicals, micelles, or polymer particles. Despite the unfavorable statistical probabilities, however, some monomer droplets capture radicals and polymerize to form microscopic or near-microscopic particles (14), and some of these particles which are entirely separate from the main particle size distribution are formed in most batch polymerizations. Polymerization in monomer droplets becomes much more significant when the size of the emulsion droplets is decreased. The use of ionic emulsifier-fatty alcohol mixtures (13) and, later, ionic emulsifier-alkane mixtures (15), allows the preparation of 0.1-0.2ym size styrene monomer droplets, which compete favorably with initiation in micelles and in the aqueous phase as the mechanism of particle nucleation. The mechanism of formation of these "mini-emulsions" has been attributed to the very low solubility of the fatty alcohols and alkanes in water (16) or to the formation of crystalline complexes between the ionic emulsifiers and fatty alcohols (17) the two mechanisms are not mutually exclusive. Thus this mechanism pertains only to special systems. 

Initiation in monomer droplets. Even though monomer droplets can capture oligoradicals, ordinarily the polymerization caused thereby is negligible because the number of droplets is serveral orders of magnitude less than that of the polymer particles. However, under conditions where the monomer is extremely well dispersed, e.g. in the presence of mixed emulsifiers, capture by the monomer droplets may become the exclusive fate of the oligoradicals, with the result that no new particles are nucleated and N is determined by the number of monomer droplets originally present (14). 

Online monitoring of different emulsion characteristics, such as conversion, comonomer composition, and particle size, have been reported in numerous studies [47-49]. hi most of the cases, estimation of reaction kinetics required calibration models and algorithms to be used, which have to account for effects of competing events, such as particle nucleation, coagulation, monomer droplets, changes in concentrations, and so on. 

Hansen and Ugelstad [37-39] suggested that all the micellar nucleation, homogeneous nucleation and monomer droplet nucleation were operative in emulsion polymerization with a concentration of surfactant greater than its CMC. This indicates that monomer-swollen micelles and particle nuclei and emulsified monomer droplets compete with one another for the incoming ohgomeric radicals from the continuous aqueous phase. Thus, the total rate of particle nucleation is given by... 

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... 

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). 

On the mechanistic side, emulsion polymerization is very different from suspension polymerization. Reactions do not normally occur in monomer droplets, hut in water phase and polymer particles.Water-solrrhle initiators are used. A typical recipe consists of 100 parts H2O, 50-120 parts monomer, 0.5 part surfactant, 0.5 part initiator, and 0.5 part chain transfer agent. There are three stages in emrrlsion polymerization. In stage I, polymer particles are nucleated from micelles. This stage usually takes a few minutes and is completed at < 10% conversion. Monomer droplets having sizes about tens of miaometers are formed in continuous water phase by agitation and stabilized by surfactant. The added amount of surfactant must be adequate and above its critical micellar concentration (CMC) so that micelles can form. 

Before the polymerization starts, the initiator molecules can either be located in monomer-swollen micelles, in monomer droplets, or in the continuous aqueous phase [35]. The distribution depends on several factors, including first of all the hydrophilic properties of the initiator. But even when oil-soluble initiators are used, radicals are very Ukely to be present in the aqueous phase [35-37]. Those radicals in the aqueous phase either (re)enter a droplet and lead to droplet nucleation, enter a micelle and lead to micellar nucleation, or start polymerizing monomer molecules dissolved in the aqueous phase. Those oligomers can either enter a droplet or precipitate (homogeneous nucleation) [38]. 

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. 

However, in the case of mini- and microemulsions, processing methods reduce the size of the monomer droplets close to the size of the micelle, leading to significant particle nucleation in the monomer droplets (17). Intense agitation, cosurfactant, and dilution are used to reduce monomer droplet size. Additives like cetyl alcohol are used to retard the diffusion of monomer from the droplets to the micelles, in order to further promote monomer droplet nucleation (18). The benefits of miniemulsions include faster reaction rates (19), improved shear stabiHty, and the control of particle size distributions to produce high soHds latices (20). 

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