Miniemulsion polymerization particle/droplet

Microemulsion and miniemulsion polymerization differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 inn)4" and there is no monomer droplet phase. All monomer is in solution or in the particle phase. Initiation takes place by the same process as conventional emulsion polymerization. 

Microemulsion and miniemulsion polymerization processes differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 ran)77 and there is no discrete monomer droplet phase. All monomer is in solution or in the particle phase. Initiation usually takes place by the same process as conventional emulsion polymerization. As particle sizes reduce, the probability of particle entry is lowered and so is the probability of radical-radical termination. This knowledge has been used to advantage in designing living polymerizations based on reversible chain transfer (e.g. RAFT, Section 9.5.2)." 2... 

NMP in miniemulsion has been more successful. In miniemulsion polymerization nuclealion lakes place directly in the monomer droplets that become the polymer particles. Particle sizes are small (<100 nm). Most w ork has used TEMPO and high reaction temperatures (120-140 °C) with S or BA as monomer. 

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] ... 

Miniemulsion polymerization involves the use of an effective surfactant/costabi-lizer system to produce very small (0.01-0.5 micron) monomer droplets. The droplet surface area in these systems is very large, and most of the surfactant is adsorbed at the droplet surfaces. Particle nucleation is primarily via radical (primary or oligomeric) entry into monomer droplets, since little surfactant is present in the form of micelles, or as free surfactant available to stabilize particles formed in the continuous phase. The reaction then proceeds by polymerization of the monomer in these small droplets hence there may be no true Interval II. 

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 majority of the recipes described in the literature are based on the anionic sodium dodecylsulfate (SDS) as a model system. The possibility of using cationic surfactants such as octadecyl pyridinium bromide for the preparation of miniemulsions was first exploited in 1976. However, the emulsions were prepared by stirring and the resulting emulsions showed broadly distributed droplet sizes [2,39,50]. Recent work on steady-state miniemulsions showed that cationic and nonionic surfactants form well-defined miniemulsions for further miniemulsion polymerization processes, resulting in narrow size distributed stable cationic and nonionic latex particles [51]. Similar molecular amounts of the simple cationic surfactant, cetyltrimethylammonium bromide or chloride... 

In miniemulsion polymerization the nucleation of the particles mainly starts in the monomer droplets themselves. Therefore, the stability of droplets is a crucial factor in order to obtain droplet nucleation. The better the droplets are stabilized against Ostwald ripening, the higher is the droplet nucleation. 

In Fig. 8 the calorimetric curve of a typical miniemulsion polymerization for 100-nm droplets consisting of styrene as monomer and hexadecane as hydrophobe with initiation from the water phase is shown. Three distinguished intervals can be identified throughout the course of miniemulsion polymerization. According to Harkins definition for emulsion polymerization [59-61], only intervals I and III are found in the miniemulsion process. Additionally, interval IV describes a pronounced gel effect, the occurrence of which depends on the particle size. Similarly to microemulsions and some emulsion polymerization recipes [62], there is no interval II of constant reaction rate. This points to the fact that diffusion of monomer is in no phase of the reaction the rate-determining step. 

PVC latex particles consisting of two size populations can be generated in a miniemulsion polymerization. The mechanism for the formation of two discrete particle families relies upon polymerization of two distinct kinds of droplets [74]. 

Polymerization in miniemulsion is a very suitable technique to avoid this problem since each droplet acts as a nanoreactor. As a result, pure polyacrylonitrile (PAN) nanoparticles were obtained in the size range 100 nmwater phase. This is no restriction for a miniemulsion polymerization process, and the use of a hydro-phobic initiator 2,2 azobis(2-methylbutyronitrile) allows the preservation of the droplets as the reaction sites by droplet nucleation (see Fig. 12). Initiation of the... 

If the monomer droplet size in a conventional emulsion polymerization can be reduced sufficiently (see below), the loci of polymerization become the monomer droplets. This system is referred to as a miniemulsion polymerization and will be discussed in detail below. The particle diameter will range from 50 to 500 nm. 

The competition for oligomeric radicals also includes particles that have been created. In miniemulsion polymerizations, the nucleation of one droplet results in the formation of one particle of equal surface area. Therefore, nucleation therein has little effect on competition for radicals. This is not so with macroemulsions, since both micellar and homogeneous nucleation result in a large shift in the surface area from micelles to particles as the particles are created and grow. 

Mouran et al. [105] polymerized miniemulsions of methyl methacrylate with sodium lauryl sulfate as the surfactant and dodecyl mercaptan (DDM) as the costabilizer. The emulsions were of a droplet size range common to miniemulsions and exhibited long-term stability (of greater than three months). Results indicate that DDM retards Ostwald ripening and allows the production of stable miniemulsions. When these emulsions were initiated, particle formation occurred predominantly via monomer droplet nucleation. The rate of polymerization, monomer droplet size, polymer particle size, molecular weight of the polymer, and the effect of initiator concentration on the number of particles all varied systematically in ways that indicated predominant droplet nucleation. 

In the case of nanoencapsulations of solids, or the incorporation of high molecular weight, highly water-insoluble additives (such as polymers, oligomers, alkyds) into polymer particles, macroemulsion polymerization will not work, since the high molecular weight material will remain in the monomer droplet as the monomer is transported out. At the end of the reaction, the additive will remain in the depleted monomer droplets, rather than in the polymer particles. Clearly, these products can only be made via miniemulsion polymerization. 

Macro- and miniemulsion polymerization in a PFR/CSTR train was modeled by Samer and Schork [64]. Since particle nucleation and growth are coupled for macroemulsion polymerization in a CSTR, the number of particles formed in a CSTR only is a fraction of the number of particles generated in a batch reactor. For this reason, their results showed that a PFR upstream of a CSTR has a dramatic effect on the number of particles and the rate of polymerization in the CSTR. In fact, the CSTR was found to produce only 20% of the number of particles generated in a PFR/CSTR train with the same total residence time as the CSTR alone. By contrast, since miniemulsions are dominated by droplet nucleation, the use of a PFR prereactor had a negligible effect on the rate of polymerization in the CSTR. The number of particles generated in the CSTR was 100% of the number of particles generated in a PFR/CSTR train with the same total residence time as the CSTR alone. 

Fig. 11 Model predictions for the number of particles in CSTR miniemulsion polymerization expressed as the number of particles divided by the number of droplets in the feed (from [64])...

Reimers [95] used polymeric costabihzer to carry out miniemulsion polymerization of MMA. Droplet nucleation was found to be the dominant nucleation mechanism in the polymerization. As a result, the nucleation was more robust, and the polymerizations were less sensitive to variations in the recipe or contaminant levels. This was evident in the rates of polymerization and in the particle numbers. The miniemulsion polymerizations were subjected to changes in initiator concentration, water-phase retarder, and oil-phase inhibitor, and were shown to be significantly more robust. 

Batch miniemulsion polymerization of MMA using PMMA as the costabilizer was carried out with SLS as the surfactant and KPS as the initiator. Solids content was kept at -30%. A low surfactant level was used with the miniemulsions to ensure droplet nucleation. The initiator concentration of the polymer-stabilized miniemulsion polymerizations was varied from 0.0005 to 0.02 Mjq, based on the total water content. An aqueous phase retarder, (sodium nitrite) or an oil-phase inhibitor (diphenylpicrylhydrazol [DPPH]), was added to both the miniemulsions and the macro emulsions prior to initiation. Particle numbers and rates of polymerization for both systems were determined. 

Results from the polymer-costabilized miniemulsion polymerizations are shown in Table 2. Droplet sizes were found to vary between 115.1 and 121.0 nm. These are in accord with measurements made by Fontenot [140] for MMA miniemulsions stabilized with hexadecane. The sizes of the particles in the final products were close to the sizes of the droplets, ranging from 102.6 to 108.1 nm, with polydispersities ranging from 1.011 to 1.027. The ratio of the number of particles to the number of droplets (N /N ) was found to be between 0.95 and 1.08. Therefore, the majority of the droplets were nucleated to form polymer particles. Droplet nucleation led to polymerization rates comparable to those for the corresponding macroemulsions. For equal concentrations of initiator, 0.01 Maq, the rates are 0.199 and 0.233 gmol/min L q for the mini- and the macroemulsion polymerizations, respectively. 

Stabilize new particles, thereby increasing the total number of particles. Since the nucleation period is lengthened, the polydispersity increases. Figure 14 shows that the dependence of the inhibitor concentration on the number of particles is 0.176 0.010. Conversion time curves indicate that an induction period results from the presence of the inhibitor. Since polymer-stabilized miniemulsion polymerization occurs via droplet nucleation, it should be less sensitive to oil-phase inhibition. Initiator radicals will enter the droplet one after the other until all of the inhibitor is used up, and the monomer polymerizes. This does not affect the number of droplets or particles. As seen in Fig. 15, the number of particles is proportional to the DPPH concentration raised to the power of 0.0031 0.0001. Therefore, the number of particles is essentially independent of the presence of inhibitor. 

Landfester et al. [ 143] studied the miniemulsion polymerization of styrene using hexadecane as the costabilizer. When styrene miniemulsions were subjected to varying sonication times (see Table 5), very similar trends are seen as for the MMA miniemulsions. The particle size and the polydispersity of miniemulsion droplets rapidly polymerized after sonication either do not depend on the amount of the costabihzer, or are very weak functions of the amount of costabilizer (see Table 6). It was found that doubhng the amount of costabilizer does not decrease the radius nor have any effect on the polydis-... 

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. 

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