Costabilizers, miniemulsion polymerization

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. 

Miniemulsion polymerization involves the use of an effective surfactant/ costabilizer system to produce very small (0.01-0.5 pm) monomer droplets. The droplet surface area in these systems is very large, and most of the surfactant is adsorbed at the droplet surface. 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 stabihze particles formed in the continuous phase. Both oil- and water-soluble initiators may be used the important feature is that the reaction proceeds by polymerization of the monomer in these small droplets, so there is no true Interval II. The mechanisms of macro- and miniemulsion polymerization are shown schematically in Fig. 1. 

In their original discovery of miniemulsion polymerization, Ugelstad and co-workers [5] used either cetyl alcohol (CA water solubility estimated at 6x10 [43]) or hexadecane (HD water solubihty estimated at 1x10 [43]) to retard monomer diffusion from submicron monomer droplets. Both CA and HD, referred to here as costabilizers, are volatile organic components and are therefore not entirely desirable in the final product. Other researchers have used polymers, chain transfer agents, and comonomers as stabiUzers, as will be discussed later. 

If Ostwald ripening is retarded by using a costabilizer, predominant droplet nucleation can be achieved. This is the basis of miniemulsion polymerization. One of the first comprehesive studies of miniemulsion polymerization was done on styrene by Choi et al. [53]. 

Although evidence exists for hquid crystal formation with fatty alcohol costabilizers, it does not for systems with long chain alkanes. Delgado et al. [89] have presented evidence that the role of hexadecane costabilizer in miniemulsion polymerizations is one of diffusional control. Rodriguez [90] and Delgado [91] have reported that no optional ratio of hexadecane to SLS exists in the preparation of miniemulsions. This provides evidence for the lack of crystal formation. Ugelstad et al. [45] have presented evidence that alkanes are more likely to follow the diffusion mechanism. 

Wang and Schork [73] used PS, PMMA and PVAc as the costabilizers in miniemulsion polymerizations of VAc with PVOH as the surfactant. They found that, while PMMA and PS were effective kinetic costabihzers (at 2-4%wt on total monomer) for this system, PVAc was not. While the polymeric costabilizers did not give true miniemulsions, Ostwald ripening was retarded long enough for predominant droplet nucleation to take place. 

Following the common practice in macroemulsion polymerization, most miniemulsion polymerizations have been run using water-soluble initiators. However, a number of researchers have looked at the possibility of using an oil-soluble initiator instead. As discussed previously, Schork and Reimers [107] and Asua et al. [108] have used LP as both the initiator and the costabilizer. In addition, Asua et al. used other oil-soluble initiators in conjunction with HD (as the costabilizer) to carry out miniemulsion polymerization of styrene. 

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. 

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-... 

Mouron et al. [105] and Wang et al. [106] have used dodecyl mercaptan (DDM) as the costabilizer in styrene and MMA miniemulsion polymerizations, respectively. Some of the results are shown in Table 8 and Table 9. For styrene (Table 8), the macroemulsion is compared with miniemulsions containing varying levels of DDM (costabilizer). In this case, the macroemulsion has a broader particle size distribution than all but one of the miniemulsions. For MMA (Table 9), miniemulsions and the equivalent macroemulsions have been compared at varying initiator concentrations. In this case, the macroemulsions... 

Reimers and Schork [144] have used polymethyl methacrylate as the costabilizer for methyl methacrylate miniemulsion polymerization. A portion of the results are shown in Table 10. In this case, the miniemulsion has a narrower particle size distribution than the equivalent macroemulsion. 

Reimers [102] carried out batch copolymerizations in both macro- and equivalent miniemulsions. MMA was used as the main monomer. The MMA was copolymerized in macroemulsion- or miniemulsion with p-methylstyrene (pMS), vinyl hexanoate (VH), vinyl 2-ethylhexanoate (VEH), vinyl n-decanoate (VD) or vinyl stearate (VS). The comonomers were copolymerized at 10%wt comonomer, 90%wt MMA. SLS was used as the surfactant and KPS as the initiator. The comonomers (all highly water insoluble) were used as the costabilizer. Miniemulsions were sonicated, while equivalent macroemulsions were only subjected to vigorous mixing. Polymerizations were carried out at 60 °C. 

Based on Luo s simulations [128], it has been found that the super-swelHng state is rather sensitive to recipe variations. Simply increasing the costabilizer level and/or using a nonionic polymeric surfactant would probably eliminate super-swelling, and hence, the instabihty. More recently, Sanderson [310] reported that two strategies could successfully form stable latex from RAFT miniemulsion polymerization ... 

Monomer miniemnlsion suitable for miniemulsion polymerization is snbmicron monomer-in-water dispersions stabilized against both coalescence by a snrfactant and Ostwald ripening by an osmotic pressnre agent, or costabilizer. The key issues in the preparation of stable monomer miniemulsion are the formnlation and the method of preparation. There have been a few review articles published over the years. ... 

Various costabilizers have been used in the miniemulsion polymerization. Cetyl alcohol and hexadecane are most often used since the original discovery of miniemulsion polymerization by Ugelstad et al. Other researchers have used initiators, polymers, chain transfer agents, or monomers as costabilizers. ... 

Miniemulsion is a special class of emulsion that is stabilized against coalescence by a surfactant and Ostwald ripening by an osmotic pressure agent, or costabilizer. Compared with conventional emulsion polymerization process, the miniemulsion polymerization process allows all types of monomers to be used in the formation of nanoparticles or nanocapsules, including those not miscible with the continuous phase. Each miniemulsion droplet can indeed be treated as a nanoreactor, and the colloidal stability of the miniemulsion ensures a perfect copy from the droplets to the final product. The versatility of polymerization process makes it possible to prepare nanocapsules with various types of core materials, such as hydrophilic or hydrophobic, liquid or solid, organic or inorganic materials. Different techniques can be used to initiate the capsule wall formation, such as radical, ionic polymerization, polyaddition, polycondensation, or phase separation from preformed polymers. 

Qi and coworkers reported the first study combining RAFT and inverse miniemulsion. Their inverse miniemulsion system comprised cyclohexane as the continuous phase, B246SF as the surfactant, and aqneous solution containing a CTA, acrylamide, and costabilizer MgS04. They found that using a water-soluble initiator, 4,4 -azobis(4-cyanovaleric acid), afforded better eontrol of the polymerization of acrylamide than using a lipophilic one, 2,2 -azobis(2-methylpropionitrile) (AIBN). RAFT control was realized up to 50% monomer conversion and after that significant deviation from RAFT control was observed. More recently, they have extended their RAFT inverse miniemulsion polymerization approach to other hydrophilic (co)polymers. ... 

In miniemulsion polymerization, the droplets are in the range from 50 to 500 nm. A combination of a surfactant (e.g. SDS) and a hydrophobe or costabilizer (for example, a long chain alkane or alcohol) is used. The droplets are formed using devices like ultrasonifiers, homogenizers or even static mixers. The miniemulsions are thermodynamically unstable and therefore are only stable for a limited period of time, ranging from hours to days. 

Microsuspension polymerization is a process used in the PVC industry to produce resins for plastisols [125], In this process, which resembles miniemulsion polymerization, a mixture of monomer and an oil-soluble initiator are dispersed in an aqueous solution of surfactants using intensive shear. The monomer droplets are polymerized yielding particles usually <2 pm, which are normally isolated by spray drying as they cannot be separated by centrifuging or filtering. These particles are solid and nonporous. The polymer particles are larger than the monomer droplets (0.1-2 pm) because the combined effect of the Ostwald ripening (as no costabilizer is used in the formulation) and droplet/particle coalescence. 

Until recently, there was only one report about the use of reactive costabilizers in miniemulsion polymerization [125]. In that study, dodecyl methacrylate (DMA) and stearyl methacrylate (SMA) were been used as cosurfactants with SDS and compared with cetyl alcohol (CA) and hexadecane (HD). It has been shown that DMA behaves like CA, whereas SMA displays a behavior similar to HD in terms of droplet size stability as well as in the particle size distribution of latexes. However, the distribution obtained using these reactive hydrophobes is in both cases somewhat narrower than for the model compounds. More recently, the same team published a study where in the polymerization of styrene in miniemulsions stabilized using DMA or SMA, small quantities of acrylic acid or methacrylic acid were added [126]. The authors were chiefly interested in the nucleation mechanism. Surprisingly, the addition of these hydrophilic monomers tends to favor nucleation within the droplets more than homogeneous nucleation, which is the dominating mechanism in the absence of these water-soluble monomers. The explanation lies in the fact that the styrene-carboxylic co-oligomers, because they are much more hydrophilic, are more reluctant to nucleate new particles. 

Miniemulsion polymerization enables to incorporate water-insoluble materials such as resins, organic pigments, polymers, etc into the polymer matrix. The additive seed allows to control the particle number and particle size during the production process. Furthermore, miniemulsion polymerizations and copolymerizations carried out with acrylic and methacrylic monomers in the presence of unsaturated alkyd resins lead to the production of stable hybrid latex particles containing grafted and crosslinked alkyd resin/acrylic products as coating polymer [114]. In the reaction, the multifunctional resin acts as a hydrophobe as well as the costabilizer of the miniemulsion. 

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