Droplets in miniemulsions

During the polymerization, the growth of droplets in miniemulsions can be suppressed. In miniemulsions the monomer diffusion is balanced by a high osmotic background of the hydrophobe, which makes the influence of the firstly formed polymer chains less important. 

Another type of interaction is confinement. We focus on the confinement of block copolymers and the resulting microphase separation. As one example, the structure of polystyrene-h/oc -poly(methyl methacrylate) (PS-f>-PMMA) in the confinement of droplets in miniemulsions is described. To better understand microphase separation, experimental results are compared to self-consistent field theory simulations. Then, we consider block copolymers bound to a solid surfaces and their response to different environmental conditions (Sect. 5). As a third example of structures and confinement, the incorporation of quantum dots into the hydrophobic region of polymersomes is demonstrated. 

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. 

The reaction described in this example is carried out in miniemulsion.Miniemulsions are dispersions of critically stabilized oil droplets with a size between 50 and 500 nm prepared by shearing a system containing oil, water,a surfactant and a hydrophobe. In contrast to the classical emulsion polymerization (see 5ect. 2.2.4.2), here the polymerization starts and proceeds directly within the preformed micellar "nanoreactors" (= monomer droplets).This means that the droplets have to become the primary locus of the nucleation of the polymer reaction. With the concept of "nanoreactors" one can take advantage of a potential thermodynamic control for the design of nanoparticles. Polymerizations in such miniemulsions, when carefully prepared, result in latex particles which have about the same size as the initial droplets.The polymerization of miniemulsions extends the possibilities of the widely applied emulsion polymerization and provides advantages with respect to copolymerization reactions of monomers with different polarity, incorporation of hydrophobic materials, or with respect to the stability of the formed latexes. 

In a first step of the miniemulsion process, small stable droplets in a size range between 30 and 500 nm are formed by shearing a system containing the dispersed phase, the continuous phase, a surfactant, and an osmotic pressure agent. In a second step, these droplets are polymerized without changing their identity. 

In 1962, Higuchi and Misra examined the quantitative aspects of the rate of growth of the large droplets and the rate of dissolution of the small droplets in emulsion for the case in which the process is diffusion controlled in the continuous phase [4]. It was proposed that unstable emulsions may be stabilized with respect to the Ostwald ripening process by the addition of small amounts of a third component, which must distribute preferentially in the dispersed phase [4]. The obtained stability in miniemulsions is said in the literature to be metastable or fully stable. The stabilization effect by adding a third component was recently theoretically described by Webster and Cates [5]. The authors considered an emulsion whose droplets contain a trapped species, which is insoluble in the continuous phase, and studied the emulsion s stability via the Lifshitz-Slyozov dynamics (Ostwald ripening). 

Colloidal stability is usually controlled by the type and amount of the employed surfactant. In miniemulsions, the fusion-fission rate equilibrium during sonication and therefore the size of the droplets directly after primary equilibration depends on the amount of surfactant. For sodium dodecylsulfate (SDS) and styrene at 20% dispersed phase, it spans a range from 180 nm (0.3% SDS relative to styrene) down to 32 nm (50 rel.% SDS) (Fig. 4a). Again, it is anticipated that rapidly polymerized latexes also characterize the parental miniemulsion. As... 

Measurement of characteristics of the emulsion droplets in concentrated media is indeed a difficult task. Some indirect methods have been used. The interfacial area and therefore the droplet size were determined by measuring the critical micelle concentration of miniemulsions [43]. Erdem et al. determined droplet sizes of concentrated miniemulsions via soap titration, which could be confirmed by CHDF measurements [44]. Droplet sizes without diluting the system can much better be estimated by small angle neutron scattering (SANS) measurements [23]. 

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. 

It was found that the chain length of the resulting polymer is inversely proportional to the square root of the initiator concentration [66], underlining that the reaction in miniemulsion is rather direct and close to an ideal radical polymerization. It could be shown that the amount of initiator used for polymerizing the latex does not have an effect on the number of nucleated droplets which shows that droplet nucleation is by far the dominant mechanism over the whole range of initiator concentrations. 

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... 

The polymerization in miniemulsion can also be performed in the presence of an oil, which is inert to the polymerization process. During polymerization, oil and polymer can demix, and many different structures such as an oil droplet encapsulated by a polymer shell, sponge like architectures, or dotted oil droplets can be formed. The formation of such structures is known from classical emul-... 

Switching from the very hydrophilic clays towards other inorganic nanoparticles, e.g., colloidal silica, leads, in the interplay with polymerization in miniemulsions, into a potential structural complexity, which covers the whole range from embedded particles (such as in the case of the calcium carbonate and carbon blacks) to surface bound inorganic layers (such as in the case of the clays). For basic research they are ideal systems to analyze complex structure formation processes in emulsions, since the original droplet shows a structure which is essentially established by molecular forces and local energy considerations, and which is ideally just solidified into a polymer structure. 

In my opinion, the field of miniemulsion is still on its rise in polymer and material science since there are numerous additional possibilities both for fundamental research and application. As a vision one may think of single molecules trapped and crystallized in each small droplet, which enables new types of physico-chemical experiments and handling of complex matter [132]. Since miniemulsions allow a very convenient and effective separation of objects in compartments of the size of 30-300 nm in diameter, some general new perspectives for polymer chemistry are opened. In miniemulsion droplets, it is in principle possible to isolate complex polymers or colloids strictly from each other and to react each single molecule for itself with other components, still working with significant amounts of matter and technically relevant mass fluxes. This... 

As a basis for hydrogels, hyperbranched polymers [41] can also be used. These polymers can be connected by click chemistry in miniemulsion droplets in order to obtain hyperbranched polyglycerol (HPG)-based particles. Such materials are of great interest for drug release because they are nontoxic [42, 43] and show similar behavior to poly(ethylene glycol)s. 

The synthesis of nanocapsules can best be obtained in miniemulsion using different approaches [107], One possibility is based on the phase separation process within a droplet during the polymerization [108], Here, vinyl monomers were polymerized in the presence of a hydrophobic oil. During the polymerization, the polymer becomes insoluble in the oil, leading to a phase separation. With properly chosen physicochemical properties of monomer and encapsulated material, a polymeric shell surrounding the liquid core can be formed. 

Kabalnov described water-based inkjet ink compositions that are miniemulsions, i.e., an aqueous vehicle having emulsified oil particles with dissolved dye molecules, where the oil droplets have a diameter of less than 1 m. In his patent, Kabalnov mentioned the advantages of miniemulsions in comparison to microemulsions, namely the surfactant nature and concentration which allow better penetration control to the printed papers, and the dye load in miniemulsions can also be increased compared to microemulsions at acceptable viscosity limits. According to this invention, the aqueous inkjet ink composition is comprised of an oil-soluble dye, a solvent, and an aqueous vehicle wherein particles of the oil-soluble dye are dissolved in low-polarity oil particles having a particle size of less than 1 m, the particles forming miniemulsions in water. 

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. 

Interval III begins when aU monomer droplets have vanished and/or the aqueous phase becomes unsaturated. Since each droplet in a macroemulsion actually absorbs radicals, they cannot disappear but rather shrink to a point where they have no excess monomer. The monomer in the aqueous phase decreases corresponding to the decrease in the particles. The conversion at which Interval III begins varies for different monomers and systems,but is typically around 40 to 50%. However, it may not be as distinguishable in miniemulsions due to early initiation of the gel effect. 

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. 

Our understanding of miniemulsion stability is limited by the practical difficulties encountered when attempting to measure and characterize a distribution of droplets. In fact, most of the well-known, established techniques used in the literature to characterize distributions of polymer particles in water are quite invasive and generally rely upon sample dilution (as in dynamic and static laser light scattering), and/or shear (as in capillary hydrodynamic fractionation), both of which are very likely to alter or destroy the sensitive equihbrium upon which a miniemulsion is based. Good results have been obtained by indirect techniques that do not need dilution, such as soap titration [125], SANS measurements[126] or turbidity and surface tension measurements [127]. Nevertheless, a substantial amount of experimental evidence has been collected, that has enabled us to estabhsh the effects of different amounts of surfactant and costabihzer, or different costabilizer structures, on stabihty. 

It is certain that we do not know what the leading effect in determining droplet stability and droplet distribution in miniemulsion is at this point both colloidal and ripening effects probably play a role. Future work is therefore needed to clarify these problems. 

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])...

An investigation of the copolymer composition demonstrated the important effect of monomer transport on the copolymerization. The droplets in the macroemulsion act as monomer reservoirs. In this system, the effect of monomer transport will be predominant when an extremely water-insoluble comonomer, such as DOM, is used. In contrast with the macroemulsion system, the miniemulsion system tends to follow the integrated Mayo Lewis equation more closely, indicating less influence from mass transfer. 

Another observation Monteiro et al. made was that a red layer was observed during Interval II, consisting of low molecular weight dormant chains, swollen with monomer. At the crossover to Interval III, this red layer coalesced, forming red coagulant. The same red layer was also observed by De Brouwer et al. [290] in miniemulsions stabilized with ionic surfactants. When polymer was used as the so-called cosurfactant, this polymer was not present in the red layer, indicating that this layer was not due to droplet coalescence. Also, the use of an... 

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