Preparation of miniemulsions

The preparation of miniemulsions for barrier dressings consisted of the follow considerations as disclosed in Georgia Tech Research Corporation International Application No. PCT/US03/06409, and listed below. 

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... 

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. 

The surfactants used in the preparation of miniemulsions can be very varied. Formulations with anionic [9], single cationic and gemini [99], and nonionic and polymeric siufactants [100] have been reported. The use of one or other surfactant type depends on the final use of the latex. 

Tiarks, F., Landdfester, K. and Antonietti, M. (2001) Preparation of polymeric nanocapsules by miniemulsion polymerization. Langmuir, 17, 908-918. 

Taniguchi T, Takeuchi N, Kobaru S, Nakahira T (2008) Preparation of highly monodisperse fluorescent polymer particles by miniemulsion polymerization of styrene with a polymerizable surfactant. J Colloid Interface Sci 327 58-62... 

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. 

The copolymer composition in miniemulsion copolymerization of vinyl acetate and butyl acrylate during the initial 70% conversion was found to be less rich in vinyl acetate monomer units [34]. Miniemulsion polymerization also allowed the synthesis of particles in which butyl acrylate and a PMMA macromonomer [83, 84] or styrene and a PMMA macromonomer [85] were copolymerized. The macromonomer acts as compatibilizing agent for the preparation of core/shell PBA/PMMA particles. The degree of phase separation between the two polymers in the composite particles is affected by the amount of macromonomer used in the seed latex preparation. 

For a successful incorporation of a pigment into the latex particles, both type and amount of surfactant systems have to be adjusted to yield monomer particles, which have the appropriate size and chemistry to incorporate the pigment by its lateral dimension and surface chemistry. For the preparation of the miniemulsions, two steps have to be controlled (see Fig. 14). First, the already hydrophobic or hydrophobized particulate pigment with a size up to 100 nm has to be dispersed in the monomer phase. Hydrophilic pigments require a hydro-phobic surface to be dispersed into the hydrophobic monomer phase, which is usually promoted by a surfactant system 1 with low HLB value. Then, this common mixture is miniemulsified in the water phase employing a surfactant system 2 with high HLB, which has a higher tendency to stabilize the monomer (polymer)/water interface. 

In contrast to the process of creating a secondary dispersion as was used for the preparation of, e.g., polyurethanes and epoxide resins, it was shown that the miniemulsion polymerization process allows one to mix monomeric components together, and polyaddition and polycondensation reactions can be performed after miniemulsification in the miniemulsified state [125]. 

Any colloidal material provides an intrinsically favorable accessibility to its surface when compared to bulk material. Therefore, the availability of receptor binding sites should be facilitated by using colloidal MIPs. Submicron scale MIPs were prepared by precipitation polymerization, emulsion polymerization, and miniemulsion polymerization. Precipitation polymerization uses the insolubility of the formed polymer microgel in a certain solvent, whereas emulsion and miniemulsion polymerization employ two solvent phases for the preparation of the colloidal polymer. The latter methods offer the opportunity for tailoring the surface of the colloids exclusively, thereby enhancing the accessibility of the binding sites. Each of the three approaches has their own characteristics and will be described in the following sections. 

Fig. 7. General scheme for the preparation of molecularly imprinted nanospheres and their use for molecular recognition. Template molecules induce the formation of binding sites during the miniemulsion polymerization. The templates are extracted from the highly crosslinked particles and are molecularly recognized by the nanospheres selective binding sites...

Fig. 9a,b. Transmission electron microscope photograph of MIP nanoparticles prepared by miniemulsion polymerization a control polymer prepared in absence of template b polymer prepared in presence of template l-BFA [30]... 

As a new approach to the preparation of water-soluble polymers in inverse miniemulsions, a redox initiation system consisting of ceric ions and carbohydrate-based surfactant Span 60 as a reducing agent has been successfully used for the... 

The crosslinking of starch at the droplet interface in inverse miniemulsion leads to the formation of hydrogels. The formulation process for the preparation of crosslinked starch capsules in inverse miniemulsion is schematically shown in Fig. 10. The influence of different parameters such as the amount of starch, surfactant P(E/B-fe-EO), and crosslinker (2,4-toluene diisocyanate, TDI) on the capsule size and stability of the system were studied. The obtained capsules were in a size range of 320-920 nm. Higher amounts of starch and surfactant result in a smaller capsule size. The TEM images of crosslinked starch capsules prepared with different amount of crosslinker (TDI) are presented in Fig. 11. The nanocapsules can be employed as nanocontainers for the encapsulation of dsDNA molecules with different lengths [114] and for the encapsulation of magnetite nanoparticles. 

Fig. 10 Preparation of crosslinked starch capsules in an inverse miniemulsion...

Landfester K, Willert M, Antonietti M (2000) Preparation of polymer particles in nonaqueous direct and inverse miniemulsions. Macromolecules 33(7) 2370-2376... 

Musyanovych A, Rossmanith R, Tontsch C, Landfester K (2007) Effect of hydrophilic comonomer and surfactant type on the colloidal stability and size distribution of carboxyl-and amino-functionalized polystyrene particles prepared by miniemulsion polymerization. Langmuir 23(10) 5367-5376... 

Holzapfel V, Musyanovych A, Landfester K, Lorenz MR, Mailander V (2005) Preparation of fluorescent carboxyl and amino functionalized polystyrene particles by miniemulsion polymerization as markers for cells. Macromol Chem Phys 206(24) 2440-2449... 

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