Microemulsion polymerization bicontinuous phases

Recently, Figoli et al. [15] reported the use of polymerized bicontinuous microemulsion (PBM) membranes as nanostructured liquid membranes for facilitated oxygen transport. The final bicontinuous microemulsion consisting of an interconnected network of water and oil channels, stabilized by the interfacial surfactant film, in which the oil (monomer) channels were polymerized to form the polymeric matrix of the liquid membranes (Fig. 7.6) and the channel width (pore size) of the membranes could be tuned between 3 and 60 nm by adjusting the composition of the cosurfactant, while the water phase remained unchanged and it was the solvent for the novel oxygen carrier. 

Qutubuddin and coworkers [43,44] were the first to report on the preparation of solid porous materials by polymerization of styrene in Winsor I, II, and III microemulsions stabilized by an anionic surfactant (SDS) and 2-pentanol or by nonionic surfactants. The porosity of materials obtained in the middle phase was greater than that obtained with either oil-continuous or water-continuous microemulsions. This is related to the structure of middle-phase microemulsions, which consist of oily and aqueous bicontinuous interconnected domains. A major difficulty encountered during the thermal polymerization was phase separation. A solid, opaque polymer was obtained in the middle with excess phases at the top (essentially 2-pentanol) and bottom (94% water). The nature of the surfactant had a profound effect on the mechanical properties of polymers. The polymers formed from nonionic microemulsions were ductile and nonconductive and exhibited a glass transition temperature lower than that of normal polystyrene. The polymers formed from anionic microemulsions were brittle and conductive and exhibited a higher Tj,. This was attributed to strong ionic interactions between polystyrene and SDS. 

As for polymerization of hydrophobic monomers in the bicontinuous phase of microemulsions, the initial structure is not preserved upon polymerization. However, a notable difference from the former systems is that the final system is a microlatex that is remarkably transparent (100% optical transmission), fluid, and stable, with a particle size remaining unchanged over years even at high volume fractions ( 60%) [20]. The microlatex consists of water-swollen spherical polymer particles with a narrow size distribution according to QELS and TEM experiments. This result is of major importance with regard to inverse emulsion polymerization, which is known to produce unstable latices with a broad particle size distribution [23]. 

Polymerization in the Continuous or Bicontinuous Phases of Microemulsion. In an attempt to prepare hydrophobic polymers used to... 

Porous polymeric materials can be prepared by the polymerization of ST in Winsor I, II, and III microemulsions (43,44). It was found that the porosity of polymer achieved by polymerization in the bicontinuous phase is higher than that in the continuous phase of the 0/W or W/0 microemulsion. This was attributed to the interconnected microdomains in the bicontinuous phase. In fact, the microstructure of polymer achieved is closely related to the nature of microemulsion (45-49). For example, porous polymer with a closed-cell structure (ie, the discrete... 

Microemulsions are thermodynamically stable systems. Oil-in-water (0/W) microemulsions are mixtures of monomer(s), water, surfactant, and, in some cases, cosurfactant. The cosurfactant is a surface-active compound that, in combination with the surfactant, reduces the interfacial tension between the monomer and the aqueous phase to very low values, ensuring the thermodynamic stability of the microemulsion. Alcohols are often used as cosurfactants. The low interfacial tension results in a frequent fluctuation in size and shape of the microemulsion droplets. In water-in-oil (W/0) microemulsions, a mixture of water-soluble monomers and water are dispersed in an organic solvent with the help of a surfactant. The use of a cosurfactant is not needed often because the monomers are surface active. The amount of surfactant required in microemulsion polymerization (>10wt%) is substantially higher than that used in emulsion polymerization. The droplet (swollen micelle) size of the both 0/W and W/0 microemulsions is in the range of 5-20 nm in diameter. Since these small droplets only weakly scatter light, the microemulsions are transparent. Bicontinuous microemulsions are sometimes formed using blends of nonionic surfactants [100]. Microemulsion polymerization has been reviewed [101]. 

The porosity of solid polystyrene produced by polymerization in a middle-phase (bicontinuous) microemulsion is greater than that obtained by polymerization in either water-continuous or oil-continuous microemulsion. The first account of a middle-phase microemulsion-based porous polymer was reported by Haque and Qutubuddin in 1988 [71]. The microemulsions were formulated with styrene, water, sodium dodecyl sulfate (SDS), and 2-pentanol or butyl cellosolve as the cosolvent. (Since butyl cellosolve has greater solubility than 2-pentanol in polystyrene, it increases the stability of SDS microemulsion.) Figure 3.14 shows the structure of polystyrene when obtained from middle-phase microemulsion polymerization at 60 °C for 36 h, the composition (wt%) before polymerization being SDS 10 %, 2-pentanol 25 %, styrene 40 %, and water 25 %. The polymerized stmcture shows pores in both micron and submicron ranges. The observed greater porosity of this solid compared to the solids obtained from polymerization of oil-continuous microemulsion (SDS 10 %, 2-pentanol 25 %, styrene 55 %, water 10 %) and water-continuous microemulsion (SDS 10 %, 2-pentanol 25 %, styrene 5 %, water 60 %) is apparently related to the fact that middle-phase microemulsions contain interconnected domains of both water-continuous and oil-continuous regions. 

Microemulsions are thermodynamically stable, clear fluids, composed of oil, water, surfactant, and sometimes co-surfactant that have been widely investigated during recent years because of their numerous practical applications. The chemical structure of surfactants may have a low molecular weight as well as being polymeric, with nonionic or ionic components [138-141]. For a water/oil-continuous (W/O) microemulsion, at low concentration of the dispersed phase, the structure consists of spherical water droplets surrounded by a monomolecular layer of surfactant molecules whose hydrophobic tails are oriented toward the continuous oil phase (see Fig. 6). When the volume fractions of oil and water are high and comparable, random bicontinuous structures are expected to form. 

While there have been efforts to polymerize other surfactant mesophases and metastable phases, bicontinuous cubic phases have only very recently been the subject of polymerization work. Through the use of polymerizable surfactants, and aqueous monomers, in particular acrylamide, polymerization reactions have been performed in vesicles (4-8). surfactant foams ), inverted micellar solutions (10). hexagonal phase liquid crystals (111, and bicontinuous microemulsions (121. In the latter two cases rearrangement of the microstructure occured during polymerization, which in the case of bicontinuous microemulsions seems inevitable b ause microemulsions are of low viscosity and continually rearranging on the timescale of microseconds due to thermal disruption (131. In contrast, bicontinuous cubic phases are extremely viscous in genei, and although the components display self-diffusion rates comparable to those... 

In recent years other colloid systems—such as microemulsions—have been found to exhibit a wide range of structures [81,82]. We can observe spontaneous phase separation, flocculation and formation of complex bicontinuous structures after the formation of these colloidal systems. It is not possible to form a colloidal system, whether in a polymeric matrix, in water, or in an organic solvent, without a supercritical input of energy, which is provided by turbulent flow conditions during the formation of microemulsions or melt fracture conditions [86] during the formation of colloidal systems in polymers. It seems that a general principle for the behaviour of multiphase systems has been found. 

Burban et al. [53] reported the preparation of microporous silica gels by polymerization of partially hydrolyzed tetramethoxysilane gels present in the aqueous phase of bicontinuous microemulsions stabilized with didodecylammonium bromide. When vacuum dried, the gels made in microemulsions had about twice as much specific surface area as conventional vacuum-dried silica gels. 

Acrylics can also be bulk polymerized. One method for bulk polymerization involves coating a mixture of acrylic monomers along with a photoinitiator onto a film and uv curing to form the adhesive in place. Also being solventless, this can be an attractive method for making thick coatings. In addition, novel phase structure can be imparted by in place curing in some instances [eg, bicontinuous microemulsions (25)]. 

Polymerization reactions have been carried out in microemulsions of all types of stmctures. As we have noted earlier, microemulsions can be of the droplet type, either with isolated water droplets dispersed in a continuous oil phase (w/o microemulsion) that usually occur in systems with high oil content or with isolated oil droplets dispersed in a continuous water phase (o/w microemulsion), typically occurring in water-rich region. Nondroplet-type microemulsions, on the other hand, feature continuous oil and water phases intertwined in dynamic extended networks and are called bicontinuous microemulsions. A monomer can be incorporated in any of the water and oil phases of microemulsions and polymerized by normal... 

One of the main objectives of using microemulsions as reaction media for polymerization is to utilize their microstructures as templates to produce polymers with similar characteristics. For example, polymerization of a large amount of hydrophobic monomers in the continuous phase of w/o microemulsions could lead to solid polymers containing the preexisting aqueous disperse phase in a swiss-cheese like formation. This would permit inclusion of materials in the disperse phase that would otherwise be insoluble in the polymer, e.g., colloidal particles of metals as catalysts. In the case of bicontinuous microemulsions, however, both hydrophobic and hydrophilic monomers can be incorporated. The morphology of the final product would then depend on the microemulsion composition and on the nature of the incorporated monomers. 

Solid porous materials can be prepared by polymerization of all three types (o/w, w/o, and bicontinuous middle phase) of microemulsions [71]. Gupta and Singh [34] obtained porous polymers by polymerization of styrene/divinylbenzene as the continuous phase in an oil-continuous (w/o) microemulsion. Which was prepared using AOT as surfactant. The polymerization was carried out thermally at 70 °C using benzoyl peroxide as initiator. The porous materials may eventually be transformed into porous membranes. Such membranes have many applications in the field of separation science, composites, medicines, and biotechnology. Depending on the pore size, the porous polymers could be used for the separation of dust as also microparticles such as virus, bacteria, pigments, colloidal particles, etc. 

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