W/O microemulsion polymerization

Unlike in conventional emulsion polymerization, no monomer droplets exist in a microemulsion polymerization system, and hence, oil-soluble initiators partition into the monomer-swollen micelles, the resultant polymer particles and the water phase. Therefore, in microemulsion polymerization, the polymerization only proceeds in the monomer-swollen micelles and the resultant polymer particles over the entire course of polymerization. Pairs of radicals produced in volumes as small as monomer-swollen micelles and polymer particles may terminate as soon as they are generated. If so, it is expected that the radicals responsible for the polymerization in the monomer-swollen micelles and the resultant polymer particles would usually be those generated from the fraction of the initiator dissolved in the water phase. In order to examine whether this expectation is correct, oil-in-water (O/W) microemulsion polymerizations of St were carried out using four kinds of oil-soluble azo-type initiators with widely different water-solubilities [209]. It was found that the rates of polymerization with these oil-soluble initiators were almost the same irrespective of their water-solubilities, when the polymerizations were carried out with the same rate of radical production for the whole system for all of the oil-soluble initiators used. Moreoever, the rate of polymerization with any of these oil-soluble initiators was only about 1/3 of that with KPS at the same rate of radical production. Considering that the rate of polymerization was pro-... 

Guo JS, Sudol ED, Vanderhoff JW, El-Aasser MS. Particle nucleation and monomer partitioning in styrene O/W microemulsion polymerization. J Polym Sci A Polym Chem 1992 30 691-702. 

Two characteristics of o/w microemulsion polymerization are different from those of conventional emulsion polymerization ... 

In comparison, the o/w microemulsion polymerization reported by Guo et al. [56, 57] can be represented by a simplified scheme with only two stages, as shown in Fig. 3.13b. In Stage I particle nucleation occurs in microemulsion droplets, the fraction of droplets initiated and converted into polymer particles being determined... 

Of course, the formation of such cross-linked microemulsion aggregates is also possible for the case of O/W microemulsions. Only here one has to use the opposite type of triblock structure, i.e., a hydrophilic central part and two hydrophobic ends. Such a triblock is given, for instance, by a poly(ethylene oxide) chain that is end-capped by two alkyl chains. This type of system was investigated for the case of EO55 that was end-capped by two oleyl chains (CL 428) in conjunction with the O/W microemulsion formed by tetradecyldimethylamine oxide (TDMAO), decane, and water [139]. For this system it was shown that for a microemulsion of a volume fraction of about 18 M) it is sufficient to add only 1.5% of the polymeric compound to increase the viscosity by more than three orders of magnitude (Fig. 9). 

First, it must be kept in mind that the formation of a microemulsion requires far more surfactant than that of an emulsion, because of the need to stabilize a large overall interfacial area. This drawback can considerably restrict the potential uses of microemulsion polymerization, since high solids contents and low amounts of surfactant are desirable for most applications. These requirements are far from being achieved at the present state of the art for polymerization in O/W microemulsions, because in most cases the amount of monomer does not exceed a few percent and that of surfactant is around 10% or more. 

Most studies have dealt either with the free radical polymerization of hydrophobic monomers—e.g., styrene [56-89], methyl methacrylate (MMA) [68,73,74,84,86,90-93] or derivatives [2,94,97], and butyl acrylate (BA) [98-100]—within the oily core of O/W microemulsions or with the polymerization of water-soluble monomers such as acrylamide (AM) within the aqueous core of W/O microemulsions [101-123]. In the latter case, the monomer is a powder that has to first be dissolved in water (1 1 mass ratio) so that the resulting polymer particles are swollen by water, in contrast with O/W latex particles, where the polymer is in the bulk state. The polymerization can be initiated thermally, photochemically, or under )>-radiolysis. The possibility of using a coulometric initiation for acrylamide polymerization in AOT systems was also reported [120]. Besides the conventional dilatometric and gravimetric techniques, the polymerization kinetics was monitored by Raman spectroscopy [73,74], pulsed UV laser source [72,78], the rotating sector technique [105,106], calorimetry, and internal reflectance spectroscopy [95]. 

The main difficulty encountered by most of the authors and one that precludes the use of higher monomer concentrations lies in retaining the optical transparency and stability of the microemulsions upon polymerization. In addition to entropic factors contributing to the destabilization of microemulsions during polymerization, the compatibility between polymer and cosurfactant also influences the system [64]. This is especially true when styrene is polymerized within O/W microemulsions that contain an alcohol because the latter is not... 

Despite these difficulties, most of the earliest studies used an alcohol in the formulation of 0/W microemulsions, and it was only in 1989 that the polymerization of hydrophobic monomers in a three-component O/W cationic system was reported by Ferrick et al. [124]. This spurred new interest, and systematic studies on ternary microemulsions based on cationic surfactants of different alkyl chain lengths ensued, mainly in the group of Gan and those of Puig and Kaler [67,77,81-87,90-93,127,128]. Nonionic surfactants were also used in ternary O/W microemulsions for the polymerizations of styrene and methyl methacrylate [68] and anionic [AOT] surfactants for the polymerization of tetrahydroflirfuryl methacrylate [95,97]. 

Figure 6 Number of polymer particles versus conversion (a) for acrylamide polymerization in AOT W/O microemulsion (from Ref 25) (b) for styrene polymerization in O/W microemulsion (from Ref 75).

Poly(p-phenylene) was made by electrochemical polymerization in o/w microemulsions [49] of benzene, sulfuric acid, and anionic, cationic, or neutral surfactant. Benzene radical cation was stabilized by the anionic surfactant, resulting in polymer with less cross-linking, smaller particle size, and a relatively narrow size distribution. With cationic surfactants, the radical cation destabilized the water droplets and led to a broader size distribution of polymer particles. 

The polymerization of methyl methacrylate (MMA) was studied by Rodriguez-Guadarrama et al. [24] in three-component o/w microemulsion made with DTAB. One-phase regions were detected at 25 and 60 in this sytem near the... 

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... 

Antonietti et al. [23] reported controlled synthesis of very fine polystyrene lat-ices with 10 nm < R < 60 nm via polymerization in ternary o/w microemulsions formed by dispersion of styrene in water, using CTAC or DTAB as surfactants. The size of the resulting particles was controlled only by the ratio of styrene to surfactant. The limited pliability of the surfactant interface resulted in a minimum droplet size which was obtained at the limit of high surfactant concentrations. 

Keywords Microemulsion polymerization Microemulsion reaction Water-in-Oil (W/O) microemulsion Oil-in-Water (0/W) microemulsion Bicontinuous microemulsion Functional membranes and inorganic/polymer nanocomposites... 

Polymerization in microemulsion systems has recently gained some attention as a consequence of the numerous studies on microemulsions developed after the 1974 energy crisis (1,2). This new type of polymerization can be considered an extension of the well-known emulsion polymerization process (3). Hicroemulsions are thermodynamically stable and transparent colloidal dispersions, which have the capacity to solubilize large amounts of oil and water. Depending on the different components concentration, microemulsions can adopt various labile structural organizations -globular (w/o or o/w tyne), bicontinuous or even lamellar -Polymerization of monomers has been achieved in these different media (4-18),... 

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