Microemulsions dispersion polymerization

There are four main types of liquid-phase heterogeneous free-radical polymerization microemulsion polymerization, emulsion polymerization, miniemulsion polymerization and dispersion polymerization, all of which can produce nano- to micron-sized polymeric particles. Emulsion polymerization is sometimes called macroemulsion polymerization. In recent years, these heterophase polymerization reactions have become more and more important... 

Because of their widely recognized solubility in SCCO2, fluoropolymers have become extensively used as modifiers in this medium (Figure 4.4). They have formed the basis of surfactants for dispersion polymerizations and water microemulsion formation, as extractants for metals and as modifiers to dissolve insoluble organic reagents, e.g. radical initiators and tin reagents. 

The next section describes measurements of interfacial tension and surfactant adsorption. The sections on w/c and o/c microemulsions discuss phase behavior, spectroscopic and scattering studies of polarity, pH, aggregation, droplet size, and protein solubilization. The formation of w/c microemulsions, which has been achieved only recently [19, 20], offers new opportunities in protein and polymer chemistry, separation science, reaction engineering, environmental science for waste minimization and treatment, and materials science. Recently, kinetically stable w/c emulsions have been formed for water volume percentages from 10 to 75, as described below. Stabilization and flocculation of w/c and o/c emulsions are characterized as a function of the surfactant adsorption and the solvation of the C02-philic group of the surfactant. The last two sections describe phase transfer reactions between lipophiles and hydrophiles in w/c microemulsions and emulsions and in situ mechanistic studies of dispersion polymerization. 

Synthesis of polymer microspheres in the presence of magnetic nanoparticles, such as suspension polymerization or its modified versions, dispersion polymerization, surface-initiated radical polymerization, acid-catalyzed condensation polymerization, emulsion polymerization, mini-/microemulsion polymerization, in situ oxidative polymerization, inverse emulsion cross-linking, emulsion/double emulsion-solvent evaporation, and supercritical fluid extraction of o/w miniemulsion... 

A major new development in a related area is the work of DeSimone et al. [26,31,50,51,75,76], who conducted dispersion polymerizations in supercritical CO2. In the early stages of the dispersion-polymerization reaction, the solutions are homogenous microemulsions containing surface-active polymers with C02-philic moieties. The monomer is soluble in the continuous phase. As the polymer grows, its solubility rapidly diminishes to form precipitated polymer particles that are stabilized by the surface-active polymer. This approach has been expanded to several different polymer systems [50]. 

Dispersed polymers are also produced by inverse emulsion polymerization, miniemulsion polymerization, dispersion polymerization and microemulsion polymerization. 

Bulk or mass polymerization" Gas-phase pol3mierization Precipitation polymerization Suspension polymerization Microsuspension polymerization Dispersion polymerization Emulsion polymerization Miniemulsion polymerization Microemulsion polymerization... 

A broad range of polymers are produced by polymerization in heterogeneous media, including polyolefins manufactured by slurry (high density polyethylene and isotactic polypropylene) and gas phase (linear low density polyethylene and high density polyethylene) polymerization coatings and adhesives produced by emulsion and miniemulsion polymerization flocculants obtained by inverse emulsion and microemulsion polymerization poly(vinyl chloride) (PVC) and polystyrene produced by suspension polymerization and toners synthesized by dispersion polymerization. As a whole, they represent more than 50% of the polymer produced worldwide [1]. 

In this chapter, emulsion, miniemulsion, microemulsion, suspension, and dispersion polymerization are discussed. Polyolefins are outside the scope of the chapter. 

Besides emulsion polymerization, all of the heterogeneous processes of polymerization use surfactants. The only exception is precipitation polymerization, such as of vinyl chloride, where the only active ingredients are the monomer and the initiator. These heterogeneous processes are suspension polymerization, dispersion polymerization, polymerization in mini- and microemulsion, and, finally, micellar polymerization. Among these processes only the suspension polymerization meet industrial importance, chiefly for vinyl chloride and styrenic compounds. 

In the second chapter (Preparation of polymer-based nanomaterials), we summarize and discuss the literature data concerning of polymer and polymer particle preparations. This includes the description of mechanism of the radical polymerization of unsaturated monomers by which polymer (latexes) dispersions are generated. The mechanism of polymer particles (latexes) formation is both a science and an art. A science is expressed by the kinetic processes of the free radical-initiated polymerization of unsaturated monomers in the multiphase systems. It is an art in that way that the recipes containing monomer, water, emulsifier, initiator and additives give rise to the polymer particles with the different shapes, sizes and composition. The spherical shape of polymer particles and the uniformity of their size distribution are reviewed. The reaction mechanisms of polymer particle preparation in the micellar systems such as emulsion, miniemulsion and microemulsion polymerizations are described. The short section on radical polymerization mechanism is included. Furthermore, the formation of larger sized monodisperse polymer particles by the dispersion polymerization is reviewed as well as the assembling phenomena of polymer nanoparticles. 

Cases (i) and (ii) can be analyzed together with other polymerization techniques in dispersed media (miniemulsion, microemulsion, dispersion, etc.) used to produce the desired functionalized latex particles and taking into consideration the reactor type used to carry out the different polymerization reactions. 

The use of other polymerization techniques in dispersed media, but different from emulsion polymerization, is also introduced in the recent literature about functionalized polymer particles. Among them there are miniemulsion, microemulsion, and dispersion polymerizations. 

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... 

Emulsifiers are used in many technical applications. Emulsions of the oil-in-water and the water-in-oil type are produced on a large scale in the cosmetic industry. Other fields of employment are polymerization of monomers in emulsions and emulsification of oily and aqueous solutions in lubricants and cutting oils. In enhanced oil recovery dispersing of crude oil to emulsions or even microemulsions is the decisive step. 

By performing in situ the polymerization of acrylamide in water/AOT/toluene microemulsions, clear and stable inverse latexes of water-swollen polyacrylamide particles stabilized by AOT and dispersed in toluene have been found [192-194], It was shown that the final dispersions consist of two species of particles in equilibrium, surfactant-coated polymer particles (size about 400 A) with narrow size distribution and small AOT micelles (size about 30 A). 

In contrast to the highly interconnected pores mentioned previously, closed pores can also be obtained by microemulsion polymerization if the initial volume fraction of the dispersed phase is kept lower than 30%. Recently two systems have been reported where the polymerization of the continuous phase and the subsequent removal of the Hquid dispersed phase resulted in the formation... 

In this review we summarize and discuss the amphiphilic properties of polyoxyethylene (PEO) macromonomers and PEO graft copolymer molecules, the aggregation of amphiphilic PEO macromonomers into micelles, the effect of organized aggregation of macromonomers on the polymerization process, and the kinetics of radical polymerization and copolymerization of PEO macromonomer in disperse (dispersion, emulsion, miniemulsion, microemulsion, etc.) systems [1-5]. 

Several morphological studies of polymeric microemulsion have involved graft and block PSt/PEO copolymers in ternary solvent mixtures containing toluene, water, and either an alcohol or an amine acting as a cosurfactant [57,58]. The co-emulsifier increases the dispersion capacity of the copolymers. 

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. 

Shang et al. (61) used microemulsion polymerization to synthesize MWCNT-PMMA composites for gas sensor applications. Better dispersion, enhanced electrical conductivity and better sensor response was observed for in-situ fabricated composites compared to composites prepared by solution mixing. Ma et al. (62) performed in-situ polymerization of MWCNT-PMMA composites in the presence of an AC electric field to study dispersion and alignment of MWCNT in PMMA matrix induced by the electric field. Experimental evidences from in-situ optical microscopy, Raman spectroscopy, SEM and electrical conductivity showed that both dispersion and alignment qualities were significantly enhanced for oxidized MWCNT compared to pristine MWCNT. 

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