Microemulsion polymerization

Microemulsion polymerization [114] involves the polymerization of oil-in-water and water-in-oil monomer microemulsions. Microemulsions are thermodynamically stable and isotropic dispersions, whose stability is due to the very low interfacial tension achieved using appropriate emulsifiers. Particle nucleation occurs upon entry of a radical into a microemulsion droplet. Microemulsion polymerization allows the production of particles smaller than those obtained by emulsion polymerization. This leads to a higher number of polymer particles, which results in a more compartmentalized system. Under these conditions, the life-time of the polymer chains increases leading to ultra-high molecular weights. Inverse microemulsion polymerization is used to produce highly efficient flocculants. 

While microemulsions can be used as potential media for polymerization in which large molecular - weight polymers with narrower molecular weight distribution (MWD) may be achieved. The microemulsion polymerization system consists of three phases an aqueous phase (containing initiator, emulsifier, coemulsifier and some amount of monomer), emulsified monomer microdroplets or the monomer swollen micelles and monomer swollen polymer particles. Water is a most important ingredient of the microemulsion polymerization system. It is inert and acts as the 

The most commonly used water - soluble initiator is potassium, ammonium or sodium salt of peroxodisulfate. Oil-soluble initiators, such as azo compounds, benzoyl peroxides, etc., are also used in microemulsion polymerization. They are, however, less efficient than water - soluble peroxodisulfates. The initiation of microemulsion polymerization is a two - step process  

1) It starts in water by the primary free radicals derived from the water - soluble initiator. 

2) The second step occurs in the monomer- swollen micelles by entered oligomeric radicals. 

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

Unlike the conventional milky white emulsion, the transparent or translucent reaction system comprising microemulsion droplets is thermodynamically 

In summary, there has been a tremendous effort devoted to the fundamental aspects of emulsion polymerization mechanisms, kinetics and processes since the early twentieth century. Representative review or journal articles concerning the conventional emulsion polymerization can be found in literature [20-25, 48-60]. The research areas related to both miniemulsion [42,61-64] and microemulsion [44-47, 65] polymerizations have received increasing interest recently. 

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Burban J FI, Fie M and Cussler E L 1995 Silica gels made by bicontinuous microemulsion polymerization AlChE J. 41 159-65... 

Manufacturing processes have been improved by use of on-line computer control and statistical process control leading to more uniform final products. Production methods now include inverse (water-in-oil) suspension polymerization, inverse emulsion polymerization, and continuous aqueous solution polymerization on moving belts. Conventional azo, peroxy, redox, and gamma-ray initiators are used in batch and continuous processes. Recent patents describe processes for preparing transparent and stable microlatexes by inverse microemulsion polymerization. New methods have also been described for reducing residual acrylamide monomer in finished products. 

Microemulsion Polymerization. Polyacrylamide microemulsions are low viscosity, non settling, clear, thermodynamically stable water-in-od emulsions with particle sizes less than about 100 nm (98—100). They were developed to try to overcome the inherent settling problems of the larger particle size, conventional inverse emulsion polyacrylamides. To achieve the smaller microemulsion particle size, increased surfactant levels are required, making this system more expensive than inverse emulsions. Acrylamide microemulsions form spontaneously when the correct combinations and types of oils, surfactants, and aqueous monomer solutions are combined. Consequendy, no homogenization is required. Polymerization of acrylamide microemulsions is conducted similarly to conventional acrylamide inverse emulsions. To date, polyacrylamide microemulsions have not been commercialized, although work has continued in an effort to exploit the unique features of this technology (100). 

The mechanism of crosslinking emulsion polymerization and copolymerization differs significantly from linear polymerization. Due to the gel effect and, in the case of oil-soluble initiators, monomer droplets polymerize preferentially thus reducing the yield of microgels. In microemulsion polymerization, no monomer droplets exist. Therefore this method is very suitable to form microgels with high yields and a narrow size distribution, especially if oil-soluble initiators are used. 

Inverse least squares, 539-41 Inverse micelles, 25487 Inverse microemulsion polymerization, 20 461... 

Microemulsion polymerization, acrylamide polymers, 1 322-323 Microemulsion publications, growth in, 16 420t... 

The procedure chosen for the preparation of lipid complexes of AmB was nanoprecipitation. This procedure has been developed in our laboratory for a number of years and can be applied to the formulation of a number of different colloidal systems liposomes, microemulsions, polymeric nanoparticles (nanospheres and nanocapsules), complexes, and pure drug particles (14-16). Briefly, the substances of interest are dissolved in a solvent A and this solution is poured into a nonsolvent B of the substance that is miscible with the solvent A. As the solvent diffuses, the dissolved material is stranded as small particles, typically 100 to 400 nm in diameter. The solvent is usually an alcohol, acetone, or tetrahydrofuran and the nonsolvent A is usually water or aqueous buffer, with or without a hydrophilic surfactant to improve colloid stability after formation. Solvent A can be removed by evaporation under vacuum, which can also be used to concentrate the suspension. The concentration of the substance of interest in the organic solvent and the proportions of the two solvents are the main parameters influencing the final size of the particles. For liposomes, this method is similar to the ethanol injection technique proposed by Batzii and Korn in 1973 (17), which is however limited to 40 mM of lipids in ethanol and 10% of ethanol in final aqueous suspension. 

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... 

Microemulsion polymerization is an emulsion polymerization with very much smaller monomer droplets, about 10-100 nm compared to 1-100 pm. Micelles are present because the surfactant concentration is above CMC. The final polymer particles generally have diameters of 10-50 nm. Although many of the characteristics of microemulsion polymerization parallel those of emulsion polymerization, the details are not exactly the same [Co et al., 2001 de Vries et al., 2001 Lopez et al., 2000 Medizabial et al., 2000]. Water-soluble initiators are commonly used, but there are many reports of microemulsion polymerization with... 

The most basic form of MIP nanomaterials is the spherical nanoparticle, obtained by a number of techniques such as microemulsion polymerization [99-101], and polymerization in diluted solutions resulting in nanospheres and microgels [102-106]. Microgels (also sometimes referred to as nanogels) are particularly interesting, since they represent soluble, though cross-linked, MIPs with a size in the low nanometer range, close to that of proteins. 

Microemulsion polymerizations follow a different mechanism from the conventional emulsion polymerizations. The most probable locus of particle nucle-ation was suggested to be the microemulsion monomer droplets [27], although homogeneous nucleation was not completely ruled out. The particle generation rate in microemulsion polymerization is given by an expression similar to Eq. (21), which was used for the miniemulsion polymerization of styrene [28] ... 

The presence of a very large number micelles indicates that radicals are captured predominantly by the monomer-swollen micelles. Each entry of a radical to a monomer-swollen micelle leads to a nucleation event and therefore the particle number increases with conversion. The particle growth is supposed to be a result of propagation of monomer and the agglomeration of primary particles. The dead monomer-swollen polymer particles and uninitiated monomer-swollen micelles serve as a reservoir of monomer. Solution or bulk polymerization kinetics seem to govern the microemulsion polymerization process [30,31]. 

Microemulsion polymerization, as the name implies, involves free-radical polymerization in extremely small size, microemulsified monomer droplets of about 1-10 nm diameter [792], The produced polymer particles tend to be small (10 nm) and have higher molar mass (106-107 g/mol) than can be obtained from conventional emulsion polymerization [792]. 

Chern,C-S. Microemulsion Polymerization in Encyclopedia of Polymer Science and Technology, Online edition, posted April 15, 2003, Wiley New York, 2002. 

S. Nyret and B. Vincent, The properties of polyampholyte microgel particles prepared by microemulsion polymerization, Polymer 38, 6129-6134 (1997). 

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. 

Shang et al. 2009 (61) MWCNT Purified Microemulsion polymerization CNT Loading levels 1 tol5wt% MWCNT-PMMA composites prepared by microemulsion polymerization at 8 wt% loading showed high sensor responses to different organic vapors such as acetone, toluene, THF, choloroform, acetonitrile, benzene They suggested the use of these composites for gas sensor applications ... 

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