Inverse emulsion polymerization, particle

In an inverse emulsion polymerization, a hydrophilic monomer, frequently in aqueous solution, is emulsified in a continuous oil phase using a water-in-oil emulsifier and polymerized using either an oil-soluble or water-soluble initiator the products are viscous latices comprised of submicroscopic, water-swollen, hydrophilic polymer particles colloidally suspended in the continuous oil phase. The average particle sizes of these latices are as small as 0.05 microns. The technique is applicable to a wide variety of hydrophilic monomers and oil media. The inverse emulsion polymerization of sodium p-vinylbenzene sulfonate initiated by both benzoyl peroxide and potassium persulfate was compared to the persulfate-initiated polymerization in aqueous solution. Hypotheses for the mechanism and kinetics of polymerization were developed and used to calculate the various kinetic parameters of this monomer. 

In an inverse emulsion polymerization, an aqueous solution of a hydrophilic monomer is emulsified in a continuous hydrophobic oil phase using a water-in-oil emulsifier. The polymerization is initiated with either oil-soluble or water-soluble initiators. Figure 2 shows a schematic representation of this system. The formation of micelles is uncertain, but is portrayed speculatively. The hydrophilic part of the emulsifier molecule is oriented toward the hydrophilic dispersed phase and the hydrophobic part toward the hydrophobic continuous phase. The initiation of polymerization proceeds by a mechanism analogous to that of the conventional system and submicroscopic particles of water-swollen hydrophilic polymer are generated in the continuous oil phase. 

Emulsion polymerization typically refers to the polymerization of a nonaqueous material in water. The polymerization of a water-soluble material in a nonaqueous continuum has been called inverse emulsion polymerization. The inverse emulsion polymerization technique is used to synthesize a wide range of polymers for a variety of applications such as wall paper adhesive, waste water fiocculant, additives for oil recovery fluids, and retention aids. The emulsion polymerization technique involves water-soluble polymer, usually in aqueous solution, emulsified in continuous oil phase using water in oil emulsifier. The inverse emulsion is polymerized using an oil- or water-soluble initiator. The product is a colloidal dispersion of sub-microscopic particles with particle size ranging from 0.05 to 0.3 pm. The typical water-soluble monomers used are sodium p-vinyl benzene sulfonate, sodium vinyl sulfonate, 2-sulfo ethyl acrylate, acrylic acid, and acrylamide. The preferred emulsifiers are Sorbitan monostearate and the oil phase is xylene. The proposed kinetics involve initiation in polymer swollen micelles, which results in the production of high molecular weight colloidal dispersion of water-swollen polymer particles in oil. 

Synthesis of gel particles in the pm-range (micro-gels) (Pelton 2000) using different techniques, e.g., thermo-sensitive micro-gels based on NIPAAm by inverse suspension polymerization (Bajpai et al. 2007) or inverse emulsion polymerization (Hirotsu et al. 1987). 

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]. 

Typical emulsion polymerizations utilize oil-soluble monomers dispersed in an aqueous media with a water-soluble initiator, whereas inverse emulsions employ water-soluble monomers dispersed in an organic medium containing an oil-soluble initiator. The insoluble polymer particles that result from these reactions are stabilized as colloids in solution by repulsive forces imparted by a small molecule ionic surfactant and/or an amphiphilic macromolecular stabilizer (56,57). These reactions produce high molecular weight, spherical polymer particles with sizes typically smaller than 1 fj,m. Since most common monomers are highly soluble in CO2, there are very few examples of C02-based emulsion poljnnerizations. However, acrylamide is a rare example of a vinyl monomer that has a low solubility in CO2 at moderate temperatures and pressures. The AIBN-initiated water-in-oil or inverse emulsion polymerization of acrylamide has been attempted at 65°C in 35.2 MPa (352 bar) CO2 (41,58) (eq. (3)). 

Inverse emulsion polymerization is used for the preparation of polymers with ultrahigh molecular masses. For this type of polymerization, the expression dispersion polymerization is often used in the literature [410]. A concentrated monomer solution (about 40% monomer in water) is dispersed under intensive stirring in aliphatic or aromatic hydrocarbons in the presence of additives (emulsifiers, protective colloids). Polymerization can be initiated by either water-soluble or oil-soluble initiators [411-418]. The advantage of this process is based on the constant viscosity of the reaction mixture, as the increase of viscosity takes place only in the dispersed phase. By the use of additives (tensides), the dispersion inverts when the emulsion is stirred into water. Precipitation from the aqueous solution yields a polymer with ultrahigh molar mass. The quality of polymer made by inverse emulsion polymerization is influenced by the following factors (1) species and concentration of initiator, (2) species and concentration of additives (emulsifiers, protective colloids), (3) type of oil phase, and (4) particle size of the dispersed water phase. Because of the easy modification of all these parameters, much attention has been given in recent years to water-in-oil emulsion polymerization of AAm and MAAm. 

When a water-miscible polymer is to be made via a suspension process, the continuous phase is a water-immiscible fluid, often a hydrocarbon. In such circumstances the adjective inverse is often used to identify the process [118]. The drop phase is often an aqueous monomer solution which contains a water-soluble initiator. Inverse processes that produce very small polymer particles are sometimes referred to as inverse emulsion polymerization but that is often a misnomer because the polymerization mechanism is not always analogous to conventional emulsion polymerization. A more accurate expression is either inverse microsuspension or inverse dispersion polymerization. Here, as with conventional suspension polymerization, the polymerization reaction occurs inside the monomer-containing drops. The drop stabilizers are initially dispersed in the continuous (nonaqueous phase). If particulate solids are used for drop stabilization, the surfaces of the small particles must be rendered hydrophobic. Inverse dispersion polymerization is used to make water-soluble polymers and copolymers from monomers such as acrylic acid, acylamide, and methacrylic acid. These polymers are used in water treatment and as thickening agents for textile applications. Beads of polysaccharides can also be made in inverse suspensions but, in those cases, the polymers are usually preformed before the suspension is created. Physical changes, rather than polymerization reactions, occur in the drops. Conventional stirred reactors are usually used for inverse suspension polymerization and the drop size distribution can be fairly wide. However, Ni et al. [119] found that good control of DSD and PSD could be achieved in the inverse-phase suspension polymerization of acrylamide by using an oscillatory baffled reactor. 

Inverse emulsion polymerization comprises emulsification of a water-miscible monomer, usually in aqueous solution, in a continuous oil medium using a water-in-oil emulsifier and polymerization using an oil-soluble or water-soluble initiator to give a colloidal dispersion of water-swollen polymer particles in oil. The average particle size of inverse latexes is usually 0.05-0.3ym in contrast to the original droplet size of 0.05-10ym. 

The mechanism and kinetics of the inverse emulsion polymerization of p-sodium styrene sulfonate was investigated using both oil-soluble and water-soluble initiators (53). Table XI gives the recipe used in these polymerizations. The p-sodium styrene sulfonate was dissolved in the water, and the solution was emulsified in the o-xylene using the Span 60 (sorbitan monostearate ICI America) emulsifier in some experiments, the benzoyl peroxide was dissolved in the o-xylene in others, the potassium persulfate was dissolved in the aqueous phase. The polymerizations were carried out at 40-70°. The rates of polymerization were measured dilatometrically, the molecular weights by solution viscosity, and the latex particle sizes by electron microscopy. 

Figure 2.15 (a) The procedure for preparation of Fe304/PS composite particles by inverse emulsion polymerization, (b) TEM images of Fe304/PS composite microspheres [24]. 

The photochemical UV radiation method was first employed by Leong and Candau [41] for the radical polymerization of acrylamide in inverse microemulsions stabilized by Aerosol OT. The polymerization was carried out using AIBN initiator and induced by UV irradiation. It was shown that the use of a microemulsion rather than an emulsion led to stable and clear microlatices d 50 nm) of uniform size, thus providing a way to overcome some of the problems of conventional inverse emulsion polymerization, such as instability of the latexes resulting in rapid flocculation and a broad particle size distribution. 

Polymer-Clay Nanocomposite Particles by Direct and Inverse Emulsion Polymerization... 

Due to the hydrophilic nature of nascent clays, one may envisage that hydrophilic clays can be encapsulated by inverse emulsion polymerization. As discussed in Section 3.2, in a direct emulsion containing hydrophilic clays, clays may be predominately located in the continuous aqueous phase and some may reside at the surface of the micelles (forming a Pickering-type emulsion in the presence or absence of surfactants) after polymerization, armored particles with clay covering the particles have been commonly observed (Figure 3.2a). 

We have tested this hypothesis in the inverse emulsion polymerization of acrylamide (A Am). A typical recipe ineluded A Am as the monomer dissolved in water, cyclohexane as the continuous phase, sodium bis(2-ethylhexyl) sul-fosuccinate (AOT) or sorbitan monooleate (Span-80) as the surfactant and MMT or LRD as typical nascent clays. Both oil- and water-soluble azo compounds, such as 2,2 -azobisisobutyronitrile (AIBN) and 2,2 -azobis[2-methyl-A-(2-hydroxyethyl)propionamide] (VA-086), were used as initiators. Span-80 appeared to lead to more stable inverse emulsions in the presence of clay than AOT. We first noticed a different kinetic behavior during polymerization in the presence of clay platelets. The conversion time history for the AAm inverse emulsion polymerization in the presence of MMT showed a significant decrease in the rate of polymerization in comparison with those without clay. With increasing concentration of clay particles, lower rates of polymerizations and lower final monomer conversions were found, together with increased retardation of the polymerization. The clay platelets in the inverse emulsions might act as diffusion barriers for monomer and/or initiator. 

Figure 3.2 Schematic representation of (a) oil-in-water (O/W) emulsion polymerization m the presence of hydrophilic clay platelets and (b) water-in-oil (W/O) inverse emulsion polymerization of hydrophilic monomer with hydrophilic clay which, hypothetically, may lead to clay encapsulated inside the latex particles.

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