A method is developed to enable emulsion polymerization to be performed under RAFT control to give living character without the problems that often affect such systems: formation of an oily layer, loss of colloidal stability, or loss of molecular weight control. Trithiocarbonate RAFT agents are used to form short stabilizing blocks from a water-soluble monomer, from which diblocks can be created by the subsequent polymerization of a hydrophobic monomer. These diblocks are designed to self-assemble to form micelles. Polymerization is initially performed under conditions that avoid the presence of monomer droplets during the particle formation stage and until the hydrophobic ends of the diblocks have become sufficiently long to prevent them from desorbing from the newly formed particles. Polymerization is then continued at any desired feed rate and composition of monomer. The polymer forming in the reaction remains under RAFT control throughout the polymerization; molecular weight polydispersities are generally low. The number of RAFT-ended chains within a particle is much larger than the aggregation number at which the original micelles would have self-assembled, implying that in the early stages of the polymerization, there is aggregation of the micelles and/or migration of the diblocks. The latexes resulting from this approach are stabilized by anchored blocks of the hydrophilic monomer, e.g., acrylic acid, with no labile surfactant present. Sequential polymerization of two hydrophobic monomers gives completely novel core-shell particles where most chains extend from the core of the particles through the shell layer to the surface.

Ab initio emulsion polymerization by RAFT-controlled self-assembly

Electrokinetic properties, colloidal stability and aggregation kinetics of polymer colloids

The encapsulation of titanium dioxide (TiO2) particles via styrene miniemulsion polymerization requires two successive dispersion steps. First, the TiO2 particles must be successfully dispersed in the monomer phase. Second, this phase must be dispersed in an aqueous surfactant solution to form stable submicron droplets. The oil/water interface between the droplets and the aqueous phase can be affected not only by the surfactant used but also the components present in the oil phase. Interfacial tensions between the phases were measured. The presence of insufficient or excess stabilizer both reduced the interfacial tensions. This was attributed to migration of the partially covered hydrophilic TiO2 particles or the excess OLOA 370 stabilizer (polybutene–succinimide pentamine) to the oil/water interface. The free surfactant in the aqueous phase and the average droplet size of the miniemulsions were characterized as a function of the process variables. The presence of TiO2 particles within the droplets limited the ability of the process to reduce the droplet size although miniemulsion droplets in the size range of 150–200 nm were still obtained for subsequent polymerizations. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4431–4440, 2000

Encapsulation of inorganic particles via miniemulsion polymerization. II. Preparation and characterization of styrene miniemulsion droplets containing TiO2 particles

The application of the miniemulsion polymerization technique to the encapsulation of titanium dioxide (TiO2) inside polystyrene latex particles was investigated. Complete encapsulation, meaning all of the polystyrene encapsulating all of the TiO2 in the colloidal particles, was not achieved. The most successful encapsulations were only achieved when the TiO2 particles, either hydrophilic or hydrophobic, were well dispersed in the styrene monomer prior to formation and polymerization of the miniemulsions. The TiO2 dispersions in styrene were prepared by sonification of the TiO2 in the presence of an adsorbing steric stabilizer OLOA 370 (polybutene–succinimide pentamine). Miniemulsions were prepared by dispersing the monomer phase (also containing hexadecane as a costabilizer and polystyrene to enhance the nucleation of the droplets) in water using sodium lauryl sulfate as surfactant. The latexes resulting from the subsequent polymerizations were characterized in terms of the encapsulation efficiencies (via density gradient column separations) and particle size. The maximum encapsulation efficiencies (83% TiO2 and 73% polystyrene) were achieved using hydrophilic TiO2 particles stabilized with 1.0 wt % OLOA 370. As the density of the particles collected from the density gradient column increased from zone to zone, both the average particle size and number of the TiO2 particles contained in each latex particle increased with the largest particle size (209 nm) containing an estimated 22 TiO2 particles. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4441–4450, 2000

Encapsulation of inorganic particles via miniemulsion polymerization. III. Characterization of encapsulation

Layer-by-Layer polyelectrolyte assemblies for encapsulation and release of active compounds.

Latexes have been cleaned for surface characterization by ion exchange and dialysis. Ion exchange is rapid and gives quantitative results, but requires purification of the ion exchange resins. Dialysis is slow and removes only part of the adsorbed emulsifier and replaces only part of the counterions with H+ ions. This paper describes a new “serum replacement” method, which gives results in much better agreement with those of ion exchange than those of dialysis, yet avoids the arduous purification of the ion exchange resins. Distilled, deionized water is pumped through the latex while the particles are confined in a stirred cell by a Nuclepore filtration membrane. This washing with water removes the solute electrolyte and adsorbed emulsifier, but does not completely replace the Na+ and K+ counterions with H+ ions. Further washing with 5 × 10−4 N hydrochloric acid, followed by water to remove excess acid, replaces these counterions completely. Seven polystyrene latexes prepared with persulfate ion initiator—three prepared with conventional emulsifier and four with sulfonated comonomeric emulsifiers—were cleaned by serum replacement, ion exchange, and dialysis. The surface charges after cleaning with serum replacement were in good agreement with those after cleaning with ion exchange to constant charge, while those after dialysis were much lower. All latexes contained only strong-acid groups; no weak-acid groups were found. For latexes prepared with comonomeric emulsifiers, serum replacement can separate the water-soluble polymer in a form suitable for analysis. The removal of solute electrolyte and adsorbed emulsifier by serum replacement can be predicted by mathematical models. The rate of removal of solute electrolyte is slower than predicted by theory for reasons not yet determined. The rate of removal of adsorbed sodium lauryl sulfate indicates that equilibrium between the adsorbed and solute species is not attained during the serum replacement process.

Cleaning latexes for surface characterization by serum replacement

The adsorption of emulsifier on latex particle surfaces often determines the stability of the latex. However, relatively few adsorption isotherms have been published because these experiments are difficult and time-consuming. This paper describes the determination of adsorption isotherms using a new serum replacement technique, which was developed to characterize latex particle surfaces by removing adsorbed emulsifier from the latex particle surfaces and solute electrolyte from the aqueous phase. The latex is confined in a cell with a Nuclepore filter membrane, and doubly-distilled, deionized water is pumped through the latex to replace the serum. This method allows complete recovery of the serum, as well as determination of the concentration profile of the desorbing species in the effluent stream. To determine adsorption isotherms, a known amount of emulsifier is added to a cleaned latex and the concentration profile of the emulsifier in the effluent stream is measured. A material balance between added emulsifier and desorbed emulsifier (from the concentration profile) gives the adsorption isotherm. This adsorption isotherm is correlated with the predictions of a mathematical model of the process. The adsorption of technical-grade sodium lauryl sulfate, Aerosol MA, and Aerosol OT on a Dow monodisperse polystyrene latex of 190 nm diameter determined at 23 ± 2° follows a Langmuir adsorption isotherm. The mole-r culac areas at saturation are 42 - 43 A2 , 39 - 45 A2, and 85 -100 A2, respectively. The adsorption of sodium lauryl sulfate on a monodisperse polymethyl methacrylate latex of 330 nm diameter also follows a Langmuir absorption isotherm and gives a molecular area at saturation of 57 A2. The adsorption of sodium lauryl sulfate on a monodisperse polyvinyl acetate latex of 165 nm diameter does not follow a Langmuir adsorption isotherm, but instead gives a curve characteristic of porous surfaces. The molecular area at saturation is 30 A2.

Rapid Measurement of Adsorption Isotherms of Emulsifiers on Latex Particles

The serum replacement technique has been used to “clean” an industrial ethyl aerylate-methyl methacrylate copolymer latex prepared in the presence of acrylic acid monomer. The “cleaning” method allowed the separation of the cleaned latex particles from the serum in a form suitable for further analysis. Both the cleaned latex particles and the serum were analyzed by several methods. Serum replacement recovered about 90% of an anionic surfactant and almost all of the nonionic surfactant used in the preparation of the latex. 17.6% of the initiators used in the preparation of the latex were found to be chemically-bound to the surface of the particles. The acrylic acid monomer found on the surface of the particles and in the aqueous phase amounted to 29.85% and 28.61%, respectively, of the total amount of acrylic acid used in the preparation of the latex. The rest of the acrylic acid was assumed to be buried inside the latex particles.

S. M. Ahmed

Papers

Cleaning latexes for surface characterization by serum replacement

Rapid Measurement of Adsorption Isotherms of Emulsifiers on Latex Particles

Application of the Serum Replacement Technique in the Characterization of an Ethyl Acrylate-Methyl Methacrylate Copolymer Latex

Desorption of Sodium Lauryl Sulfate from Polystyrene Latex Particle Surfaces