Dispersion stability, polymer particles

Besides temperature and addition of non-solvent, pressure can also be expected to affect the solvency of the dispersion medium for the solvated steric stabilizer. A previous analysis (3) of the effect of an applied pressure indicated that the UCFT should increase as the applied pressure increases, while the LCFT should be relatively insensitive to applied pressure. The purpose of this communication is to examine the UCFT of a nonaqueous dispersion as a function of applied pressure. For dispersions of polymer particles stabilized by polyisobutylene (PIB) and dispersed in 2-methylbutane, it was observed that the UCFT moves to higher temperatures with increasing applied pressure. These results can qualitatively be rationalized by considering the effect of pressure on the free volume dissimilarity contribution to the free energy of close approach of the interacting particles. 

Micron-size particles can also be prepared by polymerization of monomer dispersion in organic solvents (e.g. alcohol) in which the emerging polymer is not soluble. At the beginning of the process, the reaction mixture is homogenous but during the reaction, stabilized polymer particles precipitate. This method offers uniform particles with a diameter of 2-15 pm (40). 

During dispersion polymerization polymer particles are formed from an initially homogeneous reaction mixture by polymerization in the presence of a polymeric steric stabilizer. The process is applicable to monomers which yield polymers that are insoluble in a solvent for the monomer. Styrene has been polymerized in alcohols, with steric stabilizers such as poly(A -vinylpyrrolidone) (see Fig. 1-4 for monomer structure) or hydroxypropyl cellulose. Hydrocarbon... 

The materials we studied are non-aqueous dispersions of polymer particles. Colloidal stability of these particles in hydrocarbon solvents is conferred by a surface covering of a highly swollen polymer (the stabilizer) on a second polymer, insoluble in the medium (the core polymer), which comprises 90 % of the material (11). These particles are prepared by dispersion polymerization polymerization of a monomer soluble in the medium to yield an insoluble polymer, carried out in the presence of a soluble polymer which becomes the stabilizer. In the examples discussed here, the core polymer is formed by free radical polymerization. Hydrogen abstraction from the soluble polymer present in the reaction medium... 

Aqueous polymerization is carried out in either an emulsion or a suspension. In the emulsion polymerization, monomers are polymerized in an aqueous medium containing a water soluble free radical initiator and an emulsifier to obtain a polymer particle slurry. In the suspension polymerization, monomers are polymerized in an aqueous medium containing both a free radical initiator which is soluble in the monomer and a dispersion stabilizer inert to telomerization to achieve the dispersion of polymer particles, followed by precipitation of the dispersion. 

Some important processes for the formation of sols involve first the formation of an emulsion or a liquid aerosol. In suspension or dispersion polymerization, a monomer or monomer mixture is emulsified to a drop size approximately the same as that of the final desired particle. Polymerization is then initiated using an initiator soluble in the monomer, so that chain growth occurs within each individual drop. The result (with luck) is a dispersion of polymer particles with the same average size as the original monomer emulsion. Normally, some type of stabilizer system is employed in the emulsification stage—a surfactant or very small particles of some material such as silica. 

Third, microgravity virtually eliminates sedimentation. In emulsion polymerization and dispersion polymerization polymer particles are formed with a higher density than the surrounding medium and tend to settle. The same sedimentation issues arise with polymer production in living cells. Microgravity allows a researcher to create colloids and dispersions with stabilities not normally achievable on the ground. 

The preparation of high molecular weight, water soluble acrylamide-based flocculants as water continuous dispersions is described. This innovative method of manufacture eliminates many of the undesirable characteristics associated with the production and application of these flocculants as conventional water-in-oil emulsions or as dry powders. The monomers and their polymers, the role of the stabilizer polymer, particle characteristics, viscosity considerations, and the thermodynamic and physical stability of these polymer diversions is discussed. 

In order to overcome the difficulties associated with inverse emulsion and dry polymers, Nalco has become involved in the development and commercial practice of a unique technology for the manufacture of high molecular weight water soluble polymers based on acrylamide. This polymerization process permits the manufacture of these extremely useful polymers as water continuous dispersions. The polymer products are liquid, and so retain the virtues of ease and safety of handling, but they are manufactured in water instead of in a hydrocarbon and surfactant matrix. Thus, no oil or surfactants are released to the environment with the application of these polymers. The performance of these polymers in the various end use applications is equivalent to, or in some cases exceeds, that obtained with similar polymers produced in inverse emulsion or dry form. A discussion of this dispersion polymerization technology, the monomers and their polymers, the stabilizer polymers, particle characteristics, viscosity considerations and the thermodynamic and physical stability of the products constitutes the subject of this manuscript. 

In a classical heterogeneous dispersion polymerization, the continuous phase is organic in nature, althou in some instances water has been used as a component of the continuous phase to increase polarity. Research investigations have focused on the composition of the dispersion m um, reaction kinetics, the structure and influence of the stabilizer polymer, particle size, molecular weight and molecular weight distribution (4, S). Dispersions of poly(methyl methacrylate) (8) and poly(styrene) (9) are widely studied and among Ae best characterized systems. Recently, dispersion polymerizations conducted in supercritical carbon dioxide have also been reported (9-12). 

The inverse emulsion form is made by emulsifying an aqueous monomer solution in a light hydrocarbon oil to form an oil-continuous emulsion stabilized by a surfactant system (21). This is polymerized to form an emulsion of aqueous polymer particle ranging in size from 1.0 to about 10 pm dispersed in oil. By addition of appropriate surfactants, the emulsion is made self-inverting, which means that when it is added to water with agitation, the oil is emulsified and the polymer goes into solution in a few minutes. Alternatively, a surfactant can be added to the water before addition of the inverse polymer emulsion (see Emulsions). 

Nonaqueous Dispersion Polymerization. Nonaqueous dispersion polymers are prepared by polymerizing a methacryhc monomer dissolved in an organic solvent to form an insoluble polymer in the presence of an amphipathic graft or block copolymer. This graft or block copolymer, commonly called a stabilizer, lends coUoidal stabiUty to the insoluble polymer. Particle sizes in the range of 0.1—1.0 pm were typical in earlier studies (70), however particles up to 15 pm have been reported (71). 

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). 

Morphology of the enzymatically synthesized phenolic polymers was controlled under the selected reaction conditions. Monodisperse polymer particles in the sub-micron range were produced by HRP-catalyzed dispersion polymerization of phenol in 1,4-dioxane-phosphate buffer (3 2 v/v) using poly(vinyl methyl ether) as stabihzer. °° ° The particle size could be controlled by the stabilizer concentration and solvent composition. Thermal treatment of these particles afforded uniform carbon particles. The particles could be obtained from various phenol monomers such as m-cresol and p-phenylphenol. 

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