Inverse latexes

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). 

The formation of coagulum is observed in all types of emulsion polymers (i) synthetic rubber latexes such as butadiene-styrene, acrylonitrile-butadiene, and butadiene-styrene-vinyl pyridine copolymers as well as polybutadiene, polychloroprene, and polyisoprene (ii) coatings latexes such as styrene-butadiene, acrylate ester, vinyl acetate, vinyl chloride, and ethylene copolymers (iii) plastisol resins such as polyvinyl chloride (iv) specialty latexes such as polyethylene, polytetrafluoroethylene, and other fluorinated polymers (v) inverse latexes of polyacrylamide and other water-soluble polymers prepared by inverse emulsion polymerization. There are no major latex classes produced by emulsion polymerization that are completely free of coagulum formation during or after polymerization. 

In an inverse latex, these electrostatic forces are quite difFerent. For water-in-oil emulsions, Albers and Overbeek (J, 2, 3) found that, with ionic emulsifiers, flocculation was promoted by gravity because of the diffuse nature of the electric double layer relative to that of oil-in-water systems and, with nonionic emulsifiers,... 

The stability of the inverse latex strongly depends on an appropriate formulation. Poor chemical compatibility between oils and emulsifiers produces unstable latices, whereas a good chemical match leads to perfectly transparent and stable latices [17,18]. The latex stability was accounted for by (1) reduced gravity forces ( /, where d is the particle diameter) (2) high entropic contribution from the droplets owing to their large number and (3) low interfacial tension between polymer droplets and the continuous phase [4]. 

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. 

To find out why the hydrophilic platelets were located at the interface between a hydrophilic polymer and a hydrophobic continuous phase in the inverse latexes, cryo-TEM was used to examine the locations of the platelets in the starting inverse emulsions. Conventional TEM requires dried samples and cannot be used to characterize the starting inverse emulsions containing clay platelets. We examined a model inverse emulsion comprising water, cyclohexane and clay particles, with Span-80 as the surfactant. In the absence of clay particles, the cryo-TEM image showed that water droplets of 100-200 nm in diameter were coated with surfactant molecules. The surfactant-stabilized water droplets appeared to bundle together and formed cauliflower-like structures. When either LRD or MMT was added to the inverse emulsion, the cauliflowerlike structure appeared to remain intact, but the clay platelets were located at the interface between the water droplets and cyclohexane, not inside the water... 

Inversion ofMon cjueous Polymers. Many polymers such as polyurethanes, polyesters, polypropylene, epoxy resins (qv), and siHcones that cannot be made via emulsion polymerization are converted into latices. Such polymers are dissolved in solvent and inverted via emulsification, foUowed by solvent stripping (80). SoHd polymers are milled with long-chain fatty acids and diluted in weak alkaH solutions until dispersion occurs (81). Such latices usually have lower polymer concentrations after the solvent has been removed. For commercial uses the latex soHds are increased by techniques such as creaming. 

The polymeric latex obtained in a hydrophobic organic solvent is poorly dispersed in water because of the presence of an emulsifier with a low HLB value. For this reason, a wetting agent is added to water or emulsion prior to the dissolution. The wetting agent (a surface active substance with a high HLB value) facilitates the inversion of latex phases to produce a direct type emulsion. Usually, it belongs to oxyethylated alkylphenols, fatty alcohols, or fatty acids. 

If only electrostatic effects are responsible for polymer adsorption and flocculation, our results can be explained according to the same scheme as that used by Furusawa et Al.(20) to interprete the destabilization of negatively charged latex by a cationic polymer. In a first step, the adsorption of the polymer leads to the neutralization of the particles which are no more stabilized by electrostatic repulsions and there is flocculation (we have not studied this step since in our experiments polymer was always in large excess with respect to Al(0H)3). In a second step the adsorption inverses the charge and (we have indeed measured by... 

In solution, block copolymers display interesting colloidal and interfacial properties. They can be used as emulsifying agents in water-oil and oil-oil systems (6 ). In the later case, the oil phases are solid and they give rise to polymeric alloys (7.) or they are liquid and they allow the preparation of latexes in organic medium (8 ). However, the molecular structure of block copolymers based on polybutadiene PB (70 ) and polystyrene PS behave as thermoplastic elastomers when engaged in multiblock (PB-PS)n or triblock (PS-PB-PS) structures but never when implied in inverse triblock or diblock arrangements. Similarly the... 

Anchorage dependency has been recognized as a requirement for cell viability and the shape of a cell after adhesion is a detenninant of cell expression. Hohner and Denker examined hormone production from cells attached to substrata of both rigid and soft hydrogels of the same composition. The cells attached to the soft gel were more productive and this was attributed to the fact that the cells were more rounded. Mescher studied cytotoxic T lymphocytes and latex particles of various diameters coated with ligands for T-cell receptors. An inverse relationship between 2005 by CRC Press LLC... 

Impact strength of ABS graft resins will increase with increasing particle size of the substrate latex when the substrate surface area per grafted copolymer chain is maintained at equivalent values. At 20 wt % substrate levels the increase in impact strength appears linear with increasing particle size, but at 30 wt % substrate, nonlinearity is indicated wjien the substrate particle is less than 1100 A. The inverse relationship of tensile and impact strength is preserved. 

The nucleation mechanism of dispersion polymerization of low molecular weight monomers in the presence of classical stabilizers was investigated in detail by several groups [2,6,7]. It was, for example, reported that the particle size increased with increasing amount of water in the continuous phase (water/eth-anol), the final latex radius in their dispersion system being inversely proportional to the solubility parameter of the medium [8]. In contrast, Paine et al.[7] reported that the final particle diameter showed a maximum when Hansen polarity and the hydrogen-bonding term in the solubility parameter were close to those of steric stabilizer. 

Since the accuracy of the light scattering apparatus and the reliability of the inversion procedure had been proven by comparison of four methods on Dow polystyrene latex LS-1028-E (6), it was decided to examine a larger latex in more detail as a further study of the method of determining the internal structure of a latex by wide-angle light scattering. 

Therefore, at a constant value of Ri and K, there is a surface value above which no new particles form. In growth experiments where the surface was increased gradually, no formation of new particles could be observed above a certain surface value, and latexes appeared monodispersed. For spherical particles, capture rate per unit surface (K) is inversely proportional to particle radius in fact we have ... 

It was found that the chain length of the resulting polymer is inversely proportional to the square root of the initiator concentration [66], underlining that the reaction in miniemulsion is rather direct and close to an ideal radical polymerization. It could be shown that the amount of initiator used for polymerizing the latex does not have an effect on the number of nucleated droplets which shows that droplet nucleation is by far the dominant mechanism over the whole range of initiator concentrations. 

In the case of inverse systems, hydrophilic monomers such as hydroxyethyl acrylate, acrylamide, and acrylic acid were miniemulsified in non-polar media, e.g., cyclohexane or hexadecane [45,46]. Rather small and narrow distributed latexes in a size range between 50 nmsynthesized with nonionic amphiphilic block copolymers. Depending on the system, the surfactant loads can be as low as 1.5 wt% per monomer, which is very low for an inverse heterophase polymerization reaction and clearly underlines the advantages of the miniemulsion technique. 

Here, kd is the inverse of the Debye length. Even though the -potentials for latex spheres may exceed 25 mV and, therefore, require a more complex equation to relate to mobility (as per O Brien and White [265]), the low ionic strength (small kd) of El-FFF measurements should still ensure a proportionality between pe and . From the retention data, it is possible to obtain quantitative information regarding either the -potential of samples with known particle size eluting from the channel or the particle size, if the electrophoretic mobility is known. 

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