Latex stability surface

Anotlier model system consists of polymetliylmetliacrylate (PMMA) latex, stabilized in organic solvents by a comb polymer, consisting of a PMMA backbone witli poly-12-hydroxystearic acid (PHSA) chains attached to it [10]. The PHSA chains fonn a steric stabilization layer at tire surface (see section C2.6.4). Such particles can approach tire hard-sphere model very well [111. 

Earlier work (3) has shown that cleaned monodisperse polystyrene latexes stabilized with surface sulfate (and perhaps a few hydroxyl) groups an be used as model colloids. For example, the distribution of H ions in the electric double layer as determined by conductometric titration has been correlated with the particle diameter determined by ultracentrifugation (3). The conductometric titration gives two measures of the concentration of H+ ions the initial conductance of the latex and the amount of base required for neutralization. The number of H+ ions determined by conductance is always smaller than the number determined by titration. This difference is attributed to the distribution of the H+ ions in the electric double layer those closest to the particle surface contribute least to the overall conductance. This distribution is expressed as the apparent degree of dissociation a, which is defined as the ratio H+ ions... 

Thus the monodisperse polystyrene latex stabilized with strong-acid surface groups can be hydrolyzed to form a latex stabilized with the same number of nonionic hydroxyl groups, which in turn can be oxidized to form a latex stabilized with the same number of weak-acid carboxyl groups, thus offering model colloids with identical characteristics except for the type of chemically bound surface groups ---- strong-acid, weak-acid, non-... 

The failure of latex stability,and the resultant flocculation of the latex par tides, may cause the formation of coagulum that is recovered from the latex after polymerization as well as a buildup on the reactor surfaces. Moreover, the inherent instability of the latex may also cause flocculation during storage or transportation. 

On the other hand, several reports have been published that point out that when a polymeric surfactant acting as an electrosteric stabilizer is used, the rate of radical entry into a polymer particle should decrease due to a diffusion barrier of the hairy layer built up by the polymeric surfactant adsorbed on the surface of the polymer particles [34-36]. Coen et al. [34] found that in the seeded emulsion polymerization of St using a PSt seed latex stabilized elec-trosterically by a copolymer of acrylic acid (AA) and St, the electrosteric stabilizer greatly reduced the radical entry rate p compared to the same seed latex... 

Bibeau and Matijevic studied the stability of a PVC latex by addition of electrolytes. They also found surface ion concentrations derived by electrophoresis to be poor indicators of latex stability. Their stability results were found to compare favorably with DLVO theory predictions, using the actual surface concentration of potential-determining species as the basis for interpretation. That means taking into account both fixed charges and adsorbed emulsifier. 

When copolymerizing VCM with vinyl esters it appears to be the combination of two competing effects which determines the latex stability. A stability increasing effect seems to arise from increasing the polarity of the polymer particle surface, and a stability decreasing effect from increasing the softness of the polymer particles by internal plasticization. 

Copolymerization with vinyl acetate has a strong effect on the nature of the surface of the polymer particles, but the plasticization effect is comparatively weak. With increasing content of vinyl acetate in the copolymer the latex stability will pass through a distinct maximum before decreasing below the stability level of the homopolymer. 

The central theory for colloidal, and therefore latex, stability is because of the complimentary work of Derjaguin and Landau in Moscow and Verwey and Overbeek in Holland. This has become known as DLVO theory.The idea is to represent a total energy of interaction as the sum of individual attractive and repulsive potentials. Fig. 4 sketches out the van der Waals and electrostatic potentials, as well as the total interaction for a particular particle size, surface potential, and electrolyte concentration. 

The polymer latex stability obtained from the mini-emulsion polymerization with various ratios of SDS/CA decreases in the series l/3>l/10>l/l>l/6>l/0, which is consistent with the stability of monomer droplets reported by Ugelstad (l/3>l/2>l/l>l/6>l/0) [106]. The latex particle size decreases with increasing CA concentration. Furthermore, a two-dimensional hexagonal packing of surface-active molecules has been reported to be formed at a molar ratio of SDS/CA=l/3 in the colloidal system [107]. The good packing of the oil-water interfacial zone leads to satisfactory stability of monomer droplets, and it remains intact throughout the polymerization. 

Emulsion Polymerization. As noted with suspension polymerization, emulsion polymerization also involves the dispersion of VCM in an aqueous medium. As distinguished from suspension polymerization, however, the emulsion process involves the use of a surface active agent or soap as the emulsifier and a water-soluble catalyst or initiator instead of the monomer-soluble catalyst used in suspension processes. Although agitation is necessary, it is not as important as in the suspension processes because the emulsion is maintained by use of the soap and protective colloids to insure latex stability. 

Fig. S.9. The dependence of the CFV and LCFT upon stabilizer surface coverage for poly(12-hydroxystearic acid) stabilized latex particles in n-heptane (after Napper, 1968b).

Fig. 5.10. The dependence of the UCFT of polyfoxyethylene) stabilized latex partidtes u stabilizer surface coverage in 0-39 M MgS04 (after Napper, 1970a).

The negatively charged hydrophilic headgroup of the anionic surfactants may comprise sulfate, sulfonate, sulfosuccinate or phosphate groups attached to an extended hydrophobic backbone [82]. The nature of the hydrophilic group will influence the extent of electrostatic stabilization, the behaviour of the surfactant as a fiinction of pH, the degree of hydrolysis, and the variation of latex stability with time, electrolyte and temperature conditions. The nature of the backbone hydrophobe will influence the adsorption behaviour of the surfactant onto the latex particle surface, its cmc value, the interfacial tension (which affects monomer emulsification), and the extent of steric stabilization, among other factors. 

In many of the surfactant-free polymerizations water-soluble polymers are gloated in situ which are either grafted on to the latex particle surface or strongly adsorbed. The use of acrylamide in surfactant-free polymerization is well known and with vinyl chloride polymerization the poly(acrylamide) formed in the water phase was shown to graft on to poly(vinyl chloride) in the particle, contributing to an increased stability [93]. Poly(vinyl alcohol). PVA,... 

The presence of surfactants, besides altering the latex particle surface, can also interact with the water-soluble polymer. For instance, poly(ethylene oxide) homopolymer and block copolymers interact with sodium dodecyl sulfate surfactant [109], and hence alter the latex viscosity behaviour [110]. Other water-soluble polymers are also capable of interacting with specifle surfactants [111]. When pigmented latex dispersions are thickened with associative thickeners one must consider the interactions with some of the pigment stabilizers [112] and other additives, like coalescing aids [113]. 

An interesting observation is that in miniemulsion polymerization of VC, spherical particles up to 1-1.5 im with a smooth surface are easily prepared. According to the above scheme, this range corresponds to the limiting size for the final primary particles in stage 4. With larger particles, the sphericity is bst, and particles with different shapes and protuberances are formed. This is accompanied by a decrease in latex stability. Laiger spherical particles with smooth surfaces may, however, be obtained by a semi-continuous addition of VC. 

The most widely used theory of the stability of electrostatically stabilized spherical colloids was developed by Deryaguin, Landau, Verwey, and Overbeek (DLVO), based on the Poisson-Boltzmann equation, the model of the diffuse electrical double layer (Gouy-Chapman theory), and the van der Waals attraction [60,61]. One of the key features of this theory is the effective range of the electrical potential around the particles, as shown in Figure 25.7. Charges at the latex particles surface can be either covalently bound or adsorbed, while ionic initiator end groups and ionic comonomers serve as the main sources of covalently attached permanent charges. 

If it is assumed that the water which was added is distributed homogeneously in the mixture of hydrophobic solvent and ionomer, then the hydration of the ionic segments prevents macroscopic precipitation of the polymer. However, since the hydrophobic segments (r. 50-98% of the polyurethane ionomer) cannot be hydrated, they form associates, which can eventually develop into a microscopic precipitation. As solvent is present this has a plasticizing effect on the nucleus of latex particles which thus form a film. The hydrated ionic segments provide the stabilizing surface changes. 

Inverted raspberry-like morphologies (the mineral particles being located at the surface of the latex spheres) have also been discussed in Sections 4.4.2.2 and 4.4.2.4 about colloidal silica and layered silicates, respectively. These are mainly a consequence of the surfactant-like behavior of the inorganic particles in specific situations. This was clearly illustrated in a recent report by Landfester, who showed that silica or clays can be used as pickering stabilizers of miniemulsion jxjlymer-izations, resulting therefore in the formation of armored latexes, the surface of which was recovered by the small inorganic particles [99,131]. 

Chemically the NR produced by the Hevea brasiliensis species is almost pure cw-l,4-polyisoprene. So far none of the manufacturers of synthetic cis-, A-polyisoprene is able to achieve more than 95% of the cis isomer in their commercial products. Jitladda and coworkers have conducted extensive studies on the molecular structures of the NR molecules found in Hevea and correlated these to the biosynthesis of this in the trees. This is intrinsically linked to the end groups of the rubber molecules involving the phospholipids and proteins that are linked to the charging mechanism at the latex particle surface and hence has a direct impact on the stability of the latex. 

Polymer Areas Suspension polymerization, particle size control stabilizes surface tesnion of latexes. Modifies latex surface active properties. Polymer can be easily recovered. 

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