Latex sulfate groups

The second generation includes latices made with functional monomers like methacrylic acid, 2-hydroxyethyl acrylate [818-61 -17, acrylamide/75 -(9ti-/7, 2-dimethylaminoethylmethacrylate [2867-47-2] and sodiumT -vinyl-benzenesulfonate [98-70-4] that create in polymeric emulsifier. The initiator decomposition products, like the sulfate groups arising from persulfate decomposition, can also act as chemically bound surfactants. These surfactants are difficult to remove from the latex particle. 

Latexes initiated with persulfate normally have terminal sulfate groups, but these can be hydrolyzed to alcohols and then oxidized to carboxyl groups. Terminal alcohol groups also result from hydrogen peroxide or hydroperoxide initiated polymerization. Ottewill and Shaw (22) first showed by electrophoresis that latexes stripped of salts and emulsifiers by dialysis have both strong and weak acid sites. Van den Hul and Vanderhoff (23) then found that conductometric titrations were most effective for... 

Polystyrene latexes have been prepared using persulfate initiator for many years, but only recently have methods been developed to determine the number and loci of the sulfate surface groups. To determine these surface groups, the latex is cleaned to remove the adsorbed emulsifier and solute electrolyte, then the surface sulfate groups in the H+ form are titrated conductometrically with base. The latexes can be cleaned effectively by ion exchange (2-5) or serum replacement (6) dialysis is not effective in removing the adsorbed emulsifier and solute electrolyte (3,5,6). +... 

One possibility is hydroxyl endgroups, which may be formed by a side reaction of sulfate ion-radicals to form hydroxyl radicals (9) or hydrolysis of the surface sulfate groups. To determine if hydroxyl groups were present, the ion-exchanged latexes were oxidized by heating with persulfate and 10 silver ion at 90°, then ion exchanged and titrated conductometrically to determine the carboxyl groups. Table II (9) shows that some sul-... 

These results showed that monodisperse polystyrene particles prepared using persulfate initiator and bicarbonate buffer contain both sulfate and hydroxyl surface groups. Some sulfate groups are on the particle surface, while others are buried inside the particle. All of the hydroxyl groups are on the particle surface. These results also showed that careful control of the pH during polymerization can produce latex particles stabilized with only surface sulfate groups. 

The foregoing latexes prepared using persulfate initiator and bicarbonate buffer contained surface sulfate groups and, in some cases, surface hydroxyl groups. None of these latexes contained carboxyl groups. These latexes had final pH values of 7-8. If the bicarbonate buffer was omitted, the final pH of the latex was 2-3 because of the bisulfate ion generated by the persulfate decomposition. 

To determine the effect of different polymerization conditions on the polymer endgroups produced, polymerizations were carried out using the standard bicarbonate buffer as well as other variations. Table V (13,16) shows that the use of the persulfate-bicarbonate combination with and without emulsifier gave latexes of final pH 7-8 with only sulfate groups. The addition of 10 5 silver ion gave a latex of pH 8.5, but with weak-acid groups, presumably because of oxidation of the sulfate groups. 

Latex Particle Size, nm Surface Charge Density (sulfate group), yc/cm2 Electrophoretic Mobility in Deionized Water ym cm/sec volt... 

While most of the applications important to polymer science have been on surfaces of solids, papers have been published on liquid-phase XPS (65) and sulfate groups on polystyrene latexes have been analyzed (66). 

Common initiators for emulsion polymerization are compounds containing a peroxide bond, such as hydrogen peroxide and sodium peroxodisulfate. The most commonly used are the latter type, which contribute significantly to latex stabilization by introducing terminal sulfate groups on each polymer chain formed in the initial solution polymerization. 

All polystyrene latexes prepared by persulfate initiation may be shown to be negatively charged by electrophoresis. That is, all but one, which was discovered after years of storage at the Dow Chemical Co. to be electrically neutral, but very stable. The surface groups on this now famous latex are evidently all hydroxyl [31]. The sulfate groups originally present all must have hydrolyzed slowly upon standing ... 

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