Swelling of carboxyl-containing copolymer latexes

Abstract

Methacrylic acid copolymers (5–50%) with methyl methacrylate, styrene, and 2-ethylhexyl acrylate were prepared at 60°C by batch and semicontinuous emulsion polymerization, and the latexes were neutralized with sodium hydroxide. In some cases, ethyl acrylate or crosslinking monomers such as ethylene glycol dimethacrylate, divinylbenzene, or trimethylolpropane triacrylate were added to the recipe. The parameters varied included the Tg of the copolymer, the hydrophobicity of the base monomer, the carboxyl group distribution within the particle, and the degree of crosslinking. The variation of particle size with pH was determined using dissymmetry of light scattering, the variation of viscosity with pH using the Weissenberg Rheogoniometer in the cone-plate mode, and the loci of the carboxyl groups by a modified conductometric titration method. The serum was separated from the latex particles by serum replacement, an excess of sodium hydroxide was added to the cleaned latex, and the conductance and unreacted sodium hydroxide were measured as a function of time. The carboxyl groups were arbitrarily divided into: (i) the particle surface neutralized instantaneously; (ii) the first subsurface layer neutralized within 10 minutes; (iii) the second subsurface layer neutralized within 24 hours; (iv) the unneutralized particle core; (v) the latex serum. The three characterization techniques gave a detailed analysis of the swelling behavior of the carboxyl-containing copolymers. The swelling upon neutralization of the relatively hydrophilic methyl methacrylate copolymers was higher than that of the relatively hydrophobic styrene and 2-ethylhexyl acrylate copolymers. Batch polymerization of the methyl methacrylate copolymers gave a higher concentration of carboxyl groups at or near the particle surface; semicontinuous polymerization gave a more uniform distribution throughout the particle. Decreasing the Tg of the copolymer increased the concentration of carboxyl groups at or near the particle surface. Crosslinking of the particle had little effect on the carboxyl group distribution.