Particle size measurements of heterogeneous film-forming latexes

Abstract

Two-phase latex particles consisting of mainly polystyrene (PS) and polyisoprene (PI) in ratios varying from 70∶30 to 20∶80 were prepared using different polymerization techniques. Methacrylic acid (MAA) was used in small amounts as a comonomer. The latexes had narrow size distributions, and showed different particle morphologies depending on the monomer composition and the polymerization conditions used. In most cases the latexes were filmforming at room temperature. Particle size distributions and average particle sizes of the latexes have been determined using different particle sizing methods.

Size determination by TEM was performed after staining with osmium tetroxide (OsO₄) or uranyl acetate (UAc). The staining methods showed no significant differences in particle size averages and particle size distributions when the ratio between the PI and PS phase did not exceed 50∶50. At higher phase ratios OsO₄ staining was preferred. The glass transition temperature of the PI phase increased after OsO₄ staining, which prevented deformation of the latex particles. Particle morphologies for the heterogeneous latex particles were also determined after OsO₄ staining.

Particle sizes measured by TEM were generally smaller than the corresponding sizes measured by quasielastic light scattering (QELS). The difference in the measured diameters increased with increasing PI and PMAA content in the latex particles. The larger sizes observed by QELS are results of the presence of an immobilized water layer surrounding the particles in the aqueous environment, water absorption and swelling due to the presence of carboxylic acid groups, and adsorption of soluble carboxylated polymers forming a hydrophilic corona around the particles. By TEM the hard sphere diameters of dehydrated particles are measured.