PARTICLE-SIZE MEASUREMENTS OF HETEROGENEOUS FILM-FORMING LATEXES

Two-phase latex particles consisting of mainly polystyrene (PS) and po lyisoprene (PI) in ratios varying from 70:30 to 20:80 were prepared us ing different polymerization techniques. Methacrylic acid (MAA) was us ed in small amounts as a comonomer. The latexes had narrow size distri butions, and showed different particle morphologies depending on the m onomer composition and the polymerization conditions used. In most cas es the latexes were film-forming at room temperature. Particle size di stributions and average particle sizes of the latexes have been determ ined using different particle sizing methods. Size determination by TE M 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 rat io between the PI and PS phases did not exceed 50:50. At higher phase ratios OsO4 staining was preferred. The glass transition temperature o f the PI phase increased after OsO4 staining, which prevented deformat ion of the latex particles. Particle morphologies for the heterogeneou s latex particles were also determined after OsO4 staining. Particle s izes measured by TEM were generally smaller than the corresponding siz es 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 particle s in the aqueous environment, water absorption and swelling due to the presence of carboxylic acid groups, and adsorption of soluble carboxy lated polymers forming a hydrophilic corona around the particles. By T EM the hard sphere diameters of dehydrated particles are measured.