RHEOLOGY AND DIFFUSION OF CONCENTRATED MONODISPERSE AND BIDISPERSE POLYMER LATICES

Three monodisperse, electrostatically stabilized latices of different sizes (306, 164 and 64 nm) have been synthesized by emulsion polymerization of styrene and acrylic acid, and purified by ion-exchange. Particle size and size distribution were determined by transmission electron microscopy as well as static and dynamic light scattering from dilute dispersions in various aqueous media. Electrostatic properties were determined through titration and zeta-potential measurements. Concentrated colloidal suspensions at different volume fractions up to 50%, and various pH and ionic strengths have been studied with regard to their microscopic dynamics by fiber-optic quasielastic light scattering (FOQELS). At high ionic strength, the concentration dependence of the diffusion coefficient was in agreement with theoretical predictions for the self-diffusion of hard spheres. At low ionic strength, significant deviation from hard sphere behavior was observed and FOQELS provided an optical technique for determining the presence of a yield stress. The same samples have been studied in various rheometrical tests. At low ionic strength elastic properties dominated; at high ionic strength, samples were predominantly viscous. Intermediate viscoelasticity, at well balanced strength of interaction, could be detected. The transition from viscous to elastic character occurred over a narrow regime of ionic strength or concentration. Curves of storage modulus, G' versus frequency, omega, at different particle contents in the viscoelastic region could be shifted to a mastercurve by scaling omega with the longest relaxation time. Bimodal mixtures of small and large particles were also investigated by means of viscometry and FOQELS. When the number concentration of large spheres was small, FOQELS could be used to measure a probe-diffusion that showed a concentration dependence similar to that of viscosity and which resembled the behavior of long-time self-diffusion.