ABSTRACT
Here we study the formation of sterically stabilizing "hairy" surface layers for a series of styrene-butylacrylate and styrene-butadiene latexes copolymerized either with acrylic acid (AA), methacrylic acid (MAA), itaconic acid (IA) or acrylamide (AM) using dynamic light scattering, steady shear and high frequency rheology. This phenomenon is investigated under different conditions of pH, ionic strength, and temperature. The AA copolymerized latex has the most extended hairy layer and is very sensitive to pH and ionic strength. MAA yields a thinner hairy layer than AA due to higher hydrophobicity. IA exhibits a hairy layer thickness of about 1 nm, since it terminates polymer chain growth. For the AM copolymerized latexes high frequency viscosity reveals the existence of a thin hairy layer and the high values of the low shear viscosity and the high frequency modulus are attributed to a weak, reversible flocculation. No significant effect of particle core composition or temperature on the formation of the hairy layer was observed. High frequency rheology is most valuable for characterization of surface layers of carboxylated latexes, since it provides not only direct information about the effective volume fraction but also characterizes the strength of colloidal interactions among particles and it is applied at high particle concentration relevant during manufacturing and processing.
ABSTRACT
The sedimentation velocities and concentration profiles of low-charge, monodisperse hydroxylate latex particle suspensions were investigated experimentally as a function of the particle concentration to study the effects of the collective particle interactions on suspension stability. We used the Kossel diffraction technique to measure the particle concentration profile and sedimentation rate. We conducted the sedimentation experiments using three different particle sizes. Collective hydrodynamic interactions dominate the particle-particle interactions at particle concentrations up to 6.5 vol%. However, at higher particle concentrations, additional collective particle-particle interactions resulting from the self-depletion attraction cause particle aggregation inside the suspension. The collective particle-particle interaction forces play a much more important role when relatively small particles (500 nm in diameter or less) are used. We developed a theoretical model based on the statistical particle dynamics simulation method to examine the role of the collective particle interactions in concentrated suspensions in the colloidal microstructure formation and sedimentation rates. The theoretical results agree with the experimentally-measured values of the settling velocities and concentration profiles.