RHEOLOGICAL STUDIES ON CONCENTRATED POLYSTYRENE LATEX STERICALLY STABILIZED BY POLY(ETHYLENE OXIDE) CHAINS

The rheological properties of aqueous concentrated polystyrene latex dispersions for three particles with core diameters of 155 nm, 612 nm, and 1004 nm were investigated using steady-state shear stress-shear rate and oscillatory measurements. The relative viscosity-effective volume fraction results were fitted to the Dougherty-Krieger equation for hard spheres for dispersions with various particle sizes by adjusting the value for the adsorbed layer thickness Δ. The latter was found to decrease with an increase in volume fraction of the dispersion which is caused by considerable compression of the chains when the volume fraction of the particles approaches close-packing. The value of Δ was also found to increase with increases in particle size. From the oscillatory measurements, the complex modulus G*, the storage modulus G′, and the loss modulus G″ were obtained as a function of frequency at various latex volume fractions. The results showed that the dispersion changed from being more viscous (G″ > G′) to more elastic (G′ > G″) over a narrow range of volume fraction of the dispersion. Within this range of φ, it is likely that the chains undergo some compression and interpenetration of peripheries. When φ was increased significantly above a certain value, the system became predominantly elastic and significant interpenetration and compression of the chains occurred. If φ was further increased, the moduli increased by several orders of magnitude as did the dynamic viscosity. Under these conditions, the latex dispersion behaves as an elastic "gel" which can be fitted by a power law equation G′ = kφm where m is related to the compressibility of particles which is determined by the ratio of adsorbed layer thickness to radius of particle, Δ R. © 1992.