Rheological Properties of Disperse Systems of Spherical Particles in Polystyrene Solution at Long Time-Scales

Abstract

The dynamic and steady flow properties of disperse systems of styrene–divinylbenzene copolymer particles in a polystyrene solution have been measured over wide ranges of frequency, shear rate, and strain amplitude by means of a cone-and-plate type rheometer. The main results may be summarized as follows. 1) These systems show Newtonian behavior at extremely low rates of shear, that is, the apparent viscosity is approximately constant. This fact indicates that the systems have no yield stress, although they appear to show one if only the behavior at high shear rates is considered. 2) They show linear viscoelastic behavior at strain amplitudes less than 0.5%, but striking nonlinearities at larger strains. However, at very long time-scales, these systems are linearly viscoelastic and independent of the strain amplitude. 3) The nonlinear viscoelastic functions G1′ and G1″ decrease with increasing strain amplitude, but they are almost independent of strain for strains larger than 50%, over the entire frequency range. 4) The relaxation spectra for these disperse systems consist of two parts: one is the box type portion in the long time-scale region, where the intensity is very sensitive to strain; the other appears in the short time-scale region and is not sensitive to strain. 5) |η*| coincides well with the Newtonian viscosity at extremely low shear rates or frequencies, but the empirical law proposed by Cox and Merz does not hold at higher values. Thus, the rheological behavior of these disperse systems cannot be understood without considering the three-dimensional network structure formed by the suspended particles. On the basis of these results, a new concept for the yield stress of disperse systems is proposed and discussed.