Rheological Studies of Dispersions of Spherical Particles in a Polymer Solution

Abstract

Rheological data are reported for dispersions of crosslinked polystyrene beads (170 nm diameter) in a 0.15 weight fraction solution of polystyrene (Mw=410,000) in either of two solvents: tritolylphosphate (TCP) or 1,2‐di(2‐ethyl hexyl)phthalate (DOP). With these solvents, for fixed polymer concentration, the osmotic pressure is larger in solutions in TCP. Experiments include creep, recovery, stress‐growth at constant strain rate, and strain sinusoidal in time. Dispersions were studied, with 0.05, 0.10, 0.15, and 0.20 weight fraction wB of the beads. The dispersions exhibited solidlike behavior provided the strain did not exceed about 0.02, with an equilibrium modulus proportional to the square of the bead concentration. For larger strain, the behavior with slow deformation rates can be described by a pseudolinear constitutive equation, with a viscosity that decreases with increasing shear rate κ. The yield stress, as obtained from creep measurements, marking the transition from solidlike to fluidlike behavior is of the magnitude expected for a bead suprastructure stabilized by osmotic pressure effects related to depletion of the polymer concentration near the bead surfaces. In steady‐state flow at larger κ, the viscosity approaches a plateau value ηp independent of κ, and the recoverable strain following cessation of steady flow is also independent of κ. This behavior is interpreted in terms of remnants of the suprastructure (flocculated suspension) responsible for solidlike behavior at small strain, with the remnants increasing in number but decreasing in size with increasing κ, while their volume fractions remains about constant. It is found that ln(ηp/η0) is proportional to wB with a temperature‐independent proportionality constant which is larger for the dispersions in TCP solution. This is consistent with the larger osmotic pressure of solutions of polystyrene in TCP in comparison with those in DOP, inasmuch as the remnants of the bead suprastructure are stabilized by osmotic effects. Here η0 is the viscosity of the polystyrene solution.