The Rheological Properties of Concentrated Solutions of Styrene-Isoprene Block Copolymers I. The Solutions in n-Tetradecane and Dibutyl Phthalate

Abstract

The flow properties of concentrated solutions of styrene-isoprene block copolymers have been measured to investigate the effects of solvent, composition of the copolymers, and concentration. The solvents used in this study are n-tetradecane (nTD) and dibutyl phthalate (DBP); the former is a good solvent for the isoprene block and non-solvent for the styrene block, and the latter is the reverse.
Apparent viscosity η vs. rate of shear γ curves for solutions of the copolymers show remarkable non-Newtonian character. In the stress relaxation after cessation of steady shear flow, the shearing stress never relaxes to the zero level leaving a residual stress σ_R. In this study, we defined the relaxed stress as Δσ=σ-σ_R, and the viscosity corresponding to Δσ asΔη=Δσ/γ. Quantities η, Δη, and σ_R increase with increasing concentration of the solutions and decrease with increasing the styrene content in the copolymers. σ_R, is almost independent of γ, while η and Δη depend strongly upon γ. The value of σ_R agrees very well with that of the yield stress aσ_y calculated from Casson's equation. These results can be explained by assuming that non-soluble block chains form the aggregates which have cilia of soluble block chains on the exterior. σ_R and Δη seem to be affected mainly by the stiffness of the aggregates formed and the viscous friction between the aggregates, respectively.