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

摘要

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 σR. In this study, we defined the relaxed stress as Δσ=σ-σR, and the viscosity corresponding to Δσ asΔη=Δσ/γ. Quantities η, Δη, and σR increase with increasing concentration of the solutions and decrease with increasing the styrene content in the copolymers. σR, is almost independent of γ, while η and Δη depend strongly upon γ. The value of σR agrees very well with that of the yield stress aσy 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. σR and Δη seem to be affected mainly by the stiffness of the aggregates formed and the viscous friction between the aggregates, respectively.