Rheology of Copolymer Solutions. IV. Nonlinear Viscoelasticity of Solutions of an SBS Block Copolymer

Abstract

Nonlinear viscoelasticity was investigated for concentrated solutions of a styrene—butadiene block copolymer, having a 4-armed star-branched polybutadiene structure with a polystyrene block on each end. When the solvent was good for both polystyrene and polybutadiene, the viscoelastic behavior was similar to that of homopolymer solutions. In the case of 1-chlorohexadecane, a nonsolvent for polystyrene at temperatures below 25°C, the viscoelastic behavior of the solutions was quite different: the rate of shear-temperature and -concentration reduction rules were not applicable to the steady shear viscosity, the Cox-Merz empirical rule was not applicable, and the maximum relaxation time decreased with increasing rate of shear. It was inferred that the behavior of the copolymer solutions could be described by using the BKZ constitutive model. Model calculations performed for a solution in 1-chlorohexadecane revealed that the shear stresses for various flow histories were predicted well if the memory function was evaluated from the strain-dependent relaxation modulus. The characteristic behavior of the solutions in 1-chlorohexadecane was attributable to a strain-sensitive relaxation mode of a long relaxation time, presumably due to the molecular aggregate formed through temporary cross-links of precipitated polystyrene blocks.