Abstract
The shear stress and a normal stress difference, equivalent to the sum of the first and the second normal stress differences, were measured for a concentrated polystyrene solution in double-step shear deformations; i. e., two shear deformations were applied to the material in sequence and then the stresses were measured. When the time interval between two deformations was relatively small, the observed stresses long after the deformations were described well with the BKZ constitutive model. The stresses at short times were smaller than the values predicted with this model except for the case where two deformations have the same sign, or the same sense. A simple calculation based on the tube model of entanglement led to an expression, which gave a better prediction of the stresses. The BKZ model fails at short times probably because it does not take proper account of two different relaxation mechanisms, one due to the equilibrium process of chain contour length and the other due to the disengagement process of the chain from the deformed tube-like region.
