NONLINEAR VISCOELASTICITY OF POLYSTYRENE SOLUTIONS - STRESS DEVELOPMENT AT START OF STEADY SHEAR-FLOW AND AN EXPERIMENTAL CHECK OF SOME CONSTITUTIVE MODELS

The development of the shear stress at the start of shear flow at constant rate of shear k was measured for PS solutions in diethyl phthalate with a cone-and-plate rheometer. Ranges of molecular weight M and concentration c were 3. 10 multiplied by 10**6-7. 62 multiplied by 10**6 and 0. 112-0. 329 g/cm**3, respectively. The shear stress as a function of time t exhibited a marked maximum at large k when either M or c was relatively low. When M and c were high, the maximum was broad and low. In a few extreme cases no maximum was observed in the range of k studied. The constitutive model of Bernstein, Kearsley, and Zapas could describe approximately the shear stresses at a sudden start and on cessation of steady shear flow with a memory function evaluated from the strain-dependent relaxation modulus. The Lodge model based on a strain-rate dependent relaxation spectrum was not able to describe the strain-dependent relaxation modulus as well as the interrelation between shear stresses at a sudden start and a cessation of steady shear flow.