SLIDING PLATE RHEOMETER STUDIES OF CONCENTRATED POLYSTYRENE SOLUTIONS- NONLINEAR VISCOELASTICITY AND WALL SLIP OF 2 HIGH-MOLECULAR-WEIGHT POLYMERS IN TRICRESYL PHOSPHATE

Two high molecular weight, narrow distribution polystyrenes dissolved in tricresyl phosphate were studied using a sliding plate rheometer. S hear stress and birefringence measurements in step strain allowed the determination of N-3 for both solutions. Start-up of steady simple she ar and large amplitude oscillatory shear measurements revealed that th e critical shear stress for slip for both solutions is around 2500 Pa and that slip is dynamic in nature, allowing rheological measurements above the critical stress to be made at sufficiently high frequencies. The critical stress value was used in conjunction with data from the literature to develop a relationship between critical stress and conce ntration. Large-amplitude oscillatory shear stress measurements reveal ed that the solutions exhibit a limiting behavior at high strain rate amplitudes in the intermediate frequency range when the dependence on strain rate amplitude becomes small. Birefringence measurements showed that at high frequencies, where the time scale of the deformation is short, the oscillatory component of N-3 vanished. This was found to oc cur at lower frequencies for the lower molecular weight solution. In c ontrast, the average value of the third normal stress difference, refl ected in the zeroth harmonic, was found to vary little between the two solutions. (C) 1998 The Society of Rheology.