Sliding plate rheometer studies of concentrated polystyrene solutions: Nonlinear viscoelasticity and wall slip of two high molecular weight polymers in tricresyl phosphate

Abstract

Two high molecular weight, narrow distribution polystyrenes dissolved in tricresyl phosphate were studied using a sliding plate rheometer. Shear stress and birefringence measurements in step strain allowed the determination of N3 for both solutions. Start-up of steady simple shear and large amplitude oscillatory shear measurements revealed that the 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 concentration. Large-amplitude oscillatory shear stress measurements revealed 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 N3 vanished. This was found to occur at lower frequencies for the lower molecular weight solution. In contrast, the average value of the third normal stress difference, reflected in the zeroth harmonic, was found to vary little between the two solutions.