Nonlinear viscoelasticity and wall slip of molten polymers

Two nearly monodisperse polybutadienes (PBd) having different molecular weights were used to study nonlinear viscoelastic behavior using a sliding plate rheometer incorporating shear and normal stress transducers.Comparison of data for the shear stress growth function at the start-up of steady simple shear with predictions of the Wagner model was used to estimate the time-dependency of wall slip. Both the time-dependent slip velocity and the steady-state values were found to depend on molecular weight, thus supporting the hypothesis that "slip" actually involves a shear-induced disentanglement between molecules strongly adsorbed at the wall and those in the bulk. The critical shear stress for the onset of slip was about 230kPa for both PBds, suggesting that the critical stress is independent of molecular weight and related to the plateau modulus. Slip behavior on steel and glass substrates was quite similar. Rubber-like high-molecular-weight polybutadiene exhibited well-defined, steady, normal stresses at moderate shear rates, while molten polystyrene exhibited a steady normal stress up to much higher shear rates.Step strain data were used to determine the time and strain dependency of the nonlinear relaxation modulus. Time-strain factorability was observed, and the damping function was determined. For the moderately entangled PBd2 the experimentally determined damping function agreed with the predictions of the Doi-Edwards constitutive model and thus the Lodge-Meissner relationship. The higher molecular weight PBd3 exhibited strong wall slip