NONLINEAR VISCOELASTICITY OF POLYMER MELTS

In the reported experiments, large amplitude oscillatory shear has been employed to study the nonlinear viscoelastic properties of three polymer melts: HDPE, PS, and low density PE. The equipment used consisted of a small-gap, concentric cylinder rheometer with a controlled-speed motor unit and a rack-pinion, oscillating drive mechanism. For purposes of material characterization, plots of stress versus rate-of-strain are employed. Three material functions, obtainable from these curves, are defined and used for characterization. It was found that the extent of nonlinearity in the response is primarily a function of the strain amplitude rather than the frequency. The amplitude below which the response is practically linear depends strongly on the molecular structure. The response of HDPE was practically linear up to a strain amplitude of 10. Extensive experimental data is presented in curves.