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Several classes of multiple-mode rheological constitutive equations are tested for fitting and predicting viscoelastic flow properties of a typical low-density polyethylene melt. An optimization procedure is used to fit the phenomenological parameters of each model under consideration to experimental data taken in small-amplitude oscillatory shear flow and steady shear flow. These parameter values are then used to generate predictions for transient shear and uniaxial elongational flow experiments, and the predictions are then compared to experimental data. Model successes and failures are discussed, and the outlook for using rheological equations in real design processes is addressed.

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Transient shear properties of dilute polymer solutions are investigated in the shear rate regions where shear thickening can occur. Comparison of transient rheological and optical data with the two coupled Maxwell modes model offers insight into the physical mechanisms that give rise to this anomalous behavior. Specifically, shear thickening occurs due to the deterioration of the size and anisotropy of structures deformed at lower shear rates. Coincident theoretical and experimental discontinuities in the transient profiles of the dichroic orientation angle appear to confirm the prior statement.