Abstract
Large amplitude oscillatory shear (LAOS) and exponential shear can be used to study the nonlinear viscoelastic properties of molten polymers. Using a kinetic network theory, the sensitivity of the nonlinear viscoelasticity of a polymer melt to small changes in the linear spectrum was calculated and compared with the sensitivity of the linear response. In LAOS, the strain amplitude and phase angle of the first harmonic were no more sensitive to changes in the relaxation time than the linear counterparts. The sensitivity of the amplitudes and phase angles of all the harmonics to changes in the linear spectrum decreases with strain amplitude. In exponential shear, the sensitivity of the maximum of the exponential shear viscosity to small changes in the relaxation time was found to decrease with increasing strain rate. However, increasing strain amplitude (in LAOS) and strain rate (in exponential shear) were both found to amplify subtle differences in the kinetic parameters of the model. Thus large shearing deformations do not significantly amplify subtle changes in the relaxation spectrum, but they do amplify subtle differences in the kinetic rate constants.
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