VALIDITY OF SEPARABLE BKZ MODEL FOR LARGE-AMPLITUDE OSCILLATORY SHEAR

Large amplitude Oscillatory shear (LAOS) is a useful tool for the stud y of nonlinear viscoelasticity in polymeric liquids. To concisely desc ribe the response of a material to such a test, it is desirable to mak e use of a constitutive equation. In this way, the response can be des cribed in terms of the parameters of the rheological model. We have fo und a separable BKZ model, e.g., Wagner's equation, to be useful in th is regard for LDPE IUPAC X. The damping function determined in step sh ear experiments does not lead to accurate predictions of the LAOS resp onse. For a given low frequency, however, it is possible to fit the pa rameters of a simple damping function equation to the LAOS response at one strain amplitude and to use these parameter values to reliably pr edict the response at other strain amplitudes. Thus, the Wagner equati on provides a basis for the concise description of the response of LDP E IUPAC X to large amplitude oscillatory shear experiments at low freq uencies. Conversely, it was found that the response of a HDPE melt to large amplitude oscillatory shear cannot be concisely described by the Wagner equation.