Nonlinear viscoelastic behaviour of linear polyethylene : molecular weight effects and constitutive equation evaluations

The nonlinear viscoelastic properties of a series of blends of linear polyethylene were studied using the McGill sliding plate rheometer. A more reliable and sensitive shear stress transducer for this rheometer was designed, built and used in this work. The molecular weight dependence of a variety of nonlinear viscoelastic properties was investigated. It was determined that, as with steady state properties, the sensitivity of such properties to molecular weight diminishes with increasing shear rate. The behavior of these materials in large amplitude oscillatory shear (LAOS) was also studied. By using harmonic analysis, the frequency content of the nonlinear stress response to the sinusoidal strain was studied as a function of molecular weight, strain amplitude and frequency. The predictive abilities of the Wagner model in LAOS, exponential shear, start-up and cessation of steady shear and interrupted shear, at high shear rates were evaluated. Qualitative trends were well predicted by the model for a variety of sigmoidal and exponential damping function forms. For the first time, it has been shown that Wagner model predictions for molten thermoplastics are insensitive to the damping function form. The damping functions in simple shear and planar extension were obtained for a branched low density polyethylene (LDPE). Simple shear is similar to planar extension in a rotated reference frame and thus the two flows should have similar damping functions. It was found that the damping functions that fitted these two flows are, in fact, quite different. Thus we have shown that the contribution of kinematics cannot be simply described