Time‐dependent rheological properties and transient structural states of entangled polymeric liquids—A kinetic network model

A kinetic network model has been developed to describe the nonlinear rheological behavior of entangled polymer solutions and melts in either steady‐state or transient flows. This model is based on the concept of flow‐induced structure variation, controlled by the simultaneous existence of entanglement loss and regeneration. When steady‐state is attained, both processes occur at the same rate and the structural state (entanglement density) of the fluid remains constant with time. Under transient flow conditions, the fluid structure becomes time‐dependent as a result of unequal rates of the two competing processes affecting entanglement density. The viscoelastic response of the fluid is, in turn, influenced by the varying structural state. This is incorporated into the model through the use of structure‐dependent coefficients in the contravariant Maxwell constitutive equation for entanglement stress. Model predictions are generated for a number of transient flow programs, including the stress growth and relaxation, interrupted flow, sudden change in shear rate, and nonlinear creep experiments. Comparison with literature data supports not only the qualitative success but also the quantitative ability of this model.