5. Non-linear viscoelastic behavior laws

Among the descriptions of intermolecular interactions in molten polymers, which are responsible for the viscoelastic behavior observed, network theories provide satisfactory models, both in terms of their relative ease of use and their physical basis. These laws of behavior are based on classical theories of rubber elasticity, in which macromolecule segments are linked by temporary entanglements, continuously created and destroyed, but ensuring cooperative movement of the molecules at all times. The stress at the present time t is the sum of the contributions of different segments between entanglements created at previous times t′. However, only part of these segment contributions, a function of (t - t′), still remains at the present time. This time dependence is expressed by a relaxation time spectrum or memory function sufficient to describe the linear behavior. The deformation dependence...