In general, it is impossible to describe the linear viscoelastic behaviour of commercial polymer melts with a single mode consisting of a single elastic modulus, a single Newtonian viscosity, and a single relaxation time. Multiple modes are necessary to form a relaxation time spectrum.
In the linear region, the parameters for the spectrum (moduli, viscosities and relaxation times) are constants. However, this is not the case for the non-linear region, i.e. large deformations. In the non-linear region, the moduli, viscosities and relaxation times are a function of the stress.
The relaxation time spectrum for a commercial polymer can be represented by a number of Maxwell elements connected in parallel. At different rates of deformation, another Maxwell element will be the main working element.
For a polymer solid, an extra spring will be put in parallel which represents the equilibrium modulus Ge.