A simplified description of viscoelastic behavior of polymers as illustrated with biaxially‐oriented poly(ethylene terephthalate)

A rheological model of solid polymers is proposed. Its mechanical analog is a parallel of a linear spring and a Maxwell element with variable viscosity. The viscosity of the dashpot is allowed to change with stress both directly, by an Eyring‐type mechanism, and through free volume changes according to the Doolittle equation. Predictions of the model reproduce many of the features shown, especially after yielding, by constant velocity stress‐strain, stress‐relaxation and creep data taken at room temperature on biaxially‐oriented poly(ethylene terephthalate) over a wide range of loading rates.