Revisitation of PTT model

Nearly 30 years ago the PTT constitutive equation for molten polymers and solutions was published, and since then it has been widely used. In the intervening time much progress has been made in polymer science and mechanics, especially via network and tube theories. It is natural to unite these points of view in a revisitation of PTT-type models. We show that a new improved form of the PTT model, called the PTT-X model, can closely describe, in particular, the rheological behaviour of low-density polyethylene in (steady and transient) shear and elongational flow and in recoil from an elongational stretch. The results of an attempt to reduce the number of parameters are also described and we place the new model within the context of general network theory and tube theory.