Molecular Rheology and linear viscoelasticity

Abstract

Measurement of the linear viscoelastic properties is the basic rheological characterization of polymer melts. These properties may be evaluated in the time domain (mainly creep and relaxation experiments) or in the frequency domain. This chapter talks about mechanical spectroscopy, where the sample experiences a harmonic stimulus (either stress or strain). The linear viscoelastic data may be used as a pure rheological characterization, and the viscoelastic parameters may be related to some processing or final properties of the material under study. Moreover, linear viscoelasticity and nonlinear viscoelasticity are not different fields that would be disconnected: in most cases, a linear viscoelastic function (relaxation function, memory function or distribution of relaxation times) is used as the kernel of non linear constitutive equations, either of the differential or integral form. That means that if one could define a general nonlinear constitutive equation that would work for all flexible chains, the knowledge of a single linear viscoelastic function would lead to all rheological properties.