Non-linear Viscoelasticity of Polymer Melts

The study of the rheological behavior of polymer melts, and the existence of correlations with their structure, provides a valuable and essential tool for improving product quality and processing yields. This can be achieved in two ways:

• optimize material selection for a given processing technique and application;

• optimize a process for a given material and application.

This last point has gained in importance since the 2000s with the development of process simulation tools and software, for which the determination of a behavior law (viscous in most cases) is an obligatory step.

The fundamental concepts and results set out in the article "Linear viscoelasticity of polymer melts" in this treatise were concerned with behavior in situations of infinitesimal deformation. However, during the processing of thermoplastic polymers, from granulate to finished object, the deformation is obviously macroscopic and this situation, combined with the viscoelastic behavior of the polymer, generates a set of manifestations that are difficult to interpret within the restricted framework of linear viscoelasticity.

In the case of fluids, and in the context of current modeling possibilities, consideration of viscous behavior in shear is often sufficient, at least for common applications (injection molding of thermoplastics, paint spreading, etc.). In this context, the constitutive equations, linking all possible states of stress to all possible states of deformation, can be simplified in the particular form of flow laws. These laws then consider only the particular context of stress or deformation of the fluid (in shear) and use only part of the material parameters involved in the constitutive equation.

However, a number of features of an elastic nature, as well as the existence of processes clearly involving different kinematics (elongation in particular), show the limits of such an approach and invite us to address these aspects of the behavior of viscoelastic fluids such as polymer melts.

This article describes some of the phenomena observed in simple, but non-linear, polymer melt flow situations, the tools used to analyze shear and elongation behavior (rheometers), the links with polymer structure, and a few examples of processing applications.

The analysis of viscoelastic behavior in situations of low or high deformation, in the case of simple flows, enables us to understand the origin of a number of phenomena occurring in complex flows. These viscoelastic phenomena are sometimes at the root of a number of characteristic defects in melt-formed products, and an understanding of the causes...