Rheology of commercial polyolefins: relating the viscoelastic behaviour to microstructure

Abstract

The rheological response of commercial polydisperse polyolefins under various types of deformation has been investigated in detail. A wealth of experimental data that has been available in the literature was gathered and is presented in this work. These data include several types of commercial polymer melts of varying microstructural architecture that have been tested under various types of rheological deformation. In addition, experimental data on two commercial polyethylenes provided by Eni S.p.A., the LDPE Riblene FF20 and the HDPE Eraclene FA506, provide a direct insight into the similarities and differences of the rheological response of commercial polyolefin melts under different types of deformation. Finally, a number of molecular constitutive theories are suggested in order to explain the results on a physical basis. This research aims at gaining understanding in the relation between the molecular structure of polymers and their rheological behaviour.

The modified rubber-like liquid theory, which is based on the permanent network hypothesis, Boltzmann's superposition principle, the concept of a fading memory and the idea of the separation of time and strain effects, is used as the starting point of this study. This simple phenomenological model that was initially proposed by Lodge and was later modified by Wagner is then compared to recently suggested molecular constitutive theories that are based on direct physical criteria in order to explain the experimental observations. The goal of this study is to prove that a representative physical basis can be the starting point of a successful rheological theory. In this context, a damping function in the form of a simple power law is used to predict the rheological response of commercial polypropylene blends tested under uniaxial extensional flow. Analysis of the experimental results leads to the conclusion that a finest description of the molecular structure of these melts is necessary in order to predict the complex behaviour of polyolefin melts. The results are also compared against the predictions of the same model for commercial polydisperse PE, PP and PS melts.

A modified version of the Doi and Edwards theory has been proposed by Tsenoglou et al.\ and presented in this work for the first time. A nonlinear viscoelastic parameter is introduced into the constitutive equation. This extended model provides a physical explanation of the strain softening of commercial LDPE melts, which is observed under large magnitudes of step-strain in shear deformation, as well as the appropriate strain hardening, which is observed when polymer melts are tested against uniaxial extensional deformation. The detailed research then follows aims at the way that this parameter correlates with the branching content of the melts under consideration.

Mathematical methods that are commonly used to investigate the thermorheological complexity of commercial PE's are proposed and applied into the rheology of the commercial Riblene and Eraclene melts. The efficiency of these methods to provide an insight into the microstructural architecture of polymer melts is thoroughly examined. In addition, molecular constitutive theories that are provided in differential form, such as the eXtended Pom-Pom model, are also employed in order to make exact predictions of the rheological behaviour of melts under various types of deformation. These models are finally used in order to infer the molecular characteristics of commercial polyolefins from rheological data.