2004 -05 -26
10 :15 at ML J 21

Stress Relaxation Measurement Following Step Strains in Entangled Linear Polymer Liquids

David C. Venerus

Illinois Institute of Technology, Chicago, USA

The desire to understand the flow behavior of entangled linear polymers with narrow molecular weight distribution has attracted the attention of experimental and theoretical rheologists for many years. The most successful theoretical model to date for such systems is the tube model of de Gennes developed into a constitutive model by Doi and Edwards. One of the most widely studied rheological flows is stress relaxation following a step shear. The tube model prediction for the damping function, the ratio of the non-linear to linear shear moduli, has been found to be in good agreement with experimental data on a large number of linear, mono-disperse polymer liquids. There are, however, a significant number of cases where tube model predictions are qualitatively different from experimental stress relaxation data. Even more troubling is the fact that these differences are observed for very highly entangled polymers, where one would expect the tube model to be most applicable. It has been argued by some authors that discrepancies between experiments and tube model predictions are the result of shortcomings of the theory, while others argue that slip, either true or apparent, is the cause. In this study, we carefully examine published step strain data and attempt to establish if slip, or some other phenomena, can explain experimental data from step strain flows that are inconsistent with tube model predictions.