Filament-stretching Rheometry: A Probe for Polymer Dynamics

Lecture by Ole Hassager

Department of Chemical and Biochemical Engineering
Technical University of Denmark

Tuesday, April 28, 2009
Room 1610 Engineering Hall
Refreshments at 3:45 p.m.
Lecture at 4:00 p.m.

The nonlinear dynamics and rheology of entangled flexible polymers is an area of both industrial and fundamental interest. Landmark developments by Doi and Edwards and by Curtiss and Bird have laid the foundation for intense research on polymer melt rheology in the last 25 years. The ultimate purpose of this activity is to understand the connection between molecular structure and rheological properties, i.e., to master the field of molecular rheology. However, for entangled polymer systems, progress towards achieving this goal seems to be limited more by a lack of reliable experimental data than by a lack of model developments.

In this talk I will show how the Filament-Stretching Rheometer (FSR) developed at the DTU may be used to produce reliable data on the nonlinear extensional rheology of well defined model polymers. The FSR provides for probing polymer dynamics in a way that is not possible with the more conventional shear rheometry. Shear flows are weak in the sense that particles are separated linearly in time. Extensional flows on the other hand are strong in the sense, that particles are separated exponentially n in time. As a result, extensional flows are much more efficient than shear flows at orienting and stretching flexible chain molecules. It seems to be a generally accepted opinion, that shear rheology is easy and accurate while extensional rheology is more difficult and less reliable. I will show that, especially for highly entangled polymeric melts, the reverse is in fact true. I will present experimental results for model polymers such as monodisperse linear polymers, bi-disperse linear polymers and branched polymers, and I will indicate how the measurements contribute to our understanding of the molecular rheology of entangled polymer systems.

Ole Hassager

Ole Hassager is a professor in the Department of Chemical and Biochemical Engineering at the Technical University of Denmark (DTU). He received his M.Sc. at DTU and his Ph.D. from the University of Wisconsin-Madison. His research activities include small scale fluid mechanics, polymer physics and molecular rheology. He is director and founder of the Danish Polymer Center, which promotes interdisciplinary research and teaching in polymer chemistry and engineering. He serves on the editorial boards of the Journal of Non-Newtonian Fluid Mechanics and Rheologica Acta and is a member of the Danish National Research Council and the DTU Academic Council. His academic awards include the Statoil Award for Technical Research, and the Stichting Fund Research Award. He will be the 2009 ESR Weissenberg Award recipient in April 2009.

Professor Hassager coauthored the two-volume treatise Dynamics of Polymeric Liquids with R.B. Bird, R.C. Armstrong and C.F. Curtiss.