Modifying the pom-pom model for extensional viscosity overshoots
(2012) XVI International Congress on Rheology, — Location: Lisbon, Portugal
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- Abstract
- (en) The rheological properties of long chain branched polymers (LCBP’s) are of significant interest in industrial applications since this is the molecular structure encountered in industrial melts. Extensional experiments using Sentmanat Extensional Rheometers (SER) indicate extensional hardening but cannot access the steady state. Often, a plateau is assumed at the maximum stress measured. Applying the tube idea (i.e. in the melt state a polymer chain is confined into a tube-like region due to the mutual entanglements with other chains) to an ideal molecular structure called the pom-pom molecule McLeish, Larson and Ikson et.al were able to fit data of extensional rheology, performed by SER, of industrial LCBs. Experiments performed using another technique called Filament Stretch Rheometry (FSR) indicate a very different behaviour . In this case the extensional viscosity goes through an overshoot before it reaches its steady state. Furthermore relaxation measurements using the same technique indicate that stress relaxation after the overshoot occurs faster compered to relaxation before the overshoot. Here a modification of the original pom-pom theory that includes a stress overshoot is presented. Central to the modification is the assumption that, in non-linear flows, a fraction of the backbone-backbone entanglements can be stripped off due to chain retraction and not be immediately replaced. The strip off of strong entanglements is interpreted as dilution of the confining tube followed by some speeding up of orientation and stretch relaxation. Because the rate of entanglement stripping increases suddenly at the onset of branchpoint withdrawal, an overshoot in stress is predicted. These modifications give rise to a new constitutive equation for the rheological behaviour of LCBP’s melts. The predictions of this model for both a one mode version and a multimode version are presented revealing a reasonable agreement with the FSR measurements.
- Affiliations
Department of Applied Mathematics University of Leeds
Citations
Hawke, L. G. D., & Read, D. J. (2012). Modifying the pom-pom model for extensional viscosity overshoots. XVI International Congress on Rheology, Lisbon, Portugal. https://hdl.handle.net/2078.5/180852