Abstract
This paper describes finite-element simulations and associated experimental studies of extrudate swell for near-monodisperse and polydisperse polystyrenes. The tube-model based Rolie-Poly constitutive model, when extended to include a reduction of monomeric friction at high extension rates, makes much-improved predictions of extrudate swell at high Weissenberg number. This is especially significant for near-monodisperse polymers where rheological features are unchanged by the effects of polydispersity. Extension of this molecular rheology scheme to a polydisperse constitutive model addresses extrusion experiments on polydisperse polystyrenes inside a multipass rheometer, accounting for experimental data up to Rouse Weissenberg numbers of 50. We, therefore, show that from a measurement of polymer molecular weight distribution, it is possible to predict extrudate swell over a broad range of processing conditions for polydisperse polymers and realistic extrusion processes. Small changes in the capillary length to diameter ratio have little effect on extrudate swell in this range of Weissenberg number. This is because the capillary residence time is sufficiently long for a steady state to be reached within the polymer stretch relaxation time, which controls the most decisive physics responsible for extrudate swell.
Highlights
The vast majority of previous work on extrudate swell directly addresses systems of industrial complexity
We focus on monomeric friction reduction, but we note that both effects will reduce relaxation times at high Weissenberg number
II and III, we show that excellent agreement between experiments and simulations for prediction of extrudate swell can be made for polydisperse polymers at a range of flow rates, when both friction reduction and polydisperse effects
Summary
The vast majority of previous work on extrudate swell directly addresses systems of industrial complexity. The advent of molecular rheological theories based on entanglement models has opened up the possibility of a fully molecular, multiscale, and predictive approach to complex flow phenomena in polymer melts, such as extrudate swell. The aim of the longer program within which this work constitutes a stage is to provide such a method for predicting extrudate swell, from arbitrary die geometries, and eventually for industrially relevant polydisperse polymers, by using approximately monodisperse polymers [2], and building up in complexity by addressing bidisperse and polydisperse polymers (this work). We will apply these simulations to branched systems to show that tubemodel based theories can be used to predict a wide number of industrial polymers in relevant flow geometries
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