Influence of elongational properties on the contraction flow of polyisobutylene in a mixed solvent

Over the last decade several international programmes have been developed a round different standard fluids, one of which is the so-called S1 fluid. Th is is a solution of polyisobutylene in a mixed solvent and the aim of the p rogramme has been to study the rheology of polymer solutions from the dilut e solution to the melt. The focus of this paper will be on the flow visuali sation of contraction flows of SI through orifice dies and on the estimatio n of some of its extensional properties. The contraction ratios range from 24.4:1 and 124.3:1. The measured entry pressure drops will be correlated wi th contraction ratio and apparent wall shear rate. Experimental evidence wi ll show that, when regarded as a function of wall shear rate, the entry pre ssure drops are independent of the contraction ratios. The flow fields for different contraction ratios, at any constant apparent wall sheer rate, how ever, differ substantially. The evolution of the flow fields is monitored a nd it is shown that an initial increase in vortex size is followed by a slo wer decrease, this happening at a constant Weissenberg number. At the same Weissenberg number, small scale instabilities start occurring near the offi ce. As the shear rate is increased further, these instabilities grow in siz e until, eventually, the flow structure is destroyed. An extensional viscos ity is evaluated using a modified form of the Binding analysis for contract ion flows and we show that the results are not only in qualitative agreemen t with those from other groups, but also that the analysis is able to predi ct exactly the onset of the aforementioned flow instabilities.