Stress relaxation dynamics of an entangled polystyrene solution following step strain flow

The stress relaxation behavior of a well-entangled polystyrene following step shear deformations is investigated. New experimental data are reported for the relaxation of shear stress and first normal stress difference that are not affected by imperfect strain history or transducer compliance. Anomalous (type Q shear stress relaxation behavior is observed indicating significant discrepancies from the tube model prediction for the damping function. First normal stress difference data are used to evaluate the Lodge-Meissner relation and small deviations are observed. We find that slip between the test fluid and cone-and-plate fixtures is responsible for the observed type C behavior, but only leads to small departures from the Lodge-Meissner relation. Simulations of step strain flows with wall slip show qualitative agreement with experimental results. In addition, we examine the issue of polydispersity in the polystyrene standards used to prepare well-entangled solutions for this and similar studies. (c) 2006 The Society of Rheology.