EXPERIMENTAL STUDIES OF AN ENTANGLED POLYSTYRENE SOLUTION IN STEADY-STATE MIXED-TYPE FLOWS

Experimental measurements of birefringence and velocity gradient compo nents are reported for steady mixed type flows of a 0.076 g/cm(3) solu tion of 2.89X10(6) MW polystyrene in a mixed toluene/oligomer solvent. The flow field is produced in a co-rotating two-roll mill with a seri es of different ratios of the gap width to roller radius chosen so tha t the flow type at the stagnation point for a Newtonian fluid would ra nge from 0.0196 less than or equal to lambda less than or equal to 0.2 0, where \\E\\/\\Omega\\ = (1+lambda)/(1-lambda). Additional data are also reported, for comparison purposes, for a similar polystyrene solu tion in a simple Couette flow. Finally, the stress-optical relationshi p is used to obtain a generalized extensional viscosity as a function of strain rate. This viscosity shows a range of strain rate thinning a s predicted by reptation theory, followed at a critical Weissenberg nu mber of 0(1) based on the Rouse relaxation time by the initial stages of a region of strain rate thickening, as predicted by the Marrucci-Gr izzuti extension of reptation theory that allows for stretching of the primitive chain. Detailed comparisons with birefringence predictions from this model, using measured flow data as input, show good qualitat ive agreement but a number of quantitative differences. Most notable a re: the fact that the model predicts an orientation angle that is rota ted further from the principal strain rate direction than what is obse rved experimentally, especially for the smallest lambda values; the la ck of a distinct plateau region prior to the onset of chain stretch; a nd a rate of increase of steady state chain stretching with Weissenber g number that is significantly weaker in the experiments than what is predicted via the model. (C) 1998 The Society of Rheology.