Nonlinear rheology of highly entangled polymer solutions in start-up and steady shear flow

Orientation angle and stress-relaxation dynamics of entangled polystyrene (PS)/diethyl phthalate solutions were investigated in steady and step shear flows. Concentrated (19 vol %) solutions of 0.995, 1.81, and 3.84 million molecular weight (MW) PS and a semidilute (6.4 vol %) solution of 20.6 million MW PS were used to study the effects of entanglement loss on dynamics. A phase-modulated flow birefringence apparatus was developed to facilitate measurements of time-dependent changes in optical equivalents of shear stress (n(12) approximate to C sigma) and first normal stress differences (n(1) = n(11) - n(22) approximate to CN1) in a planar-Couette shear-flow geometry. Flow birefringence results were supplemented with cone-and-plate mechanical rheometry measurements to extend the range of shear rates over which entangled polymer dynamics are studied. In slow (tau (-1)(d0) > <(gamma )overdot > ) steady shear-flow experiments using the ultrahigh MW polymer sample (20.6 X 10(6) MW PS), steady-state n(12) and n(1) results manifest unusual power-law dependencies on shear rate [n(12,delta delta) similar to <(gamma )overdot > (0.4) and n(1,delta delta) similar to <(gamma )overdot > (0.8)]. At shear rates in the range tau (-1)(d0) < <(gamma )overdot > < tau (-1)(R), steady-state orientation angles chi (SS) are found to be nearly independent of shear rate for all but the most weakly entangled materials investigated. For solutions containing the highest MW PS, an approximate plateau orientation angle XP in the range 20-24 degrees is observed; chi (p) values ranging from 14 to 16 degrees are found for the other materials. In the start-up of fast steady shear flow ( <(gamma )overdot > greater than or equal to tau (-1)(R)), transient undershoots in orientation angle are also reported. The molecular origins of these observations were examined with the help of a tube model theory that accommodates changes in polymer entanglement density during flow. (C) 2001 John Wiley & Sons, Inc.