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 m olecular weight (MW) PS and a semidilute (6.4 vol %) solution of 20.6 milli on MW PS were used to study the effects of entanglement loss on dynamics. A phase-modulated flow birefringence apparatus was developed to facilitate m easurements of time-dependent changes in optical equivalents of shear stres s (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 mechanic al rheometry measurements to extend the range of shear rates over which ent angled polymer dynamics are studied. In slow (tau (-1)(d0) > <(gamma )overd ot > ) 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 independ ent of shear rate for all but the most weakly entangled materials investiga ted. For solutions containing the highest MW PS, an approximate plateau ori entation angle XP in the range 20-24 degrees is observed; chi (p) values ra nging 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 o f a tube model theory that accommodates changes in polymer entanglement den sity during flow. (C) 2001 John Wiley & Sons, Inc.