Abstract
AbstractOrientation 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(n12≈ Cσ)and first normal stress differences(n1= n11− n22≈ 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(τ>) steady shear‐flow experiments using the ultrahigh MW polymer sample (20.6 × 106MW PS), steady‐staten12andn1results manifest unusual power‐law dependencies on shear rate[n12,ss∼0.4andn1,ss∼0.8]. At shear rates in the rangeτ<< τ, steady‐state orientation anglesχSSare 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 angleχpin the range 20–24° is observed;χpvalues ranging from 14 to 16° are found for the other materials. In the start‐up of fast steady shear flow(γ˙ ≥ τ), 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. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2275–2289, 2001
Published Version
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