Stress overshoot of polymer solutions at high rates of shear

Overshoot of shear stress, σ, and the first normal stress difference, N1, in shear flow were investigated for polystyrene solutions. The magnitudes of shear corresponding to these stresses, γσm and γNm, for entangled as well as nonentangled solutions were universal functions of γ˙τeq, respectively, and γNm was approximately equal to 2γσm at any rate of shear, γ˙. Here τeq = τR for nonentangled systems and τeq = 2τR for entangled systems, where τR is the longest Rouse relaxation time evaluated from the dynamic viscoelasticity at high frequencies. Only concentrated solutions exhibited stress overshoot at low reduced rates of shear, γ˙τeq 1, γσm and γNm were approximately proportional to γ˙τeq. At very high rates of shear, the peak of σ is located at t = τR, possibly indicating that the polymer chain shrinks with a characteristic time τR in dilute solutions. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1917–1925, 2000