Hydrolyzed starch ‐ polyacrylonitrile graft copolymer solutions: Effect of shear history on transient and steady ‐ state viscoelastic behavior

Both viscosity ζ and primary normal stress difference (P11 - P22) of water solutions of hydrolyzed starch-polyacrylonitrile graft copolymers have been investigated, with a cone and plate rheometer, as a function of time and sample history at a variety of shear rates. At a constant shear rate the highly viscoelastic fluids show strong stress overshoot effects, both ζ and (P11 - P22) rising to a maximum and then falling to a steady-state value. Both equilibrium levels and peak heights strongly depend on sample history. Unexpectedly, oscillatory shearing of certain samples at low frequency and amplitude, before steady shearing at constant shear rate, had an extreme effect on observed peak heights of both ζ and (P11 - P22) but little on equilibrium levels. Magnitude of the effect depends on the time for which the oscillations are imposed. Such behavior may be characteristic of some high molecular weight polymers in solution and useful in identifying long-range entanglements which strongly influence the elastic properties of these solutions.