A transition occuring in ideal elastic liquids during shear flow.

Dilute solutions of high molecular weight polyisobutylene dissolved in kersoene and low molecular weight polybutene have previously been reported to behave as ideal elastic liquids ("Boger fluids"). We report here rheological properties for similar solutions, having, however, higher molecular weights for polyisobutylene. At low shear rates, these solutions exhibit the expected Boger- type rheological behavior, and obey the Oldroyd-B constitutive equation. However, above a critical shear rate which depends upon molecular weight, prolonged shearing in a cone-and-plate or parallel-plate rheometer induces a time-dependent increase in the solution viscosity and elasticity. We find that the dependence of this transition on the Weissenberg number and the gap width or cone angle is consistent with a viscoelastic instability predicted by Phan-Thien for Oldroyd-B fluids. This instability appears to be of some generality for Boger fluids, since we have also observed it in a new monodisperse Boger fluid (polystyrene in low molecular weight polystyrene and dioctyl phthalate). Furthermore, this transition may have previously been observed (though not identified) by Jackson, et al, using the Boger fluid polyacrylamide in maitose.