Exponential shear flow of linear, entangled polymeric liquids

A previously proposed reptation model is used to interpret exponential shea r flow data taken on an entangled polystyrene solution. Both shear and norm al stress measurements are made during exponential shear using mechanical m eans. The model is capable of explaining all trends seen in the data, and s uggests a novel analysis of the data. This analysis demonstrates that expon ential shearing flow is no more capable of stretching polymer chains than i s inception of steady shear at comparable instantaneous shear rates. In fac t, all exponential shear flow stresses measured are bounded quantitatively by stress measurements taken during inception of steady shear. Information taken from the model about chain stretching suggests that normal stress mea surements are strong indications of stretching, whereas shear stress measur ements are indicative of both chain stretching and segment orientation. (C) 2000 The Society of Rheology. [S0148-6055(00)00605-2].