Prediction of melt elasticity from viscosity data

A simple method for predicting the primary normal stress function, θ, from viscosity data is proposed. A relation between θ and the viscosity function, θ, based on the Goddard‐Miller rheological equation of state, is given. An easy‐to‐use generalized chart for estimating θ from θ is presented; in order to use this chart, viscosity data must be available from the zero‐shear‐rate value of the “power law region.” The method was applied to six polymer melts, three polymer solutions, and an aluminum soap solution for which θ had been measured experimentally. Even though θ varied by a factor of 103 over the examined range of shear rate, the theory and experiment seldom differed by more than a factor of 2. The generalized chart for θ was combined with Tanner's theory to prepare a graph which may be used to estimate jet swell from viscosity data. Comparison with experimental data of jet swell for 4 polymer melts shows good agreement.