Measurements of shear viscosity and normal stresses in entangled polymers

We present a simple method to measure the viscosity as well as both the first and second normal stress differences of polymers in nonlinear shear flow. It is based on the use of cone-partitioned plate (CPP) geometry in order to measure over a wide range of shear rates without edge fracture problems. In particular, we employ a modular CPP setup with two different diameters of the inner transducer, mounted on a rotational strain-controlled rheometer. This setup overcomes limitations of previous approaches based on CPP, such as moderate temperatures, the need for multiple measurements with different volume of samples, and yields data over a wide temperature range by performing a two-step measurement on two different samples. The method has been successfully tested with two entangled polystyrene solutions at elevated temperatures. Results compare favorably with the limited literature data, especially on the second normal stress difference, as well as with predictions obtained with a recent tube-based model of entangled polymers accounting for shear-flow-induced molecular tumbling. Limitations and possible improvements of the proposed simple experimental protocol are also discussed.