A double partition cone-plate piezorheometric tool to measure normal stress differences

The nonlinear rotational rheometry of viscoelastic materials such as polymer melts and solutions can become technically challenging due to the intervention of edge fracture instabilities when the shear rate increases. By using a cone-partitioned plate (CPP) fixture, i.e., an inner plate connected to the transducer and a nonmeasuring external partition fixed on rheometer's upper stage, one can significantly delay edge fracture in the inner plate and measure the shear stress over a wide range of shear rates, reaching a Rouse–Weissenberg number of 40 or more. However, to extract both the first and the second normal stress differences, two different CPP geometries and/or loadings are needed. To bypass this cumbersome approach, which is also prone to sample loading errors, it has been proposed to use two partitions, the so-called CPP3 geometry. In this arrangement, the inner partition serves as a second surface to measure the normal force signal. Earlier attempts were promising but measurements were too complicated and with limited temperature control. Here, we present CPP3+, a new fixture to measure the normal forces over a wide temperature range, which is based on the CPP3 concept. Our approach involves a piezoelectric sensor that is coaxially mounted on the inner partition, and the entire tool fits into the temperature control oven of a strain controlled rheometer. The stiffness of the piezoelectric sensor rules out any force-rebalance complexities, while its fast response provides high quality transient signals. We discuss the design and implementation of this simple and robust fixture and present data with entangled polymer solutions and melts to demonstrate the proof of concept. Still, the CPP3+ cannot be used above 80 °C, and edge fracture may affect the normal stress signal (and, in particular, the sensitive N2 data), something that could be mitigated in the future.