A cone-partitioned plate rheometer cell with three partitions (CPP3) to determine shear stress and both normal stress differences for small quantities of polymeric fluids

Edge fracture hampers the measurement of the two normal stress differences N-1 and N-2 in a cone-partitioned plate rheometer with two partitions (CPP2). A third nonmeasuring partition has therefore been added, solely to shield off edge fracture (CPP3). The partial normal forces on the inner two partitions are much better balanced than on the CPP2. The new partitioned plate rheometer cell has been downsized to fit an MCR300. Potentially, N-1 and N-2 can now be obtained for all rheometric tests that can be performed with that rheometer. This publication reports on the technical features of the CPP3 cell. Step shear-rate tests in the range 0.1 < Wi < 3 and a strain of gamma = 40 have been performed with a poly(dimethyl siloxane) (PDMS) standard to proof the rheometric functionality. The characteristic relaxation time of the PDMS is comparable to the axial response time, in spite of a cone angle of 0.15 rad. This means that transient normal force data are affected by instrument compliance. Steady state N-1 and N-2 however are measured correctly. N-1 compares with data from a CPP2 and the MCR300, but can be determined with much less polymer. The scattering of the steady state second normal stress difference N-2 is substantially reduced compared to the CPP2. A critical evaluation of the pros and cons of the CPP3 is given, based on the results of this study. (C) 2013 The Society of Rheology. [http://dx.doi.org/10.1122/1.4797458]