Predefinition of measurement standard to determin viscosity by rotational rheometers

Tests in oscillatory mode must be carried out. The steps to be executed with a plate as geometry and a Peltier plate as temperature controller are as follows:

1. Strain or stress sweep at different temperatures (both in the liquid phase and during the phase transition). A frequency about 1 Hz is advisable. With this step, the linear viscoelastic region will be determined. With melted PCM, lower frequencies should perhaps be used.
2. Frequency sweeps in the molten and in the phase transition region. The frequency sweep must be carried out at a stress or strain within the linear viscoelastic region (defined previously in step 1). In any case, it would not be advisable to use high stress or strain as this could affect the measurements. It is important to work far from the minimum angular displacement value of the rheometer. This frequency sweep will provide information about the rheological behavior of the sample in the molten and in the phase transition region (Newtonian or non-Newtonian, if the Cox–Merz rule is fulfilled). If the sample is Newtonian, step 3 may be tested at any frequency. Frequencies below 1 Hz are recommended to avoid problems of inertia in the case of measuring with control stress rheometers, especially with melted PCM.
3. Once the linear viscoelastic region is determined and the frequency test executed at different temperatures, an oscillatory temperature ramp test (or by steps) must be executed (both for melting and solidification). If the PCM is non-Newtonian, the test will be carried out at 0.01 Hz and if it is Newtonian, 1 Hz will be sufficient. Low gaps must be used to avoid temperature gradients in the sample. Different gaps should be tested. When the results do not vary, an appropriate gap will have been found.