Single Frequency vs Time Sweep
Screen for change over time
This assay evaluates rheological stability and changes (i.e. curing, physical degradation) over time under highly controlled conditions, typically temperature. Similar to other frequency modulated assays, this assay should be performed within the material's LVER (Linear ViscoElastic Region) determined with an amplitude sweep also performed at the same temperature(s) to ensure the sample is rheologically intact during the assay. This assay is particularly helpful to screen samples that may lose volatiles (including water) over time resulting in potentially confounding results and conclusions. Using a solvent trap enclosure along with enriching the sample environment with the volatiles and applying a low viscosity silicone oil around the plate edges are often helpful.
Like the other oscillatory assays, the single frequency sweep measures viscoelastic properties such as stiffness (complex modulus, G*), solid nature (elastic or storage modulus (G')), liquid nature (viscous or loss modulus (G")), phase angle, complex viscosity, and tan delta (G"/G') across a frequency range.
The figure shows complex moduli (G*) for differently prepared polymeric changing over 3 hours at 180C with 4N compression force under nitrogen.