Assay Rheological Behavior and Changes Over Time

A time sweep evaluates rheological stability and changes (i.e. curing, degradation, volatile loss) over time under highly controlled conditions.  This assay is performed at a constant temperature in either rotational (constant shear stress or shear rate) or oscillatory (single frequency under constant stress or strain) modes.

For time sweeps performed in the oscillatory mode, this assay should be performed within the material's LVER (Linear ViscoElastic Region) determined with an amplitude sweep that was performed at the same temperature(s) to ensure the sample is rheologically intact at the beginning of 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') over time.

The figure shows complex moduli (G*) for differently prepared polymers changing over 3 hours at 180C with 4N compression force under nitrogen.