DWS Microrheology of Aqueous Polymer Solutions





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It is important to note that for the higher polymer concentration the solution is highly viscoelastic and thus can only be characterized using DWS microrheology. DLS microrheology (available with the NanoLab 3D) requires samples with lower viscosity and the absence of low frequency elastic moduli. This is due to the limited sensitivity to small particle displacements and the lacking possibility of performing ensemble averages in all common DLS instrument configurations. DWS with Echo technology (LS Instruments patented) however can easily access samples such as the 1.0 wt% polymer solution studied here. Even the 0.1 wt% sample would only barely be suitable for DLS microrheology and would require measurement times of hours to obtain accurate data in the low frequency regime.




We applied DWS microrheology to two different polymer solutions with concentrations 0.1 and 1.0 wt% adding about 1% in volume of polystyrene tracer beads.  Measurements were carried out using sealed glass cuvettes for a duration of 6 minutes per sample.

The agreement with mechanical rheometry is found to be good to excellent over the range of frequencies accessible to both methods. The validity of the Cox-Merz rule for polymer solutions has been successfully established for both samples.  The DWS data extends the frequency range accessible to the MHz range and requires only small sample values 0.2-0.3 mL.



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