PDMS Measurements Under Pressure Influence of the Used Gas for Pressurization

Sample preparation is particularly important for rheological measurements. This is especially true for measurements of viscosity under pressure. If viscosities of samples in reactors or aggregates are to be measured, it must be ensured that the sample in the measuring cell also corresponds to the material to be tested in the application. Often it is not sufficient to simply simulate temperature and pressure conditions.


Usually increased pressure has only a small influence on the viscosity of liquids. However, this does not apply if the system is approaching a phase boundary, or if the gas used for pressurization not only dissolves to a saturation concentration in the sample, but is completely miscible. An example of this is the silicone oil (polydimethylsiloxane, PDMS) system with carbon dioxide (CO2).[1]

If PDMS is pressurized with nitrogen, the viscosity increases slightly compared to measurements at atmospheric pressure. Initially this can also be observed for the use of CO2. If, on the other hand, the measurements are repeated after longer waiting times, the viscosity has dropped considerably in some cases. Due to the small boundary surface and the long diffusion paths, it takes a long time to reach equilibrium concentration in the measuring geometry. In order to simulate the real flow behavior of PDMS within a reactor or aggregate under gas pressure at the rheometer, time-dependent effects must be taken into account.

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