Automatic Range Adjustment - Extending the Measuring Range in Strain-Controlled Oscillatory Rheometry
When characterizing polymeric materials, it is of great interest to measure rheological properties over a wide range of temperatures and frequencies. Usually, the accessible temperature and frequency ranges are limited, due to limitations of rheometers and of measuring geometries, especially when samples undergo transitions during the tests. Choosing the right configuration regarding optimal measuring system and motor mode is essential. Additionally, flexible and automatically-adjusted measurement parameters can be quite helpful.
Rheology is an important analysis method in many fields and industries and is continuously facing new challenges to increase its spectrum of feasible measurements. Improvements to the software and firmware allow even the most challenging tests to be carried out. In this report, a practical feature called “Automatic Range Adjustment” (or ARA for short) is highlighted.
Polymeric samples such as adhesives, polymer melts, or waxes which are exposed to a wide temperature range may change their properties from a glassy solid to a rubbery material, or even to liquid-like melts. Since the maximum torque capabilities as well as the torque sensitivity of a rheometer are both limited, choosing appropriate parameters for strain-controlled temperature ramp tests is often based more on experience than on profound knowledge. As an example, to perform a strain-controlled test within the optimum torque range of the instrument, when the sample is in glassy state, a parallel plate measuring system with a very small diameter would be preferred. Conversely, at higher temperatures, in the rubber elastic or liquid state, a measuring system with a larger diameter would be beneficial.
In order to reduce this issue, a measuring system with an intermediate diameter can be used in combination with an automatic adaptation of the commanded strain amplitude (the ARA feature). The commanded strain is adapted such that the torque value stays above or below a user-defined lower or upper torque limit, respectively. This allows the instrument to stay within the optimum torque range in any possible case. This greatly reduces trial and error runs to find the ideal commanded parameters during measurement method development.
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