Most rheologists would prefer a rheometer that delivered state-of-the-art performance in both modes, since it would cut capital costs, and increase experimental flexibility with no performance loss. Such devices are now commercially available. George Dallas reports.
Controlled stress and controlled rate rotational rheometers are increasingly used to provide materials scientists with the viscoelastic properties of products, and to predict their performance in end-use applications.
Either version should supply similar results for most materials, but experience indicates that certain measurements are better, or more easily, made by a particular design.
In controlled stress rheometers, a torque (stress) is applied to the sample, and the resulting displacement (strain or shear rate) is measured, usually by a high resolution optical encoder. In controlled rate rheometers, a displacement or speed (strain or shear rate) is applied to the sample and the resultant stress is measured by a torque transducer. The former is the choice for measuring viscoelastic properties of dispersions (especially weakly structured ones), for apparent yield stress measurements, and for analysis of materials, whose end-use involves stress driven processes (eg sagging, sedimentation).
Creep and recoverable compliance, important measurements in stability studies, are also best made this way. In addition, they produce quality data at shear rates several orders of magnitude lower than available from controlled rate models.
In contrast, the more intuitive and earlier developed controlled rate rheometer has advantages for tests specifically mimicking processes involving relatively high shear rates (eg stirring), assuming the torque generated is within the measurement range of the transducer. Analysis of polymer melts is also synonymous with controlled rate rheometers, as is the measurement of stress relaxation moduli.
Over the past decade, attempts have been made to develop the desired dual-purpose rheometer, either by development of ahybrid' motors, or by utilising modern electronics and computer control to simulate aast loop' controlled rate operation in a controlled stress instrument. This latter approach is the subject of this document.
The afast loop' approach has long been viable due to the inherent simplicity of the controlled stress drive system and its adaptability to computer control. However only recently has the necessary combination of a high torque range, low inertia motor, optimised air bearing technology, high speed electronics and sophisticated software been developed.
In 2001, TA Instruments introduced the AR 2000 Advanced Rheometer (Fig. 1), whose Mobius Drive system combines the two classical approaches into a aseamless' system that provides the desired performance and mode flexibility. It allows the rheologist to focus on testing for performance and system response (real world situations), since it automatically engages the drive as required by experimental conditions. The Mobius system incorporates an ultra-low inertia motor (15 µNm.s2, triple air bearing, a 2 000 000: 1 torque range, and high-speed electronics. Its ability to swiftly and accurately respond and track changes in input speed or position comparable to that of dedicated high performance controlled rate rheometers is documented in Fig. 2, where the data clearly shows the rheometer's ability to perform the rapid step changes in shear rate (angular velocity) needed to model thixotropic behaviour and to control the shear rate through a ramp(1). In addition the wide torque range of the drive system also produces superior stress relaxation results. It extends the G(t) data well beyond the time where data from the controlled rate rheometer would become too noisy for accurate measurement due to the limited range of its torque transducer. Fig. 3 shows high quality stress relaxation data from a PDMS sample performed on the AR 2000.
It is important that the engineering effort to duplicate controlled rate results does not compromise the controlled stress performance of the rheometer. Results from the AR 2000 confirm no loss in performance. For example, creep (step stress) data was acquired as fast as a point every millisecond, and used to reveal interesting short time scale sample properties. In Fig. 4, aringing' behaviour from gels and emulsions can be modelled to derive viscoelastic parameters from less than 1 second of data.
These results indicate that the AR 2000 has successfully met the performance and mode flexibility criteria required by the professional rheologist. In addition, initial comparisons indicate that the AR 2000 is superior to the ahybrid' motor design in terms of response of the drive system to input speed and position control commands, and the quality of stress relaxation data generated. u
ENQUIRY No 50
George Dallas is with TA Instruments, New Castle, DE. USA. www.tainst.com
Reference: 1. G Dallas and R Smith, Applied Rheology, Vol.11, No. 6, Nov/Dec 2001.
