Experimental Determination of the Storage and Loss Moduli

In order to obtain an accurate picture of the relaxation behavior of a polymer it is essential to have oscillatory shear data over the broadest possible range of frequencies. 

Both controlled-angular displacement (strain-controlled) and controlled-torque (stress-controlled) rotational rheometers are used, with the former giving superior performance at high frequencies and the latter better precision at low frequencies. There have also recently appeared on the market instruments said to be capable of operating in both modes. Controlled torque instruments can also be used to make creep and creep recovery tests, which are described in the next section. In order to obtain a linear viscoelastic characterization that includes the terminal zone, it is sometimes useful to combine data from oscillatory flow with those from a creep experiment, and this is also discussed in the following section. Rheometrical methods are described in some detail in several books [3,8,9] 

The fixtures in contact with the sample, which constitute the rheometer geometry , may be either a cone and a plate or two parallel plates. For linear viscoelasticity measurements, parallel disks are nearly always used, as the loading of samples and setting of the gap are much simplified. Factors that must be taken into consideration in order to obtain rehable data are  

Squeezing a sample either in the initial loading or when changing the gap, also compresses the sample, and the stresses introduced must be allowed to relax before meaningful data can be obtained. Commercial rheometers usually provide an indication of the normal thrust on the fixtures that can be used to track this relaxation process. 

Care must be taken to ensure that the moduli determined represent linear behavior. If the actual output signal is observed on an oscilloscope, one can inspect the wave form to verify that it is a sinusoid. However, this is rarely done. And the software provided by rheometer manufacturers will calculate and display values of the storage and loss moduli, even if the 

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