Small deformation rheometry

Small deformation rheometry refers to testing procedures that do not cause structural damage to the sample. Constant stress rheometers, such as dynamic mechanical analyzers or oscillatory constant stress rheometers, are often used. 

In continuous tests the deformation is large so the structure of the sample is destroyed. In oscillatory rheometry the deformation is small so the structure remains intact. Both viscous and elastic properties can be measured simultaneously. The elastic, solid-like component of the response (the storage modulus, G ) is in phase with the deformation and the viscous, liquid-like component (the loss modulus G") is out of phase. The ratio G /G is a measure of the relative importance of the viscous and elastic components. Thus, for example, ice cream that has a high G and a low G"/G is more solid-like than liquid-like. 

In normal capillary rheometry for polymer melts, the flowing stream exits into the atmosphere, and the driving static pressure in the reservoir is taken to he AP. In such cases, end effects involving viscous and elastic deformations at the entrance and exit of the capillary should be taken into account when calculating the true shear stress at the capillary wall, particularly if the ratio of capillary length to radius L/R) is small. 

Dynamic mechanical methods (typically oscillatory parallel plate rheometry) are commonly used to measure the dynamic mechanical properties from the liquid state to the solid state. By using small-amplitude oscillatory deformations (linear viscoelastic regime), the dynamic storage and loss moduli can be obtained. From these quantities, the viscosity and modulus can be calculated (71) (see Dynamic Mechanical Analysis). 

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