Viscoelasticity – measuring elasticity and viscosity
In physics, elasticity is the ability of a body to resist a distorting influence and to return to its original size and shape when that influence or force is removed. Consider the common rubber band. Stretch it, and it returns to its original shape. At the (macro)molecular level, the chains within the rubber can be stretched and will return to their original shape.
This is also true for metals. Consider, for example, the spring–whose elastic properties are its very raison d’etra. However, we know that if you drastically elongate the rubber band or the spring, it may have deformed to the point where the molecular structure is broken and it does not return to its original state. In other words, no material is perfectly elastic, although some are more elastic than others. To reverse a sports metaphor, they bend but do not break.
Now consider a plastic material such as ABS plastic. Known for its impact resistance and sturdy construction, and yet when heated it becomes a fluid that can be molded or extruded. How else would be get those neatly shaped plastic building bricks?
Viscosity is the measure of a fluid’s resistance to deformation. Put your finger into water, and the water will return to its original structure. Run your finger across the frosting on a birthday cake, and you’re likely to be in trouble!
Materials that exhibit both viscous and elastic characteristics when deformed under stress are known to be viscoelastic. And this would include any polymeric material—from butter to bicycle helmets. (Yes, butter, a lipid, consists of long chains of identical, repeating molecules known as monomers, i.e., a polymer. Understanding the viscoelastic properties of lipids is a key discipline in food science.)
Rheology is a branch of physics that studies the deformation and flow of materials, both solids and liquids, and is therefore an excellent tool to analyze the mechanical properties of polymers in their various physical states. A key function in plastics production is characterizing its flow properties. When will it melt? When will it break? When it cools, will it be rigid or soft? Can it be molded? How can it be extruded? Can it be made stronger? Or cheaper?
Understanding the viscoelastic properties of a polymeric material is essential in optimizing plastics formulations and blends, and in managing the processing properties of the material. The molecular structure of a polymer melt determines which behavior is dominant (viscous or elastic). For example, too much elasticity can lead to flow anomalies and unwanted effects during many common processing steps.
Rheology viscoelastic tests
Rheological application specialists here recently published a lengthy examination of viscoelastic tests that can be performed with rotational rheometers. The paper explains how the obtained results relate to different processing conditions as well as to the final product properties. Anyone involved in polymer engineering, production or quality control will find this to be a valuable resource in understanding viscoelastic properties in order to optimize formulations and blends, adapt a process to the properties of a material, or avoid problems caused by flow anomalies.
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