viscoelasticity

Polymers can be runny and springy at the same time. In more technical terms, they display a combination of viscous and elastic behavior. The viscoelastic properties of a material are often measured with an instrument called a rheometer.

The partially viscous nature of some polymers is what makes them useful for dampening vibration. By definition, viscous materials dissipate energy when they are deformed. A material that was completely elastic would not be damaged by vibration, but would not lessen it. A material that was completely viscous would be permanently deformed by vibration. Caterpillar, one of the world's leading producers of earth moving equipment, hires some of its Materials Engineers almost exclusively for the purposes of developing rubber formulations that work well for minimizing unwanted vibration in the equipment that they manufacture. They have developed some extremely clever, proprietary rubber blends that dramatically improve the performance of their products.

Muscle tissue behaves in a viscoelastic manner. When people stretch, they are temporarily deforming their own muscles so that they will be more flexible.

Although it is not very strong for most viscoelastic materials, a shape-memory effect occurs when they are deformed at a temperature below their glass transition temperature then heated. Temperfoam, often called viscoelastic foam, takes advantage of this useful phenomenon.

The degree of crosslinking, the current temperature, degree of entanglement, and intermolecular forces all play a role in the degree of viscoelasticity. In general, the greater the crosslink desnsity, the more elastic a polymer will be.

The spring and dashpot model is one of the classic methods for thinking about and describing viscoelasticity.

• Springs absorb and then release energy. They represent the elastic component of a viscoelastic material.
• Dashpots dissipate energy. They represent the viscous component of a viscoelastic material.
• Springs and dashpots can be connected together in series or in parallel, just like the components of electrical circuitry.

When stretched, this system will only return partially to it's original shape. Energy will be lost when the system is stretched, but not as it returns to its original shape. Warm cheese and chewing gum behave this way.

            ___
               | 
NNNNNNNN======]|===========
            ___| 
Spring      Dashpot

When stretched, this system will return almost exactly to its original shape. Energy will be lost when the system is stretched, and as it returns to its original shape. Materials that are used to dampen vibration should ideally behave this way. Muscles also behave this way.

        Spring 
        |NNNNNNNNNNN|
        |           |
========|  ___      |======= 
        |     |     |
        |====]|=====|
           ___|
        Dashpot     

The above models are actually an oversimplfication. Most real materials behave like a combination of a series and and parallel spring and dashpot setups. It is generally good to think of a material as X percent spring and dashpot in series and Y percent spring and dashpot in parallel.

                            Second Spring
            ___             |NNNNNNNNNNN|
               |            |           |
NNNNNNNN======]|============|  ___      |======= 
            ___|            |     |     |
Spring      Dashpot         |====]|=====|
                               ___|
                            Second Dashpot