Viscoelasticity

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Viscoelasticity is a material property that combines both viscous and elastic characteristics. Many materials have such viscoelastic properties. Especially materials that consist of large molecules show viscoelastic properties. Polymers are viscoelastic because their macromolecules can make temporary entanglements with neighbouring molecules which causes elastic properties^[1]. After some time these entanglements will disappear again and the macromolecules will flow into other positions where new entanglements will be made (viscous properties).

A viscoelastic material will show elastic properties on short time scales and viscous properties on long time scales. These materials exhibit behavior that depends on the time and rate of applied forces, allowing them to both store and dissipate energy.

Viscoelasticity has been studied since the nineteenth century by researchers such as James Clerk Maxwell, Ludwig Boltzmann, and Lord Kelvin.

Several models are available for the mathematical description of the viscoelastic properties of a substance:

• Constitutive models of linear viscoelasticity assume a linear relationship between stress and strain. These models are valid for relatively small deformations only.
• Constitutive models of non-linear viscoelasticity are based on a more realistic non-linear relationship between stress and strain. These models are valid for relatively large deformations.

The viscoelastic properties of polymers are highly temperature dependent. From low to high temperature the material can be in the glass phase, rubber phase or the melt phase. These phases have a very strong effect on the mechanical and viscous properties of the polymers.

Typical viscoelastic properties are:

• A time dependant stress in the polymer under constant deformation (strain).
• A time dependant strain in the polymer under constant stress.
• A time and temperature dependant stiffness of the polymer.
• Viscous energy loss during deformation of the polymer in the glass or rubber phase (hysteresis).
• A strain rate dependant viscosity of the molten polymer.
• An ongoing deformation of a polymer in the glass phase at constant load (creep).

The viscoelasticity properties are measured with various techniques, such as tensile testing, dynamic mechanical analysis, shear rheometry and extensional rheometry.