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I recently stumbled upon the concept of viscoelasticity. Not being a chemist, but being interested in the topic, I started to read the wikipedia article.

The main thing I extracted from it is:
Solids are elastic, fluids are viscous, viscoelastics materials are both viscous and elastic.

I didn't get it, maybe due to my poor background. Is viscoelasticity a state of matter or just a property of materials? In the latter case, can we talk about viscoelastic solid and viscoelastic fluids?
I know the state of matter isn't an element nor compound or material property but it changes as temeprature and pressure change. The strength of the bonds between atoms/molecules and the "value" of their kinetic/vibrational energy is what determines how we see them.

Anyway, I couldn't really fit viscoelastic material in my chemistry mental scheme, so I'm here in search of some clarifications.

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    $\begingroup$ Would you consider a semi-crystalline polymer between its glass transition and its melting point a solid or a liquid? Above its melting point it would be considered a liquid, but would still exhibit viscoelastic behavior. $\endgroup$
    – Chet Miller
    Apr 6, 2018 at 17:00

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Yes, you can talk about viscoelastic solids and viscoelastic liquids, and it is possible to distinguish the two by oscillatory mechanical tests (e.g. with a rheometer). You usually apply a sinusoidal deformation over time to your specimen and measure the time-dependent response.

In case of a true elastic solid, the deformation will induce a force which follows the deformation without time delay. It is like if you pull a spring: you will instantly feel the force required to do so. In other words: The phase angle which describes how far the sinusoidal material response is shifted from the sinusoidal deformation is 0°.

In case an ideally viscous liquid, you will only feel a force if you 'deform' with some velocity. Therefore, the force you feel while oscillatory deformation will follow the deformation with some time delay. The maximum force will be measured when the velocity of deformation is at its maximum. This results is a phase angle of 90°. It is like stirring in a pot of water: the faster you stir, the more force is required.

In a viscoelastic case, your phase angle is somewhere between 0° and 90°. If it is between 0° and 45° you are closer to an elastic solid and you would call your material a viscoelastic solid. If it is between 45° and 90° it is a viscoelastic liquid.

In a viscoelastic solid, a bigger fraction of deformation energy is stored elastically than is dissipated. In a viscoelastic liquid, the opposite is true. As mentioned by Karl, if a material behaves like a viscoelastic solid or liquid also depends on the frequency or velocity of deformation as well as on the deformation itself.

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    $\begingroup$ +1 But not exactly. A viscoelastic material behaves elastic in some frequency regime(s), and viscous ("lossy") in others. The distinction solid/liquid is usually made in the low frequency + small deformation limit. $\endgroup$
    – Karl
    Nov 16, 2019 at 15:52
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There are two basic models that describe the two fundamental branches of viscoelastic behaviour:

  • The Maxwell model is made up of a spring and a dashpot in series: For fast deformations, the dashpot does not really move, and your spring gives pure elastic behaviour. For slow deformations, the spring stays relaxed, and the damper fluid in your dashpot turns all energy into heat.
  • The Voigt-Kelvin/Kelvin-Voigt model is a spring and dashpot in parallel: Even at the slowest/smallest deformation, the spring never relaxes until you let go and let it return your sample to the original shape. And the faster you try to deform it (e.g. oscillatory), the more heat your damper produces.

The former, e.g. a molten polymer, is a liquid, the latter, e.g. crosslinked rubber, is a solid.

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Viscoelastic materials are generally considered solids, but often are noncrystalline (like the vinyl in records). The 'visco' property is akin to ductility in metals, that a slowly deformed piece of material cannot be made to relax to its original shape.

Presumably, some near-neighbor bonds (hydrogen bonds?) reform in relatively short times when strained.

The vinyl record will hold its shape indefinitely under small forces, but if you bend it, the large force applied causes internal strain only for a short time, the strain relaxes and the material permanently takes the as-bent shape. Rapid bending (slap the record) will shatter it. Silly putty, if you are familiar with that material, is also viscoelastic, but won't hold its shape indefinitely, and should be considered a very viscous liquid, except that rapid bending will break it (in propogating-crack fashion).

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    $\begingroup$ Vinyl (PVC) is partially crystalline (ten percent or so), otherwise the record would still shatter if you hit it, but it would already deform standing upright in your bookshelve, and any music pressed into it would vanish after twenty minutes. Polymer melts otoh. are viscoelastic liquids. $\endgroup$
    – Karl
    Nov 16, 2019 at 15:37

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