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Viscoelasticity |
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  1
Materials, Chemicals, Time Needed |
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- Hammer
- Bouncing Putty from WACKER's Experimental Kit
The length of the experiment
depends on what is to be performed with the Bouncing Putty.
It may range from several seconds
(hitting it with a hammer or bouncing it off the floor
or wall) to about 30 minutes (until it starts to flow). |
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| 2
Procedure and Observations |
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| If the ball of putty is thrown
onto the floor or against the wall, it bounces back like
a rubber ball. If a length of the putty is
quickly pulled apart by its two ends, it tears cleanly in two.
However,
if it is slowly pulled apart, it turns into a long gooey string,
just like normal dough. |
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When the two ends are brought
into contact, they fuse to each other.
No matter what shape it has,
slowly kneading
the putty will transform it into any other shape. When the
putty is being kneaded, it
will be found that the more force used, the greater the material
resists (deformation resistance).
All these properties of
Bouncing Putty can be seen by clicking on the Videos in
the header for this experiment. |
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3
Discussion of Results |
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Bouncing Putty is made from a
viscoelastic material. This means that in some cases (exposure
to slow-acting force), it behaves like a liquid. In others, such
as exposure to rapid force, it acts like an elastic material.
The viscoelasticity in this case is due to intermolecular forces
in the putty.
Bouncing Putty is a silicone in which approx. 0.5 mol % silicon
atoms in the siloxane chain have been replaced by boron atoms.
Each boron atom is part of the main chain in the silicone molecule,
but it also bears a fairly long side chain with a siloxane backbone.
These side chains are tangled up in each other and this makes
it difficult for one silicone molecule to slide past another.
The viscoelastic behavior may be explained as follows: A short-acting
force on the material only causes the tangled side chains to
bend slightly, after which they quickly return to their original
position. The main chains of adjacent silicone molecules do not
slide past each other. The material is behaving elastically.
However, if the force is maintained, entire silicone macromolecules
will slide past each other despite the entanglements of the side
chains. A simple way of viewing this is to imagine that the side
chains are bent back far enough until one main chain can slide
past another. When entire silicone molecules slide past each
other, the material flows. In that case, the Bouncing Putty is
behaving like very sticky honey – it is highly viscous.
These explanations are explained in more detail with formulae
and diagrams in the Flash animation in the header of this page. |
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4
Tips and Comments |
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- The experiment makes for an
interesting demonstration of the phenomenon of viscoelasticity.
Bouncing
Putty arouses a great deal of interest because of the material’s
strange behavior.
- Explanations of the observations could be
used to illustrate the relationship between the molecular
structure of a material
and its properties.
- Since the experiment is safe and simple,
it is ideal for the classroom. The pupils could try out
their own experiments
in
addition to those described here.
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5
Supplementary Information |
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| In silicone molecules whose backbone
consists of siloxane chains, conformational mobility of the parts
of the molecule is provided by the freedom to rotate about the
Si-O bonds. This also applies to the siloxane side chains in
Bouncing Putty, which are bound to the boron atoms as described
above (3 Discussion of Results). |
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| 6
References |
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| W. Held et al., Learning by Doing – School
Experiments with WACKER Products (handbook accompanying WACKER's Experimental Kit), Wacker Chemie AG, Munich, 2007 |
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