1. Experiment is contained in the WACKER's Experimental Kit.

Yes

 2. Experimental procedure has been modified

Yes, extended

 3. A separate experimental procedure has been devised

Yes

 4. Video clip available

Yes ( as wmv or mov)

 5. Flash animation available

Yes

 6. Other materials: Slide DH7

Viscoelasticity

TopDown 1 Materials, Chemicals, Time Needed
  • 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).

TopDown 2 Procedure and Observations

First, shape the putty into a ball.
If the ball is slowly compressed with a hammer, it flattens out into a disk. However, if it is struck suddenly with the hammer, the hammer bounces back as if it had just struck a hard rubber ball, without any change in the shape of the putty. If the ball of putty is simply left on a flat surface, it will flow out into the shape of a disk within 30 minutes.

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.

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.

TopDown 3 Discussion of Results

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Bouncing Putty is a silicone in which approx. 0.5 mol % silicon atoms in the siloxane chain have been replaced by boron atoms.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.

TopDown 4 Tips and Comments

  • 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.

TopDown  5 Supplementary Information

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).
TopBottom  6 References
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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