Silly Putty
Silly Putty? More like Super Putty!

Concepts
Polymers, properties of solids and liquids, adhesion

Materials
  • 1 package of silly putty
  • White index card
  • Pencil
  • Colored pencil
  • Black ink pen
  • Blue ink pen

Procedure
Part 1: Solubility of Silly Putty
1. Students should press the Silly Putty against the white index card, peel it off and then look at the side of the putty that was pressed against the card. This will act as the control of the experiment.
2. Next students should draw four identical happy faces on the index card; one with each writing utensil: pencil, colored pencil, black ink pen, blue ink pen.
3. One at a time, students should first roll the Silly Putty into a ball and then press it against a happy face drawing. Then peel off the Silly Putty and observe the underside of the putty (the part pressed against the drawing) to see if the image is transferred.
4. Students should record which writing utensils were absorbed by the Silly Putty and of the ones absorbed compare how well it transferred.
5. Challenge students to suggest explanations for their observations.

Part 2: Investigation of the Mechanical Properties of Silly Putty
1. Allow student groups 10 – 15 minutes to examine the mechanical properties of Silly Putty. For example, students could roll it, bounce it, stretch it, and roll it into a ball and let it sit several minutes.
2. Ask groups to summarize and discuss their observations
3. Ask what happens when Silly Putty is pulled slowly (it stretches) versus pulled rapidly (it snaps).
4. Challenge students to suggest explanations for their observations.


Explanation
Silly Putty is a polymer that was discovered in 1941 as General Electric Laboratories was attempting to make a synthetic rubber using silicon instead of carbon. A silicone polymer, which contains silicon-oxygen chains instead of the carbon-carbon chains found in many polymers, was heated with boric oxide to a temperature of about 200°C. When the polymer was cooled the material now known as Silly Putty was .

Adhesion is the tendency of dissimilar particles or surfaces to cling to one another. Silly Putty is a pretty good adhesive and can readily cause the graphite found in pencils to cling to it. Graphite is a solid with a layered structure and the topmost layers can easily be removed by pressing the Silly Putty against it. Ink from a pen does not adhere to Silly Putty for the simple fact that the liquid ink soaks (is dissolved) into the paper. Most inks found in pens consist of a liquid dye (such as blue or black) dissolved in a quickly evaporating solvent. Once the liquid dye is dissolved in the paper it is very difficult for the Silly Putty to remove it.

figure15.jpg
Figure 15: All smiles - pencil adheres to Silly Putty but pen does not.

Silly Putty has several unusual mechanical properties: it is viscoelastic, meaning it can be stretched and shaped and mashed back together again. Additionally, it can be rolled into a spherical ball, but, if left to sit for a period of time, it will spread out and flatten. It can be stretched if pulled slowly, but a rapid pull causes the two pieces to break with a snap. Thus, Silly Putty is a non-Newtonian fluid, a fluid that doesn’t behave like most fluids. The viscosity of most fluids only depends on the temperature – the reason why we warm maple syrup up. In non-Newtonian fluids, the viscosity also depends on the amount of force that is applied. These properties of Silly Putty can be related to its molecular structure. The basic structure is a silicon-oxygen chain with boron atoms in place of some of the silicon atoms. Because each boron atom in the Silly Putty polymer is attached to a hydroxyl (-OH) group, there are many opportunities for hydrogen bonding (see figure below). The hydrogen bonds are much weaker than the covalent cross-linking bonds and as a result can be easily broken. If pulled slowly, not as many bonds are broken and Silly Putty tends to flow instead of snap.


figure16.jpg
Figure 16: Hydrogen bonding in Silly Putty.

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