Experiment 3

A Silly Polymer

Cross-Linking a Polymer to Create Everyone's Favorite Childhood Toy, Silly Putty

Objective: The objective of this experiment is to cross-link a polymer and observe the changes in the physical properties as a result of this cross-linking. The changes in physical properties of a cross-linked polymer are also studied as the temperature is varied.

Review of Scientific Principles:

If a substance springs back to its original shape after being twisted, pulled, or compressed, it is most likely a type of polymer called an elastomer. The elastomer has elastic properties (i.e., it will recover its original size and shape after being deformed). An example of an elastomer is a rubber band or a car tire.

The liquid latex (Elmer's glue) which you use contains small globules of hydrocarbons suspended in water. The silly putty is formed by joining the globules using sodium borate (a cross-linker). The silly putty is held together by very weak intermolecular bonds that provide flexibility around the bond and rotation about the chain of the cross-linked polymer. If the cross-linked bonds in a polymer are permanent, it is a thermosetting plastic, even if above the glass-transition temperature (T_g). If the bonds are non-permanent, it can be considered either thermoplastic or an elastomer.

Time: A 20-25 minute period is required to perform the mixing/making of the silly putty.

Materials and Supplies:

• 55 % Elmer's glue solution in water
• 4 % borax solution (sodium borate)
• Styrofoam cups
• zip lock bags
• food colors

Procedure:

1. Wear goggles and lab aprons.
2. Pour 20 ml of the Elmer's glue solution into a Styrofoam cup.
3. Add 10 ml of the cross-linker (borax solution) to each cup.
4. Immediately begin stirring the solutions together using the wooden stick.
5. After a couple of minutes of mixing, the silly putty should be taken out of the cup and kneaded in the hands. Don't worry about the material sticking to your gloves as these pieces will soon mix with the larger quantity with which you are working. Continue to knead until the desired consistency is reached.
6. Using a ruler to measure, drop the ball from a height of 30 centimeters. To what height does it rebound?
7. Stretch the silly putty slowly from each side.
8. Compress the silly putty back into a ball.
9. Pull the silly putty quickly from each side and compare the results.
10. Place the silly putty on some regular news print and press down firmly.
11. Remove the silly putty from the news print and make observations.
12. Repeat the same procedure on a comic section of the newspaper. The silly putty is non-toxic and safe to handle so you can put it in a zip-lock bag and take it home.
13. Follow good laboratory procedure and wash your hands with soap and water when you have finished the experiment.

Video Clip

Data and Analysis:

Height of the rebound _________ cm.

Observations of pulling the silly putty slowly:

Observations of pulling the silly putty quickly:

Observations of the silly putty on newsprint:

Observations of the silly putty on the comic's section of the newspaper:

Questions:

1. How do the physical properties of the glue, water mixture change as a result of adding the sodium borate?
2. What would be the effect (your thoughts) of adding more sodium borate solution?
3. What is the ratio of the height of the drop to that of the rebound distance?
4. Who in the class had the ball with the most elasticity?
5. How did you come to the conclusion of whose ball was most elastic?
   At Home:
   -Place your ball in the refrigerator for 10 minutes. Recheck the bouncing portion of this experiment.
6. What are your observations?
7. Why do you think this was observed?
   -Now place your ball about 6 inches from a light bulb for about 5 minutes and again recheck the bouncing portion of this experiment.
8. What are your observations?
9. Why do you think this happened?

Explain the Following:

1. Why does a car tire appear to be flat in the summer even though the gas inside is hotter than in the winter.
2. Why does a basketball bounce differently inside a gym than it does outside on a cold wintry day.
3. Why will a tire sometimes bump during the winter as a car is moving, only to smooth out its ride after the car has been traveling for a distance.

Teacher Notes:

Objective: The objective of this experiment is to investigate cross-linking using a similar technique as was used in the making of slime. The same parameters are worked again with a formal and a quantitative measurement used to describe elasticity. The added home investigation of the effect of temperature on the elasticity also includes concepts of molecular motion and intermolecular bond strength.

Review of Scientific Principles:

If a substance springs back to its original shape after being twisted, pulled, or compressed it is a type of polymer called an elastomer. The elastomer has elastic properties. It will recover its original size and shape after being deformed.

The liquid latex used contains small globules of hydrocarbons suspended in water. Joining these globules forms the mass with which the students will be working. The covalent bonds along the chain are strong, but the bonds between chains are normally weak. However, additives such as borax allow the formation of strong "cross-links" between chains, such as C-B-C. As the number of cross-links increases, the material becomes more rigid and strong.

Natural sources of this liquid latex are milkweed, rubber trees, pine trees, aloe plants, and many desert plants. This latex is used to quickly mend and repair any damage to the outer covering of the plant.

• The materials used in this experiment are all non toxic. It is a good idea always to exhibit good laboratory technique when working with the students. Make sure the laboratory.

Experimental:

There are many variations of this experiment.


1. The original silly putty was prepared using sodium silicate and mixing this with borax.
2. A variation also exists using laundry starch and mixing it with borax.
3. Similar variations also exist by sprinkling the borax evenly and gently over the solution of latex then working it with the hands. This does not require as much kneading to dehydrate the sample.

Time: - About 15 minutes are required to ready solutions, cups and tongue depressors.
10-15 minutes will be required in lab for testing and clean up.
The students will require 10-15 minutes of work at home in order to finish all of the experimental work on this laboratory and the write up.

Answers to Questions:

1. The liquid type of starting material should jell and become more viscous as cross-linking occurs.
2. The material will become more solid or rigid.
3. Student answer. This is only a method of measuring elasticity of the polymer. Stretching gives a similar means of comparison.
4. Student answer.
5. Greatest rebound to drop height ratio.
6. Here the student will be studying the effect of temperature variation on elasticity. Students are sometimes surprised if they place their sample into a freezer rather than a refrigerator. The results are that the ball will shatter rather than bounce.
7. The ball should be more elastic.
8. Contrary to what some students will predict, should the ball become too warm, the resulting ball will deform rather than continue to increase in elasticity.
9. The ball deformed rather than rebounding.

-All of the answers to the questions in the EXPLAIN THE FOLLOWING section involve the use of principles previously presented in this laboratory.