Bouncing Balls

Developers:

Joseph Dinich
Beck School
Cherry Hill, New Jersey

Noel G. Harvey
Exploratory Plastics Research
Robin Queenan
Plastics Research
Rohm and Haas Company
Bristol, PA

Grade
Level:

7 through 9

Discipline:

Chemical Technology

Goal:

Students will make a polymer to demonstrate its properties and develop an awareness of the wide variety and uses of polymers.

Specific Objectives:

The students will:

1. Define the term polymer.
2. Make a polymer and identify its properties.
3. Recognize that a chemical reaction took place.

Background:

The term polymer comes from the Greek words �poly,� meaning �many,� and �meros� meaning �parts.� The word �parts� refers to a grouping of atoms called a molecular unit, known as a monomer. A molecular unit or monomer alone is not a polymer. Many monomers strung together in a series form a unique polymer. Polymers can be thought of as long chains which are capable of entangling with each other much like a bowl of spaghetti. In some systems, chemical bonds (called �crosslinks�) between distinct polymer chains may form. Most early polymers consisted of fewer than 200 monomers. Today�s polymers may contain thousands of monomers. The numerous ways in which these monomers can be linked may be very complex. They include single chains, parallel chains, intertwining chains, spirals, and loops. The type of monomers, the number that are joined together, and the degree of entanglements determine the physical properties of the polymers. All living things contain polymers. Nylon, polyethylene, wood, proteins, and most plastics and rubbers are all polymers. As a result of the very high molecular weight and the entanglements between chains, polymers have excellent chemical resistance and are very durable.

Silicon is a very interesting type of atom. Find its position on the periodic table of the elements. Like carbon, silicon makes four chemical bonds and can branch out on that many directions to make long chains. In sodium silicate, the silicon atom is bonded to four oxygen atoms and is not linked in any chains. The ethyl alcohol molecule is very simple, and has just two carbon atoms. When sodium silicate and ethyl alcohol are put together, the silicate particles begin to link up with each other to form long chains as the ethyl groups (sometimes known as �R�) replace oxygen atoms in the silicate ion. Some become crosslinked between chains. Water molecules and heat are byproducts of the formation of the polymerization bond.

The large molecule is a solid. It is a type of silicone polymer.

Materials:

20 mL sodium silicate solution 5 mL ethyl alcohol 2 50-mL beakers stirring rods paper towels

small plastic sandwich bags disposable plastic gloves (2) 25-mL graduated cylinders paper clips goggles

Management Tips:

1. Wear goggles and disposable plastic gloves.
2. Provide students with background information on polymer chemistry.
3. Use paper clips or paper chain links to represent monomers. Linking many of these together will model a polymer. These chains of paper clips represent a polymer of many monomer units. In addition, tangling together several distinct chains will illustrate the phenomenon of polymer chain entanglement. This entanglement is aided by physical mixing. The students will be doing this when they rotate the ball in their hands, resulting in the rubbery properties.
4. The difficulty of breaking apart polymer chains can also be illustrated by having the children clasp hands (forming the chemical bonds between monomer units) and �tangle� amongst each other. This activity can be used to emphasize the durable nature of plastics and the lack of degradability.
5. Identify some natural polymers (cotton, silk, wool, wood protein, amino acids, hair) and synthetic polymers (rubber, plastic wrap, nylon) and their uses. Some synthetic polymers are found in artificial body parts, rugs, cars, sneakers, rugs, bullet proof vests, skateboards.
6. This experiment can be used in cooperative learning groups.

Procedure:

1. Define the term polymer and list some examples and their uses.
2. Model a polymer with paper clips. Use this model to demonstrate that the polymer is formed from a large number of monomer units, and the chain entanglement concept.
3. Use paper clip model and the �children holding hands� model (described in tips) to demonstrate the resistance of the monomer units to disengaging, and the difficulty in untangling the chains, which lead to the durability of polymers.
4. Measure 20 mL of sodium silicate solution and pour it into a beaker. Avoid contact with the skin.
5. Observe the liquid and record its characteristic properties.
6. Measure 5 mL of ethyl alcohol in a clean graduated cylinder and pour it into another beaker. CAUTION: Alcohol is flammable.
7. Observe the alcohol and record its characteristic properties.
8. Pour the alcohol into the cup with the sodium silicate solution.
9. Using a circular motion, stir with a stirring rod until the substance formed is solid. Does the cup feel warmer?
10. Wearing plastic gloves, place the polymer between the palms of your hands and gently rotate until a spherical ball that no longer crumbles is formed. BE PATIENT - discover a technique. Moisten the ball occasionally by holding it in a small stream of water from the faucet.
11. Bounce your ball (squeeze it, stretch it, step on it - find out about it!).
    Observe and compare the properties of your ball to the properties of ethyl alcohol and sodium silicate.
12. Compare your ball with those of the other members of the class. How many properties can you compare? (size, height of bounce, etc)
13. Store the ball in a plastic bag. If it crumbles, it can be reformed.

Questions:

1. What is a polymer?
2. How do you know that a chemical reaction took place when the two liquids were mixed?
3. Why did rotating the polymer in the palms of your hand make it more rubbery?
4. What factors (if any) made a difference in properties among the balls?
5. Plastics are polymers. Why do plastic trash bags and plastic wrappers stretch when you try to tear them open.

Extended Activities:

1. Compare your polymer ball to a store-bought super ball.
2. Using litmus paper, check the acid/base nature of the ethyl alcohol and sodium silicate solution. Check their solubility in water.
3. Determine the density of the three substances.

References:

This activity was adapted from an experiment entitled �Making a Super Ball� from the book Chemical Demonstrations, Volume II, 2nd Edition, Lee R. Summerlin, Christie L. Borgford, and Julie B. Ealy, �American Chemical Society, 1988.

D. Hurd, M. Silver, A. BornnBacher, C.W. McLaughlin Physical Science Prentice-Hall, Englewood Cliffs, NJ 1988.