The Young Scientist Program

http://ysp.wustl.edu

Washington University Medical School

Funding provided by The Leon Lowenstein Foundation

Polymer Chemistry I

Chemistry Teaching Team

References (if any): adapted from materials provided by Laura Kosbar, PhD, IBM T. J. Watson Research Center, Yorktown Heights, NY.

Main Concepts:

Synthesis of polymers

Module Overview: A polymer is made up of long chains of molecules that have crosslinked to each other. The more these long chains are crosslinked, the stiffer the polymer becomes. On the other hand, a less crosslinked polymer will have more elasticity and flexibility. Synthetic polymers make up many items we use every day: fabrics, plastics, and rubbers. Polymers also occur in nature: wood, proteins, and deoxyribonucleic acid (DNA).

Materials:

Safety goggles

Gloves

Sodium silicate (“water glass”) solution

Ethyl alcohol

Dixie cups

Coffee stirrers

Paper towels

Food coloring

Plastic sandwich bags

Safety: wear safety goggles and plastic disposable gloves for this exercise. Sodium silicate is an irritant; avoid contact with skin.

Methods/Protocol:

I. We are going to do an exercise to help us think about how polymers work.

Have the students assemble in a large open space. Tell them they are all currently small molecules, such as water molecules, and let them move around randomly. Change the temperature- when it gets hotter, the “molecules” have more energy and move faster, and when it gets colder, the molecules have less energy and move very slowly. Make it very cold, so that all of the students stop.

Now, have them join hands so that they form 3 or 4 separate chains. Let the chains move around. They will have restricted mobility because the chains MUST stay together. They are now linear polymers. If we reduce the temperature, it is harder for the chains to move [kind of like a clump of cold spaghetti]. If we raise the temperature, movement becomes easier.

Finally, have the YSP volunteers [or more students] form “crosslinks” by joining two different chains together. Now let the students try to move around- which should be difficult! Explain that more crosslinks corresponds to less flexibility, and that the extent of crosslinking is a good predictor of polymer behavior.

II. Making a Polymer Ball

1. Measure 20 mL of sodium silicate solution and pour it into a small paper cup. Add a few drops of food coloring.

2. Add 5 mL of ethyl alcohol to the cup and stir with a coffee stirrer until the mixture begins to solidify.

3. Place the polymer in the palm of your gloved hand and gently press with the palm of the other hand until it holds together in a ball. You may want to moisten the ball occasionally by holding it under a stream of water from the faucet.

4. Store the ball in a small plastic bag. The ball may continue to harden for the next several hours.

Discussion Questions:

1. How is the ball different from the two substances (sodium silicate and ethyl alcohol) that it was made from?

The reactants were liquid, and came together to form a solid, gooey substance. We could tell that a reaction had occurred because we saw a state change – in this case, the mixture solidified.

2. The element silicon is an important ingredient in this polymer. What about silicon makes it good at making polymers?

Silicon [Si], like carbon [C], likes to form four chemical bonds. Thus, silicon can branch out in many directions and make long chains.

3. The ball may be still gooey and soft at the end of the demonstration. What needs to happen for it to firm up?

If the polymer chains are crosslinked to one another more, the polymer will become firmer. We demonstrated this by making a polymer chains of students, then ‘crosslinking’ them to each other. When we were crosslinked together, it was harder to move around.