We are surrounded by polymers in our environment, many of which are synthetic, i.e. manmade. In this synthetic category are many fabrics, including rayon, nylon, dacron, and polyester. The polyvinylchloride in some pipes and polyvinyl floor tiles are also synthetic polymers, as is the teflon in our frying pans. However, polymers have existed on earth even before life itself. In our bodies, our muscles and bones are made of the polymer we call protein. Our genetic material, DNA and RNA, is also a polymer. The cellulose we find in all plant material is a polymer, as are the complex carbohydrates we find in bread and pasta. Rubber is a good example of a naturally occurring polymer.

What is a polymer? What do all these very different materials have in common? Polymers form when many of the same molecule, or similar molecules, called monomers, are attached together like beads on a string. The beads are called monomers and the "necklace" is the polymer. The links between the beads in polymers are chemical bonds. Most polymers are comprised of many long strings of beads. To make the polymer stronger, many times these long strings are connected with "cross-linkers" to make a stronger mesh like material from the polymers.

Scientists have been able to create polymers with very different properties by using different monomers and different cross-linking reagents. In this lab we have the students form people polymers to demonstrate some of the different properties between monomers, straight chain polymers and crosslinked polymers. The students also have the opportunity to explore and create for themselves several cross-linked polymers from straight chain polymers. [While there are many great demonstrations of forming the polymers directly from monomers, I have not found any suitable for a middle school classroom. They use chemicals which cannot be handled safely in your classrooms.]

The purpose of this particular actively is to demonstrate the different in mobility between monomers, straight-chain polymers and cross-linked polymers.

Monomer

Polymer

This demonstration works best in an old-fashioned classroom with desks in rows.

1. Depending on the size of the class, invite 10 - 12 students to be part of the demonstration. e.g. For 12 students, form two polymers chains of 5 students each, using the pencils as the linkers between the people monomers.
2. Allow the other two students to be monomers.
3. Have all these students then walk from the front of the class room to the back. The polymers are not allowed to break any of the links. The monomers should get the back more easily than the long chains of polymers, but the polymers still slink to the back fairly easily, though a little more slowly.
4. Now use the two monomers as crosslinkers, either as one two person cross-link or two one person

The students could have to explain in their own words what happened and why

The students could be lead in a discussion as to why this is the case. Have then talk about why it is easier to walk with only one link rather then many. Where else is this prevalent in their lives?

The purpose of this experiment is to explore the strength of a crosslinked polymer.

Crosslink

Polymers can be made out of everyday household items and can be a lot of fun. The next three experiments are messy, creative and fun, as well as good examples of Polymers.

In each of these experiments a polymer with different properties is made. In each we start with a straight chain polymer. In both the guar gum of Part 1 and the corn starch of Part 2, the monomers are simple sugar-like molecules. The monomer of corn starch is glucose, the monomers of guar gum are two other simple sugars similar to glucose. However the monomers are connected differently in the two polymers. For example one could say in the corn starch the sugars are holding hands, while in the guar gum, a hand may be holding onto a foot.

Corn starch is familiar to students as a product that is derived from the flour one gets from grinding corn kernels. Guar gum may seem foreign, but is a very similar product. It is derived from the flour (called guar flour) of the seeds of another plant, called cyamopsis. Corn starch is frequently used in cooking as a thickening agent. Guar gum is a much better thickening agent, with five to eight times the thickening power of corn starch. In fact it is a common natural food additive, especially in lower priced ice creams. You can ask your students to look for it among the ingredients in food products. Borax, is a cleaning agent, available in most super markets.

In part 3, the monomer is an alcohol, like ethanol. All of the straight chain polymers have lots of O-H groups which can hydrogen bond with water. That is why the powdery polymers gets thicker and takes on different properties in water. They are collecting many layers of structured water around them though hydrogen bonds. In Part 1 and Part 3, we add a cross-linker to the polymer which changes its properties dramatically.

each student will need

• 9 ounce plastic cup
• Tongue depressor or coffee stirrer
• Water
• Guar gum (available from us or chemical supply house)
• Borax (a saturated solution in water)

Procedure:

1. Give each student a cup and tongue depressor
2. Put about 1/2 teaspoon of guar gum into each cup
3. Put about 1/2 cup of water into the cups (that's about 2/3 filled with water), have the students stir this to dissolve the guar gum as you add the water, and then set it aside.

5. We recommend you do Experiment 2: Corn Starch during the wait for the guar gum to thicken.

6. Add some (1 T or a good squirt.) of the saturated borax solution to each cup, and have the students stir.
7. With the addition of the cross-linking borax, the thickened guar gum turns into."slime." The students can explore the properties of the polymer. Does it bounce, stretch, break, float or sink in water? Afterwards they can take the slime home in a plastic bag.

(Wonder Science, p 278 does a slightly different spin on this experiment.)

Corn starch and water is one of the most fascinating natural polymers. If you don't tell them what it is they tend to do more experimental things with it instead of just playing. This mixture is bizarre because it has pressure dependent viscosity. This means that the more pressure you exert on it the less it flows. For example, you can pick it up by squeezing it, but if you let it sit in your hand it will drip out like a liquid. It will dry out while the students are playing with it and the corn starch powder will be all over the place. But it is well worth the mess! After letting them play for a coup of minutes, you can return to the slime activity."

each group will need

• Corn starch (a premixed solution of about 1 cup of corn starch and about 1/4 cup of water)
• Foil disposable baking pans
• Tongue depressors

You can make just one of these polymers or try all three crosslinking reagents to compare the properties of the polymer formed.

5. 3 For the epsom salts polymer, for one Tablespoon of glue, use about 1/2 tsp. of epsom salts dissolved in 1/2 tsp. of water.

Have the students shows you the various polymers that they made since the main objective for this lesson was to have fun exploring the wonders of science.

Ask the students to write a story about a polymer that they would like to invent and what it would be used for. When Laura had her students do this, she was impressed with some of the stories, Not only were the stories very creative but also they demonstrated that the students learned the key concepts of the monomers forming polymer chains which are then crosslinked by a different molecule.

There are also additional activities from WonderScience: pp. 177 -201: Section 6, Materials Science; Unit 23, Polymers, Unit 24, Plastics, Unit 25, Rubber.

SCIENCE CONTENT STANDARD #5: DESIGNED WORLD

This lesson on polymers satisfies Benchmark 1 for grades 5-8: "Discuss how and why science and technology have created synthetic (non-natural) materials".

This lab is a great way to get the students really excited about science. Many may be scared of the lab or the concepts that come with learning science, but this lab shows that science does not have to be frightening. It can be fun, interactive, and entertaining. The Extensions section also provides a way for the students to practice their writing skills.