Experiment 1
Crunch and Munch Lab
Desk Top Building of Polymer Chain Components
Objective: The objective of this lab is to introduce the concepts and vocabulary of "polymers" with simple models.
Review of Scientific Principles:
Polymers (Greek-POLY...many and MEROS...parts) have existed since the beginning of life. Both "natural" and "synthetic" polymers are an integral part of our life. Most of the natural and synthetic materials with which we come in contact are wholly or partly polymeric in nature.
Polymers (plastics) are large molecules (macromolecules) made up of repeating units called "mers" or more correctly "monomers". These "units" are chemical molecules. To introduce the common terms used in polymers, we will use the models shown in this desktop experiment.
Time: This laboratory experiment requires about 40 minutes.
Materials and Supplies:
30 half toothpicks
Procedure:
- Remove the initiators from the bag that you were given. Record your bag number. Add a toothpick and than a monomer to each initiator. Continue to add toothpicks and monomers to chain until all the monomers have been used. (Don't eat the experiment.)
Different polymers have different types, shapes, and numbers of monomers. The initiators are used only in addition polymerization reactions like those we are modeling in this experiment (there are also condensation
types of polymerization). The initiators which start the polymerization reaction are a group of chemicals called "free radicals". These chemically unstable groups are formed by tearing apart a normally stable molecule so that there is an unpaired electron (pairing produces stability) in some part of the chemical segment.
- Using toothpicks, connect the partial chains together at the ends which do not have "initiators" located on them. Continue the connection until all the partial chains have been used.
- Using the ends of the crosslinker with the attached toothpick, connect the chains together (cross-link). The connection of chains together along their body is called cross-linking. The synthetic process has an origin as far back as "vulcanization" in which sulfur was used to cross-link natural rubber in making and patching tires. In later experiments, we will be using borax as a cross-linking agent.
Questions:
- Describe (define) a polymer in your own words.
- Draw your polymer and the polymers of two other people who have different numbers on their bags. It should be noted here that normal polymers have literally tens to hundreds of thousands of monomers making up a chain instead of the sparingly few that you have been given to use.
- As the number (concentration) of initiators increase, what happens to the length of the chains? (Note: You will have to compare the above structure to those of other students.)
- How (predict) do the "strength" and "flexibility" of the polymers change as the number (concentration) of cross-linkers increases.
a) A "branched polymer" is formed when one chain is attached along the body of another chain. A branched polymer resembles the branches of a tree. Redraw your structure so that it shows branching.
b) What did you have to do with one of the terminal ends in order to create the branching requested for your polymer?
- Below is the structure of benzoyl peroxide (used in acne medicines). Separate the molecule to show two identical free radicals.
- Below is the polymer of PVC, Poly(Vinyl) Chloride. Circle the repeat unit of this if is was made from ethylene.
Teacher Notes:
Objective: The objective of this laboratory is to learn the vocabulary of polymer synthesis through the making of models. This is a very simplistic modeling lab. The terms used will have meaning and purpose as a result of this desk top lab.
Review of Scientific Principles:
We will be using cheeze snack foods to present models in addition polymerization and cross-linking of the polymer chains. All of these materials may be obtained from a grocery or discount store. Starch or Styrofoam peanuts may be used instead of food products if the maturity of the class is in question.
Students at this point have no background for condensation
polymerization and it is suggested that nothing but its existence be noted at this time. A more "in-depth" presentation will be made later in the module.
Free radicals are introduced as initiators to the polymerization process. The formation of a sample radical and its action on a monomer may be described as:
In the presence of UV light or other high energy sources, a monomer may also form a radical. In this section the cross-linker and monomer were considered as totally different. This self initiating and/or self cross-linking of the monomer should
not be presented to the student at this time. The ethylene above can alter the double bond forming the unstable radical (shown bellow) as it is struck by UV light.
A popular example of a harmful radical is one formed by the types of Chloro-Fluoro-Carbons that we use as refrigerant gases.
Of course, the very reactive ozone of the ozone layer of the atmosphere may cause the same reaction, also forming the unstable radicals.
Time: The preparation time for this lab is about 30 minutes.
Procedure:
Three classes of bags should be filled and numbered as follows:
| Bag # | Cheeze balls | Cheeze Puffs | Cheeto Crunchies |
|---|
| 1 | 4 | 20 | 2 |
|---|
| 2 | 7 | 20 | 4 |
|---|
| 3 | 10 | 20 | 7 |
|---|
Answers to Questions:
- A high molecular weight macromolecule made up of multiple repeating units.
- Students should have only one attachment on each initiator.
- The greater the number of initiators or concentration of initiators, the shorter will be the length of the straight chain of the polymer.
- As the concentration of cross-linkers increase, the flexibility/fluidity of the polymers will decrease. This explanation can be likened to the fact that as the number of steps on a ladder increase, so will the stability of the ladder.
- Those students having an odd # of initiators will already have a branched polymer in this model. In the normal polymerization "branching" will occur as part of the normal process regardless of the number of initiators. Students that had an even number of initiators could do one of two things:
a) they could remove one of the initiators from one of chains to make the connection.
b) they could break one of the chains making a branch with each segment.
