Plastics and Polymers: Part 1

Transcription

1 Plastics and Polymers: Part 1 Author: Brittland DeKorver and Doris Pun Institute for Chemical Education and Nanoscale Science and Engineering Center University of Wisconsin-Madison Purpose: To learn about plastics and other polymers. Learning Objectives: 1. Learn how to categorize different plastics based on physical and chemical properties. 2. The method of making simple polymers. 3. Learn how plastic can be decomposed or recycled. Next Generation Science Standards (est. 2013): PS1.A: Structure and Properties of Matter (partial) PS1.B: Chemical Reactions National Science Education Standards (valid ): Standard B: Physical Science o Properties and changes in properties of matter o Transfer of energy Standard D: Science and Technology o Abilities of technological design o Understanding about science and technology Standard F: Science in Personal and Social Perspectives o Science and technology in local, national and global challenges Grade Level: 3-8 Time: 60 minutes Materials: Construction paper strips (range of colors for polymer models) Tape Elmer s glue Popsicle sticks Food coloring Borax (sodium borate) Plastic bags Paper cups Safety goggles Gloves Petri dish Pipettes Beakers Aqualoc/sodium polyacrylate powder/instant snow Polyvinyl alcohol 4% solution (1/3 cup of OVA in 1 qt of water, heat to dissolve) Polyurethane solutions (available from Flinn Scientific #C0335)

2 Safety: Students and mentors should wear safety goggles and gloves while handling the Borax and during the polyurethane activity. New polyurethane foam or the two monomer parts should not be touched by bare skin. Wash skin immediately if contact occurs. Do not inhale any of the chemicals, including the sodium polyacrylate powder. Preparation ahead of time: 1. Make Borax solution. Mix 1 teaspoon Borax with 1 cup of water. Scale up as needed. 2. Rehearse sodium polyacrylate demo beforehand so that you can be confident and convincing. 3. Make polyvinyl alcohol solution by heating 1/3 cup of PVA in 1 quart of water until the alcohol is completely dissolved. Allow to cool to room temperature before use. If a slimy layer is formed while cooling, skim and discard. This solution can be stored for several months in a sealed container. Introduction: Plastics are found everywhere. They are found in bags, water bottles, durable reusable containers, video game consoles, plumbing, automobiles, synthetic heart valves, and even rocket and missile solid fuels. Plastics are synthetic, or man-made polymers. A polymer is a molecule made up of a repeating unit, or monomer. It is created by a polymerization, or a chemical reaction where monomers combine to form long chains (or other shapes such as coils, stars, and sheets). Cross-linking is the synthesis of polymers by bridging polymer chains together. Polymers are characterized based upon their physical properties. The monomer or monomers that make up a polymer will affect the physical properties of the polymer. Not only the identity of the monomer or monomers, but the way the monomers are arranged, whether they are single chains, branched or linked chains, or star-shaped dendrimers, will also affect the physical properties of the polymer. Some polymers, called copolymers, are made of more than one type of monomer. This can create even more variation in the type of polymer. For example, polyethylene can have long chains of molecules that lead the plastic to form a high-density material that is rigid. However, if the polyethylene chains have branched long and short chains, the material is flexible and has low-density. Tell the students they are going to learn about different types of polymers and they ll make their own polymers.

3 Procedures: 1. Construction Paper Polymer Models. a. Define polymers as molecules that are made up of many repeating units. Explain that the construction paper links will represent the repeating units. b. Show students how to make links out of rectangular strips of construction paper by joining the ends with tape. c. Tell students to make a molecule with at least ten links. Do not give them further instructions about the order or arrangement of the links. d. Encourage variety in their construction: some students may make the model using only one color, others may use more; some may make long straight chains, others may bunch the chains together. e. Give them five minutes and then have each student display their polymer to the whole group. f. Discuss the differences in construction and talk about polymers having different structures. i. Monomer each color represents a different type of monomer ii. Polymer chain use only one type of monomer a. Straight chain b. Branched polymer (comb, star, etc shaped) iii. Co-polymer use 2 types of monomers a. Alternating co-polymers b. Block co-polymers make chains of each type of monomer then join chains together c. Random co-polymers iv. Cross-linked polymer bridge two straight chains together g. Mentors should participate in chain linking with students, especially to create structures that provide more examples of variance. (I.e. if no student has made a branched polymer, a mentor could do it.) 2. Making Borax-glue Polymer (Gluep) Adapted from Fun with Chemistry. 1 a. Remind students to put on gloves and safety glasses. b. In a plastic bag, mix 1 tablespoon of glue with 1 tablespoon of water; close and mix well. c. Optional: Add one drop of food coloring. d. Add 1 tablespoon of Borax solution (the cross-linker) to the bag. e. Tie bag and massage mixture together. f. Observe the slime-like substance forming. g. Test the properties of the slime. Remind students to wash hands after playing with it. 1 Fun with Chemistry, Vol. 2; Sarquis M.; Sarquis, J., Eds. ICE Publ , Institute for Chemical Education, University of Wisconsin Madison: Madison, 1993.

4 h. Students may take their Borax-glue polymer home in the plastic sandwich bag. 3. Sodium Polyacrylate, a Superabsorbent Polymer a. Demo: Aqualoc i. Carefully sprinkle about 0.5 g Aqualoc in a clear dish or container without the students observing. ii. Show the students the container, holding it so that it appears to be empty. iii. Ask for a volunteer to help. iv. Show the students another dish with approximately ¼ cup of water. Slosh the water around for effect. Ask what would happen if you tip the container upside down over the volunteer s head. v. Have the volunteer stand in front of the other students. Standing behind the volunteer, quickly pour the water into the dish with Aqualoc, swirl gently to distribute the water, and tip the dish at a 90 angle (or greater) over the volunteer s head. vi. Practicing beforehand will allow you to judge the appropriate relative amounts with confidence. The amounts listed here are very conservative estimates and will not exceed the absorbance of the polymer. vii. Allow students to extract sodium polyacrylate from diapers. 1. Cut a diaper in half. 2. Place into a re-sealable bag, quart or gallon sized. 3. Shake bag until white powder has collected in the bottom of the bag. 4. Allow students to test powder or take it home. b. Instant Snow i. Show students instant snow powder. Select one student to measure one gram of the powder. ii. Have students measure 5-10 g of water and pour it over the snow, one at a time. Assign one person to keep track of how much water has been added. iii. Allow students to touch the instant snow and make other observations. iv. Instant snow can be reused by drying it in an oven at a temperature no higher than 250 C. 4. Demo: Polyurethane Polymer a. Put on goggles and gloves. b. Pour equal amounts of monomers A and B into paper cups. c. Mix A into B. d. Stir with wooden stick until it starts to expand.

5 e. Allow students to make observations. (Optional: a gloved student may touch the outside of the cup before the foam begins to form to observe the temperature change.) f. Allow a 24 hour cure time before allowing students to touch. Discussion: The glue is made up of long chains of hydrogen and carbon atoms, called polyvinyl acetate (not to be confused with PVA, polyvinyl alcohol!). 2 Borax molecules are made up of two parts: sodium and borate ions. The borate ions can attach two polymer chains like the rungs on a ladder. The two chains are linked by the borate ions. 3 This process is called cross-linking. When the long polymer chains are unlinked, they can slip and slide around each other and are somewhat liquidy. Once the cross links are added, the chains don t move as easily and the substance becomes more solid-like. However, the cross links are somewhat weak, which is why the substance has some flexibility. This slime is classified as an elastomer, or a substance that springs back to its original shape after manipulation. Chemically, an elastomer consists of nonpermanent crosslinked bonds (ie. weak intermolecular bonds). Instant snow is a polymer made by cross-linking the molecules of sodium polyacrylate polymer. At first, adding water will form small fluffy clusters, appearing like powdery snow, but further addition of water allows for the polymer clusters to stick to one another, forming a gel or slush. Because of the high concentration of sodium ions, water is absorbed by the polymer through osmosis. Plants use osmosis to move water through their cells. Sodium polyacrylate can move and absorb water so well, it is commonly found in disposable diapers. Polyurethanes are commonly found in floatation devices, furniture, packaging, insulations, and many other items. These polymers are usually the flexible and rigid foam, foam seals, adhesives, surface coatings, and elastomeric wheels and tires. Polyurethanes are produced by the reaction of a polyalcohol with a polyisocyanate in the presence of a catalyst. The foam solution A contains the polyalcohol and the catalyst. A catalyst is a substance that speeds up a reaction without being changed. Solution B contains diisocyanate monomers and oligomers (short chains). The reaction, like most polymerization reactions is exothermic, or releases heat. A small amount of water is also present in the solution and reacts with solution B, which produces carbon dioxide, CO2, gas. The CO2 causes the polyurethane foam to expand and create pores, making a lightweight polymer. 2 Yueh-Huey Chen, Jing-Fun Yaung J. Chem. Educ. 2006, 83, JCE Editorial Staff J. Chem. Educ. 1998, 75, 1432A 1432B

6 Evaluation: What are polymers? Show me two things in the classroom that are polymers. This lesson is the product of the Institute for Chemical Education and the Nanoscale Science and Engineering Center at the University of Wisconsin-Madison. This Material is based upon work supported by the National Science Foundation under grant number DMR SCIENCountErs Lessons are licensed under a Creative Commons Attribution- NonCommercial 4.0 International License. Permissions beyond the scope of this license may be available by ing ice@chem.wisc.edu.