Summary

In this lesson students are introduced to polymeric materials by exploring polymers (mostly plastics) used in automobiles. Students will learn about the features that all polymeric materials have in common and the features that distinguish one polymer from another on the molecular level. Students will learn how the molecular differences translate into property differences. The selection of a polymer with the right properties for any particular application is of critical importance in an automobile.

Grade Level

High school

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
• HS-ETS1-3: Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural and environmental impacts.
• Scientific and Engineering Practices:
  • Engaging in Argument from Evidence
  • Obtaining, Evaluating, and Communicating Information

Objectives

By the end of this lesson, students should be able to

• Identify polymeric and non-polymeric materials.
• Understand that all polymers attain their properties by virtue of their size (high molecular weight).
• Understand that differences in polymer properties result from differences in the specific chemical structure of the polymer and the impact of those differences on the intermolecular forces between polymer chains

Chemistry Topics

This lesson supports students’ understanding of

• Polymers
• Intermolecular Forces
• Molecular Structure
• Physical Properties

Time

Teacher Preparation: Time will vary, see Teacher Notes.

Lesson: Minimum times listed:

• Engage: 5 min
• Explore: 25 min
• Explain: 15 min
• Elaborate: 10 min
• Evaluate: 5 min

Materials

• Internet enabled device
• Student handout

Safety

• There are no safety concerns for this activity.

Teacher Notes

• This resource could be used as a post-AP Chemistry exam activity.
• The time for teacher preparation will vary, as it is needed to acquire samples of different plastic materials. Time will depend on how many samples the teacher wants to provide and what is on hand.
• This lesson could be a one or two period (60 minute) lesson. It will be more effective as a two period lesson that allows students time to research, study their findings, and present to the class. The times listed above are absolute minimum times needed.
• This activity provides an opportunity for students to apply their knowledge of intermolecular forces (IMFs) to understand the varied physical properties of some synthetic polymers (plastics). Students should have a reasonable command of intermolecular forces. Two resources that may help students develop this understanding are listed below: Lessons on IMFs using computer simulations POGIL lesson on IMFs

• Engage: Show picture of a car with plastic components identified. Note that many plastics are used in cars. Solicit reasons why students think plastics are used in cars (easy to make, cheap, lightweight for fuel economy, have unique properties, etc.). Explain that plastics are a subset of the class of materials called “polymers” and that polymers achieve their mechanical properties by virtue of their tremendous size. Merely by being very long, polymer molecules become entangled with one another, leading to their useful properties. Note that there are many different types of plastics with vastly different properties. The property differences result directly from different compositions at the molecular level. If possible, bring samples of different plastics to show students. Some examples might include:
  • water bottles: polycarbonate or poly(ethylene terephthalate) — check the molding for identity
  • wash bottles: polyethylene
  • hardware (nuts, bolts, wall anchors, spacers): nylon (polyamide)
  • electrical tape or wire insulation: poly(vinyl chloride)LEGOs, computer keys, power tool housings: ABS (acrylonitrile-butadiene-styrene copolymer)
  • pill case with hinged lid: polypropylene
  • other plastic samples may be available from a local plastics distributor. Check online or the Yellow Pages for a distributor in your area.
• Explore: Students will brainstorm and search the web to identify which plastics are used in cars, where they are used, their chemical structures, and the property requirements of their application.
• Explain: Students will research the plastic used for a selected automotive component. Looking at chemical structures of each polymer, they will try to identify the type of intermolecular forces exhibited between polymer chains.
• Elaborate: Based on chemical structure and intermolecular forces, students will try to explain why each particular polymer is used for its application. Considerations might be mechanical strength, chemical resistance (gasoline, oil, alcohol washer fluid, water, etc.), cost, clarity, toughness, etc. During student discussion, the teacher should group polymers into hydrogen bonding polymers, polar polymers, and nonpolar polymers.
• Evaluate: Students will present a one-page summary of their findings for presentation to the class. The format could be written (for display using a document camera or poster) or a PowerPoint-style slide.

• Resource material: https://plastics-car.com/

• Possible Extension Activity: After students have completed the WebQuest, the teacher may wish to provide students with the opportunity to explore the behavior of some different plastic materials. Two properties that might be explored are high temperature behavior (e.g. melting point or softening temperature) and solvent resistance. It is beyond the scope of this activity to list all possible variations and their attendant safety and practical considerations, but teachers may wish to obtain a variety of different plastic materials (like the list in the Engage section of this activity) and allow students to expose the samples to various solvents such as water, alcohol (e.g. methyl or ethyl), acetone, hydrocarbon (e.g. hexane, mineral spirits), etc.

• Sample Responses to Student Questions
  Pre-lab:

a) Polymer: a very large molecule made of many repeating units, called monomers, linked together in a long chain
b) Intermolecular force: the force of attraction of one molecule on another
c) Macromolecule: a very large molecule; synonym for polymer
d) London dispersion force: an intermolecular force that results from temporary shifts in the electron cloud of a molecule. The shift in the electron cloud gives the molecule a momentary dipole (positive end and negative end) that is causes a neighboring molecule to have a momentary dipole and results in an attraction.
e) Dipole-dipole force: an intermolecular force of attraction between two polar molecules.
f) Hydrogen bond: the strongest intermolecular attraction; results when a hydrogen atom is directly bonded to a highly electronegative atom (N, O, or F).

Results:

Procedure 1, 2:

Procedure 3:

Procedure 4:

Polycarbonate:

This molecule has London dispersion forces and dipole-dipole forces for its intermolecular forces. There is no hydrogen bonding.

Analysis:

1. How many plastic items were you able to list?
   I listed 11 items.
2. How many different plastics were used to make these items?
   I found six different plastics.
3. From your pre-lab work, what do all plastics have in common at the molecular level?
   All plastics are very long molecules made of smaller units called monomers.
4. What features of the plastic you studied make it the “right plastic for the job”?
   Polycarbonate is strong, tough, and has good clarity. It can withstand being hit by gravel or small stones without cracking. It also doesn’t yellow from sunlight very fast. It can be molded into complex shapes to fit the car.