Plastic Surgery

Abstract:

This is an inquiry-driven lesson with many layers that is structured utilizing the 7 E’s learning cycle approach (elicit, engage, explore, explain, elaborate, evaluate, extend). Students observe chemical reactions, determine the atomic and molecular forces responsible for those reactions, and use the scientific method to make predictions and construct a data-driven explanation of the results.

Students will be provided with multiple plastic polymers to investigate. They will start with simple identification procedures like density to identify common plastics and gain an appreciation of the molecular factors that influence the plastics' properties. They will also test and hypothesize about other properties like elasticity. They will investigate the effects of temperature change on plastics and discover that temperature change may create a substance with new properties even when brought back to the original temperature. An examination of the recycling scheme (numbers 1-5) will also be discussed.

Elicit

Students are asked one of the following eliciting questions. Choose a question appropriate to your grade and skill level.

  1. What could be some things that plasticsare used for?
  2. How are plastics different from each other?
  3. What kinds of clothes do you think are made from plastics? What advantages does this offer? What drawbacks are there?
  4. Why are there different numbers on plastics? What do they represent?

Engage

Students use a hairdryer to heat the surface of a milk jug, which is HDPE (high density polyethylene). Because of the regular packing of the polymer chains, HDPE has a crystalline structure at room temperature. HDPE is cloudy and opaque because the many small crystals refract light. When heated, the crystals melt, and HDPE becomes an amorphous solid that is transparent. This chemical property is the same in water. When water is an made up of many ice crystals, it is more opaque than liquid water, which has no crystalline structure.

Explore

As a discrepant event demonstration, the heat released from the catalytic decomposition of H2O2 is used to shrink a soda bottle. To do this demonstration, any size of PETE soda bottle may be used, such as a 2 liter, or 24 ounce bottle. Fill it with a small amount of 30% H2O2. Then, add the catalyst MnO2 or KI. The H2O2 will rapidly break down, and a vent of steam shoots out of the top of the bottle. The heat of this reaction shrinks the bottle. Keep an identical bottle for comparison.

Students then can observe this characteristic on their own by shrinking polystyrene. Polystyrene is most easily attainable from CD cases. They can shape and decorate samples and then shrink them using a heat source such as a hot plate or an oven. Data can be collected and analyzed here, if desired.

Why do these materials shrink when heated? To make these objects, the plastic is stretched and quickly cooled. So, upon heating in the classroom, these stretched out molecules, which have previously been “stuck” in their conformation, contract. So, the overall sample contracts along some directions, and it looks like it shrinks.

Explain

As a concrete model of polymer packing, students can use paperclips of different sizes or colors to represent main chains and side chains of polymer molecules. Students will observe that linear chains can pack the tightest (as HDPE does), and branched chains leave large spaces that decrease the density (as LDPE, or low density polyethylene, does).

Facilitate a discussion about the findings of both of the demonstrations. From this students should be able to explain how the arrangement of atoms is fundamental to the observed physical properties of substances.

Elaborate

Students will develop an experiment using a large sample of common polymers. Polymers that can be used are blocks of LDPE, HDPE, nylon, grocery store bags (HDPE), high end department store bags (LLDPE, or linear low density polyethylene), and dry cleaning bags (LDPE). Independent variables can be the polymer tested, or its density. Dependent variables could be elasticity, tensile test(the amount of stretching force it takes to deform the polymer, which is called necking), shearing (breaking) distance, the nature of the shear (clean break or jagged break), or any other forms of stress. Students will collect data, graph and analyze it.

Procedure suggestions

  • Students can measure the density of the polymers using solid blocks of polymer in the lab, or students can attempt to look up this information.
  • Students can subject the plastic sheets to stress in a variety of ways:
  • stretching the plastics by hand to a specific distance
  • stretching the plastics by hand until a specific event (necking or shearing) occurs and measuring that distance
  • anchoring a plastic sheet and then attaching a weight to it and letting the plastic get stretched
  • putting a weight in the middle of a plastic sheet that is anchored on two or more sides

All of these methods have many flaws;however, these flaws are an excellent way for students to recognize, discuss, invent, and/or test methodological solutions to minimize error.

  • Silly putty (which is a mixture and is mainly silicone polymers, such as dimethyl siloxane) doesn’t neck and shear in exactly the same way as the polymers in this lab, but it is an opportunity for the students to experience similar effects hands on, which will get them thinking about possible ways to measure the effect. If silly putty is stretched, it forms a smaller cross-sectional area and it also can shear in a clean break. These properties are chemically different than what occurs in the polymers in this lab, but it can be a good analogy for the students.

Evaluate

Students will analyze and graph their data and make a disposition about their hypothesis. Students will then present their findings,critique the strengths and weaknesses of their procedure, propose improvements to their procedure, and suggest further research that could be done.

Extend

Students will discuss and or do research into the recycling number system. What are the challenges in recycling the material?

Standards Addressed:

This module addresses multiple content standards according to MPS, Wisconsin, and national benchmarks. Specifically, these involve science connections, science inquiry, and the structure of atoms and matter. Below is a thorough list of the standards addressed at these levels.

Science Connections

Milwaukee Public Schools (MPS) Learning Target:

Conduct scientific studies using models, scientific methods, including data collection, analysis, and other research tools to study systems similar to those found in nature or human made.

WisconsinState Academic Standards:

Science Connections

A12.3Give examples that show how partial systems, modes, and explanations are used to give quick and reasonable solution that are accurate enough for basic needs

A12.5Show how the ideas and themes of science can be used to make real life decisions about careers, work place, life-styles, and use of resources

A12.7Re-examine the evidence and reasoning that lead to the conclusion drawn from the investigations using the scientific themes

National Content Standards:

Unifying Concepts and Processes: systems, order, organization; evidence, models, explanation; change, constancy, measurement; evolution and equilibrium; form and function

Nature of Science

MPS Learning Target:

Explain how scientific knowledge is attained. Describe the ethical problems that may result when doing scientific research. List scientists from various backgrounds and tell about their contributions.

WisconsinState Academic Standards:

B12.4Show how basic research and applied research contribute to new discoveries, inventions, and applications

B12.5Explain how science is based on assumptions about natural world and themes that describe the natural world

National Content Standards:

G. History and Nature of Science:Science as a Human Endeavor, Nature of Scientific Knowledge, Historical Perspectives

Science Inquiry

MPS Learning Target: Research scientific concepts

Undertake scientific investigations using appropriate scientific methods, technology, and tools. Use computer-based technology, mathematics, and scientific vocabulary to communicate results.

WisconsinState Academic Standards

C12.1When studying science content, ask questions suggested by current social issues, scientific literature, and observations of phenomena, build hypotheses that might answer some of these questions, design possible investigations, and describe results that might emerge from such investigations

C12.2Identify issues from an area of science study, write questions that could be investigated, review previous research on these questions, and design and conduct responsible and safe investigations to help answer the questions

C12.3Evaluate the data collected during an investigation, critique the data collection procedure and results, and suggest any ways to make any needed improvements

National Content Standard- A. Science as Inquiry:

Abilities to do Scientific Inquiry (Identify Questions and Concepts that Guide Investigations, Design and Conduct Scientific Investigations, Use Technology and Mathematics to Improve Investigations and Communications, Formulate and Revise Scientific Explanations and Models Using Logic and Evidence, Recognize and Analyze Alternative Explanations and Models, Communicate and Defend a Scientific Argument) and Understandings about Scientific Inquiry

Structure of Atoms and Matter

MPS Learning Target:

Diagram the structure, and analyze the chemical reactions of elements. Explain the forces that hold the atom together, and how nuclear interactions change it. Describe the difference between chemical reactions and atomic reactions.

WisconsinState Academic Standards

D12.1 Describe atomic structure and the properties of atoms, molecules, and matter during physical and chemical interactions

D12.2 Explain the forces that hold the atom together and illustrate how nuclear interactions change the atom

D12.4 Explain how substances, both simple and complex, interact with one another to produce new substances

D12.5 Identify patterns in chemical and physical properties and use them to predict likely chemical and physical changes and interactions