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 plastics are 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.
Ball and stick models showing
the packing of HDPE.
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
Ball and stick models showing
the packing of LDPE.
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.
Above: Ball and stick model of a PETE chain.
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:
o stretching the plastics by hand to a specific distance
o stretching the plastics by hand until a specific event (necking or
shearing) occurs and measuring that distance
o anchoring a plastic sheet and then attaching a weight to it and letting
the plastic get stretched
o 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
Above: A piece of polyethylene showing the property of
get them thinking about possible
necking. The polymer chains of polyethylene begin as
ways to measure the effect. If silly
being highly entangled. When the polyethylene is
putty is stretched, it forms a smaller
stretched, these chains are forced to into a
cross-sectional area and it also can
conformation where they are more and more “pullet
shear in a clean break. These
taught” or “lined up.” This process happens at a
properties are chemically different
specific point, called a neck, which can be identified by
than what occurs in the polymers in
a smaller cross-sectional area than the rest of the
this lab, but it can be a good
material. As stretching continues it is easier to
analogy for the students.
prorogate the neck than to begin a new one somewhere
else on the material. Eventually, a point will be reached
where the polymer chains cannot be stretched further,
and the material will shear.
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
MPS, Wisconsin, and national
involve science connections,
atoms and matter. Below is a
addressed at these levels.
Science Connections
The recycling number system.
1: polyethylene terephthalate
2: high density polyethene
3: polyvinyl chloride
4: low density polyethene
5: polypropylene
6: polystyrene
7: other
Milwaukee Public Schools (MPS)
Conduct scientific studies
including data collection,
study systems similar to those found in nature or human made.
multiple content standards according to
benchmarks. Specifically, these
science inquiry, and the structure of
thorough list of the standards
Learning Target:
using models, scientific methods,
analysis, and other research tools to
Wisconsin State Academic Standards:
Science Connections
A12.3 Give examples that show how partial systems, modes, and explanations are used to give quick and
reasonable solution that are accurate enough for basic needs
A12.5 Show 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
A12.7 Re-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.
Wisconsin State Academic Standards:
B12.4 Show how basic research and applied research contribute to new discoveries, inventions, and
applications
B12.5 Explain 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 computerbased technology, mathematics, and scientific vocabulary to communicate results.
Wisconsin State Academic Standards
C12.1 When 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
C12.2 Identify 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
C12.3 Evaluate 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.
Wisconsin State 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