Activities
A Silly Polymer
Cross-Linking a Polymer to Create Everyone's Favorite Childhood
Toy, Silly Putty
Objective: The objective of this experiment is to
cross-link a polymer and observe the changes in the physical
properties as a result of this cross-linking. The changes
in physical properties of a cross-linked polymer are also
studied as the temperature is varied.
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Review of Scientific Principles:
If a substance springs back to its original shape after being
twisted, pulled, or compressed, it is most likely a type of polymer
called an elastomer. The elastomer has elastic properties (i.e., it will
recover its original size and shape after being deformed). An
example of an elastomer is a rubber band or a car tire.
The liquid latex (Elmer's glue) which you use contains small
globules of hydrocarbons suspended in water. The silly putty is
formed by joining the globules using sodium borate (a cross-linker).
The silly putty is held together by very weak intermolecular bonds
that provide flexibility around the bond and rotation about the
chain of the cross-linked polymer. If the cross-linked bonds in
a polymer are permanent, it is a thermosetting plastic, even if
above the glass-transition temperature (Tg). If the
bonds are non-permanent, it can be considered either thermoplastic
or an elastomer.
Time: A 20-25 minute period is required to perform the
mixing/making of the silly putty.
Materials and Supplies:
- 55 % Elmer's glue solution in water
- 4 % borax solution (sodium borate)
- Styrofoam cups
- zip lock bags
- food colors
General Safety Guidelines:
- Since borax solid (a bleaching agent) and solution will burn
the eyes, goggles and aprons should be worn.
- Hands should always be washed after kneading the silly putty
and finishing the experiment.
- Some people are allergic to Borax (may have skin reaction)
Procedure:
- Wear goggles and lab aprons.
- Pour 20 ml of the Elmer's glue solution into a Styrofoam cup.
- Add 10 ml of the cross-linker (borax solution) to each cup.
- Immediately begin stirring the solutions together using the
wooden stick.
- After a couple of minutes of mixing, the silly putty should
be taken out of the cup and kneaded in the hands. Don't worry
about the material sticking to your gloves as these pieces will
soon mix with the larger quantity with which you are working.
Continue to knead until the desired consistency is reached.
- Using a ruler to measure, drop the ball from a height of 30
centimeters. To what height does it rebound?
- Stretch the silly putty slowly from each side.
- Compress the silly putty back into a ball.
- Pull the silly putty quickly from each side and compare
the results.
- Place the silly putty on some regular news print and press
down firmly.
- Remove the silly putty from the news print and make observations.
- Repeat the same procedure on a comic section of the newspaper.
The silly putty is non-toxic and safe to handle so you can put
it in a zip-lock bag and take it home.
- Follow good laboratory procedure and wash your hands with
soap and water when you have finished the experiment.
Data and Analysis:
Height of the rebound _________ cm.
Observations of pulling the silly putty slowly:
Observations of pulling the silly putty quickly:
Observations of the silly putty on newsprint:
Observations of the silly putty on the comic's section of the
newspaper:
Everyone knows Silly Putty bounces. When you shape it into a
ball and bounce it off of a hard, smooth surface, Silly Putty
bounces better than just about any regular rubber ball.
Cooling Silly Putty improves its "bouncability." Shape
your Silly Putty into a ball and bounce it. Measure how high it
bounces. Then, place the ball in the freezer for about an hour.
While it's still cold, bounce it the same way you did when it
was warm. And the result?
Silly Putty has a rebound of 80 percent, meaning it will bounce
back 80 percent of the height it's dropped -- pretty impressive,
no?
Questions:
- How do the physical properties of the glue, water mixture
change as a result of adding the sodium borate?
- What would be the effect (your thoughts) of adding more sodium
borate solution?
- What is the ratio of the height of the drop to that of the
rebound distance?
- Who in the class had the ball with the most elasticity?
- How did you come to the conclusion of whose ball was most
elastic?
At Home:
-Place your ball in the refrigerator for 10 minutes. Recheck
the bouncing portion of this experiment.
- What are your observations?
- Why do you think this was observed?
-Now place your ball about 6 inches from a light bulb for about
5 minutes and again recheck the bouncing portion of this experiment.
- What are your observations?
- Why do you think this happened?
Explain the Following:
- Why does a car tire appear to be flat in the summer even though
the gas inside is hotter than in the winter.
- Why does a basketball bounce differently inside a gym than
it does outside on a cold wintry day.
- Why will a tire sometimes bump during the winter as a car
is moving, only to smooth out its ride after the car has been
traveling for a distance.
Teacher Notes:
Objective: The objective of this experiment is to investigate
cross-linking using a similar technique as was used in the making
of slime. The same parameters are worked again with a formal and
a quantitative measurement used to describe elasticity. The added
home investigation of the effect of temperature on the elasticity
also includes concepts of molecular motion and intermolecular
bond strength.
Review of Scientific Principles:
If a substance springs back to its original shape after being
twisted, pulled, or compressed it is a type of polymer called
an elastomer. The elastomer has elastic properties. It will recover
its original size and shape after being deformed.
The liquid latex used contains small globules of hydrocarbons
suspended in water. Joining these globules forms the mass with
which the students will be working. The covalent bonds along the
chain are strong, but the bonds between chains are normally weak.
However, additives such as borax allow the formation of strong
"cross-links" between chains, such as C-B-C. As the number of
cross-links increases, the material becomes more rigid and strong.
If the rigidity of a polymer is noticed to decrease when a critical
temperature is reached, the polymer is called a thermoplastic.
If the bonds between polymer molecules are very strong, the material
decomposes before any softening occurs. Such a material is called
a thermoset plastic.
Natural sources of this liquid latex are milkweed, rubber trees,
pine trees, aloe plants, and many desert plants. This latex is
used to quickly mend and repair any damage to the outer covering
of the plant.
General Safety Guidelines:
- The materials used in this experiment are all non toxic. It
is a good idea always to exhibit good laboratory technique when
working with the students. Make sure the laboratory.
Experimental:
There are many variations of this experiment.
- The original silly putty was prepared using sodium silicate
and mixing this with borax.
- A variation also exists using laundry starch and mixing it
with borax.
- Similar variations also exist by sprinkling the borax evenly
and gently over the solution of latex then working it with the
hands. This does not require as much kneading to dehydrate the
sample.
Time: - About 15 minutes are required to ready solutions,
cups and tongue depressors.
10-15 minutes will be required in lab for testing and clean up.
The students will require 10-15 minutes of work at home in order
to finish all of the experimental work on this laboratory and
the write up.
Answers to Questions:
- The liquid type of starting material should jell and become
more viscous as cross-linking occurs.
- The material will become more solid or rigid.
- Student answer. This is only a method of measuring elasticity
of the polymer. Stretching gives a similar means of comparison.
- Student answer.
- Greatest rebound to drop height ratio.
- Here the student will be studying the effect of temperature
variation on elasticity. Students are sometimes surprised if
they place their sample into a freezer rather than a refrigerator.
The results are that the ball will shatter rather than bounce.
- The ball should be more elastic.
- Contrary to what some students will predict, should the ball
become too warm, the resulting ball will deform rather than
continue to increase in elasticity.
- The ball deformed rather than rebounding.
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