Effect of Temperature on the Length of Elastomers: Rubber
Band Thermodynamics
Objectives: To examine and explain the effect of temperature on elastomers (rubber
bands)
Introduction:
Stretch a rubber band and allow it to contract while it is in contact with your upper lip. Is
there any temperature change as the rubber band is stretched and allowed to contract?
Record your observations below. Based on your observations, predict (and write down)
the effect of temperature on the length of a rubber band while stressed.
Observations:
Hypothesis:
Materials:
Rubber band –thin and small
200-500 g weight
1000 mL graduated cylinder
Aluminum foil
Blow dryer or heat gun
Rod
Ruler
Procedure: See picture below for guidance in setting up the experiment.
1. Hook a small, thin rubber band around a 200 – 500 g weight. The rubber band
should be moderately stretched (2 – 4 times its original length) with the weight
attached to it.
2. Loop the rubber band/weight around a rod and place the rod on top of the
graduated cylinder so that the rubber band with the weight is hanging inside the
cylinder.
3. Place a ruler inside the graduated cylinder and record where a predetermined spot
on the rubber band is on the ruler (we used the loop of the rubber band where it is
connected to the weight). Leave the ruler in the graduated cylinder.
4. Place the aluminum foil over top of the graduated cylinder.
5. Using the heat gun or blow dryer, heat the rubber band through the glass of the
graduated cylinder for 1 min.
6. Record where the predetermined spot on the rubber band is located on the ruler.
7. Allow the rubber band to cool to room temperature and again record the location
of the predetermined spot on the rubber band using the ruler.
8. Repeat the procedure for another rubber band.
Observations:
Before Heating
After Heating
Material
Length of stressed
rubber band before
heating
Length of stressed
rubber band after
heating
Rubber band trial 1
9.0 in
9.6 in
Length of stressed
rubber band after
allowing to cool to
room temperature
9.0 in
Potential discussion uestions:
1. How does the rubber band (elastomer) differ in behavior from most other
materials?
2. How are the results obtained in the experiment consistent with the observations
made of the temperature of the stretched and relaxed rubber band (from the
introduction section)?
3. Explain your observations of the rubber band behavior in terms of entropy.
Discussion
As students stretch the rubber bands across their upper lips, they should feel the rubber
bands become warmer. As the rubber bands contract, the students should feel them
become cooler. The rise in temperature of the rubber band as it is stretched is mostly due
to work (being converted to heat) acting on the rubber material to stretch. In terms of
thermodynamics and the Gibb’s Free Energy equation, ΔG = ΔH – TΔS, ΔH is negative
when the rubber band is stretched and positive when the rubber band is contracted. In
order to feel the heat from the rubber band, the student needs to physically stretch the
rubber band, meaning the reaction is not spontaneous and ΔG is positive. When the
rubber band contracts, the reaction is spontaneous and ΔG is negative. In examining the
Gibb’s Free Energy equation, ΔS must be negative when the rubber band is stretched if
ΔG is positive and ΔH is negative. When the rubber band is contracted, ΔG is negative
and ΔH is positive, causing ΔS to be positive. In a stretched state, the polymers of the
rubber band have fewer conformations that they can assume and are straightened out and
aligned with each other, leading the polymer to be ordered. This gives a negative entropy
value (ΔS). When the rubber band is contracted the polymers are entangled, meaning
they are disordered and have a positive ΔS value.
When the rubber band is stretched, heating the rubber molecules causes them to move
around more. As a result, the polymers become less aligned and more naturally
disordered. This causes the rubber band to shrink. In other words, the molecules become
more tangled, leading the rubber band to contract.