The Entropic Properties of Rubber Bands

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The Entropic Properties of Rubber Bands

• Pamela Roley
• September 13, 2005
• Chem 440a

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Materials expand when heated

• Pavement expands as it is heated
• Conversely, pavement contracts as it cools
• This lead to potholes!!

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Why does pavement expand?

• THERMAL EXPANSION
• Coefficient of Thermal Expansion, a
• For a rubber band a 60 x 10-6 F-1
• Measured Expansion a?T

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What about Rubber Bands?

• Stretched rubber bands actually CONTRACT when
  heated
• Wait..but Rubber Bands have a positive
  coefficient of thermal expansion
• ( For a rubber band a 60 x 10-6 F-1 )
• Whats going on???

First lets do an experiment!
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Step 1 Stretch your rubber band

• Stretch your rubber band vigorously
• Repeat for about a minute
• Place the rubber band against your lip

What do you notice about your rubber band?
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Whats going on?

• Your rubber band should feel warm
• This is an exothermic process dQ lt0
• What is driving the change?? ? ENTROPY
• Stretching the rubber band decreases its entropy
• Entropy is the amount of disorder in a system
• Decreasing entropy means the rubber band is
  becoming more ordered

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Step 2 Allow your rubber band to contract

• Hold your rubber band stretched for about a
  minute
• Allow your rubber band to contract
• Place rubber band against your lip

What do you notice about your rubber band now?
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Whats going on now?

• Your rubber band should feel cool
• This is an endothermic process dQ gt 0
• As the rubber band contracts entropy increases
• The rubber band is becoming more disordered
• 2nd Law of Thermodynamics the universe is
  always moving towards increased entropy

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Why does contraction of a rubber band increase
entropy?
   
   

• Rubber bands are composed of cross-linked polymer
  chains
• Polymer chains are like spaghetti
• In a large pot spaghetti noodles have room to
  spread out and order themselves
• In a small pot they are more disordered

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Stretching a Rubber Band

• Polymer chains in stretched rubber bands have
  more space to order
• Contracted rubber bands are less ordered
• Polymer chains are crosslinked

Fig. 1 A stretched and retracted rubber band4
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What will happen if you stretch a rubber band and
then heat it?
It will contract
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Why do stretched rubber bands contract when
heated?

• Rubber bands are entropic springs
• More entropically favorable for rubber bands to
  contract
• Added T provides energy to polymer chains so they
  can become more disordered

Fig. 2 Stretched rubber bands contract as T is
increased
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Entropic Springs

• When springs are stretched they have a restoring
  force to return to their original shape
• Restoring force in rubber bands is entropyhence
  entropic springs
• Restoring force is proportional to temperature
• Greater the temperature increase greater the
  restoring force
• Restoring force must overcome thermal expansion
  force

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Summary

• Rubber bands are composed of cross-linked polymer
  chains
• Stretching a rubber band is an exothermic process
  because heat is given off as entropy decreases
• Contracting a rubber band is endothermic because
  the rubber band takes heat in from the
  environment in order to become more disordered or
  gain entropy
• When a stretched rubber band is heated it will
  contract because entropy will be the restoring
  force causing the rubber band to contract to its
  original shape
• For this reason rubber bands are called entropic
  springs

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Questions
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References

• Ball, D. Physical Chemistry, Thomson, Brooks/Cole
  2003.
• Bassam, Z., Shakhashiri, Z. CHEMICAL
  DEMONSTRATIONS A Handbook for Teachers of
  Chemistry, Volume 1, The University of Wisconsin
  Press, Madison, Wisconsin
• Division of Engineering, Brown University, EN222
  Mechanics of Solids, http//www.engin.brown.edu/c
  ourses/En222/Notes/Hyperelast/Hyperelast.htm
• Donald, A. University of Cambridge, Rubber
  Elasticity, http//www.poco.phy.cam.ac.uk/teaching
  /A_Donald/Polymers_3.htm
• Fried, J. Polymer Science Technology, 2nd Ed,
  2003.
• PhysLink.com Physics and Astronomy Online
  www.physlink.com/Education/AskExperts/ae478.cfm