I am looking for information about loss of elasticity in stretched
rubber bands. My son is doing a science project in which he is
stretching them for a controlled length of time and then measuring
them. I need backgroud information on this topic.

Hi alanhatt

  That is a good project, to measure viscoelastic properties of polymer
  materials.
  It may be good idea to try different materials (PVC, polyethylen ..)
  which one finds around in a typical household, perhaps temperature effect. t

  Of course, rubber bands are not made of (natural) rubber any more, but
  all polymers (common name plastics) behave the same way:
  
  If they are not crosslinked, they flow -- meaning there are more plastic
  then elastic and they do not return to original shape after deformation.
  If polymer chain are crosslinked, material becames elastic - meaning
they return to original shape. (The comment posted below is not
correct).

 Here are few links to look at:

 vulcanisation
     http://www.bouncing-balls.com/chemistry_tech_conservation/vulcanization.htm  

  vulcanisation and history of rubber         (Mr. Godyear)
http://en.wikipedia.org/wiki/Vulcanization
   
crosslinking
         http://www.psrc.usm.edu/macrog/xlink.htm  

  creep tester
     http://www.wff.nasa.gov/~code820/outreach/TODWD.htm

 have fun

  hedgie

Hi alanhatt,
I don't really know what kind of background information you are
looking for but it looks like your son will be doing a good job in
acquiring some first-hand information on the subject.  I feel that I
could give you pages of information of why the rubber band will lose
its elasticity over time but don't you feel that this would detract
from what he may learn from coming to his own conclusions.  I will
give him a clue or two so he may start in the right direction.
1. No solid is 100% elastic.
2. The closer a solid returns to its original shape after being
distorted, the greater its  elasticity.
3. The greater the molecules in a substance is distorted and the
longer they are distorted, the less elasticity will be noticed in the
substance.

Regards and I wish him well in his project. 
I wish him well in his project.