Entropy and Rubber Bands
Entropy and Rubber Bands
Can someone please explain why rubber bands contract when they are hot and expand when they are cold? This is opposite of most substances. The research I found talks about the polymer chain movement and how in order for the polymer chain to increase movement in middle of the rubber band, the chain shrinks up at the end, thereby actually causing contraction of the rubber band when it is hot. Do you have further explanation?
-
Craig_Bridge
- Former Expert
- Posts: 1297
- Joined: Mon Oct 16, 2006 11:47 am
What grade are you in?
The following explaination maybe way above an appropriate response for your grade level. If it is, then let us know what grade you are in and we can try to find some grade appropriate explainations. The words I put in quotes are ones you should try and do some reasearch on if you don't understand my explaination.
Elastic materials (such as "rubber") are complex "three dimensional solids". When you put one dimension in "tension" at least one of the other dimensions typically goes into "compression" because of internal forces exerted by the "elastomeric bonds".
If you heat an elastomer that has "shape memory" (but not enough to destroy the elastomeric bonds and their shape memory), the shorter dimensions (width and thickness) will typically expand and put the internal elastomeric bonds into tension forcing the longer dimension to go into compression from these internal forces.
Cooling an elastomer significantly below its original thermal shape set temperature typically makes it brittle and any force will permanently destroy elastomeric bonds.
Cooling an elastomer from above its original thermal shape set temperture down to its original thermal shape set will return the material to its original shape absent any external forces.
Try searching variations of "Thermal properties of Elastomeric Materials" and "shape memory materials" if you want to find more on the materials science involved.
I'm not sure that they actually do what you are stating with respect to cooling. "Rubber" is a fairly generic term for both rare natural rubber and the far more common various synthetic elastomeric materials.Can someone please explain why rubber bands contract when they are hot and expand when they are cold?
The following explaination maybe way above an appropriate response for your grade level. If it is, then let us know what grade you are in and we can try to find some grade appropriate explainations. The words I put in quotes are ones you should try and do some reasearch on if you don't understand my explaination.
Elastic materials (such as "rubber") are complex "three dimensional solids". When you put one dimension in "tension" at least one of the other dimensions typically goes into "compression" because of internal forces exerted by the "elastomeric bonds".
If you heat an elastomer that has "shape memory" (but not enough to destroy the elastomeric bonds and their shape memory), the shorter dimensions (width and thickness) will typically expand and put the internal elastomeric bonds into tension forcing the longer dimension to go into compression from these internal forces.
Cooling an elastomer significantly below its original thermal shape set temperature typically makes it brittle and any force will permanently destroy elastomeric bonds.
Cooling an elastomer from above its original thermal shape set temperture down to its original thermal shape set will return the material to its original shape absent any external forces.
Try searching variations of "Thermal properties of Elastomeric Materials" and "shape memory materials" if you want to find more on the materials science involved.
-Craig