[paul.fr]

i am guessing they are made of rubber! [] so what is it about rubber that is so stretchy? and why do rubber bands snap back in to their shape when you stop stretching them, or pick them up after firing them at someone/thing?

[eric l]

As (almost) always :  check Wikipedia on "elastomeres"
http://en.wikipedia.org/wiki/Elastomer

[daveshorts]

Rubber is made up of very very long molecules which are cross linked (held) together fairly infrequently. Imagine a 3D net with huge holes then tangled up with itself.

[diagram=177_0]
the red blobs are supposed to be cross links (in normal rubber this is done with sulphur atoms)

When the molecules are warm they are vibrating - wriggling. If you imagine a straight piece of rope and then wiggling it, it will get shorter
[diagram=176_0]

So normally the rubber molecules are all wiggley.

Because they are crosslinked together occasionally they can't flow past each other completely so, if you stretch the the rubber they will straighten out, then if you let go the vibrations will make them shorter again.

[paul.fr]

Cheers, Dave. Nice pics also.

[iko]

Elastin

http://chem.csusb.edu/~dpedersn/C436/elastin.gif

Elastin is our structural 'rubber' protein!
Same mechanism, different materials.

ikod

[iko]

The cross-links between elastin protein chains (monomers) are made by a special enzyme called lysyl oxidase that requires copper as a catalyst or coenzyme.  Experimental copper deficiency leads to elastin fibers derangement and aortic rupture (!).
One of the causes of copper deficiency in humans is an excessive assumption of zinc, sometimes used for acneic dermatosis (for months or years) or chronic zinc intoxication. Too much zinc interferes with copper adsorption.

Cardiovascular disease from copper deficiency:  a history.

Klevay LM.
U.S. Department of Agriculture, Grand Forks Human Nutrition Research Center, ND 58202, USA.

Although the nutritional essentiality of copper was established in 1928, a preoccupation with hematology delayed the discovery of cardiovascular disease from copper deficiency for more than a decade. Anatomical studies of several species of deficient animals revealed, interalia, aortic fissures and rupture, arterial foam cells and smooth muscle migration, cardiac enlargement and rupture, coronary artery thrombosis and myocardial infarction. Abnormal biochemistry in deficiency probably contributes to these lesions, e.g., decreased activities of lysyl oxidase and superoxide dismutase which result in failure of collagen and elastin crosslinking and impaired defense against free radicals.
Copper deficiency also decreases copper in hearts and other organs and cells and increases cholesterol in plasma. Abnormal physiology from deficiency includes abnormal electrocardiograms, glucose intolerance and hypertension. People with ischemic heart disease have decreased cardiac and leucocyte copper and decreased activities of some copper-dependent enzymes. Copper depletion experiments with men and women have revealed abnormalities of lipid metabolism, blood pressure control, and electrocardiograms plus impaired glucose tolerance. The Western diet often is as low in copper as that proved insufficient for these people. Knowledge of nutritional history can be useful in addressing contemporary nutritional problems.

J Nutr. 2000 Feb;130(2S Suppl):489S-492S.

 


Copper deficiency leads to structural and
mechanical troubles not only in humans...

http://www.omafra.gov.on.ca/english/crops/field/news/croppest/2006/02cpo06a2f2.JPG

[WylieE]

When the molecules are warm they are vibrating - wriggling. If you imagine a straight piece of rope and then wiggling it, it will get shorter


So normally the rubber molecules are all wiggley.

You can test this out:  Take a good thick rubber band and hold it between your hands and stretch it out.  Then immediately put it up to your lips- notice the temperature.  Then let it relax and feel it again- notice the change in temperature.  It is a fairly dramatic change.
Colleen

[paul.fr]

You can test this out:  Take a good thick rubber band and hold it between your hands and stretch it out.  Then immediately put it up to your lips- notice the temperature.  Then let it relax and feel it again- notice the change in temperature.  It is a fairly dramatic change.
Colleen

#

wow, that's great Colleen.  []