How does elasticity work at a molecular level?



I refer to rubber bands and other elastic materials.

In: Engineering

At a molecular level a rubber band is made of long polymer chains, this is to say long chains of atoms. These chains can be deformed but like to revert to their original shapes, they also are able to slide past each other.
These properties are what allow for elasticity to occur.

Now if the rubber is treated with heat or chemically, links between the chains are formed which inhibit these sort of movements and the rubber may be more like a solid plastic.

All materials have some level of elasticity, even metals and ceramics, only they deform very little, and if you push them too far they permanently deform or crack.

The most basic forms of elasticity come from atomic bonds themselves stretching. This is a kind of elasticity present in all materials.

For rubbers specially, there’s another layer on top of this that allows for much greater stretching. Rubber molecules are very long and chain-like, i.e. the molecular can bend and fold in lots of places. When you stretch a rubber, you’re straightening out all of these chains, making the material much longer. But the natural state of the molecules is to be kinda jumbled up, so the material will want to return to that state by contracting.

This part is usually not explained well, so I’ll try my best. All materials have a natural thermal energy to them. This means all their molecules are moving and vibrating. When a long chain molecule moves and vibrates, it more or less stays as a bunch of somewhat jumbled chains, some get a little more jumbled, some a little less, so the material stays constant. But once you stretch it, there’s only way for the molecules to move, and that’s by folding and shortening. If the chain is already full straightened, it can’t straighten more. So the natural thermal energy in the rubber will make it contract.

Hope that helps!

>I refer to rubber bands and other elastic materials.

Consider a box of uncooked spaghetti. The individual dried noodles themselves can flex a little bit, but in general are pretty rigid.

Now consider how they behave when cooked. They form a tangle that can stretch and deform but clump together if you pull too much. Also note that the length of thin noodles like spaghetti or ramen influences their tendency to get tangled and clump together when you try to eat them with a fork.

Likewise molecules of *Elastomers* like rubber have a long, randomly coiled, tangled, springy structure like cooked ramen.

In contrast hard plastics like nylon or polyethylene have long, straight molecules that are arranged all in rows like dried spaghetti in a box.

Further evidence of this kind of tangled structure can be seen in rubber balloons filled with helium. Helium atoms are very small and chemically inert. Because of this they can slowly diffuse through the gaps and holes between individual tangled rubber molecules and escape into the air, likewise air molecules can slowly diffuse inwards. The latter may be confusing but is largely driven by the fact that there’s a low concentration of nitrogen inside the balloon, rather than the small difference in pressure. This explains why rubber helium balloons will shrink and lose buoyancy as the helium slowly diffuses out through the thin rubber.