From what I know already rubber bands are made up of long straight molecules (alkanes I assume?) and when unstretched these are a mess of coiled and cross linked. And by stretching them you untangle this mess. That I understand!

Now I have 2 questions:

1)Why can these strands be pulled into straight formations? When I pull my tangled headphones the knot only gets tighter? Im gonna say its because those two situations which may on the surface look very similar are actually not. Tangled headphones are 'touching' and have friction (which are just electrostatic forces already, I know) preventing the sliding. Due to the great amount of these interactions with all the different parts of the headphones touching it requires a much greater force. Whilst the chains are on a much smaller scale and so can actually move freely past each other as long as you provide enough energy to overcome the electrostatic forces because nothing is actually touching. (That is a truly poor explanation but what I'm ultimately trying to say is that the reason headphones can't do it is because the forces are much larger but individual chains can because they are just molecules that can actually slide over one another because they're not actually touching and simply exist in a close proximity to each other)

2) When you pull the chains apart why does the force increase? Electrostatic forces decrease with the distance apart so shouldnt the force be less? Why is it greater? Is it possibly to do with the way the chains want to actually fold due to their bonds? Which makes them once straight pull back towards that coiled and knotted formation? I could see this being heavily reliant on what atoms are present and their electron configurations.

This is a really interesting question and I've loved thinking about it. Sorry if my attempts at explanation arent great. I don't get the chance to talk them through often. Looking forward to your ideas!

  • The ideal elastomer is an entropic (rather than an enthalpic) spring; that is, temperature, rather than bonds, causes the resistance that you feel (at least for small deformations). Put another way, there are many, many more ways for rubber's long molecules to be kinked and folded than straight; and this imbalance imparts a resistance to extension. You may be interested in my answer here. – Chemomechanics Aug 3 '17 at 0:45
  • In addition, the knotting that you describe appears to be sufficiently rare in conventional polymers that the exception is notable. – Chemomechanics Aug 3 '17 at 0:48
  • The Wikipedia article on rubber elasticity is very comprehensive and answers your question better than I could... – Floris Aug 3 '17 at 10:56

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