Rubber and Rubber Balloons: Paradigms of Thermodynamics

I have just finished reading a book “Rubber and Rubber Balloons: Paradigms of Thermodynamics” by Ingo Müller and Peter Strehlow.

It is a nice easy-going undergrad reading exploring rubber and its properties from a thermodynamic point of view. The authors apply the theory to rubber balloons to familiarize reader with such concepts as stability, bifurcation, hysteresis, phase transitions and probably a couple of others I missed. Huh, you wouldn’t expect all of these to be observed with a bunch of simple party balloons, would you?

Once again the book is very accessible in terms of math machinery used, making it perfect as a supplementary reading for students interested in thermodynamics and taking their course in general physics. The equipment required to reproduce the experiments also doesn’t seem to be very sophisticated allowing for some neat demonstrations.

For the details I refer you to the book. Apart from rubber balloons, which are no doubt fun, the rubber itself is a very interesting material. It were its remarkable properties which made me at last find some time and read a book about rubber.

The thing with rubber is that the elastic forces you experience are entropic, that is when you stretch a rubber band you (roughly speaking) do not increase its internal energy, you decrease its entropy. That’s because rubber molecules are long twisted chains and when you expand rubber you straighten them, thus ordering (decreasing their entropy). A simple kinetic theory of rubber based on entropic reasoning is presented in the book. For quick introduction on rubber thermodynamics I suggest you John Baez’s post about entropic forces.

However the explanation based on entropy (or number of available states) may seem unsatisfactory to you because it doesn’t directly provide a visual atomistic/mechanical explanation for elastic rubber forces. Meanwhile the picture is quite simple. Imagine a long chaotically wobbling chain. Obviously, it cannot be straight if it wobbles and more it wobbles more twisted and short it becomes. The chain is a rubber molecule and wobbling is thermal motion. From this picture it is also clear why rubber shrinks when heated.

To assist the explanation I have prepared an animation featuring a model of a single rubber molecule, it is available as .mp4 and .gif (gif may have problems with fps). The modeling was done in Step.

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