Stretching an uninflated toy balloon causes it to release heat and become hot. Allowing it to contract causes it to absorb heat and become cool. The cooling is particularly striking and surprising. [Incidentally John Gough in 1802 tried to rationalize this in terms of there being less room in the stretched rubber for heat - which he regarded as caloric, a material fluid. This was a good, imaginative idea, although wrong.]

One might have thought that allowing the molecules to regain their natural shape would result in a release of heat as they achieve lower internal energy. In fact the opposite is the case. The molecules have lower internal energy (that is they lie in deeper, narrower potential energy minima) in the stretched balloon than in the relaxed one!

Of course free energy must fall in a spontaneous process like relaxation of the stretched balloon.

What drives contraction must be an increase in entropy, large enough to overwhelm the increase in internal energy. For the isolated (adiabatic) balloon the source of the increased internal energy is the vibration that represents heat, so the sample cools.

The structural rationale is clear. The stretched form is low in energy for two reasons:

Conformation. Extending the chain converts gauche conformations to anti ones, which are lower in energy.

Intermolecular packing. The extended chains can pack more tightly together and give local crystallization, which dominates the lowering of energy and occurs as the stretching reaches its limit.

Of course the stretched form is also low in entropy, since it is so highly ordered both in terms of conformation and in terms of packing.

Contraction of stretched rubber (and of other important biological or artificial polymers) belongs to the class of spontaneous processes that offend a chemist's intuition by absorbing, rather than releasing, heat. All of these processes, which include evaporation of a liquid, expansion of a gas, and dissolution of many solids, are driven by entropy.

Incidentally, stretching a metal spring, where deformation increases the internal energy, does not release heat. This is a normal case dominated by internal energy rather than by entropy.