Rubber Physics (Part 3)
Rubber heats up when it expands and cools when it contracts. What is the physical cause for this strange effect?
For example, if you hold a rubber band in both hands and expand it very quickly, you will find that it heats up significantly. After cooling it to room temperature and then quickly shrinking it back to its original length, you'll notice that it cools even further.
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1 solution
Rubber consists of long-chain polymer molecules, most of which are rolled up and entangled. In this form, the molecules are maximally disordered and therefore have a large configuration entropy. By applying an external force, the molecules are stretched to length and are at least partially detangled. The stretched shape of the molecule has fewer configuration possibilities and therefore only a small configuration entropy.
If we unravel the molecules, the potential energy of the atoms does not change, since the bond lengths between the atoms remain the same and the interaction between different polymer chains is weak. Therefore, you would not have to spend any energy to stretch the rubber. It would almost dissolve between our hands if we use only apply a small amount of force. The main reason why we still have to spend energy to strech the rubber is the change in the configuration entropy of the molecules .
With the rapid stretching of the rubber band, this has no possibility to exchange heat with its environment. Therefore, there is an adiabatic process in which the entropy remains constant. But we have already noticed that the configuration entropy has to decrease due to the strain. Therefore, there must be another entropy contribution that compensates for this entropy decrease. The only possibility is to increase the temperature of the rubber so that the atoms vibrate more, thus causing more disorder. The energy required for this is supplied to the rubber via the mechanical work during stretching. Therefore, during stretching, a force must be applied to heat the material (a so-called entropy force). Unlike friction forces, that also convert mechanical motion into heat, the energy flow can be reversed in the case of the entropy force. When the rubber contracts, the mechanical work done is released again. This energy comes from the thermal motion of the atoms, causing the material to cool down.