Gases and Rubber

Abstract

We demonstrate that the entropy is “driven” toward a maximum by the random character of the thermal motion. That drive is capable of exerting forces. Thus the expansive pressure of a gas and the elastic force of a stretched rubber strap are both entropic forces. The study of these cases — gases and rubber — can provide a thorough appreciation of the “mechanism” by which entropy grows. At the same time it becomes clear that the growth is merely probable, albeit very probable, but not strictly deterministic.

Entropy is a measure of disorder; this aspect of entropy is best understood by considering a polymeric rubber molecule. The knowledge of entropy of a rubber molecule implies knowledge of the entropy of a rubber strap which, in turn, allows us to calculate the thermal equation of state of rubber. Altogether this chain of arguments is known as the kinetic theory of rubber. Different as gases and rubber may be in appearance, thermodynamically those materials are essentially identical. A joker with an original turn of mind has once commented on this similarity by saying that “rubbers are the ideal gases among the solids”. Both exhibit entropic elasticity.