The Elasticity of Rubber

Abstract

Abstract In order to explain the elasticity of rubber, several authors assume that long chain molecules have a tendency to form spirals, and that when an external force displaces them from this state of equilibrium, they tend to return to it. These concepts lead in turn to two different points of view. 1. It may be assumed that there are forces of attraction between the different parts of a single molecule, for example, between the double bonds. Upon stretching, these forces of attraction are overcome. The free energy of the stretched thread thus consists of the potential energy of the molecular parts which have been separated from one another. 2. On the other hand, the molecule may be said to have a certain rigidity which, in its final analysis, depends upon the rigidity of the tetrahedric valences of carbon. In this case it is to be assumed that stretching brings about a deformation of the angles between these vectors. The first hypothesis has been developed by several authors. Recently Mack has assumed that there are forces of attraction between the hydrogen atoms in the rubber chains. He attributes the phenomena of elasticity to a mechanism of “evaporation and condensation of hydrogen.” None of these hypotheses explains the elastic properties which are found in substances such as elastic ligaments, and especially in elastic sulfur, which have a wholly different structure from that of rubber. Moreover these two hypotheses, which involve a change in the total potential energy, are incompatible with the fact that the tension of substances such as rubber, which are stretched and held at a constant length, increases with the temperature. Now a rise of temperature ought to diminish the work necessary to overcome on the one hand the forces of attraction and on the other hand the forces which keep the atoms in a position of established equilibrium. The result would be that the tension of a stretched rubber thread would diminish with an increase in temperature.