The Thermoelastic Properties of Rubber

Abstract

Abstract In an earlier communication it was shown that, within a certain range of elongations, viz., 100 to 300 per cent, and at constant elongation, the stress in stretched rubber is directly proportional to the absolute temperature. Within this range, therefore, the stress is analogous to the pressure of an ideal gas, which at constant volume is likewise proportional to the temperature. This behavior of rubber and of other rubberlike substances is said to be ideal. However, a sample of rubber the behavior of which is ideal at elongations of 100 per cent and higher deviates from this behavior when its elongation is less than 100 per cent or greater than 500 per cent. One of the reasons for these deviations from ideal behavior at high deformations has already been discussed in a previous publication; as a result of stretching, rubber begins to crystallize. The crystallites do not contribute to the elastic force, but if the temperature is raised, they fuse and thereby increase the stress, which then increases more rapidly than that of an ideal rubber. The deviations from ideal behavior at small deformations have not in any way been studied thoroughly, so the first part of the present work is concerned with these deviations from ideal behavior. First of all it is well to discuss some experimental facts. The thermal coefficient of the force F at constant elongation Δl, (∂F/∂T)Δl, is negative for low elongations (up to about 10 per cent), i.e., the force decreases with rise in temperature, and the coefficient of linear expansion in the direction of traction remains positive, like that of an ordinary material. At elongations of approximately 10 per cent, the stress is independent of the temperature; at this point the coefficient of linear expansion in the direction of stretching is, therefore, equal to zero. At higher deformations, the stress increases with rise in temperature, a phenomenon which indicates a negative coefficient of linear expansion.