An Experimental Study of the Thermodynamics of Rubber in Extension and Torsion

The stress‐strain properties of three different rubbers have been measured using two different techniques, in order to evaluate the relative internal energy contribution to the total stress f e / f and the corresponding value of Me/M over a range of strain and temperatures. Two different experimental techniques have been designed; one is to allow measurement in simple extension of the length of a rubber sample in relation to the temperature at constant force, and the other is to measure the variation of torsional couple M in a cylinder of rubber (subjected to combined torsion about the axis and extension in the axial direction) with temperature. The thermoelastic results obtained for natural rubber and butyl rubber do not contain any surprising features, and are found to be in good agreement with other reported experimental results for these rubbers. Values of f e / f and Me/M obtained for cis‐trans polybutadiene rubber show different trends between the two techniques.