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The internal energy and the entropy components of the elastic restoring force in rubbers were determined for natural rubber up to an extension ratio of about 3.0. Four different experimental measurements were necessary to determine these components: (1) the force-temperature coefficient at constant temperature and length; (2) the force-pressure coefficient at constant temperature and length; (3) the thermal expansion coefficient at constant length; and (4) the isothermal compressibility at constant length. The force-temperature and the force-pressure coefficients were functions of strain whereas the expansion coefficients and the isothermal compressibilities were independent of strain. These measurements gave an internal energy contribution of 23% for natural rubber independent of the strain over the range of extensions studied.

To describe the thermal as well as the elastic behavior of rubbers a new phenomenological description of elastomers based on a generalized measure of strain was developed. The incompressible form of the strain energy function correctly described the elastic data on various elastomers (natural rubber, styrene-butadiene rubber, chlorinated ethylene-propylene copolymer rubber) in both homogeneous and non-homogeneous deformation fields. For a given rubber the same set of parameters fitted the data in simple tension, simple compression, equal biaxial tension and pure shear up to the point of rupture.

The compressible form of the strain energy function also described the thermoelastic data on natural rubber. The thermoelastic data on chlorinated ethylene-propylene copolymer rubber, taken out of the literature, were also predicted. From the new strain energy function it was possible to determine the interchain interactions. For natural rubber the interchain energy effects were found to be small (4%) as compared to the intrachain energy effects.

The experimental results on natural rubber established the range of validity of the statistical mechanical (molecular) and the continuum mechanical (phenomenological) theories. The temperature coefficient of the unperturbed dimensions of natural rubber determined from the internal energy component of the force yielded the conformational energies associated with the cis-polyisoprene chain.

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