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The first order approximation of the theory of the so-called simple material describes the viscoelastic behavior of soft polymers in large deformations with twelve time-dependent material functions and three material constants, all functions of the three invariants of the deformation tensor. Restricting consideration to deformations in which time shift invariance is preserved, a series of models was developed which describe the time dependence of the stress through the Boltzmann superposition integral incorporating into it a suitable nonlinear measure of strain. The theory was developed in its most general three-dimensional form. Its predictions for homogeneous deformations were tested in a series of experiments on an uncrosslinked styrene-butadiene copolymer.

For the prediction of the viscoelastic behavior of soft polymers the simplest form of the theory requires only one time function, the relaxation modulus. In addition, it requires a strain parameter which is a characteristic material constant. The dependence of this parameter on temperature and other material and experimental variables was examined on hand of estimates from published data as well as from the experimental results reported here.

Sensitive tests in which small (theoretically infinitesimal) deformations were superposed on a finite stretch, allowed a distinction to be made between the two simplest forms of the theory which, in principle, should apply to solidlike and to liquidlike behavior, respectively. Although uncrosslinked styrene-butadiene copolymer behaves in many respects like a liquid, it showed unprecedented excellent agreement with the predictions of the solid model at room temperature. It was concluded that under this condition the liquidlike character of the copolymer is not strong enough to satisfy the special requirements of the liquid model.

The new theory allows specific predictions to be made concerning the contribution of internal energy to the restoring force in rubberlike materials. It represents an advance over former theories in that it permits the contribution to be obtained on uncrosslinked as well as on crosslinked soft polymers. Literature data were reviewed and examined in the light of the theory.

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