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Two models, one for materials possessing a well‐defined reference state (solid‐like behavior, model S), the other for materials lacking such reference (liquid‐like behavior, model L) were tested with the aid of experimental data obtained in various strain histories in simple tension on an uncrosslinked styrene–butadiene rubber (SBR) gum rubber over the temperature range from −20 to 23°C.The models resulted from the introduction of the nonlinear n‐measure of strain into the Boltzmann superposition integral. Both gave excellent predictions of the response of the SBR to step and to ramp functions of strain. Published data on the response of polyisobutylene to an exponential stretch ratio excitation were in similarly good agreement with both models. Sensitive tests in which small deformations were superposed on a finite stretch allowed a distinction to be made in favor of model S.In uncrosslinked materials the strain parameter n was found to be small (typically about 0.5) and sensibly independent of temperature in the range investigated. By contrast, in crosslinked rubbers, n is typically between 1 and 2 at room temperature and generally decreases with temperature. The dependence of n on the nature of the material, crosslink density, temperature, and crosslinking conditions was examined.

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