Significance of the Equation of State for Rubber

Abstract

The physical significance of stress‐strain curves and of isometrics obtained by the relaxation method is discussed and clarified. Stress‐strain curves taken at various temperatures give the correct dependence of stress upon temperature if they are taken sufficiently fast so that stress relaxation does not mask the temperature dependence. Isometrics obtained after previous relaxation of the sample are shown to depend upon duration and temperature of the relaxation by a numerical factor only. The basis for this behavior is the factorization of the stress into a factor depending upon extension and temperature only which corresponds to the equation of state and another factor depending upon the temperature T* and the duration of the relaxation process. For simple stress relaxation, the same factorization holds with T* equal to T. A general theory is formulated for time dependent elastic phenomena by generalizing Boltzmann's theory. The theory explains why factorization does not hold for creep, in agreement with experiment.