A Comparison of the Transient Stress-Strain Response of Rubber to Its Linear Dynamic Behavior

To assess the utility of the Cox-Merz rule, master curves of the small strain, dynamic shear modulus are compared to the transient mechani­cal response of rubbers stretched over a seven-decade range of strain rates (< 10³ s^-1). The experiments were carried out on 1,4- and 1,2-polybutadiene and a styrene-butadiene copoly­mer. These rubbers have glass transition tempera­tures, T_g, equal to –93.0, 0.5, and 4.1°C, so that the experiments extended into the rub­bery plateau, the beginning of the glass transition zone, and the glassy regime, respectively. For the 1,4-poly­butadiene, in accord with previous results, strain and strain rate effects were decoupled. For the other two materials encroachment of the segmental dynamics did not allow separation of the effects of strain and rate. We find that for rubbery polymers near T_g, the use of linear dy­namic data to pre­dict stresses, strain energy, and other mechanical prop­erties at higher strain rates entails large error. For example, the strain rate associated with an up­turn in the mod­ulus due to onset of the glass transition was three orders of magni­tude higher for large tensile strains than for linear oscillatory shear.