Effect of Finite Extensibility on the Viscoelastic Properties of a Styrene-Butadiene Rubber Vulcanizate in Simple Tensile Deformations up to Rupture

Stress-strain and rupture data were determined on an unfilled styrene-butadiene vulcanizate at temperatures from −45 to 35° C and at extension rates from 0.0096 to 9.6 min−1. The data were represented by four functions: (1) the well-known temperature function (shift factor) aT; (2) the constant-strain-rate modulus, F (t, T) reduced to temperature T0 and time t/aT, i.e., T0F (t/aT)/T (3) the time-dependent maximum extensibility λm (t/aT); and (4) a function Ω(χ) where χ=(λ−1)λm0/λm, in which λ is the extension ratio and λm0 is the maximum extensibility under equilibrium conditions. The constant-strain-rate modulus characterizes the stress-time response to a constant extension rate at small strains, within the range of linear response; λm is a material parameter needed to represent the response at large λ; and Ω(χ) represents the stress-strain curve of the material in a reference state of unit modulus and λm=λm0. The shift factor aT was found to be sensibly independent of extension. At all values of t/aT for which the maximum extensibility is time-independent, the relaxation rate was also found to be independent of λ. These observations indicate that the monomeric friction coefficient is strain-independent over the ranges of T and λ covered in the present study. It was found that λm0=8.6 and that the largest extension ratio at break (λb)max is 7.3. Thus, rupture always occurs before the network is fully extended.