Ultimate Tensile Properties of Elastomers. III. Dependence of the Failure Envelope on Crosslink Density

Ultimate tensile properties of amorphous unfilled elastomers can be characterized by a failure envelope which results from a plot of log log σbT0vs.log (λb− 1) where σb is the stress at break based on the cross section area of an unstressed specimen, λb is the corresponding ultimate extension ratio, and T0 and T are, respectively, an arbitrarily selected reference temperature and the test temperature expressed in °K. Values of σb and λb are normally obtained from tests made at different extension rates at a series of temperatures. To determine the effect of crosslink density on the failure envelope, a study was made of eight vulcanizates of a hydrofluorocarbon elastomer, Viton A-HV. The number of effective network chains, νe, in these vulcanizates varied from 0.38× 10−4to 9.46× 10−4mole/cm.3 Ultimate tensile properties were determined at temperatures between −5 and 230° C and at seven to ten extension rates at each temperature. The data gave a master failure envelope on a plot of λbσb273/Tvs.log Ee(λb−1) 273/T, where Ee is the equilibrium tensile modulus. This behavior shows that λbσbT0/T is a function of Ee(λb−1); thus the maximum value of (λb−1) is inversely proportional to νe. Ultimate tensile properties of one vulcanizate were also determined under constant loads at four temperatures and the data were found to yield the same envelope as those from tests made at constant rates of extension.