Viscoelastic properties of sulfonated ethylene–propylene terpolymer neutralized with zinc cation

In the reported experiments, stress-relaxation measurements in uniaxial extension, in the terminal zone, were made of sulfonated ethylene-propylene terpolymer (EPDM), neutralized with zinc cation. For all the polymers, the decay of the modulus with elapsed time was gradual and featureless, reflecting a broad distribution of relaxation times. For polymers containing zinc stearate measured at 80 degree C, the amount of permanent set was low for relaxation up to about 20 h, but increased rapidly with additional hours of stretch. Composite curves were constructed by matching the stress-relaxation response at short times, moduli measured at long times fell above the composite curve. The shapes of the composite curves were similar, regardless of the level of EPDM sulfonation or the presence of zinc stearate. The apparent Arrhenius activation energy for short-time relaxation at similar levels of the modulus was about 35 kcal/mol at 140 degree C for the unplasticized polymers and about 70 kcal/mol at 90 degree C for the polymers containing zinc stearate.