Studies of Viscoelastic Relaxation of Some Nitrile Rubbers under Compressive Stress

Stress relaxation in rubbers is usually supposed to be due to chemical aging phenomena of a mainly elastic material. Considerable physical viscoelastic processes can, however, be observed in the rubbery region, depending upon the type of rubber, crosslink density, type of crosslink, filler, and so on. Thus, in chemical stress relaxation experiments chemical and physical mechanisms are superimposed and can seldom be distinguished. Stress relaxation curves registered at different temperatures contain contributions from both types of mechanism. From a practical, as well as a theoretical, point of view it is therefore essential to find methods of distinguishing between the two relaxation processes. This would be possible if the relaxation curves were obtained during periods of time so short that the chemical relaxation can be neglected. The long-term physical relaxation is then obtained by shifting the curves by, for instance, the method of reduced variables. This technique has been utilized by Curro and Salazar. We now present an alternative procedure by which the physical viscoelastic stress relaxation behavior is determined from dynamic data. The physical relaxation curves are calculated from values of E′(ω) and E″(ω) obtained at different frequencies and temperatures. In this method of determining the physical relaxation, no change of sample is needed, nor is it necessary to allow the sample to relax between measurements. The stress relaxation behavior under compression of three nitrile rubbers has been studied, and it is shown that physical mechanisms dominate just above room temperature, while chemical mechanisms dominate at higher temperatures.