| Abstract: | Stress relaxation was studied in torsion under superposed hydrostatic pressure using a newly constructed device. Two materials, a styrene–butadiene rubber and a butadiene rubber, were studied in the range from ?60 to 30°C at pressures up to 500 MPa. The time–temperature–pressure superposition of the data can be described by the FMT form of the free-volume theory which uses input from relaxation experiments at high pressure. The applicability of the Havlí?ek–Ilavský–Hrouz form of the Adam–Gibbs theory was also examined. This theory, which does not require information from high-pressure relaxation experiments, offers less flexibility than the FMT theory and is less successful in predicting superposition. The data are also examined in light of an adaptation of the Simha–Somcynsky equation of state to the free-volume theory. Prediction of the effect of pressure by the theory requires a single adjustable parameter. It was possible to calculate the free-volume parameters of several polymers without use of information from high-pressure experiments. |
