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Stress relaxation measurements were made in uniaxial tension under superposed hydrostatic pressures up to 5 kbar at temperatures ranging from -25 to 50[degrees]C. Two lightly filled (Hypalon-40 and Viton-B) and one highly filled elastomer (Neoprene) were studied because their pressure transition lie within the range of the apparatus. The construction and operation of the apparatus are discussed. Measurements on Hypalon and Viton were made either by varying the temperature while maintaining the pressure constant at 1, 1,000, and 2,000 bars, or by holding the temperature constant while varying the pressure from atmospheric to 4,600 bars. The viscoelastic response of Neoprene was measured at 25[degrees]C and pressures up to 4,600 bars.

The measurements were converted to a time dependent shear modulus. Time-temperature and time-pressure superposition was then applied to the reduced data to obtain master curves at 1, 1,000, and 2,000 bars. By introducing either the Murnaghan or the Tait equation of state into the free volume theory, an expression was obtained which describes the shift factors, log a[subscript T,P'] resulting from the empirical shifts into the master curves at atmospheric pressure. This equation then gave an excellent prediction of the empirically found shift factors resulting from forming the master curves at 1,000 and 2,000 bars.

Because the measurements made as function of temperature and those made as a function of pressure must be consistent, certain ambiguities in the free volume theory have been removed. This leads to an essential improvement in the theory.

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