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A strain energy density function is proposed which is based on a generalized measure of strain. The function has the form W=(2G/n)IE+BIEm where G, B, n, and m are material constants, and IE is the first invariant of the (generalized) Lagrangean strain (λan−1)/n. The function fits data on natural rubber and on a synthetic rubber in various homogeneous stress fields up to the point of break. The powers n and m are sensibly independent of temperature, while the two moduli G and B depend linearly on temperature, over the range investigated.

Stress relaxation measurements were made as a function of temperature and hydrostatic pressure on two lightly filled elastomers (Hypalon 40 and Viton B), one highly filled elastomer (Neoprene WB), and on an EPDM rubber. The latter was not piezorheologically simple. The lightly filled elastomers showed piezorheologically simple behavior, i.e., their response curves under different hydrostatic pressures could be superposed empirically by a simple horizontal shift along the logarithmic axis. The filled elastomer was piezorheologically simple only in the rubbery region and in the beginning of the transition region. The dependence of the empirical shift distances, log ap, on P could not be described by either the Ferry-Stratton or the Bueche-O'Reilly equation. By considering the bulk modulus to be linearly related to pressure, a new equation has been developed for log aT,P which describes the pressure dependence well and contains the WLF equation as a limiting case. Published data on the response of poly(vinyl chloride) under superposed hydrostatic pressure are shown to obey the new equation also. The theoretical importance of the new equation lies in the fact that combination of the usual isobaric measurements at atmospheric pressure as function of temperature with isothermal measurements as function of pressure allows, in principle, all the molecular parameters required by the free volume theory to be determined unambiguously.

Two‐phase polymeric materials such as polymer blends, block copolymers, and graft copolymers, are thermorheologically complex. Mechanical response curves obtained on such materials at different temperatures cannot, in principle, be brought into superposition by a simple shift along the logarithmic time or frequency axis. The shift factors become functions of time or frequency in addition to temperature. A general treatment of time‐temperature superposition in thermorheologically complex materials is developed and a model is proposed from which, for a two‐phase material, the amount of shift can be calculated which is necessary to bring a point on a mechanical response curve obtained at a given temperature and time or frequency into superposition at another temperature. The mechanical responses of the constituent homopolymers and their temperature functions must be known.