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An apparatus has been constructed that permits the measurement of time‐dependent changes in pressure near the point of vitrification. The same instrument is used for measuring steady‐state PVT properties, which are necessary for a proper analysis of the dynamic measurements. The former experiments are referred to as pressure‐jump volume‐relaxation (PJVR) measurements and serve as a direct probe of the structural relaxation process that occurs in all glasses. Experiments have been performed on polystyrene from 110 to 150°C and up to 2 kbar using pressure steps of 500 bars. The qualitative observations are analogous to those obtained at atmospheric pressure by rapid changes in temperature, namely (1) nonlinearity, (2) asymmetry, and (3) memory effects associated with complicated temperature or pressure histories. Each of these effects is accounted for semiquantitatively by a phenomenological order‐parameter model that has been extended to include the effect of pressure. Deviations between theory and experiment increase as temperature and pressure increase, this being manifest mostly in a predicted recovery curve (expansion isobar) that recovers the equilibrium volume more quickly than the experimental data; the contraction isobars are in most cases predicted within experimental error. The adjustable parameters of the model are found to vary somewhat with pressure and temperature, apparently due to variations in δ and Δκ. The activation volume suggests that 10–20 monomer segments are involved in the recovery process, assuming that the activation volume itself represents only a fraction of the dynamic unit (as observed in molecular glasses).

The KAHR model of structural relaxation has been extended to include the effects of pressure upon the retardation times of glass‐forming materials. The previously used methodology is applied with a continuous distribution of retardation times of the fractional exponential form. Several forms of the pressure dependence are examined. The combined temperature and pressure changes on structural recovery of glasses are addressed in this paper.

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