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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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The application of laser light scattering techniques to characterize the structural and dynamical properties of bulk polymers near the glass transition temperature is reviewed. Preliminary results of a study of poly(n‐butyl methacrylate) are presented. As in an earlier study of atactic polystyrene, the results suggest the coexistence of two relaxation mechanisms near Tg: a fast, single exponential decay; a slow process characterized by a wide distribution of relaxation times. The slow relaxation mechanism is evidently the main‐chain glass transition process; it appears possible that the fast decay is a manifestation of side‐chain motion. However, in contrast to the polystyrene system where the fast mechanism is always of small amplitude in the temperature range amenable to study (Tg < T < Tg + 40°), the fast mechanism in the poly(n‐butyl methacrylate) system exhibits a sudden transition to high amplitude (∼50 percent of the total observable relaxation amplitude) at T ∼ Tg + 30°. At this point therefore, we suggest that a strong resonant coupling of the fast process with the slow relaxation is occurring.

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An apparatus has been constructed that permits the measurement of time-dependent changes in volume following a sudden change in pressure near the point of vitrification. The same instrument is used for measuring steady state pressure, volume, and temperature properties that are necessary for a proper analysis of the dynamic measurements. The former experiments are referred to as pressure-jump volume relaxation (PJVR) measurements and are considered a direct probe of the structural relaxation process that occurs in all glasses.' This type of experiment relies on the nonequilibrium nature of the glassy state and the relatively slow approach toward equilibrium near the vitrification temperature and pressure.'Experimentally, the measurements are performed as shown in FIGURE 1. Following equilibration on the liquidus line (point A), the sample is subjected to a rapid change in pressure to enter the nonequilibrium glassy state (point B). The pressure is then held constant during the relaxation of the volume toward a new equilibrium state (point C). The cycle is completed by executing another pressure step of equal magnitude, but opposite in sign, resulting in another nonequilibrium state (point D), followed by recovery toward the initial volume (point A). Experiments are then repeated a t different starting pressures and temperatures.The present experiments have been performed on polystyrene ( Tg at zero pressure is 100° C) at temperatures from 110 to 150° C and at pressures of 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 t e m p e r a t~r e ,~ namely, 1) nonlinearity with respect to the magnitude of the departure from equilibrium, 2) asymmetry with respect to the sign of the departure from equilibrium, and 3) memory effects associated with complicated temperature or pressure histories. Each of these effects can be accounted for by a phenomenological order parameter model that has been extended to include the effect of p r e~s u r e .~ This model assumes that a distribution of relaxation times exists and that these relaxation times depend on the external variables (temperature and pressure) as well as on the instantaneous departure of the system from equilibrium. This can be represented as a product of independent shift factors 7 = roa , . a d . a p (1) where ro is a reference relaxation time, a , = exp[-B(T -T,)], a6 = exp[ -( 1x)O6/Aa], and ap = exp(bpP). Note that T, is a reference temperature, 0 is proportional to an activation energy, x is an adjustable parameter that reflects the n~nlinearity,~ 6 = ( V / V,) -I , V, is the equilibrium volume, Aa is the change in the 330