SynopsisHydrostatic pressure usually increases the glass transition temperature T, of a polymer glass by decreasing its free volume; if the pressurizing environment is soluble in the polymer, however, one might expect an initial decrease in Tg with pressure as the polymer is plasticized by the environment. Just such a minimum in the Tg of polystyrene (PS) is observed as the pressure of COz gas is increased over the range 0.1-105 MPa from both ultrasonic (1 MHz) measurements of Young's modulus E and static measurements of the creep compliance J . A time-temperature-pressure superposition law is obeyed by PS which allows a master curve for the compliance to be constructed and shift factors to be determined. A master curve for E is then obtained by using the Boltzmann superposition principle. The compliance J reaches a maximum, and E and T, reach minima, at a COz pressure of ca. 20 MPa at both 34 and 45OC, which are above the critical temperature (31OC) of COz. At the minimum, T, is 41 at 45°C and 36 at 34OC, the larger depression at 34OC evidently corresponding to the higher solubility of COz at the lower temperature. The plasticization effect due to COz can be isolated by subtracting the effect of hydrostatic pressure alone from the experimental data. The results leave no doubt that at high pressures COZ gas is a severe plasticizer for polystyrene.

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