Effects of high‐pressure CO2 on the glass transition temperature and mechanical properties of polystyrene

Abstract

Hydrostatic pressure usually increases the glass transition temperature Tg 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 CO 2 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 Tg reach minima, at a CO 2 pressure of ca. 20 MPa at both 34 and 45°C, which are above the critical temperature (31°C) of CO 2. At the minimum, Tg is 41 at 45°C and 36 at 34°C, the larger depression at 34°C evidently corresponding to the higher solubility of CO 2 at the lower temperature. The plasticization effect due to CO 2 can be isolated by subtracting the effect of hydrostatic pressure alone from the experimental data. The results leave no doubt that at high pressures CO 2 gas is a severe plasticizer for polystyrene.