Three independent studies1-3 have been reported on sorption of carbon dioxide in PEMS. In all three cases, a high molecular weight polymer was used. Koros et al.1 observed, in the pressure range 0-2 MPa, a dual mode sorption shapes of the isotherms concave to the pressure axis. However, at higher pressures, the isotherms change to linear or even nonlinear form convex to the pressure axis2,3 that was explained in terms of the effect of plastisization exerted by carbon dioxide. Figure 12 shows the temperature dependence of the pressure pg at which the glass transition in the polymer takes place, as well as the corresponding concentration Cg of cardon dioxide. It is seen that the data of Refs. 2 and 3 are in a good agreement. The both curves reach the temperature axis as it can be anticipatedm the in vicinity of the glass transition temperature for the pure PEMA.
Figure 13 shows Van't Hoff plot for the solubility coefficients taken from the graph2,3 or calculated by the dual mode sorption parameters. The values reported in all three works are in good agreement.
The least squares treatment of the data gave the following equation:
log(S/cm3(STP) cm-3 atm-1) = -4.353 + 1.4331/(T/K).
The corresponding enthalpy of sorption Δs = —27.5 kJ/mol. It can be recommended for the prediction of the initial slope of the isotherms. For the pressure of carbon dioxide up to 2 MPa the dual mode sorption parameters reported by Koros et al.1 can be used. For the higher pressure, the works of Chiou and Paul2 and Kamiya et al.3 give rather close values of solubility, but the sorption isotherm obtained in Ref. 3 should be, tentatively, recommended since they cover the wider range of pressure and temperature.