Pressure dependence of diffusion coefficient for CO 2 in glassy polymers

The pressure dependence of the apparent diffusion and permeation coefficients were observed by using the permeation time‐lag method for CO 2 in glassy poly(ethylene terephthalate) (PET), polystyrene (PS) and poly(vinyl chloride) (PVC) below 1 atm. The results show that the permeation coefficient is constant whereas the diffusion coefficient increases with pressure. According to the theoretical prediction of Paul, it can be concluded that the adsorbed CO 2 in these glassy polymers is completely immobilized and does not participate directly in the diffusion. A computer was used in the numerical calculation to determine the true diffusion coefficient from the model of Paul, et al . A comparison of the curves calculated with these constants and experimental values gave excellent agreement for the three glassy polymers. But there is a large difference between the values of one of the parameters obtained by this time‐lag method and the sorption method. Relations between this difference and the magnitude of the parameters are discussed.