CO2-induced plasticization phenomena in glassy polymers

A typical effect of plasticization of glassy polymers in gas permeation is a minimum in the relationship between the permeability and the feed pressur e. The pressure corresponding to the minimum is called the plasticization p ressure. Plasticization phenomena significantly effect the membrane perform ance in, for example, CO2/CH4 separation processes. The polymer swells upon sorption of CO2 accelerating the permeation of CH4. As a consequence, the polymer membrane loses its selectivity. Fundamental understanding of the ph enomenon is necessary to develop new concepts to prevent it. In this paper, CO2-induced plasticization phenomena in 11 different glassy polymers are investigated by single gas permeation and sorption experiments . The main objective was to search for relationships between the plasticiza tion pressure and the chemical structure or the physical properties of the polymer. No relationships were found with respect to the glass transition t emperature or fractional free volume. Furthermore, it was thought that pola r groups of the polymer increase the tendency of a polymer to be plasticize d because they may have dipolar interactions with the polarizable carbon di oxide molecules. But, no dependence of the plasticization pressure on the c arbonyl or sulfone density of the polymers considered was observed. Instead , it was found that the polymers studied plasticized at the same critical C O2 concentration of 36+/-7 cm(3) (STP)/cm(3) polymer. Depending on the poly mer, different pressures (the plasticization pressures) are required to rea ch the critical concentration. (C) 1999 Elsevier Science B.V. All rights re served.