SORPTION OF CARBON-DIOXIDE IN FLUORINATED POLYIMIDES

Sorption isotherms of CO2 for ten fluorinated polyimides measured at 3 5-degrees-C and up to about 25 atm are analyzed according to the dual- mode sorption model. Sorption properties for these polyimides are comp ared with those for other glassy polymers including unfluorinated poly imides. The glassy polymers with higher glass transition temperatures T(g) tend to show greater CO2 sorption. Introduction of a - C (CF3)2 - linkage into the repeat unit of the main chain increases the sorption by 20-80%. For glassy polymers, including the fluorinated and unfluor inated polyimides, the Langmuir affinity constant b and Henry's law so lubility constant k(D) are correlated with the content of functional ( carbonyl or sulfonyl) groups [FG], and a composite parameter reflectin g the magnitude of both [ FG ] and free-space fraction V(F), respectiv ely, with some exceptions. The Langmuir capacity constant C(H)' is cor related with T(g), but there are two correlation lines; one for unfluo rinated polyimides and a different one for other glassy polymers inclu ding fluorinated polyimides. The slope of the former group is smaller probably because of smaller differences in thermal probably because of smaller differences in thermal expansion coefficients in rubbery and glassy states. Most fluorinated polyimides show greater solubility Of CO2 than unfluorinated polyimides and other glassy polymers, because o f their larger C(H)' and k(D). (C) 1993 John Wiley & Sons, Inc.