Sorption and Diffusion of Organic Vapors in Two Fluoroelastomers

Immersion experiments with Aflas (I), poly(tetrafluoroethylene-co-propylene), and Fluorel (II) [poly(vinylidene fluoride-co-perfluoropropylene)], showed greater swelling of I in nonpolar liquids and much higher swelling of II in polar liquids: over 100% (wt/wt) in two ketones and a phosphate ester. Sorption isotherms determined for toluene and acetone at 25 and 35°C were fitted with the Flory-Rehner relation, employing a concentration-dependent solvent-polymer interaction parameter. The fitted K parameters indicated that the degree of crosslinking in II was lower than in I. However, the high swelling of II by polar solvents is attributed primarily to the polar nature of II resulting from the asymmetric CF(CF₃) moiety. Diffusion coefficients determined from sorption kinetics, corrected for nonisothermal effects, and converted to solvent self-diffusion coefficients were fitted with the Fujita free-volume relation. The values were much higher for I than II with acetone and also slightly higher for I with toluene. The estimated zero-concentration values were 1.5 E-09 cm²/s for Aflas-acetone, 0.3 E-09 cm²/s for Fluorel-acetone, and even lower for toluene. The low diffusion coefficients, which contribute to the superior barrier performance of these elastomers, is due, in part, to the high glass transition temperatures of I and II, -7 and -21°C, respectively.