Comparison of Hydrogen Sulfide Transport Properties in Fluorinated and Nonfluorinated Polymers

Permeability coefficients of hydrogen sulfide in fluorinated polymers are anomalously low when compared to its permeability in nonfluorinated polymers. As a result, fluoropolymer membranes have unusual selectivities for gas pairs involving H 2 S. For example, while the mixture CO 2 /H 2 S selectivity is 0.66 for nonfluorinated, rubbery poly(dimethylsiloxane), this selectivity is 8.0 for a fluoroelastomer composed of tetrafluoroethylene, perfluoromethyl vinyl ether, and perfluoro-8-cyano-5-methyl-3,6-dioxa-1-octene (TFE/PMVE/8CNVE) and 27 for a glassy, cyclic perfluoroether (Cytop) under similar test conditions. The low H 2 S permeability in fluoropolymers is caused primarily by unexpectedly low H 2 S solubility in the fluorinated polymer matrices. For instance, H 2 S solubility in a Teflon AF copolymer of tetrafluoroethylene and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole is approximately 5 times lower than the value predicted by a correlation of solubility with gas critical temperature. This low solubility is not related to penetrant size or condensability effects but rather is caused by unfavorable molecular interactions between H 2 S and fluoropolymers. As a result of this unusual solubility behavior, fluorinated glassy polymers are more resistant to H 2 S-induced plasticization than nonfluorinated glasses. These findings may prove useful in emerging membrane applications that involve H 2 S, such as natural gas treatment.