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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 H2S. For example, while the mixture CO2/H2S 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 H2S permeability in fluoropolymers is caused primarily by unexpectedly low H2S solubility in the fluorinated polymer matrices. For instance, H2S 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 H2S and fluoropolymers. As a result of this unusual solubility behavior, fluorinated glassy polymers are more resistant to H2S-induced plasticization than nonfluorinated glasses. These findings may prove useful in emerging membrane applications that involve H2S, such as natural gas treatment.

The effect of pressure on solubility and the influence of temperature on solubility, permeability, and diffusivity of C3F8 and its hydrocarbon analogue, C3H8, are reported in rubbery poly(dimethylsiloxane) (PDMS) and glassy poly(1-trimethylsilyl-1-propyne) (PTMSP). The solubility of C3F8 is lower than that of C3H8 in both polymers at all temperatures and pressures investigated. The isosteric enthalpy of mixing C3F8 with PDMS and PTMSP is higher than that of C3H8 due to less favorable polymer−fluorocarbon interactions in the case of C3F8, and it decreases with increasing C3F8 concentration. Assuming a coordination number of 10, the energy associated with mixing C3F8 molecules and PDMS segments is 4.5 kJ/mol more than that required to mix C3H8 molecules with PDMS segments, in the limit of infinite dilution. The isobaric activation energy of permeation (EP) for C3F8 is positive for both polymers, and that for C3H8 is negative in both polymers. This result is particularly interesting for PTMSP since all previous studies of activation energy of gas permeation in PTMSP report values that are near zero or negative; this study provides the first report of a positive EP value in PTMSP. In PDMS, differences in both activation energy of diffusion (ED) and enthalpy change on sorption contribute significantly to the difference in EP values of C3H8 and C3F8. For PTMSP, the difference in EP values for C3F8 and C3H8 stems mainly from a substantially larger ED value for C3F8 than for C3H8.

The addition of nanoscale, nonporous fumed silica [FS] particles to size-selective poly(2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole-co-tetrafluoroethylene) [AF2400] systematically increases penetrant permeability coefficients, similar to behavior previously observed in vapor-selective polyacetylenes, but contrary to results in traditional filled polymer systems. Permeability coefficients of large penetrants increase more than those of small molecules in filled AF2400, thereby decreasing the size selectivity of this polymer. AF2400 is readily plasticized by n-butane, whereas AF2400 containing 40 wt % FS exhibits antiplasticization behavior, suggesting that filler addition alters AF2400 to allow n-butane molecules to be accommodated in the polymer without significant swelling and subsequent plasticization of the matrix. Both filled and unfilled AF2400 have essentially the same gas solubility coefficients, so all of the increase in penetrant permeability in filled AF2400 is a result of increased diffusion coefficients. There is reasonable agreement between diffusion coefficients obtained from transient sorption and steady-state data, both of which increase regularly with increasing FS content. Positron annihilation lifetime spectroscopy reveals that FS addition increases the size of free volume elements in AF2400. Thermal analysis of filled AF2400 shows that FS has no detectable effect on the polymer's glass transition temperature, indicating that FS has little impact on long-range chain mobility.

Penetrant permeability coefficients in high-free-volume, glassy poly(1-trimethylsilyl-1-propyne) [PTMSP] increase systematically with increasing concentration of nonporous, nanoscale fumed silica [FS]. For example, the permeability of PTMSP containing 40 wt % FS to methane is 180% higher than that of the unfilled polymer. Gas and vapor solubility in the nanocomposites are unaffected by FS at concentrations of up to 50 wt %. Penetrant diffusion coefficients in PTMSP increase with increasing FS content, and the enhanced permeability in the nanocomposites is due to this rise in diffusivity. These results are qualitatively similar to behavior previously observed when FS was added to another stiff-chain polyacetylene, poly(4-methyl-2-pentyne) [PMP]. However, in contrast to PMP, the permeability of PTMSP to relatively small gases increases more upon filling than that of larger penetrants. This results in a reduction in vapor/permanent-gas selectivity for filled PTMSP. In fact, mixed-gas n-butane/methane selectivity is 64% lower in PTMSP containing 50 wt % FS than in pure PTMSP. These results, combined with penetrant diffusion coefficients on the order of 10^-3 cm²/s in filled PTMSP, suggest an escalating influence of free phase transport mechanisms such as Knudsen diffusion as FS concentration in the polymer increases.

In contrast to the performance of traditional filled polymer systems, penetrant permeability coefficients in high-free-volume, glassy poly(4-methyl-2-pentyne) (PMP) increase systematically and substantially with increasing concentration of nonporous, nanoscale fumed silica (FS). For instance, the permeability of PMP containing 40 wt % FS to methane is 2.3 times higher than that of the unfilled polymer. Gas and vapor uptake in the PMP/FS nanocomposites is essentially unaffected by the presence of up to 40 wt % FS, while penetrant diffusion coefficients increase regularly with increasing filler content. This increase in diffusivity is responsible for elevated permeability in the PMP/FS nanocomposites. The addition of FS to PMP augments the permeability of large penetrants more than that of small gases, consistent with a reduction in diffusivity selectivity. Consequently, vapor selectivity in the nanocomposites increases with increasing FS concentration. Activation energies of permeation in PMP decrease with increasing FS content, suggesting that penetrant diffusive jumps require less energy at higher filler concentrations. Positron annihilation lifetime spectroscopy (PALS) reveals that FS subtly increases the free volume in PMP available for molecular transport. The accessible free volume measured by PALS correlates favorably with relative penetrant permeability in the nanocomposites. Transmission electron microscopy confirms that the FS nanoparticles are relatively well dispersed in PMP.

The gas permeability of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) containing nanoparticulate fumed silica increases with increasing filler content. This unusual phenomenon is explored using ¹²⁹Xe NMR spectroscopy to examine the effect of filler on the free volume of the PTMSP host matrix. The ¹²⁹Xe NMR chemical shift decreases regularly with increasing fumed silica concentration, consistent with an increase in the average size of free volume elements or cavities through which molecular transport can occur. A relationship between the chemical shift and gas permeability in the filled polymer is reported.

The sorption and dilation properties of a series of n-alkanes and the corresponding perfluorinated compounds have been examined in two amorphous copolymers of tetrafluoroethylene (TFE) and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (BDD), commercially available under the names Teflon AF1600 and AF2400. The analysis was made at three different temperatures:  25, 35, and 45 °C, to test the effect of temperature on solubility and to evaluate the sorption enthalpies. The partial molar volumes of most penetrants have also been determined in both copolymers. The experimental data have been satisfactorily compared with the sorption isotherms predicted or correlated using the nonequilibrium lattice fluid model.

Sorption and dilation isotherms are reported for a series of gases (N2, O2, CO2), hydrocarbon vapors (CH4, C2H6, C3H8), and their fluorocarbon analogs (CF4, C2F6, C3F8) in poly(dimethylsiloxane) (PDMS) at 35°C and pressures up to 27 atmospheres. The hydrocarbons are significantly more soluble in hydrocarbon-based PDMS than their fluorocarbon analogs. Infinite dilution partial molar volumes of both hydrocarbons and fluorocarbons in PDMS were similar to their partial molar volumes in other hydrocarbon polymers and in organic liquids. Except for C2H6 and C3H8, partial molar volume was independent of penetrant concentration. For these penetrants, partial molar volume increased with increasing concentration. The Sanchez–Lacombe equation of state is used to predict gas solubility and polymer dilation. If the Sanchez–Lacombe model is used with no adjustable parameters, solubility is always overpredicted. The extent of overprediction is more substantial for fluorocarbon penetrants than for hydrocarbons. Very good fits of the model to the experimental sorption and dilation data are obtained when the mixture interaction parameter is treated as an adjustable parameter. For the hydrocarbons, the interaction parameter is approximately 0.96, and for the fluorocarbons, it is approximately 0.87. These values suggest less favorable interactions between the hydrocarbon-based PDMS matrix and the fluorocarbon penetrants than between PDMS and hydrocarbons. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3011–3026, 1999