@article{alentiev_shantarovich_merkel_bondar_freeman_yampolskii_2002, title={Gas and vapor sorption, permeation, and diffusion in glassy amorphous teflon AF1600}, volume={35}, ISSN={["0024-9297"]}, DOI={10.1021/ma020494f}, abstractNote={Sorption and permeation parameters of light gases, C1−C12 hydrocarbons, and C1−C7 perfluorocarbons were determined in a random, amorphous, glassy copolymer containing 65 mol % 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (BDD) and 35 mol % tetrafluoroethylene (TFE) (TFE/BDD65 or AF1600). AF1600 results were compared to those of another copolymer, AF2400, which contains 87 mol % BDD. As the amount of bulky, packing-disrupting BDD increases, solubility coefficients increase systematically, primarily due to increases in the nonequilibrium excess volume of the glassy polymer. Permeability and diffusivity also increase with increasing BDD content. AF1600 is easily plasticized by larger, more soluble penetrants and is susceptible to penetrant-induced conditioning. As penetrant size increases, permeability and diffusion coefficients decrease. The rates of decrease of permeability and diffusivity with increasing penetrant size, which characterize permeability and diffusivity selectivity, are intermediate bet...}, number={25}, journal={MACROMOLECULES}, author={Alentiev, AY and Shantarovich, VP and Merkel, TC and Bondar, VI and Freeman, BD and Yampolskii, YP}, year={2002}, month={Dec}, pages={9513–9522} }
 @article{de angelis_merkel_bondar_freeman_doghieri_sarti_2002, title={Gas sorption and dilation in poly(2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole-co-tetrafluoroethylene): Comparison of experimental data with predictions of the nonequilibrium lattice fluid model}, volume={35}, ISSN={["0024-9297"]}, DOI={10.1021/ma0106090}, abstractNote={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.}, number={4}, journal={MACROMOLECULES}, author={De Angelis, MG and Merkel, TC and Bondar, VI and Freeman, BD and Doghieri, F and Sarti, GC}, year={2002}, month={Feb}, pages={1276–1288} }
 @article{merkel_toy_andrady_gracz_stejskal_2002, title={Investigation of Enhanced Free Volume in Nanosilica-Filled Poly(1-trimethylsilyl-1-propyne) by 129Xe NMR Spectroscopy}, volume={36}, ISSN={0024-9297 1520-5835}, url={http://dx.doi.org/10.1021/ma0256690}, DOI={10.1021/ma0256690}, abstractNote={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.}, number={2}, journal={Macromolecules}, publisher={American Chemical Society (ACS)}, author={Merkel, T. C. and Toy, L. G. and Andrady, A. L. and Gracz, H. and Stejskal, E. O.}, year={2002}, month={Dec}, pages={353–358} }
 @article{merkel_gupta_turk_freeman_2001, title={Mixed-gas permeation of syngas components in poly(dimethylsiloxane) and poly(1-trimethylsilyl-1-propyne) at elevated temperatures}, volume={191}, ISSN={["0376-7388"]}, DOI={10.1016/S0376-7388(01)00452-5}, abstractNote={The permeability of poly(dimethylsiloxane) (PDMS) and poly(1-trimethylsilyl-1-propyne) (PTMSP) to a simulated syngas feed containing H2, CO, CO2, and H2S was determined as a function of temperature up to 240°C (464°F). The permeation properties of rarely studied CO and H2S were found to be consistent with their molecular properties (i.e. critical temperature) in both rubbery PDMS and high free volume, glassy PTMSP. At room temperature, PDMS and PTMSP are more permeable to the more condensable gases CO2 and H2S than to H2. However, both polymers become hydrogen selective at elevated temperatures. Activation energies of permeation are highest for H2 in both polymers and decrease regularly with increasing gas condensability. PTMSP exhibits evidence of accelerated physical aging at high temperature.}, number={1-2}, journal={JOURNAL OF MEMBRANE SCIENCE}, author={Merkel, TC and Gupta, RP and Turk, BS and Freeman, BD}, year={2001}, month={Sep}, pages={85–94} }
 @article{merkel_bondar_nagai_freeman_pinnau_2000, title={Gas sorption, diffusion, and permeation in poly(dimethylsiloxane)}, volume={38}, ISSN={["0887-6266"]}, DOI={10.1002/(SICI)1099-0488(20000201)38:3<415::AID-POLB8>3.0.CO;2-Z}, abstractNote={The permeability of poly(dimethylsiloxane) [PDMS] to H2, O2, N2, CO2, CH4, C2H6, C3H8, CF4, C2F6, and C3F8, and solubility of these penetrants were determined as a function of pressure at 35 °C. Permeability coefficients of perfluorinated penetrants (CF4, C2F6, and C3F8) are approximately an order of magnitude lower than those of their hydrocarbon analogs (CH4, C2H6, and C3H8), and the perfluorocarbon permeabilities are significantly lower than even permanent gas permeability coefficients. This result is ascribed to very low perfluorocarbon solubilities in hydrocarbon-based PDMS coupled with low diffusion coefficients relative to those of their hydrocarbon analogs. The perfluorocarbons are sparingly soluble in PDMS and exhibit linear sorption isotherms. The Flory–Huggins interaction parameters for perfluorocarbon penetrants are substantially greater than those of their hydrocarbon analogs, indicating less favorable energetics of mixing perfluorocarbons with PDMS. Based on the sorption results and conventional lattice solution theory with a coordination number of 10, the formation of a single C3F8/PDMS segment pair requires 460 J/mol more energy than the formation of a C3H8/PDMS pair. A breakdown in the geometric mean approximation of the interaction energy between fluorocarbons and hydrocarbons was observed. These results are consistent with the solubility behavior of hydrocarbon–fluorocarbon liquid mixtures and hydrocarbon and fluorocarbon gas solubility in hydrocarbon liquids. From the permeability and sorption data, diffusion coefficients were determined as a function of penetrant concentration. Perfluorocarbon diffusion coefficients are lower than those of their hydrocarbon analogs, consistent with the larger size of the fluorocarbons. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 415–434, 2000}, number={3}, journal={JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS}, author={Merkel, TC and Bondar, VI and Nagai, K and Freeman, BD and Pinnau, I}, year={2000}, month={Feb}, pages={415–434} }
 @article{merkel_bondar_nagai_freeman_2000, title={Sorption and transport of hydrocarbon and perfluorocarbon gases in poly(1-trimethylsilyl-1-propyne)}, volume={38}, ISSN={["0887-6266"]}, DOI={10.1002/(SICI)1099-0488(20000115)38:2<273::AID-POLB1>3.0.CO;2-X}, abstractNote={Pure gas solubility and permeability of H2, O2, N2, CO2, CH4, C2H6, C3H8, CF4, C2F6, and C3F8 in poly(1-trimethylsilyl-1-propyne) (PTMSP) were determined as a function of pressure at 35°C. Permeability coefficients of the perfluorinated penetrants are approximately an order of magnitude lower than those of their hydrocarbon analogs, and lower even than those of the permanent gases. In striking contrast to hydrocarbon penetrants, PTMSP permeability to fluorocarbon penetrants decreases with increasing penetrant size. This unusual size-sieving behavior in PTMSP is attributed to low perfluorocarbon solubilities in PTMSP coupled with low diffusion coefficients relative to those of their hydrocarbon analogs. In general, perfluorocarbon penetrants are less soluble than their hydrocarbon analogs in PTMSP. The difference in hydrocarbon and perfluorocarbon solubilities in high free volume, hydrocarbon-rich PTMSP is much smaller than in hydrocarbon liquids and liquidlike polydimethylsiloxane. The low solubility of perfluorocarbon penetrants is ascribed to the large size of the fluorocarbons, which inhibits their dissolution into the densified regions of the polymer matrix and reduces the number of penetrant molecules that can be accommodated in Langmuir sites. From the permeability and sorption data, diffusion coefficients were calculated as a function of penetrant concentration. With the exception of H2 and the C3 analogs, all of the penetrants exhibit a maximum in their concentration-dependent diffusion coefficients. Resolution of diffusion coefficients into a mobility factor and a thermodynamic factor reveals that it is the interplay between these two terms that causes the maxima. The mobility of the smaller penetrants (H2, O2, N2, CH4, and CO2) decreases monotonically with increasing penetrant concentration, suggesting that the net free volume of the polymer–penetrant mixture decreases as additional penetrant is added to PTMSP. For larger penetrants mobility either: (1) remains constant at low concentrations and then decreases at higher penetrant concentrations (C2H6, CF4, and C2F6); (2) remains constant for all concentrations examined (C3H8); or (3) increases monotonically with increasing penetrant concentration (C3F8). Presumably these results reflect the varying effects of these penetrants on the net free volume of the polymer–penetrant system. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 273–296, 2000}, number={2}, journal={JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS}, author={Merkel, TC and Bondar, V and Nagai, K and Freeman, BD}, year={2000}, month={Jan}, pages={273–296} }
 @article{yampol'skii_berezkin_popova_korikov_freeman_bondar_merkel_2000, title={Thermodynamics of gas and vapor sorption by amorphous glassy AF teflons}, volume={42}, number={6}, journal={Polymer Science. Series A}, author={Yampol'skii, Y. P. and Berezkin, V. G. and Popova, T. P. and Korikov, A. P. and Freeman, B. D. and Bondar, V. I. and Merkel, T. C.}, year={2000}, pages={679–688} }
 @article{merkel_bondar_nagai_freeman_yampolskii_1999, title={Gas sorption, diffusion, and permeation in poly(2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole-co-tetrafluoroethylene)}, volume={32}, ISSN={["0024-9297"]}, DOI={10.1021/ma990685r}, abstractNote={The solubility and permeability of H2, O2, N2, CO2, CH4, C2H6, C3H8, CF4, C2F6, and C3F8 in TFE/BDD87, a random copolymer prepared from 87 mol % 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole [BDD] and 13 mol % tetrafluoroethylene [TFE], are reported as a function of temperature and pressure. Sorption isotherms of all penetrants except hydrogen are concave to the pressure axis and are well-described by the dual-mode model. Hydrogen exhibits linear sorption isotherms. In contrast to previous results in hydrocarbon-rich polymers, the solubility of perfluorocarbon penetrants is higher in TFE/BDD87 than that of their hydrocarbon analogues. The solubility of all penetrants in TFE/BDD87 decreases with increasing temperature. Enthalpies of sorption become more negative as penetrant size increases. Fluorocarbon enthalpies of sorption at infinite dilution are significantly more exothermic than those of their hydrocarbon analogues, suggesting more favorable interactions between fluorocarbon penetrants and perflu...}, number={25}, journal={MACROMOLECULES}, author={Merkel, TC and Bondar, V and Nagai, K and Freeman, BD and Yampolskii, YP}, year={1999}, month={Dec}, pages={8427–8440} }
 @article{de angelis_merkel_bondar_freeman_doghieri_sarti_1999, title={Hydrocarbon and fluorocarbon solubility and dilation in poly(dimethylsiloxane): Comparison of experimental data with predictions of the Sanchez-Lacombe equation of state}, volume={37}, ISSN={["0887-6266"]}, DOI={10.1002/(SICI)1099-0488(19991101)37:21<3011::AID-POLB11>3.0.CO;2-V}, abstractNote={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}, number={21}, journal={JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS}, author={De Angelis, MG and Merkel, TC and Bondar, VI and Freeman, BD and Doghieri, F and Sarti, GC}, year={1999}, month={Nov}, pages={3011–3026} }
 @article{merkel_bondar_nagai_freeman_1999, title={Hydrocarbon and perfluorocarbon gas sorption in poly(dimethylsiloxane), poly(1-trimethylsilyl-1-propyne), and copolymers of tetrafluoroethylene and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole}, volume={32}, ISSN={["0024-9297"]}, DOI={10.1021/ma9814402}, abstractNote={Sorption of a series of gases, perfluorocarbon vapors (CF4, C2F6, and C3F8) and their hydrocarbon analogues in poly(dimethylsiloxane) [PDMS], poly(1-trimethylsilyl-1-propyne) [PTMSP], and two random copolymers of tetrafluoroethylene [TFE] and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole [BDD] are reported as a function of penetrant pressure at 35 °C. Sorption isotherms for all penetrants in rubbery PDMS are linear or slightly convex to the pressure axis, while those in the glassy polymers are concave and are well described by the dual mode model. Fluorocarbon sorption levels are lower than sorption levels of their hydrocarbon analogues in the hydrocarbon-based PDMS and PTMSP matrixes, while the reverse is true in the fluorinated TFE/BDD copolymers. Exceptionally low fluorocarbon solubilities in PDMS are ascribed to poor penetrant/polymer energetic interactions.}, number={2}, journal={MACROMOLECULES}, author={Merkel, TC and Bondar, V and Nagai, K and Freeman, BD}, year={1999}, month={Jan}, pages={370–374} }