SynopsisPermeability coefficients P for He, and C3H, in 12 different silicone polymer membranes were determined at 35.0"C and pressures up to 9 atm. Values of F for CO,, CH,, and C,H, were also determined a t 10.0 and 55.0"C. In addition, mean diffusion coefficients D and solubility coefficients S were obtained for CO,, CH,, and C3H, in 6 silicone polymers a t 10.0, 35.0, and 55.0"C. Substitution of increasingly bulkier functional groups in the side and backbone chains of silicone polymers results in a significant decrease in P for a given penetrant gas. This is due mainly to a decrease in E, whereas S decreases to a much lesser extent.Backbone substitutions appear to have a somewhat lesser effect in depressing than equivalent side-chain substitutions. The selectivity of a silicone membrane for a gas A relative to a gas €3, i.e., the permeability ratio &A)/&€%), may increase or decrease as a result of such substitutions, but only if the substituted groups are sufficiently bulky. The selectivity of the more highly permeable silicone membranes is controlled by the ratio S(A)/S(H). whereas the selectivity of the less permeable membranes depends on both the ratios D(A)/D(B) and S(A)/S(R). The permeability as well as the selectivity of one silicone membrane toward CO, were significantly enhanced by the substitution of a fluorine-containing side group that increased the solubility of CO, in that polymer.

The solubility of methane, carbon dioxide, and propane in five silicone polymers was measured at 10.0, 35.0 and 55.0°C and at pressures up to 26 atm. The polymers were poly(dimethyl siloxane), poly(methyl propyl siloxane), poly(methyl octyl siloxane), poly(trifluoropropyl methyl siloxane), and poly(phenyl methyl siloxane). At a given temperature and pressure, the solubility of the penetrant gases decreases with increasing bulkiness of the polymer side chains, and with decreasing critical temperature of the penetrant. The solubility of carbon dioxide in poly(trifluoropropyl methyl siloxane) appears to be anomalously high, possibly because of specific penetrant/polymer interactions. The temperature and pressure dependence of the solubility coefficients for the penetrant/polymer systems studied are described, and different methods of correlating these coefficients are compared.

The solubility of carbon dioxide, methane, and propane in poly(dimethyl silmethylene) [(CH3)2SiCH2]x and poly(tetramethyl silhexylene siloxane) [(CH3)2Si (CH2)6Si (CH3)2O]x was measured in the temperature range from 10.0 to 55.0°C and at elevated pressures. The present results are compared with similar measurements made with other silicone polymers. At a given temperature and pressure, the solubility of the above three gases is highest in poly(dimethyl siloxane) (Me2SiO)x. The gas solubility is decreased by either backbone‐chain or side‐chain substitutions of functional groups in (Me2SiO)x which increase the stiffness of the polymer chains and decrease the specific or fractional free volume of the polymers. It is conjectured that a decrease in the free volume of silicone polymers has a greater effect in decreasing the gas solubility than differences in gas/polymer interactions [with the exception of specific interactions (e.g., between CO2 and polar groups in the polymer)]. © 1993 John Wiley & Sons, Inc.

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