Structure‐permeability relationships in silicone polymers

Abstract

Permeability coefficients P for He, O 2, N 2, CO 2 CH 4, C 2 H 4, C 2 H 6, and C 3 H 8 in 12 different silicone polymer membranes were determined at 35.0°C and pressures up to 9 atm. Values of P for CO 2, CH 4, and C 3 H 8 were also determined at 10.0 and 55.0°C. In addition, mean diffusion coefficients D and solubility coefficients S were obtained for CO 2, CH 4, and C 3 H 8 in 6 silicone polymers at 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 D, whereas S decreases to a much lesser extent. Backbone substitutions appear to have a somewhat lesser effect in depressing P than equivalent side‐chain substitutions. The selectivity of a silicone membrane for a gas A relative to a gas B, i.e., the permeability ratio P(A)/P(B), 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(B), whereas the selectivity of the less permeable membranes depends on both the ratios D(A)/D(B) and S(A)/S(B). The permeability as well as the selectivity of one silicone membrane toward CO 2 were significantly enhanced by the substitution of a fluorine‐containing side group that increased the solubility of CO 2 in that polymer.