Gas permeability of a new silicone ring polymer: Isotactic poly(phenyl silsesquioxane)

The permeability of isotactic poly(phenyl silsesquioxane) (PPSQ) to O₂ and N₂ at 25.0°C and to CO₂ and CH₄ at 35.0°C was measured at low pressures by the “time‐lag” method. PPSQ is a glassy silicone polymer with an 8‐atom ring structure of alternating Si and O atoms. The CO₂/CH₄ selectivity of PPSQ is markedly higher than that of other silicone polymers, whereas its O₂/N₂ selectivity appears to be only slightly higher. PPSQ is not as effective a “molecular sieve” as would be expected from its ring structure, possibly because its relatively large mean interchain distance suggests that this polymer has a large mean free volume.