The permselectivity of polyorganosiloxanes containing ester functionalities

The permeabilities of carbon dioxide, methane, oxygen and nitrogen in a range of polyorganosiloxane membranes containing a side-chain ester functionality of 0-21.4 mol% of Si atoms, have been determined by a continuous flow method over the temperature range 35-100°C. The ratio of the CO₂ and CH₄ permeabilities, or permselectivity, increases with increase in ester functionality. As measurement of the diffusivities at 35°C shows that there is little change in the relative diffusivity of CO₂ to CH₄ with increase in ester functionality, the greater permselectivity results from an increase in the relative solubility of CO₂ to CH₄. This is confirmed by measurements of the relative solubility using a vacuum microbalance. Moreover, since the activation energy for CO₂ permeation is much lower than that for CH₄, the permselectivity becomes greater as the temperature is lowered. In contrast, the ratio of the oxygen/nitrogen permeabilities decreases slightly with increase in ester functionality as a result of a decreasing relative solubility, the relative diffusivity again remaining virtually unchanged.