The permselectivity of polyorganosiloxanes containing ester functionalities

Abstract

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 CO 2 and CH 4 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 CO 2 to CH 4 with increase in ester functionality, the greater permselectivity results from an increase in the relative solubility of CO 2 to CH 4. This is confirmed by measurements of the relative solubility using a vacuum microbalance. Moreover, since the activation energy for CO 2 permeation is much lower than that for CH 4, 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.