Fundamentals of advanced polymer membranes for vapor separations

Polymers for gas separation membranes can now be designed to sieve penetrant molecules based primarily on penetrant size or based primarily on penetrant solubility in the polymers. Using these two approaches, polymers may be tailored which are more permeable to small components in a gas mixture or more permeable to larger components in the mixture. Both approaches have commercial utility. Size sieving materials are used commercially for the separation of supercritical gas mixtures (O₂/N₂, H₂/N₂, CO₂/CH₄, etc.). A simple theory presented herein provides a rationale for the selectivity/permeability tradeoff curves that characterize gas separations using size-sieving polymers. Results from the theory suggest that the most fruitful route to producing better materials for such separations is to increase backbone chain stiffness, increase interchain spacing, and increase solubility selectivity. For solubility selective polymers, which are more permeable to large components in a gas mixture, rubbery poly(dimethylsiloxane) has been developed as a commercial membrane for the removal of VOCs from mixtures with light gases. However, new materials are being developed for this applications. In particular, disubstituted polyacetylenes appear to offer an entirely new class of solubility selective polymers based on stiff chain glassy polymers that are endowed with extraordinarily high levels of fractional free volume. These materials provide another route to highly permeable, vapor-selective polymers.