Plasticization has been an important issue in gas separation membranes. Plasticization indicates permeability change by the feed pressure. Swelling by a penetrant such as carbon dioxide has been accepted as the main cause. In glassy membrane whose chains are rigid, plasticization has been widely studied. However, the mechanism of plasticization is not focused in the studies and still questionable. In contrast, rubbery membranes have flexible chains which are prone to the swelling. Therefore, rubbery membrane is an appropriate material to magnify the swelling phenomenon and study the mechanism.<br/>Since PEBAX series is a block copolymer containing various range of rubbery to glassy ratio, the range of product can be used to clarify the effect of monomers or compositions. In this study, PEBAX 2533 which contains 80% of rubbery domain was used to investigate the mechanism of swelling and PEBAX 1657 which contains 60% of rubbery domain was used to compare.<br/>To minimize the impact of hydrostatic pressure, a rest test was conducted. A membrane was exposed to 4 bar of carbon dioxide less than an hour and kept in the membrane cell for 12 hours after ventilation or nitrogen purge. To see a directional effect, a reverse test was conducted after the rest test. A membrane after the rest test was flipped as the membrane facing the feed side during the rest test faced the permeate side. FT-IR, SEM, XRD, and XPS were used to analyze the change of membrane.<br/>In a rest test, the permeability of membranes under not purged or permeate side purged condition increased more than 50%. However, when feed or both side had been purged, less than 20% of permeability increase was observed. The swelling has a directional effect along the flux of operation. The exposure time of carbon dioxide at 4 bar affected the degree of swelling. The membranes whose permeability increased more than 100% lost the carbon dioxide/nitrogen selectivity. However, when the membrane was flipped and nitrogen was introduced at 4 bar, some of the permeability was recovered. When only carbon dioxide was introduced, the permeability was not recovered. Furthermore, without flipping, the permeability recovery was limited. According to the results, swelling was induced by the stress caused by the continuous flow of carbon dioxide and the swollen chains can be compressed by non-swelling penetrant. The membrane network underwent a reversible and irreversible swelling and the ratio was dependent on the degree of swelling.

Fundamental study of swelling mechanism by carbon dioxide in PEBAX 2533 membrane for gas separation

2019 Fall National Meeting

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