Partially pyrolized gas-separation membranes made from blends of copolyetherimides and polyimides
AbstractHerein, a new approach to achieve gas separation materials with improved resistance to plasticization (obtained by partial pyrolysis of new polyimide blends) is presented. Blends of an aromatic polyimide (formally, 6FDA-6FpDA) and aliphatic-aromatic copolyetherimides based on the same polyimide in combination with other aliphatic polyimides, having polyethylene oxide moieties, were obtained. After their synthesis and characterization, a selective elimination of the polyether by thermal treatment was carried and the obtained partially-pyrolyzed materials were tested as gas separation membranes.
For neat blends, permeability was shown to decrease when PEO content increases. This effect is due to polyethylene oxide (PEO) chains encroaching on free volume because phase segregation was minimized. When these blends were subjected to thermal treatment at relatively low temperatures in air or in N2, it was clearly observed that all the polyether moieties were selectively eliminated from the structure, which gave rise to an increase of permeability. Higher permeabilities were obtained after a thermal treatment at 390 °C in N2. Degradation of PEO chains after a 290 °C treatment in air produced materials with permeability values lower than those observed under N2. In both cases, even though PEO was selectively and wholly eliminated, permeability was not totally recovered to give the values observed for neat 6FDA-6FpDA due probably to a combination of shrinkage and crosslinking of the membrane produced during the thermal process. Crosslinking provides the materials with an increased resistance to plasticization. In particular, the best improvement against plasticization proceeded from the thermal treatments of blends at 290 °C in air.