Poly (styrene - co - maleic anhydride) and Polystyrene Grafted with Poly(ether amines): Synthesis, Characterization and Gas Separation Performance
The present work highlights the synthesis, characterization and membrane properties of new graft copolymers obtained either by direct amidation of poly(styrene-co-maleic anhydride) or by sulphonation of polystyrene and subsequent amidation with different poly(ether amide)s “jeffamines®” as grafts. These materials were tested as membranes for CO2 and CH4 separation. Firts, direct amidation of poly(styrene-co-maleic anhydride. The characterization results obtained by 1H-NMR, 13C-NMR, GPC, DSC, and TGA demonstrated the successful synthesis of graft copolymers with PEG contents from 5 to 26 wt.% (series-ML,-MM and -MH) using poly(styrene-co-maleic anhydride) and Jeffamine. The graft copolymers' thermal analyses indicated the final products to be amorphous materials with glass transition temperatures decreasing for an increasing content of PEG bonded to the matrix poly(styrene-co-maleic anhydride). A second group of graft copolymers (series GSMA and GPSS) were synthesized by sulfonation of poly(styrene) and poly(styrene-co-maleic anhydride), respectively, and subsequent amidation with Jeffamine. The successful synthesis was proven by elemental analysis (EA), 1H-NMR, GPC, FT-IR, and thermal analyses (DSC and TGA). Polymers of PEG contents up to 35 wt.% with a degree of sulfonation up to 24 wt.% were obtained. An increase of the PEG content in the graft-copolymer lead to a decrease of the glass transition temperature in comparison to the sulphonated polystyrene. Three series of composites membranes with different PEG contents were prepared by casting and their CO2, N2, and CH4 permeabilities as well as their CO2/N2 and CO2/CH4 selectivities were determined. Generally, the overall CO2/N2 and CO2/CH4 selectivity increased with higher poly(ethylene glycol) contents. The membranes made from SMA and Jeffamine (M-2070) (series-MH) with PEG contents of 20 – 26 wt.% exhibited a CO2 permeability of 120 - 280 Barrer and a CO2/N2 selectivity of 34 – 42 at 35°C and 1 atm of feed pressure, which can be regarded as competitive to membranes reported in literature and to the theoretical upper bound limit defined by Freeman´s theory. The high seIectivity of the CO2/N2 pair was explained by the high solubility of CO2 in the PEG. Due to the incorporation of PEG the glass transition temperature decreased, the CO2 permeability increased stronger than the N2 permability, and in consequence the CO2/N2 selectivity was enhanced. In case of the CO2/CH4 pair the selectivity was further influenced by a decrease of the CH4 permeability. Hence, it may be presumed that the transport of CH4 through the membrane is controlled rather by diffusion rather than by solution. The high CO2 permeability can be explained by two concurrent contributions, the solubility and the diffusivity. The solubility of CO2 is higher than that of N2 due to the significant differences in the condensability of both gases. . At the same time, the graft copolymers consisting of flexible and polar poly(ethylene glycol) (PEG) segments are very attractive for the polar CO2 molecules.
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