Synthesis and Characterization of Poly(arylene ether sulfone)s for Reverse Osmosis Water Purification and Gas Separation Membranes
Crosslinking is an effective technique for increasing the salt rejection of water purification membranes and the selectivity of gas separation membranes. An abundance of monomers, telechelic oligomers, and novel polymers were synthesized for use as separation membranes. These materials were often imbued with crosslinking functionalities to increase their performance during testing at the University of Texas-Austin.
Crosslinking of sulfonated poly(arylene ether sulfone) oligomers was studied systematically with regard to end-group functionality, polymer composition, and polymer hydrophilicity. Sulfonated bisphenol A based poly(arylene ether sulfone) random copolymers were synthesized with reactive amine endgroups and further functionalized with a tetra-epoxy resin, acryloyl chloride, phenylethynyl phthalic anhydride, and maleic anhydride. The reaction between amine terminated oligomers and a tetra-epoxy produced large, ductile membranes with gel fractions approaching 99%, the highest reported for crosslinked sulfonated polysulfone oligomers. This crosslinking reaction was studied by synthesizing two series of oligomers, one based on a bisphenol A monomer and the other based on a 4,4’-biphenol monomer. Both series were synthesized with 40, 50 and 60% degrees of sulfonation, so that hydrophilicity and composition could be studied with regard to water purification properties. All six oligomers were produced with a gel fraction exceeding 90%, and the membranes were evaluated at the University of Texas-Austin. The crosslinked oligomers demonstrated relatively constant salt rejection across a range of hydrophilicity values, which proved that crosslinking restricted the large amount of swelling that non-crosslinked sulfonated polysulfones undergo. The crosslinked oligomers had the best water purification properties reported for sulfonated polysulfone, with similar water permeabilities and an order of magnitude higher selectivity (Pw/Ps = 1.69 ± 0.13 x 103) than analogous linear copolymers (Pw/Ps = 3.67 ± 0.53 x 102). An additional series of linear sulfonated copolymers were also synthesized based upon a hydroquinone bisphenol, which also had superior water purification properties (1.06 ± 0.06 L μm m-2 h-1 bar-1, Pw/Ps = 2.44 ± 0.15 x 103) compared to previously synthesized linear copolymers.
Poly(arylene ether)s were also investigated for use as gas separation membranes. A poly(arylene ether ketone) and poly(arylene ether sulfone) were both synthesized with moieties capable of oxidation and/or photocrosslinking through benzylic hydrogen abstraction by an excited ketone. The polymers produced tough, ductile films. Gas transport properties of the linear polymers and crosslinked polymer were compared. The O2 permeability of one exemplary non-crosslinked poly(arylene ether) was 2.8 Barrer, with an O2/N2 selectivity of 5.4. Following UV crosslinking, the O2 permeability decreased to 1.8 Barrer, and the O2/N2 selectivity increased to 6.2.