Diol-Crosslinked Electrospun Composite Anion Exchange Membranes

Abstract

A dual fiber electrospinning procedure was employed to prepare composite anion exchange membranes, where an interconnected network of hexanediol-crosslinked polysulfone polyelectrolyte fibers with quaternary ammonium or imidazolium fixed charge groups was embedded in a polyphenylsulfone matrix. Chloromethylated/iodomethylated polysulfone nanofibers containing 1,6-hexanediol were electrospun simultaneously with polyphenylsulfone fibers and the resultant mat was processed into a dense and defect-free anion exchange membrane by: (i) physical compaction, (ii) heating to create diol crosslinks, (iii) exposure to chloroform vapor which softened the polyphenylsulfone and allowed it to fill the voids between crosslinked fibers, and (iv) functionalization of the chloromethyl/iodomethyl fibers with trimethylamine, 1-methylimidazole, or 1,2-dimethylimidazole. The presence of the polyphenylsulfone matrix and polyelectrolyte crosslinks controlled water swelling and allowed for good mechanical properties when the ion exchange capacity and hydroxide ion conductivity were high. For example, a room temperature water-equilibrated membrane containing 35 wt% polyphenylsulfone and 65 wt% crosslinked polysulfone fibers with 1,2-dimethylimidazolium fixed charge sites and 8% crosslinking degree exhibited a room temperature OH− ion conductivity of 49 mS/cm, a gravimetric swelling of 96%, and a stress at break of 16 MPa. Composite membranes showed reasonably good chemical stability in 1.0 M KOH at 50°C, with a 20% loss in OH− conductivity after 3 days.