Formation of ultrathin, defect-free membranes by grafting of poly(acrylic acid) onto layered polyelectrolyte films

Deposition of hyperbranched poly(acrylic acid) (PAA) films on porous supports is an attractive method for synthesizing derivatizable, ultrathin composite membrane skins. We previously synthesized these films by sputtering a thin gold layer onto a porous alumina support and then grafting several layers of PAA to a self-assembled monolayer of mercaptoundecanoic acid on the gold. This paper demonstrates grafting of PAA onto PAA/poly(allylamine hydrochloride) (PAH) films that were prepared by alternating polyelectrolyte deposition. This procedure overcomes the inconvenience associated with sputtering of gold and allows synthesis of defect-free membranes using only one grafted PAA layer. Membrane skins consisting of 2.5 bilayers of PAA/PAH plus 1 layer of grafted PAA (total thickness of similar to 14 nm) effectively cover the underlying pores of porous alumina supports without filling them, as shown by field-emission scanning electron microscopy (FESEM) images. When derivatized with H(2)NCH(2)(CF(2))(6)CF(3), these composite membranes show an ideal selectivity of 2.4 for O(2) over N(2), demonstrating the absence of defects in this system. The chemistry developed for the grafting process also allows modest derivatization of PAA/PAH films with H(2)NCH(2)(CF(2))(6)CF(3) to increase hydrophobicity.