Novel polyamide-imide/cellulose acetate dual-layer hollow fiber membranes for nanofiltration

A novel dual-layer nanofiltration (NF) hollow fiber membrane was fabricated by the simultaneous co-extrusion of Torlon® polyamide-imide and cellulose acetate dopes through a triple-orifice spinneret in a dry-jet wet phase inversion process. For the first time, the nanopores of dual-layer hollow fiber membranes were molecularly designed by controlling the phase inversion process with the aid of various non-solvent additives into the polymer solutions. Compared to ethanol and 2-propanol, the addition of methanol into the dope led to a significantly decreased pore size but dramatically increased pure water permeability. The improved NF performance may be attributed to (1) a controllable thin selective outer layer; (2) a less resistant interface between the outer and inner layers; and (3) a fully porous substructure with reduced transport resistance. In addition to non-solvent additives, spinneret temperature also plays an important role in designing dual-layer hollow fiber membranes with desirable NF performance. When the spinneret temperature was increased from 25 °C to 50 °C, the mean effective pore radius and the pure water permeability were simultaneously decreased, which was due to the formation of a denser surface skin and a more compact interface between the two layers. In addition to exhibiting a higher rejection of divalent anions than monovalent anions, and a lower rejection of divalent cations at pH 7.0, the newly developed NF dual-layer hollow fiber membranes with methanol as additive has a relatively high pure water permeability of 11.93 l m−2 bar−1 h−1 with a mean effective pore radius of 0.63 nm. These concepts hold great potential for the design of tailor-made NF membranes for various industrial applications.