Self-Assembled Block Copolymer Thin Films as Water Filtration Membranes

Nanoporous membranes containing monodisperse pores of 24 nm diameter are fabricated using poly (styrene-b-lactide) block copolymers to template the pore structure. A 4 mu m thin film of the block copolymer is cast onto a microporous membrane that provides mechanical reinforcement; by casting the copolymer film from the appropriate solvents and controlling the solvent evaporation rate, greater than 100 cm(2) of a thin film with polylactide cylinders oriented perpendicular to the thin dimension is produced. Exposing the composite membrane to a dilute aqueous base selectively etches the polylactide block, producing the porous structure. The ability of these pores to reject dissolved poly(ethylene oxide) molecules of varying molecular weight matches existing theories for transport through small pores.