Isoporous membranes with gradient porosity by selective swelling of UV-crosslinked block copolymers

Selective swelling of amphiphilic block copolymers (BCPs) is an effective and nondestructive pore-making strategy. Here we coupled swelling-induced pore generation with UV crosslinking to fabricate BCP isoporous membranes with gradient porosity. Polystyrene- block-poly(2-vinylpyridine) (PS- b-P2VP) solutions were coated onto macroporous supporting membranes to achieve composite films, which were then annealed in solvent vapor for the perpendicular alignment of the P2VP phases near the surface of the coating BCP layer. After swelling of BCP in hot ethanol and drying, isopores of ~8 nm formed at the surface of BCP layer following the selective swelling-induced pore-formation mechanism. Then UV exposure and subsequent secondary swelling at stronger condition of the membranes were conducted to enlarge the inner pores while maintaining the surface structures. With balanced UV crosslinking and secondary swelling, the finally obtained membranes showed ordered perpendicular pores at the outmost layer and gradient porosity with enlarged interconnected pores inside the BCP layer. Due to the gradient structures, the membranes exhibited much higher flux while the surface structures and retention remained essentially unchanged. Moreover, compared to the membranes without UV treatment, the membranes showed better performances in discriminating polyethylene glycol molecules with different molecular weights and still kept a sensitive pH-responsive property.