Tunable nanoporous membranes with chemically-tailored pore walls from triblock polymer templates

Membranes derived from self-assembled block polymers have shown promise as highly selective and highly permeable filters, but the complex synthetic routes and limited pore functionalities of existing systems need to be improved if these materials are to serve as a platform for the next generation of nanostructured membranes. Here, the facile synthesis of a polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (PI-PS-PDMA) triblock polymer using a controlled reversible addition-fragmentation chain transfer (RAFT) polymerization mechanism is reported. This material is then processed into a membrane using a self-assembly and non-solvent induced phase separation (SNIPS) technique, which creates an asymmetric, porous structure consisting of a selective layer that contains a high density of PDMA-lined pores (9.4×10¹³poresm^-2) with an average diameter of 8.1nm, as determined using solute rejection tests. Solvent flow experiments demonstrate that the PI-PS-PDMA membrane has a pH-independent permeability of 6Lm^-2h^-1bar^-1. The PDMA moiety lining the pore walls is converted, through simple hydrolysis in the solid state, to yield a poly(acrylic acid)-lined (PAA-lined) structure. The permeability of the PI-PS-PAA membrane is pH-dependent, and ranges from 0.6Lm^-2h^-1bar^-1 for solutions with a pH greater than 4 to 16Lm^-2h^-1bar^-1 for a solution at pH 1. Solute rejection tests demonstrated a pore size of 3.4nm for the PI-PS-PAA membrane, which is the smallest pore size reported to date for membranes fabricated from self-assembled block polymers. The facile synthesis of the PI-PS-PDMA material, the scalable SNIPS membrane fabrication protocol, and the simple conversion chemistry of the pore functionality demonstrate that these nanostructured membranes are a strong platform for applications within the range of water purification, pharmaceutical separations, sensors, and drug delivery.