Development of a High-Flux Thin-Film Composite Nanofiltration Membrane with Sub-Nanometer Selectivity Using a pH and Temperature-Responsive Pentablock Co-Polymer

Producing block co-polymer-based nanofiltration (NF) membranes with sharp molecular weight cutoffs via an efficient method exhibiting persistent size-based separation quality is challenging. In this study, this challenge was addressed by reporting a facile approach to fabricate pentablock co-polymer (PBC)-based thin-film composite (TFC) NF membranes. The PBC, consisting of temperature-responsive Pluronic F127 (PEO-b-PPO-b-PEO) middle blocks and pH-responsive poly(N,N-(diethylamino)ethyl methacrylate) end blocks, were synthesized by atom-transfer radical polymerization. This polymer was then attached electrostatically to the surface of polysulfone/sulfonated polyether-sulfone support membranes fabricated using a non-solvent-induced phase separation technique. The conformational changes of the PBC chains in response to pH and temperature determined the pure water flux and neutral solute (PEG 1000) rejection performance of TFC membranes. Permeability of the membranes increased from 13.0 ± 0.63 to 15.9 ± 0.06 L/m2·h·bar and from 6.7 ± 0.00 to 13.9 ± 0.07 L/m2·h·bar by changing the solution pH from 4 to 8.5 and temperature from 4 to 25 °C, respectively. The pH- and temperature-responsive conformational changes did not affect the PEG 1000 rejection and membrane pore radius, which remained constant at ∼89% and ∼0.9 nm, respectively. This important finding was attributed to the high grafting density of co-polymer chains, resulting in spatial limitations among the grafted chains. The pore size of ∼0.9 nm achieved with the proposed membrane design is the smallest size reported so far for membranes fabricated from block co-polymers. TFC membranes demonstrated high stability and maintained their flux and rejection values under both static (storage in an acidic solution for up to 1 month) and dynamic (filtering PEG 1000 solution over 1 week) conditions. Pentablock co-polymers enable a NF membrane with a sharp molecular weight cutoff suitable for size-selective separations. The membrane fabrication technique proposed in this study is a scalable and promising alternative that does not involve complex synthetic routes.