Dual stimuli-responsive polysulfone membranes with interconnected networks by a vapor-liquid induced phase separation strategy.

Dual pH- and thermo-responsive polysulfone (PSf) membranes with three-dimensionally interconnected networks are fabricated by introducing poly(acrylic acid-co-N-isopropylacrylamide) (P(AA-NIPAm)) into the membrane surfaces and pore walls during membrane formation via a vapor-liquid induced phase separation (V-LIPS) process. After introducing the copolymers of P(AA-NIPAm), the fabricated membranes exhibit impressive open network pores on the surfaces, meanwhile their cross-sectional structure turns from classical asymmetric finger-like structure into bi-continuous nanopores throughout the whole thickness of membrane, due to high solution viscosity and low mass transfer rate of VIPS process. Furthermore, pure water permeation tests show that the pure water permeation (Lp) and the molecular weight cutoff (MWCO) of the fabricated PSf/P(AA-NIPAm) membranes increases sharply as the solution pH decreases from 12.5 to 1.5 and the feed temperature increases from 25 to 50 °C, attributing to the increasing pore size. With the decreasing mass ratio of AA to NIPAm, the pH-responsive coefficient decreases, while the temperature- responsive coefficient increases. In particular, for the fabricated membrane with the mass ratio of AA to NIPAm of 3 to 2, Lp changes from ∼16.0 to ∼821.4 L m-2 h-1 bar-1 and MWCO increases from ∼223.1 to ∼1493.2 kDa, as the filtration experiments are operated with feed pH and temperature of 12.5/25 °C and 1.5/50 °C respectively. The results proposed in this study provide a novel mode for design and development dual responsive porous membranes in situ, which will enable good separation of various materials and expand the scope of membrane applications.

Microstructures and performances of pegylated polysulfone membranes from an in situ synthesized solution via vapor induced phase separation approach.

In situ pegylated (PEGylated) microporous membranes have been extensively reported using poly(ethylene glycol) (PEG)-based polymers as blending additives. Alternatively, free standing PEGylated polysulfone (PSf) membranes with excellent hydrophilicity and antifouling ability were directly fabricated from polysulfone/poly(ethylene glycol) methyl ether methacrylate (PSf/PEGMA) solutions after in situ cross-linking polymerization without any treatment via vapor induced phase separation (VIPS) process for the first time. The microstructures and performances of the resulting membranes shifted regularly by adjusting exposure time of the liquid film in humid air. With increasing exposure time, plenty of worm-like networks formed and distributed on membrane surfaces, meanwhile cross-sectional morphology changed from asymmetric finger-like microporous structure to symmetric cellular-like structure, resulting in better mechanical stability. As the exposure time raised from 0 to 5 min, the surface coverage of carboxyl groups increased from ∼1.1 to 4.0 mol%, leading to the decrease in water contact angle from ∼63 to 27° and the increase in water flux from ∼110 to 512 L m-2 h-1. In addition, at prolonged exposure time, increasing hydrophilic PEG chains migrated to membrane surfaced and repelled the adsorption and deposition of protein, resulting in better antifouling ability. The findings of this study offer a facile and high efficient strategy for flexible design and fabrication of the in situ PEGylated membranes with desirable structures and performances in large scale.