CO2-responsive membranes prepared by selective swelling of block copolymers and their behaviors in protein ultrafiltration

Gas-Responsive Smart Membrane Separation

Membrane separation technology is one of the most important techniques in modern separation science. To understand the self-regulation mechanisms of cell membranes and mimic their working principles, a plethora of artificial membranes with responsive abilities to external stimuli have been engineered and prepared, whose smart sieving functions continue to attract attention and are applied in various fields. Among all the known stimuli, gas as a new trigger mode exhibits certain biocompatibility and offer irreplaceable advantages compared to other stimuli, such as cleanliness, ease-of-handling, and nondestructive, which make gas-responsive membranes as one of the most promising, smart separation materials. In this review, we summarize recent breakthroughs in the development of gas-responsive membranes, outline the novel strategies on membrane fabrication, and highlight their advanced applications in controlled cargo release, size-/charge-based substance separation, oil-water separation, and self-cleaning. We also outlook the perspectives on the potential research directions and opportunities in the future.

Optimizing the Synthesis of CO2-Responsive Polymers: A Kinetic Model Approach for Scaling Up

The kinetic model is a crucial tool for optimizing polymer synthesis protocols and facilitating the scaled-up production processes of the CO2-responsive polymer poly(N-[3-(dimethylamino)propyl]-acrylamide)-b-poly(methyl methacrylate)(PDMAPAm-b-PMMA), which is supposed to be implemented in direct air capture (DAC) technology. This study presents a simulation of the kinetic model developed for the Reversible Addition-Fragmentation Chain-Transfer (RAFT) polymerization of N-[3-(dimethylamino)propyl]-acrylamide (DMAPAm), alongside an investigation into the kinetics of this polymerization using the simulation as an analytical tool, as well as the application of the simulation for the upscaling of RAFT polymerization. Ultimately, the kinetic model was validated through two kinetic experiments, confirming its reliability. It was subsequently employed to optimize the synthesis recipe and to predict the properties of PDMAPAm homopolymers, thereby supporting the upscaling of PDMAPAm-b-PMMA diblock copolymer synthesis. In the end, the preliminary results of the CO2-responsiveness of the diblock copolymer were determined with a simple experiment.

Interfacial Constructing Poly(ionic liquids) on Nanoporous Block Copolymers for Antifouling Ultrafiltration

The remarkable flexibility in structural tunability and designability of poly(ionic liquids) (PILs) has garnered significant attention. Integration of PILs with membranes, novel properties, and functionalities is anticipated for applications in the fields of membrane separation. Here, we develop a facile method to prepare PIL-functionalized membranes in a one-step process by combining selective swelling-induced pore generation and ionic liquid functionalization. The block copolymer of poly(2-dimethylaminoethyl methacrylate)-block-polystyrene (PDMAEMA-b-PS, abbreviated as SDMA) films is immersed in a mixture of ethanol and bromopropane. In addition to the formation of nanoporous structures, an interfacial quaternization reaction between the PDMAEMA blocks and bromopropane occurs to generate poly(methacrylatoethyl propyl dimethylammonium bromide), resulting in the PIL-Br-functionalized membrane (SIL-Br) during the swelling process. It is noteworthy that bromopropane acting as a reactant also promotes the process of selective swelling. The water permeability of the resulting SIL-Br membrane is several times higher than that of the SDMA membrane, which is attributed to the increased pore size and significantly higher hydrophilicity of the SIL-Br membrane. In addition, the anion exchange of SIL-Br with l-proline (l-Pro) readily forms SIL-Pro-functionalized membranes (SIL-Pro), which exhibit exceptional electrical neutrality. Antifouling tests demonstrate that both SIL-Br and SIL-Pro have excellent resistance to proteins compared to the non-PIL-functionalization SDMA membrane, implying their great potential as antifouling membranes for water treatment.

Effects of Carbon Spacer Length on Conformational Transitions and Protein Adsorption of Polyzwitterions

The properties of polyzwitterions are closely linked to their carbon spacer length (CSL) between oppositely charged groups. A thorough understanding of the effect of CSL on the properties of polyzwitterion-functionalized membranes is important for their fouling resistance and separation performances. In this work, polyzwitterion-functionalized membranes with different CSLs are prepared by coupling selective swelling-induced pore generation with zwitterionization, and the investigation is focused on comprehending the molecular mechanisms underlying protein resistance and conformational transitions within polyzwitterions under varying CSLs. The zwitterionized films show an enhancement in the surface negative potential with the increase of CSL, attributed to the negatively charged groups distanced from the positively charged groups. Quartz crystal microbalance with dissipation (QCM-D) demonstrates that zwitterionized films with different CSLs display distinct levels of resistance to protein adsorption. The trimethylamine N-oxide-derived polymer (PTMAO, CSL = 0) zwitterionized film shows the highest resistance compared to the poly(3-[dimethyl(2'-methacryloyloxyethyl] ammonio) ethanesulfonate (PMAES, CSL = 2) zwitterionized film and the poly(sulfobetaine methacrylate) (PSBMA, CSL = 3) zwitterionized film, owing to its electrical neutrality and pronounced hydrophilicity. Moreover, analysis of the anti-polyelectrolyte behaviors reveals that PTMAO does not undergo a significant conformation transition in deionized water and salt solutions, while the conformations of PMAES and PSBMA display to be more salt-dependent as the CSL increases, attributed to their increased polarization and dipole moment. As a result, the permeability of zwitterionized membranes exhibits enhanced salt responsiveness with the increase in CSL. The findings of this study are expected to facilitate the design of adsorption-resistant surfaces desired in diverse fields.

Engineering Dual CO2- and Photothermal-Responsive Membranes for Switchable Double Emulsion Separation

Stimulus-responsive membranes demonstrate promising applications in switchable oil/water emulsion separations. However, they are unsuitable for the treatment of double emulsions like oil-in-water-in-oil (O/W/O) and water-in-oil-in-water (W/O/W) emulsions. For efficient separation of these complicated emulsions, fine control over the wettability, response time, and aperture structure of the membrane is required. Herein, dual-coated fibers consisting of primary photothermal-responsive and secondary CO2-responsive coatings are prepared by two steps. Automated weaving of these fibers produces membranes with photothermal- and CO2-responsive characteristics and narrow pore size distributions. These membranes exhibit fast switching wettability between superhydrophilicity (under CO2 stimulation) and high hydrophobicity (under near-infrared stimulation), achieving on-demand separation of various O/W/O and W/O/W emulsions with separation efficiencies exceeding 99.6%. Two-dimensional low-field nuclear magnetic resonance and correlated spectra technique are used to clarify the underlying mechanism of switchable double emulsion separation. The approach can effectively address the challenges associated with the use of stimulus-responsive membranes for double emulsion separation and facilitate the industrial application of these membranes.

Nanocellulose-based membranes with pH- and temperature-responsive pore size for selective separation

Smart gating membranes have drawn much attention due to the controllable pore structure. Herein, a smart gating membrane with dual responsiveness was prepared from bacteria cellulose (BC) grafted with pH- and temperature-responsive polymers. By external stimulation, the average pore size of the membrane can be controlled from 33.75 nm to 144.81 nm, and the pure water flux can be regulated from 342 to 2118 L·m-2·h-1 with remarkable variation in the pH range of 1-11 and temperature range of 20-60 °C. The adjustability of pore size is able to achieve the gradient selective separation of particles and polymers with different sizes. In addition, owing to the underwater superoleophobicity and the nanoscale pore structure, the membrane separation efficiencies of emulsified oils are higher than 99 %. Moreover, the controllable pore size endows the membrane with good self-cleaning performance. This nanocellulose-based smart gating membrane has potential applications in the fields of controllable permeation, selective separation, fluid transport, and drug/chemical controlled release systems.