Effects of hydrophilicity of blended submicrogels on the microstructure and performance of thermo-responsive membranes

Thermo-Sensitive Microgel/Poly(ether sulfone) Composited Ultrafiltration Membranes

Thermo-sensitive microgels known as PMO-MGs were synthesized via surfactant free emulsion polymerization, with poly(ethylene glycol) methacrylate (OEGMA₄₇₅) and 2-(2-methoxyethoxy) ethyl methacrylate (MEO₂MA) used as the monomers and N, N-methylene-bis-acrylamide used as the crosslinker. PMO-MGs are spherical in shape and have an average diameter of 323 ± 12 nm, as determined via transmission electron microscopy. PMO-MGs/poly (ether sulfone) (PES) composited ultrafiltration membranes were then successfully prepared via the non-solvent-induced phase separation (NIPS) method using a PMO-MG and PES mixed solution as the casting solution. The obtained membranes were systematically characterized via combined X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, Fourier transform infrared spectroscopy and contact angle goniometer techniques. It was found that the presence of PMO-MGs significantly improved the surface hydrophilicity and antifouling performance of the obtained membranes and the PMO-MGs mainly located on the channel surface of the membranes. At 20 °C, the pure water flux increased from 217.6 L·m^-2·h^-1 for pure PES membrane (M00) to 369.7 L·m^-2·h^-1 for PMO-MGs/PES composited membrane (M20) fabricated using the casting solution with 20-weight by percentage microgels. The incorporation of PMO-MGs also gave the composited membranes a thermo-sensitive character. When the temperature increased from 20 to 45 °C, the pure water flux of M20 membrane was enhanced from 369.7 to 618.7 L·m^-2·h^-1.

Advanced stimuli-responsive membranes for smart separation

Membranes have been extensively studied and applied in various fields owing to their high energy efficiency and small environmental impact. Further conferring membranes with stimuli responsiveness can allow them to dynamically tune their pore structure and/or surface properties for efficient separation performance. This review summarizes and discusses important developments and achievements in stimuli-responsive membranes. The most commonly utilized stimuli, including light, pH, temperature, ions, and electric and magnetic fields, are discussed in detail. Special attention is given to stimuli-responsive control of membrane pore structure (pore size and porosity/connectivity) and surface properties (wettability, surface topology, and surface charge), from the perspective of determining the appropriate membrane properties and microstructures. This review also focuses on strategies to prepare stimuli-responsive membranes, including blending, casting, polymerization, self-assembly, and electrospinning. Smart applications for separations are also reviewed as well as a discussion of remaining challenges and future prospects in this exciting field. This review offers critical insights for the membrane and broader materials science communities regarding the on-demand and dynamic control of membrane structures and properties. We hope that this review will inspire the design of novel stimuli-responsive membranes to promote sustainable development and make progress toward commercialization.

Designable Micro-/Nano-Structured Smart Polymeric Materials

Smart polymeric materials with dynamically tunable physico-chemical characteristics in response to changes of environmental stimuli, have received considerable attention in myriad fields. The diverse combination of their micro-/nano-structural and molecular designs creates promising and exciting opportunities for exploiting advanced smart polymeric materials. Engineering micro-/nano-structures into smart polymeric materials with elaborate molecular design enables intricate coordination between their structures and molecular-level response to cooperatively realize smart functions for practical applications. In this review, recent progresses of smart polymeric materials that combine micro-/nano-structures and molecular design to achieve designed advanced functions are highlighted. Smart hydrogels, gating membranes, gratings, milli-particles, micro-particles and microvalves are employed as typical examples to introduce their design and fabrication strategies. Meanwhile, the key roles of interplay between their micro-/nano-structures and responsive properties to realize the desired functions for their applications are emphasized. Finally, perspectives on the current challenges and opportunities of micro-/nano-structured smart polymeric materials for their future development are presented.

Smart Self-Cleaning Membrane via the Blending of an Upper Critical Solution Temperature Diblock Copolymer with PVDF

Poly(2,2,2-trifluoroethyl methacrylate)-b-poly(imidazoled glycidyl methacrylate-co-diethylene glycol methyl ether methacrylate) (PTFEMA-b-P(iGMA-co-MEO₂MA)) containing an upper critical solution temperature (UCST) polymer chain was prepared and blended with poly(vinylidene fluoride) (PVDF) to produce a thermoresponsive membrane with smart self-cleaning performance. The successful preparation of the membrane was demonstrated by attenuated total reflection-Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy characterization. The membrane shows UCST performance, and its flux changes with the filtrate temperature as the UCST polymer chain stretches out and contracts in response to various temperatures. In addition, the UCST polymer chain can disrupt the foulant and push it away from the membrane when the temperature is above the UCST and thus enables membranes to exhibit a smart self-cleaning behavior. To the best of our knowledge, this work is the first report of a smart self-cleaning membrane based on the blending of a diblock copolymer containing a UCST polymer chain with PVDF.

Stimuli-Responsive Zwitterionic Core-Shell Microgels for Antifouling Surface Coatings

Fouling on filtration membranes is induced by the nonspecific interactions between the membrane surface and the foulants, and effectively hinders their efficient use in various applications. Here, we established a facile method for the coating of membrane surface with a dual stimuli-responsive antifouling microgel system enriched with a high polyzwitterion content. Different poly(sulfobetaine) (PSB) zwitterionic polymers with defined molecular weights and narrow dispersities were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization and integrated onto poly(N-vinylcaprolactam) (PVCL) microgels via a controlled dosage of a cross-linker, adapting a precipitation polymerization technique to obtain a core-shell microstructure. Increasing the PSB macro-RAFT concentration resulted in a shift of both upper critical solution temperature and lower critical solution temperature toward higher temperatures. Cryogenic transmission electron microscopy at different temperatures suggested the formation of a core-shell morphology with a PVCL-rich core and a PSB-rich shell. On the other hand, the significant variations of different characteristic proton signals and reversible phase transitions of the microgel constituents were confirmed by temperature-dependent ¹H NMR studies. Utilizing a quartz crystal microbalance with dissipation monitoring, we have been able to observe and quantitatively describe the antipolyelectrolyte behavior of the zwitterionic microgels. The oscillation frequency of the sensor proved to change reversibly according to the variations of the NaCl concentration, showing, in fact, the effect of the interaction between the salt and the opposite charges present in the microgel deposited on the sensor. Poly(ethersulfone) membranes, chosen as the model surface, when functionalized with zwitterionic microgel coatings, displayed protein-repelling property, stimulated by different transition temperatures, and showed even better performances at increasing NaCl concentration. These kinds of stimuli-responsive zwitterionic microgel can act as temperature-triggered drug delivery systems and as potential coating materials to prevent bioadhesion and biofouling as well.