Self-Cleaning Membranes from Comb-Shaped Copolymers with Photoresponsive Side Groups

Photo-responsive anti-fouling polyzwitterionic brushes: a mesoscopic simulation

The antifouling effects of a toothbrush-shaped photo-responsive polyzwitterionic membrane were studied via dissipative particle dynamics simulations in this work. The results reveal that the membrane modified by spiropyran methacrylate brushes displays photo-switchable and antifouling capability due to the photo-induced ring-opening reaction. Namely, surface morphology and hydrophilicity change in response to visible or UV light irradiation, which can be observed visually by protein adsorption and desorption. Further study indicates that: (1) brush-modification density can influence the structure and properties of the membrane. With low modification density, systems cannot establish an intact selective layer, which hinders the antifouling ability; as the modification density increases, the intact selective layer can be formed, which is conducive to the expression of photo-responsiveness and antifouling capability. (2) Factors of toothbrush-hair length and grafting ratio can influence the establishment of a light-responsive surface: as the grafting ratio and toothbrush-hair length increase, the light-responsive surface is gradually formed, meanwhile, the antifouling ability can be continuously reinforced under UV light irradiation. (3) As the brushes switch into a zwitterionic merocyanine state under UV exposure, the selective layer swelling becomes stronger than that with a hydrophobic spiropyran state under visible exposure. This is owing to the enhanced interaction between zwitterionic brushes and water, which is the root of the antifouling effect. The present work is expected to provide some guidelines for the design and development of novel antifouling membrane surfaces.

Self-indicating polymers: a pathway to intelligent materials

Self-indicating polymers have emerged as a promising class of smart materials that possess the unique ability to undergo detectable variations in their physical or chemical properties in response to various stimuli. This article presents an overview of the most important mechanisms through which these materials exhibit self-indication, including aggregation, phase transition, covalent and non-covalent bond cleavage, isomerization, charge transfer, and energy transfer. Aggregation is a prevalent mechanism observed in self-indicating polymers, where changes in the degree of molecular organization result in variations in optical or electrical properties. Phase transition-induced self-indication relies on the transformation between different phases, such as liquid-to-solid or crystalline-to-amorphous transitions, leading to observable changes in color or conductivity. Covalent bond cleavage-based self-indicating polymers undergo controlled degradation or fragmentation upon exposure to specific triggers, resulting in noticeable variations in their structural or mechanical properties. Isomerization is another crucial mechanism exploited in self-indicating polymers, where the reversible transformation between the different isomeric forms induces detectable changes in fluorescence or absorption spectra. Charge transfer-based self-indicating polymers rely on the modulation of electron or hole transfer within the polymer backbone, manifesting as changes in electrical conductivity or redox properties. Energy transfer is an essential mechanism utilized by certain self-indicating polymers, where energy transfer between chromophores or fluorophores leads to variations in the emission characteristics. Furthermore, this review article highlights the diverse range of applications for self-indicating polymers. These materials find particular use in sensing and monitoring applications, where their responsive nature enables them to act as sensors for specific analytes, environmental parameters, or mechanical stress. Self-indicating polymers have also been used in the development of smart materials, including stimuli-responsive coatings, drug delivery systems, food sensors, wearable devices, and molecular switches. The unique combination of tunable properties and responsiveness makes self-indicating polymers highly promising for future advancements in the fields of biotechnology, materials science, and electronics.

Biomimetic redox-responsive smart coatings with resistance-release functions for reverse osmosis membranes

Membrane fouling induces catastrophic loss of separation performance and seriously restricts the applications of reverse osmosis (RO) membranes. Inspired by the mussel structure, polydopamine (PDA) and cystamine molecules (CA) with excellent anti-fouling properties were used to prepare accessible, biocompatible, and redox-responsive coatings for RO membranes. The PDA/CA-coated RO membranes exhibit a superior water flux of 65 L m-2 h-1 with a favourable NaCl rejection exceeding 99%. The water permeability through the PDA/CA-coated membrane is much higher than that of most membranes with similar rejection rates. Due to the formed protective hydration layers by PDA/CA coatings, anti-fouling properties against proteins, polysaccharides and surfactants were evaluated separately, and ultralow fouling properties were demonstrated. Moreover, the disulfide linkages in CA molecules can cleave in a reducing environment, yielding the degradation of PDA/CA coatings, thereby removing the foulants deposited on the coatings. The degradation endows the coated membranes with satisfying longtime anti-fouling properties, where the flux recovery reaches up to 90%. The construction of redox-responsive smart coatings not only provided a promising route to alleviate membrane fouling but can also be upscaled for use in numerous practical applications like sensors, medical devices, and drug delivery.

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.

Synthesis and Surface Properties of Photoresponsive Gemini Surfactants: Implication for Remediating PAHs-Contaminated Groundwater

The efficient utility of surfactants remains a daunting task for groundwater remediation. In this study, we have synthesized a conventional photoresponsive surfactant 4-[4-[(4-butylphenyl)azo]phenoxy]butyldimethylethylammonium bromide (AzoPB) and a gemini photoresponsive surfactant N¹,N²-bis[4-[4-[(4-butylphenyl)azo]phenoxy]butyl]-N¹,N²-tetramethylethane-1,2-diammonium bromide (AzoPBT) for solubilizing PAHs in groundwater. The two surfactants' photosensitivity, surface properties, and solubilization/release ability for phenanthrene (Phe) and acenaphthylene (Ace) were studied in detail. Under UV-light irradiation for 15-20 s, the two surfactants can be converted from trans to cis, while cis-to-trans isomerization can be achieved under visible-light irradiation for 1 min. Compared to AzoPB, AzoPBT exhibited strong surface properties such as lower critical micelle concentration (0.52 mM), surface tension (γ, 28.94 mN·m^-1), minimum area (A_min, 1.72 × 10^-8 nm²), and higher maximum adsorption (Γ_max, 96.55 mol·m^-2). The solubility of Phe and Ace in the AzoPBT aqueous solution (12.84 and 14.27 mg/L) was much higher than that in the AzoPB aqueous solution (7.51 and 8.77 mg/L) and gradually increased as the surfactant concentration increased in both aqueous solutions. Compared to AzoPB, gemini surfactant AzoPBT exhibited stronger solubilization ability. After four cycles of cis-trans isomerization conversion, AzoPBT could still reduce the hydrophobicity of Phe in natural groundwater, although the solubility of Phe decreased slightly. Additionally, the release capacity of AzoPBT was significantly higher than that of AzoPB during the cyclic solubilization-release process. The results indicated that gemini photoresponsive surfactants should be preferable to conventional photoresponsive surfactants for groundwater remediation due to their higher solubilization and release efficiency for Phe in the cyclic solubilization and release process, which can improve repair efficiency, minimize secondary pollution, and reduce remediation costs.

Adaptive 2D and Pseudo-2D Systems: Molecular, Polymeric, and Colloidal Building Blocks for Tailored Complexity

Two-dimensional and pseudo-2D systems come in various forms. Membranes separating protocells from the environment were necessary for life to occur. Later, compartmentalization allowed for the development of more complex cellular structures. Nowadays, 2D materials (e.g., graphene, molybdenum disulfide) are revolutionizing the smart materials industry. Surface engineering allows for novel functionalities, as only a limited number of bulk materials have the desired surface properties. This is realized via physical treatment (e.g., plasma treatment, rubbing), chemical modifications, thin film deposition (using both chemical and physical methods), doping and formulation of composites, or coating. However, artificial systems are usually static. Nature creates dynamic and responsive structures, which facilitates the formation of complex systems. The challenge of nanotechnology, physical chemistry, and materials science is to develop artificial adaptive systems. Dynamic 2D and pseudo-2D designs are needed for future developments of life-like materials and networked chemical systems in which the sequences of the stimuli would control the consecutive stages of the given process. This is crucial to achieving versatility, improved performance, energy efficiency, and sustainability. Here, we review the advancements in studies on adaptive, responsive, dynamic, and out-of-equilibrium 2D and pseudo-2D systems composed of molecules, polymers, and nano/microparticles.

MEUF for removal and recovery of valuable organic components present in effluents: A process intensified technology

In recent years, the domain of the research space in novel separation process has been led by membrane systems as a panacea providing multifarious benefits of high separation efficiency, elimination of extreme process conditions, sustainability, and environment friendliness coupled with high operational flexibility. In this niche area, often, ultrafiltration is touted as a robust separation technique due to its high separation efficiency, membrane stability, and lower operating costs. The only drawback of relatively large pore size can be overcome by combining surfactant addition, leading to development of integrated processes termed as Micellar Enhanced Ultrafiltration. MEUF processes isolate and selectively separate valuable organics present in effluent streams. The process characteristics fit the bill as a typified example for process intensification Technology interventions for recycling of surfactants can enhance the cost-competitiveness of the process. This has the potential to develop into a broad-spectrum effluent treatment option with a change of surfactants for target contaminants. Here, in this review, we attempt to critically examine the unique features of this technology, development of spin-offs with wide-ranging applications. Specifically applications in removal of hazardous, and persistent components like dissolved organics have been critically studied. The focus was to highlight the crux of the novel technologies highlighting the efficacy and the underlying concept of process intensification. PRACTITIONER POINTS: Role of MEUF as a sustainable process intensifying separation technique for removal and recovery of organics. Novel process development using MEUF. Comparative performance analysis to assess efficacy. Discussions on future integrative process development. Sustainability aspect of MEUF with possibility of byproduct recovery.

Preparation and properties of a polyurethane film based on novel photochromic spirooxazine chain extension

A smart polyurethane material with good photoresponsiveness and reversible surface wettability based on a photoresponsive chain extender (SO–OH).

Photoswitchable surface wettability of ultrahydrophobic nanofibrous coatings composed of spiropyran-acrylic copolymers

HYPOTHESIS

Light-controlling of surface characteristics in polymeric coatings has been a significant research area because of its potential application in development of smart surfaces. Wettability of light-responsive polymeric coatings based on spiropyran photochromic compound could be tuned by light irradiation. This is mainly because of spiropyran isomerization between the hydrophobic and hydrophilic states.

EXPERIMENTS

Light-responsive latex nanoparticles containing spiropyran moieties were synthesized by semi-continuous emulsion copolymerization of acrylate monomers, which have different chain flexibility depending on the copolymer composition. Photochromic properties of spiropyran in stimuli-responsive latex nanoparticles displayed dependence of photochromism intensity and its kinetics to flexibility of the polymer chains in addition to the polarity of media. Photoswitchable surface wettability of the spiropyran-containing acrylic copolymer coatings was investigated, where the photo-responsive coatings were prepared by solution casting and electrospinning methods.

FINDINGS

The photoswitchable coatings prepared by solution casting and electrospinning methods showed significant differences in their physical characteristics and especially surface wettability. The polymeric coatings displayed water droplet contact angles in the range of 60-93°, which could reversibly be switched to 55-86° upon UV light (365 nm) illumination as a result of isomerization of the hydrophobic spiro form to the zwitterionic merocyanine form. The nanofibrous coatings prepared by electrospinning method displayed higher contact angles in the range of 120-136°, which was switched to 78-105° upon UV light irradiation. The developed photo-responsive coatings displayed highly-efficient photoswitching between the two hydrophobic and hydrophilic states as a response to UV and visible light irradiation. The photoswitchable nanofibrous coatings displayed ultrahydrophobic characteristics, where the colored water droplets were stable on their surface and could easily be adsorbed by a cellulosic tissue. In summary, the photoswitchable nanofibrous coatings could be applied for design and development of ultrahydrophobic materials with the ability of photo-controlling of surface wettability by light irradiation with tunable intensity.

Preparation and Insights of Smart Foams with Phototunable Foamability Based on Azobenzene-Containing Surfactants

Smart foams with tunable foamability exhibit superb applications in many fields such as colloidal and interface science. Herein, we have synthesized an azobenzene-containing surfactant with excellent photoresponsiveness by a simple thiol-maleimide click reaction between thioglycolic acid and 4-(N-maleimide) azobenzene (MAB). The structure and the photoresponsive behavior of the novel surfactant are characterized. Depending on the solution concentration, the synthesized surfactant demonstrated various speeds for the trans/cis photoisomerization varying from 9 to 24 s for the given concentration range and excellent reversible photoisomerization cycling stability (more than 20 cycles) upon light irradiation. Based on these conformational switches, a series of phototriggered obvious surface properties (e.g., critical micelle concentration (CMC), surface tension (γ), and surface excess concentration (Γ)) changes of the surfactant are achieved. More specifically, the smart foam system with tunable foamability is realized. As-formed smart foams with rapid photocontrolled reversible foaming/defoaming transition and excellent cycling stability make them very attractive candidates for applications in wastewater treatment, green textile, oil extraction, and emulsification.

Membrane Surface Engineering with Bifunctional Zwitterions for Efficient Oil-Water Separation

Chemical modification provides a solution to the membrane fouling problem in oily water purification. However, complicated synthesis processes and harsh reaction conditions are frequently encountered with this approach. Here we developed two bifunctional zwitterionic materials, i.e., n-aminoethyl piperazine propanesulfonate (P-SO₃-NH₂) and 1,4-bis (3-aminopropyl) piperazine propanesulfonate (P-2SO₃-2NH₂), by a clean method and grafted them onto membrane surface via a fast single-step reaction. These materials endow the resultant membrane a more hydrophilic and smoother surface, significantly improving the water permeability, fouling resistance and recyclability of membrane in forward osmosis oily water reclamation. The water fluxes produced by the P-2SO₃-2NH₂ modified membrane are 47% (from 20.0 to 29.3 LMH) and 60% (from 16.0 to 25.6 LMH) higher than those of the unmodified membrane when DI water and an oily emulsion (1500 ppm) as the respective feeds. A higher water flux recovery is also achieved for the P-2SO₃-2NH₂ modified membrane (94%) than that of the nascent membrane (82%) after a 12-h experiment. These promising findings coupled with a facile and efficient membrane modification approach provide inspiration for both membrane exploration and oily water treatment.

Electric Assisted Salt-Responsive Bacterial Killing and Release of Polyzwitterionic Brushes in Low-Concentration Salt Solution

Polyzwitterionic brushes with strong antipolyelectrolyte effects have shown great potential as versatile platforms for the development of switchable friction/lubrication and bacterial absorption/desorption surfaces. However, the surface property switches of these brushes are usually triggered by high salt concentrations (>0.53 M), thereby greatly limiting their applications in biological fields where the salt concentration for mammals is ?0.15 M. To solve this problem, an electric field was used to assist the salt-responsive process of the polyzwitterionic brushes to achieve bacterial release at low concentrations of the salt solution. Briefly, poly(3-(dimethyl (4-vinylbenzyl) ammonium) propyl sulfonate) (polyDVBAPS) brushes grafted on ITO surfaces were prepared by surface initiated atom transfer radical polymerization. The bacterial release of this surface was conducted under an electric field, where anions were migrated and enriched around the brush-grafted ITO surface as anode. The local high concentration ion led to the conformation change of the brush and release of the attached bacteria. The effect of salt type, salt concentration, electric field strength, and conducting time on the bacterial release properties were investigated. The results indicated that under an electrical field of 3 V/mm, polyDVBAPS showed release capacities of ?93% for E. coli and ?81% for S. aureus in 0.12 M NaCl electrolyte solution. Furthermore, by the introduction of a bactericidal agent, i.e., Triclosan (TCS), an antibacterial surface with dual functions of killing and release was fabricated. This surface could kill ?90% and release 95% of attached E. coli in a 0.12 M NaCl solution by the application of a 3 V/mm electric field. This work demonstrated the feasibility of triggering a salt-responsive behavior of polyzwitterionic at low salt concentration by assistance of electric field, which would greatly extend the applications of polyzwitterionic, in particular in biological applications.

Co-Deposition of Stimuli-Responsive Microgels with Foulants During Ultrafiltration as a Fouling Removal Strategy

In this study, we show that codeposition of temperature responsive microgels in the foulant cake layer and cleaning of the cake upon stimuli-induced size change of the microgels is an effective method of fouling removal. Humic acid in CaCl₂ solution was used as a model foulant and poly( n-isopropylacrylamide) (p(NIPAm)) and poly( n-isopropylacrylamide- co-sulfobetainemethacrylate) (p(NIPAm- co-SBMA)) were used as temperature responsive microgels. Filtrations were done below the lower critical solution temperature (LCST) and temperature was increased to above the LCST for cleaning. As an extra cleaning a temperature swing of above, below and then again above the LCST was applied. P(NIPAm) was found to be ineffective in cleaning the foulant deposit despite the 20-fold change in its volume with temperature change at LCST. P(NIPAm- co-SBMA) microgels, on the other hand, provided high fouling reversibility on hydrophilic poly(ether sulfone)(PES)/poly(vinylpyrrolidone) (PVP) and hydrophobic PES membranes. Better fouling reversibility with these microgels was observed at low and high solution ionic strength. While the use of microgels alone increased fouling reversibility to some extent, even in the absence of temperature stimulus, 100% reversibility could only be obtained when a temperature switch was applied in the presence of microgels, showing the effect of microgels' volume change in cleaning.

Reversible Photo- and Thermoresponsive, Self-Assembling Azobenzene Containing Zwitterionic Polymers

Commercially available azo dyes bearing amino groups were grafted to zwitterionic copolymers composed of cyclic anhydride functionality. The zwitterionic copolymers were prepared for the first time by polymerizing sulfobetaine (SB) monomer with maleic anhydride (MA) under conventional free radical polymerization as well as reversible addition-fragmentation chain transfer (RAFT) polymerization. Poly(SB- co-MA) self-assembled in deionized water. Azobenzene grafted zwitterionic poly((SB- co-MA)- g-Azo) exhibited multiresponsive behavior. As confirmed by UV-vis spectroscopy, trans → cis isomerization of the azo group was responsible for the photo- and thermal responsive behavior. The photoisomerization was reversible, and no photoaging was detected during the repeated exposure to UV and visible light. The water-soluble nature of photoresponsive azo dye grafted copolymers makes it suitable for applications in biological systems.

Hydrophobic Antifouling Electrospun Mats from Zwitterionic Amphiphilic Copolymers

A porous material that is both hydrophobic and fouling-resistant is needed in many applications, such as water purification by membrane distillation. In this work, we take a novel approach to fabricating such membranes. Using the zwitterionic amphiphilic copolymer poly(trifluoroethyl methacrylate- random-sulfobetaine methacrylate), we electrospin nonwoven, porous membranes that combine high hydrophobicity with resistance to protein adsorption. By changing the electrospinning parameters and the solution composition, membranes can be prepared with a wide range of fiber morphologies including beaded, bead-free, wrinkly, and ribbonlike fibers, with diameters ranging between ∼150 nm and 1.5 μm. The addition of LiCl to the spinning solution not only helps control the fiber morphology but also increases the segregation of zwitterionic groups on the membrane surface. The resultant electrospun membranes are highly porous and very hydrophobic, yet resist the adsorption of proteins and retain a high contact angle (∼140°) even after exposure to a protein solution. This makes these materials promising candidates for the membrane distillation of contaminated wastewater streams and as self-cleaning materials.

Recent Advances in the Fabrication of Membranes Containing "Ion Pairs" for Nanofiltration Processes

In the face of serious environmental pollution and water scarcity problems, the membrane separation technique, especially high efficiency, low energy consumption, and environmental friendly nanofiltration, has been quickly developed. Separation membranes with high permeability, good selectivity, and strong antifouling properties are critical for water treatment and green chemical processing. In recent years, researchers have paid more and more attention to the development of high performance nanofiltration membranes containing “ion pairs”. In this review, the effects of “ion pairs” characteristics, such as the super-hydrophilicity, controllable charge character, and antifouling property, on nanofiltration performances are discussed. A systematic survey was carried out on the various approaches and multiple regulation factors in the fabrication of polyelectrolyte complex membranes, zwitterionic membranes, and charged mosaic membranes, respectively. The mass transport behavior and antifouling mechanism of the membranes with “ion pairs” are also discussed. Finally, we present a brief perspective on the future development of advanced nanofiltration membranes with “ion pairs”.

Solid-state photochromic behavior of pyrazolone 4-phenylthiosemicarbazones

Pyrazolone 4-phenylthiosemicarbazones show excellent solid-state photochromic properties, thermal stability and fatigue resistance upon the UV/Vis irradiation.