CO2-responsive polymeric materials: synthesis, self-assembly, and functional applications

Ultrahigh Metal-Organic Framework Loading and Flexible Nanofibrous Membranes for Efficient CO2 Capture with Long-Term, Ultrastable Recyclability

In the global transition to a sustainable low-carbon economy, CO₂ capture and storage technology plays a key role in reducing emissions. Metal-organic frameworks (MOFs) are crystalline materials with ultrahigh porosity, tunable pore size, and rich functionalities, holding the promise for CO₂ capture. However, the intrinsic fragility and depressed processability of MOF crystals make the fabrication of the flexible MOF nanofibrous membrane (NFM) rather challenging. Herein, we demonstrate an effective strategy for the versatile preparation of self-supported and flexible HKUST-1 NFM with ultrahigh HKUST-1 loading (up to 82 wt %) and stable and uniform HKUST-1 growth through the combination of electrospinning, multistep seeded growth, and activation process. The loading rate of MOF is the highest level among the reported analogues. Significantly, the HKUST-1 NFM exhibits a prominent CO₂ adsorption capacity of 3.9 mmol g^-1, good CO₂/N₂ selectivity, and remarkable recyclability. The CO₂ capacity retains ∼95% (3.7 mmol g^-1) of the initial value after 100 adsorption-desorption cycles, indicating that the HKUST-1 NFM has long-term and ultrastable recyclability and a significant practical value. Thus, the low-cost and scalable production pathway is able to convert MOF particles into self-supported and flexible NFMs, and thereby, they are better applied to the efficient postcombustion CO₂ capture.

Switchable Single-Walled Carbon Nanotube-Polymer Composites for CO2 Sensing

We report a chemiresistive CO₂ sensor based on single-walled carbon nanotubes (SWCNTs) noncovalently functionalized with a CO₂ switchable copolymer containing amidine pendant groups that transform into amidinium bicarbonates in response to CO₂. To fabricate a robust surface-anchored polymer-SWCNT dispersion via spray coating, we first designed and synthesized a precursor copolymer, P(4VP-VBAz), bearing both 4-vinylpyridine (4VP) groups and azide groups. The SWCNT dispersant group, 4VP, is capable of debundling and stabilizing nanotubes to improve their solubility in organic solvents for solution processing. Well-dispersed P(4VP-VBAz)-SWCNT composites are covalently immobilized onto a glass substrate functionalized with alkyl bromides, and then the amidine moieties are subsequently attached to form the resulting CO₂-switchable P(Q4VP-VBAm)-SWCNT composites via a copper(I)-catalyzed azide-alkyne cycloaddition click reaction at the film surface. The amidine groups are strong donors that compensate or pin carriers in the SWCNTs. In the presence of CO₂ under humid conditions, the generated amidinium bicarbonates from the polymer wrapping increase the concentration and/or liberate the hole carriers in the nanotubes, thereby increasing the net conductance of the composites. The amidinium moieties revert back to the amidines when purged with a CO₂-free carrier gas with a reversible decrease in conductance. We also demonstrate high selectivity to CO₂ over the other atmospheric gases such as O₂ and Ar.

Active control of properties of concrete: a (p)review

Concrete properties to a large extent depend on mix design and processing, currently leaving only limited options to actively modify concrete properties during or after casting. This paper gives a (p)review on a more advanced active control of properties of concrete, based on the application of external signals to trigger an intended response in the material, either in fresh or hardened state. Current practices in concrete industry that could be considered as active control are briefly summarized. More advanced active control mechanisms as studied in other fields, e.g. based on hydrogels and other functional polymers, are reviewed and some principles are listed. A specific focus is further given on potential methods for active rheology control. Based on the concepts developed in other fields, substantial progress could be made in order to achieve active control of fresh and hardened concrete properties. However, several challenges remain, like the stability and functioning of the responsive material in a cementitious environment, the applicability of the control signal in a cementitious material, and the economy, logistics and safety of a control system on a construction site or in precast industry. Finding solutions to these challenges will lead to marvelous opportunities in general, and for 3D and even 4D printing more particularly.

Controlled Movement of a Smart Miniature Submarine at Various Interfaces

Smart miniaturized aquatic devices have many important applications, but their locomotion at different interfaces remains a challenge. Here, we report a smart miniaturized submarine moving at various air/liquid or oil/water interfaces. The microsubmarine is fabricated by a CO₂-responsive superhydrophobic copper mesh and is driven by the Marangoni effect. The microsubmarine can not only transfer among different interfaces reversibly but also move horizontally at the interfaces freely. The unique locomotion of the device is attributed to a CO₂-triggered switch between superhydrophobicity and underwater superoleophobicity. Moreover, the microsubmarine exhibits good stability and excellent oil repellence at the oil/water interface. Our study provides a strategy for fabricating smart aquatic devices that have potential applications in environment monitoring, water purification, channel-free microfluidics, and so on.

Facile Fabrication of CO2-Responsive Nanofibers from Photo-Cross-Linked Poly(pentafluorophenyl acrylate) Nanofibers

CO₂-responsive nanofibers were facilely prepared from photo-cross-linked poly(pentafluorophenyl acrylate) (PPFPA) nanofibers via "amine-active ester" chemical modification. Photo-cross-linked PPFPA nanofibers were modified with histamine under mild conditions to generate cross-linked poly(histamine acrylamide) (PHAAA) nanofibers featuring a CO₂ responsiveness. As expected, the prepared cross-linked PHAAA nanofibers can exhibit a CO₂-responsive behavior to induce a reversible transition from hydrophobic to hydrophilic upon alternating addition and removal of CO₂ on the surface of nanofibrous membranes. Based on this finding, we could demonstrate that cross-linked PHAAA nanofibers can be employed for reversible absorption and release of protein using bovine serum albumin (BSA) as a model.

Frontiers in poly(ionic liquid)s: syntheses and applications

We review recent works on the synthesis and application of poly(ionic liquid)s (PILs). Novel chemical structures, different synthetic strategies and controllable morphologies are introduced as a supplement to PIL systems already reported. The primary properties determining applications, such as ionic conductivity, aqueous solubility, thermodynamic stability and electrochemical/chemical durability, are discussed. Furthermore, the near-term applications of PILs in multiple fields, such as their use in electrochemical energy materials, stimuli-responsive materials, carbon materials, and antimicrobial materials, in catalysis, in sensors, in absorption and in separation materials, as well as several special-interest applications, are described in detail. We also discuss the limitations of PIL applications, efforts to improve PIL physics, and likely future developments.

Amidoxime-modified chitosan for pigment red 224 enrichment through reversible assembly

An amidoxime-modified chitosan, featuring favorable porosity and super-lipophilic properties, was successfully prepared for pigment red 224 enrichment.

CO2-Switchable-hydrophilicity membrane (CO2-SHM) triggered by electric potential: faster switching time along with efficient oil/water separation

CO₂-Switchable-hydrophilicity membrane (CO₂-SHM) triggered by electric potential which could be effectively used for oil/water separation.

Graft-modified cellulose nanocrystals as CO2-switchable Pickering emulsifiers

Cellulose nanocrystals (CNC) grafted with <25 wt% PDEAEMA or PDMAPMAm were used as CO₂-switchable Pickering emulsifiers for the reversible emulsification/demulsification of oil and water.

Synthesis and characterization of CO2-sensitive temperature-responsive catalytic poly(ionic liquid) microgels

A class of poly(ionic liquid) microgels exhibiting CO₂-switchable temperature-responsive volume phase transition behavior have been synthesized and used for CO₂ fixation.

CO2-Acidolysis of iminoboronate ester based polymersomes

The iminoboronate-terminalized star-like prodrug N₃-(OEG-IBCAPE)₄ was prepared to investigate the CO₂-acidolysis of polymersomes with a tunable release feature of CAPE molecules.

Thermoresponse and self-assembly of an ABC star quarterpolymer with O2 and redox dual-responsive Y junctions

The stimuli-tunable LCST-type phase transition and self-assembly behaviors of a multi-responsive 3-miktoarm star bearing O₂/redox-sensitive and H-bond-switchable Y junctions were revealed.

Balsam-Pear-Skin-Like Porous Polyacrylonitrile Nanofibrous Membranes Grafted with Polyethyleneimine for Postcombustion CO2 Capture

Amine-containing sorbents have been extensively studied for postcombustion carbon dioxide (CO₂) capture because of their ability to chemisorb CO₂ from the flue gas. However, most sorbents are in the form of powders currently, which is not the ideal configuration for the flue gas separation because of the fragile nature and poor mechanical properties, resulting in blocking of the flow pipes and difficult recycling. Herein, we present a novel approach for the facile fabrication of flexible, robust, and polyethyleneimine-grafted (PEI-grafted) hydrolyzed porous PAN nanofibrous membranes (HPPAN-PEI NFMs) through the combination of electrospinning, pore-forming process, hydrolysis reaction, and the subsequent grafting technique. Excitingly, we find that all the resultant porous PAN (PPAN) fibers exhibit a balsam-pear-skin-like porous structure due to the selective removal of poly(vinylpyrrolidone) (PVP) from PAN/PVP fibers by water extraction. Significantly, the HPPAN-PEI NFMs retain their mesoporosity, as well as exhibit good thermal stability and prominent tensile strength (11.1 MPa) after grafting, guaranteeing their application in CO₂ trapping from the flue gas. When exposed to CO₂ at 40 °C, the HPPAN-PEI NFMs show an enhanced CO₂ adsorption capacity of 1.23 mmol g^-1 (based on the overall quantity of the sample) or 6.15 mmol g^-1 (based on the quantity of grafted PEI). Moreover, the developed HPPAN-PEI NFMs display significantly selective capture for CO₂ over N₂ and excellent recyclability. The CO₂ capacity retains 92% of the initial value after 20 adsorption-desorption cycle tests, indicating that the resultant HPPAN-PEI NFMs have good long-term stability. This work paves the way for fabricating NFM-based solid adsorption materials endowed with a porous structure applied to efficient postcombustion CO₂ capture.

An eco-friendly approach for heavy metal adsorbent regeneration using CO2-responsive molecular octopus

Perennial problems of adsorption in wastewater treatment include adsorbent recycling, generation of waste sludge and secondary pollution because harmful concentrated acids, bases or strong chelators are often used for adsorbent regeneration and adsorbate recovery. We report, for the first time, an eco-friendly regeneration concept demonstrated with a CO₂-responsive octopus-like polymeric adsorbent. Various heavy metals can be scavenged at very high Q_e by such adsorbent through coordination. Most importantly, the rapid and complete regeneration of the adsorbent and recovery of the heavy metal ions can be readily achieved by CO₂ bubbling within a few minutes under mild conditions, i.e., room temperature and atmospheric pressure. The adsorbent can then be restored to its adsorptive state and reused upon removal of CO₂ by simply bubbling another gas. This eco-friendly, effective, ultra-fast and repeatable CO₂-triggered regeneration process using CO₂-responsive adsorbent with versatile structure, morphology or form can be incorporated into a sustainable closed-loop wastewater treatment process to solve the perennial problems.

CO₂-Responsive Graft Modified Chitosan for Heavy Metal (Nickel) Recovery

Chitosan was chemically functionalized with poly(diethylaminoethyl methacrylate) (PDEAEMA) using a grafting to approach to produce a CO₂-responsive material for adsorbing metals from wastewater streams. A need for improved economical and greener approaches to recover heavy metals from wastewater streams exists due to increasing resource scarcity. Chitosan is currently used as an adsorbent for heavy metals but suffers from some properties that can be disadvantageous to its effectiveness; it is difficult to effectively disperse in water (which limits available surface area) and to regenerate. We set out to improve its effectiveness by grafting CO₂-responsive tertiary amine containing polymers onto the chitosan backbone, with the goals of preparing and assessing a new type of adsorbent based on a novel concept; using carbon dioxide switchable polymers to enhance the performance of chitosan. PDEAEMA chains prepared by nitroxide-mediated polymerization were grafted onto chitosan functionalized with glycidyl methacrylate. In carbonated water, the grafted chitosan displayed improved dispersibility and exhibited a Ni(II) adsorption capacity higher than several other chemically functionalized chitosan variants reported in the literature with the regenerated material having a higher capacity than all physical and chemical derivatives reported in the literature. The results of this study validate the continued development of this material for applications in heavy metal removal and recovery from wastewater streams.

Partially Acetylated or Benzoylated Arabinose Derivatives as Structurally Simple Organogelators: Effect of the Ester Protecting Group on Gel Properties

Sugar-based low-molecular-weight gelators (LMWGs) have been used for various applications for a long time. Herein, structurally simple, ester-protected arabinosides are reported as low-molecular-weight organogelators (LMOGs) that are able to gel aromatic solvents, as well as petrol and diesel. Studies on the mechanical strength of the gels, through detailed rheological experiments, indicate that gels from the 1,2-dibenzoylated arabinose gelator possess better mechanical properties than those from the 1,2-diacetylated gelator. These results are interpreted in terms of the tendency of the former to form fibers with comparatively lower diameter than those of the latter, based on detailed field-emission SEM and AFM studies. Investigations of the interactions responsible for the self-assembly of gelators through IR spectroscopy and wide-angle X-ray scattering reveal that the primary interactions responsible are hydrogen bonds between the hydroxyl groups and ester C=O, which is absent in the solid state of the gelators. In addition, π interactions present in the 1,2-dibenzoylated derivative result in a more regular arrangement, which, in turn, leads to better mechanical properties of the gels compared with those of the 1,2-diacetylated gelator.

Dynamic Covalent Chemistry of Carbon Dioxide: Opportunities to Address Environmental Issues

Extraction and purification of basic chemicals from complex mixtures has been a persistent issue throughout the development of the chemical sciences. The chemical industry and academic research have grown over the centuries by following a deconstruction-reconstruction approach, reminiscent of the metabolism process. Chemists have designed and optimized extraction, purification, and transformation processes of molecules from natural deposits (fossil fuels, biomass, ores), in order to reassemble them into complex adducts. These highly selective and cost-effective techniques arose from developments in physical chemistry but also in supramolecular chemistry, long before the term was even coined. Thanks to the extremely diverse toolbox currently available to the scientific community, artificial molecular systems of increasing complexity can be built and integrated into high-technology products. If humanity has proven through the ages how gifted it can be at this deconstruction-reconstruction game, which has transformed the natural world to a human-shaped one, it has been confronted for more than a century by a new challenge: the deconstruction and reconstruction from a new type of deposit, the waste resulting from the mass production of disposable manufactured goods. In this Account, we will explore the potential contribution of controlled molecular and supramolecular self-assembly phenomena to the challenge of selective and efficient capture of valuable target molecules from mixtures found in postconsumer waste. While it may appear paradoxical to add more molecular ingredients to an already compositionally complex system in order to address a purification issue, we will compare the selectivity, yield, and cost of such an atypical procedure with traditional physical techniques. In the context of carbon dioxide capture or release, we will specifically focus on the coupling between this reversible covalent fixation of the gas by amines and an additional chemical equilibrium. This equilibrium may involve covalent or noncovalent bond formation between a supplementary species and either the unloaded reactant or the CO₂-loaded product. Thereby, this new reactive species may act as a CO₂ capture agonist or antagonist by either thermodynamically favoring the carbamation or decarbamation direction. Indeed, superagonism, the increase of CO₂ loading per amine site upon carbamation beyond the theoretical limit of 0.5, can be achieved using tightly bound cationic metal counterions. In all cases, upon binding and adduct formation, a mutual selection process occurs between one member of the CO₂-based dynamic combinatorial library and one agonist or antagonist, which can itself be contained in a complex mixture of analogues. If this adduct is the only species that, upon formation, can self-aggregate into a separate solid phase, selection and binding are accompanied by translocation, rendering the purification procedure operationally straightforward. This general strategy, based on a simple design of coupled molecular systems, may easily be implemented within new, disruptive technologies for selective extraction of target molecules, thereby providing a substantial environmental and economic benefit.

Gas-Responsive Polymers

Gas-responsive polymers have inspired much interest over the past ten years. Gas triggers can interact with functionalities on polymer chains and, thus, modulate their chain structures, architectures, and aggregation states. This review summarizes the latest research progresses in the theme of developing different gas triggers for fine control over some critical properties of polymers, as well as their potential applications in various areas. We focus on the interactions/reactions between gases and gas-responsive functionalities of polymers and highlight some state-of-art developments, which provided good insight and understanding of each particular gas-responsive polymer. We also offer a perspective point of view on future research directions on gas-responsive polymers, both in fundamental studies and in potential application developments.

CO2-Responsive fluorescent hyperbranched poly(ether amine)s

Hyperbranched polymers bearing a fluorophore display gas-controlled fluorescence switching as a result of reversible protonation/deprotonation of amino groups in their structure.

Temperature-/CO2-dual-responsiveness of a zwitterionic “schizophrenic” copolymer

A zwitterionic “schizophrenic” copolymer with dual-responsiveness to temperature and carbon dioxide self-assembles to undergo a reversible phase transition in a weakly alkaline borate buffer solution.

CO2-Stimulated morphology transition of ABC miktoarm star terpolymer assemblies

CO₂-Regulated self-assembly of star terpolymers star-[poly(ethylene glycol)-polystyrene-poly[2-(N,N-diethylamino)ethyl methacrylate]] (μ-PEG-PS-PDEA) was studied and an unusual vesicle/microsphere-to-lamella transition upon CO₂ stimulation was observed.

RAFT polymerization to form stimuli-responsive polymers

Stimuli-responsive polymers respond to a variety of external stimuli, which include optical, electrical, thermal, mechanical, redox, pH, chemical, environmental and biological signals. This paper is concerned with the process of forming such polymers by RAFT polymerization.

Post-synthetic modification of polyvinyl alcohol with a series of N-alkyl-substituted carbamates towards thermo and CO2-responsive polymers

Intensive efforts have been devoted to the synthesis of thermoresponsive polymers with terminal N-alkyl-substituted groups.

Facile preparation of hybrid vesicles loaded with silica nanoparticles via aqueous photoinitiated polymerization-induced self-assembly

We report a photoinitiated dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA) in the presence of silica nanoparticles using a poly(ethylene glycol) methyl ether (mPEG) macromolecular chain transfer agent (macro-CTA).

Grafting CO2-responsive polymers from cellulose nanocrystals via nitroxide-mediated polymerisation

CO₂-Responsive polymers are grafted onto cellulose nanocrystals using nitroxide-mediated polymerisation.

CO2-Responsive polymer materials

This paper reviews the chemical fundamentals of CO₂-responsive polymers as well as the latest reported “smart” material systems switched by CO₂.

CO2-Responsive graft copolymers: synthesis and characterization

The synthesis and self-assembly study of CO₂-responsive graft copolymers fabricated from a “graft-to” strategy based on pentafluorophenyl esters as grafting sites.