Design rationale of thermally responsive microgel particle films that reversibly absorb large amounts of CO2: fine tuning the pKa of ammonium ions in the particles

Design Rationale for CO2 Separation Membranes with Micropatterned Surface Structures

Here, we report the design rationale of CO2 separation membranes with micropatterned surface structures. Thin film composite (TFC) membranes with micropatterned surface structures were fabricated by spray coating amine-containing hydrogel particles on the top of micropatterned porous support membranes, which were synthesized by a polymerization-induced phase separation process in a micromold (PIPsμM). The pore size of the support membranes was optimized by tuning the proportion of good and poor solvents for the polymerization process so that the microgels would be assembled as a defect-free separation layer. The relationship between the size of the micropatterned structures on the surface of the support membrane and the thickness of the separation layer was optimized to maximize the surface area of the separation layer. The rationally designed micropatterned TFC membrane showed a CO2 permeability (835.8 GPU) proportional to the increase in surface area relative to the flat membrane with a high CO2/N2 selectivity of 58.7. The rational design for micropatterned TFC membranes will enable the development of inexpensive and high-performance functional membranes not only for CO2 separation but also for other applications such as water treatment and membrane reactors.

Nanoparticle-Based Tough Polymers with Crack-Propagation Resistance

Although thin elastomer films of polymer nanoparticles are regarded as environmentally friendly materials, the low mechanical strength of the films limits their use in various applications. In the present study, we investigated the fracture resistance of latex films composed of acrylic nanoparticles where a small quantity of a rotaxane crosslinker was introduced. In contrast to conventional nanoparticle-based elastomers, the latex films composed of the rotaxane-crosslinked nanoparticles exhibited unusual crack propagation behavior; the direction of crack propagation changed from a direction parallel to the crack to one perpendicular to the crack, resulting in an increase in tear resistance. These findings will help to broaden the scope of design of new types of tough polymers composed of environmentally friendly polymer nanoparticles.

CO2 Capture and Low-Temperature Release by Poly(aminoethyl methacrylate) and Derivatives

Poly(aminoethyl methacrylate) (PAEMA), poly(ethylene oxide)-block-(aminoethyl methacrylate) (PEO-PAEMA), and their guanidinylated derivates, poly(guanidine ethyl methacrylate) (PGEMA) and poly(ethylene oxide)-block-(guanidine ethyl methacrylate) (PEO-PGEMA), were prepared to study their capabilities for CO₂ adsorption and release. The polymers of different forms or degree of guanidinylation were thoroughly characterized, and their interaction with CO₂ was studied by NMR and calorimetry. The extent and kinetics of adsorption and desorption of N₂ and CO₂ were investigated by thermogravimetry under controlled gas atmospheres. The materials did not adsorb N₂, whereas CO₂ could be reversibly adsorbed at room temperature and released by an elevated temperature. The most promising polymer was PGEMA with a guanidinylation degree of 7% showing a CO₂ adsorption capacity of 2.4 mmol/g at room temperature and a desorption temperature of 72 °C. The study also revealed relations between the polymer chemical composition and CO₂ adsorption and release characteristics that are useful in future formulations for CO₂ adsorbent polymer materials.

Assembly of Defect-Free Microgel Nanomembranes for CO2 Separation

The development of robust and thin CO₂ separation membranes that allow fast and selective permeation of CO₂ will be crucial for rebalancing the global carbon cycle. Hydrogels are attractive membrane materials because of their tunable chemical properties and exceptionally high diffusion coefficients for solutes. However, their fragility prevents the fabrication of thin defect-free membranes suitable for gas separation. Here, we report the assembly of defect-free hydrogel nanomembranes for CO₂ separation. Such membranes can be prepared by coating an aqueous suspension of colloidal hydrogel microparticles (microgels) onto a flat, rough, or micropatterned porous support as long as the pores are hydrophilic and the pore size is smaller than the diameter of the microgels. The deformability of the microgel particles enables the autonomous assembly of defect-free 30-50 nm-thick membrane layers from deformed ∼15 nm-thick discoidal particles. Microscopic analysis established that the penetration of water into the pores driven by capillary force assists the assembly of a defect-free dense hydrogel layer on the pores. Although the dried films did not show significant CO₂ permeance even in the presence of amine groups, the permeance dramatically increased when the membranes are adequately hydrated to form a hydrogel. This result indicated the importance of free water in the membranes to achieve fast diffusion of bicarbonate ions. The hydrogel nanomembranes consisting of amine-containing microgel particles show selective CO₂ permeation (850 GPU, α_CO2/N2 = 25) against post-combustion gases. Acid-containing microgel membranes doped with amines show highly selective CO₂ permeation against post-combustion gases (1010 GPU, α_CO2/N2 = 216) and direct air capture (1270 GPU, α_CO2/N2 = 2380). The membrane formation mechanism reported in this paper will provide insights into the self-assembly of soft matters. Furthermore, the versatile strategy of fabricating hydrogel nanomembranes by the autonomous assembly of deformable microgels will enable the large-scale manufacturing of high-performance separation membranes, allowing low-cost carbon capture from post-combustion gases and atmospheric air.

Non-close-packed arrangement of soft elastomer microspheres on solid substrates

Unlike rigid microparticles, soft and deformable elastomer (rubber) microspheres were found to exhibit a non-close-packed arrangement on solid substrates after the evaporation of water from their dispersions. The microscopic observation revealed that individual microspheres are ordered in regular intervals at the air/water interface of a sessile droplet and remain fixed on the substrate without being affected by the capillary forces during evaporation due to their deformability. Moreover, using the Langmuir-Blodgett method, thin films of non-close-packed structures could be successfully generated over large areas. Our findings may potentially help to control the arranged structures of elastomer microspheres, which can be expected to improve the nano-science and technology for the precise control for e.g. surface patterning.

Triply-responsive OEG-based microgels and hydrogels: regulation of swelling ratio, volume phase transition temperatures and mechanical properties

Facile methods to coordinate swelling ratio, volume-phase transition temperatures and mechanical properties for pH-, thermal-, and cationic-responsive microgels and hydrogels.

Salt-Enhanced CO2-Responsiveness of Microgels

Here, we report a distinct mechanism for harnessing CO₂-responsiveness through enhancing CO₂ capture ability. The finding is demonstrated on the microgels that are composed of oligo(ethylene glycol) and sulfonate moieties. Laser light scattering studies on dilute aqueous dispersion of these microgels indicated a low CO₂-responsivity, which can be significantly enhanced by adding NaCl and other salts. This salt-enhanced CO₂-responsiveness of microgels can be elucidated by the antipolyelectrolyte behavior and its superposition of forming cross-links physically with CO₂ as an intermediate. Further results of the filtration experiments on microgel translocation through pores suggest the feasibility of the explanation. The finding is also supported by the CO₂ capture-release experiments on the dispersion, which can reversibly absorb and desorb CO₂.

Thermocells Driven by Phase Transition of Hydrogel Nanoparticles

Thermoelectric conversion of low temperature, delocalized, and abundant thermal sources is crucial for the development of the Internet of Things (IoT) and/or a carbon-free society. Thermocells are of great interest in thermoelectric conversion of low-temperature heat due to the low cost and flexibility of components. However, significant improvement of the conversion efficiency is required for the practical use of the cells. Here, we report thermo-electrochemical cells driven by volume phase transition (VPT) of hydrogel nanoparticles (NPs). Entropically driven VPT of poly(N-isopropylacrylamide) NPs containing carboxylic acids and amines generates a pH gradient of up to 0.049 and -0.053 pH K^-1, respectively, around physiological temperature. The pH gradient triggers the proton-coupled electron transfer (PCET) reactions of quinhydrone on the electrodes, resulting in the highly efficient thermoelectric conversion with a Seebeck coefficient (S_e) of -6.7 and +6.1 mV K^-1. Thermocells driven by phase transition of hydrogels provide a nontoxic, flexible, and inexpensive charger that harvests carbon-free energy from abundant energy sources such as solar, body and waste heat.

Carbon Dioxide and Nitrogen-Modulated Shape Transformation of Chitosan-Based Composite Nanogels

Chitosan/poly[N-(3-(dimethylamino)propyl)methacrylamide]/poly(acrylic acid) (CS/PDMAPMA/PAA) composite nanogels (CPACNGs) were fabricated in the solution of chitosan by surfactant-free emulsion polymerization. N-(3-(Dimethylamino)propyl)methacrylamide (DMAPMA) and acrylic acid (AA) were initiated by 2,2'-azobis-2-methyl-propanimidamide to graft from the backbone of chitosan. Nanogels were formed by noncovalent forces, including of hydrogen bonds, hydrophobic, and electrostatic interaction. Nanogels were characterized by transmission electron microscopy, scanning electron microscope dynamic light scattering, X-ray photoelectron spectroscopy, Fourier transform infrared spectrometer spectra, and 1H NMR. Spherical nanoparticles were observed in the latex system. Nanogels exhibited an excellent CO2 responsivity and CO2/N2 reversible response and switchability and had a faster response rate. The morphological shape transformation of nanogels was modulated by bubbling with CO2 and N2. The responsive mechanism was explored by determining the pH and electrical conductivity. In addition, nanogels were successfully emulsified by bubbling with CO2, and then a phase transition was achieved by bubbling with N2 in the organic solvent/water mixture.

Effect of Charge Groups Immobilized in Hydrogel Microspheres during the Evaporation of Aqueous Sessile Droplets

In contrast to conventional dispersions of solid microspheres, dilute dispersions containing soft hydrogel microspheres (microgels) exhibit unique drying behavior due to their selective adsorption at the air/water interface of sessile droplets. So far, the impact of the size, chemical composition, and softness (degree of cross-linking) of microgels has been investigated. In the present study, we present the impact of charged groups introduced in the microgels on the adsorption and assembly behavior of these microgels at the air/water interface using a series of microgels with different amounts and distribution of charged groups. A series of experiments under different conditions (pH value and ionic strength) afforded information that clarified the adsorption, interpenetration, and deformation behavior of such charged microgels at the air/water interface. The results indicate that the adsorption and the deformation of charged microgels at the air/water interface are suppressed by the presence of charged groups. Moreover, charged microgels adsorbed at the interface are more dynamic and not highly entangled with each other; i.e., even though the more dynamic charged microgels are arranged at the interface, these arranged structures are disrupted upon transferring onto the solid substrates. Our findings of this study can be expected to promote the further development of applications, e.g., foams and emulsions stabilized by microgels, that crucially requires an in-depth understanding of the adsorption behavior of charged microgels at the air/water interface such as coatings.

Reversible p Ka Modulation of Carboxylic Acids in Temperature-Responsive Nanoparticles through Imprinted Electrostatic Interactions

The acid dissociation constants (p K_a values) of Brønsted acids at the active sites of proteins are reversibly modulated by intramolecular electrostatic interactions with neighboring ions in a reaction cycle. The resulting p K_a shift is crucial for the proteins to capture, transfer, and release target ions. On the other hand, reversible p K_a modulation through electrostatic interactions in synthetic polymer materials has seldom been realized because the interactions are strongly shielded by solvation water molecules in aqueous media. Here, we prepared hydrogel nanoparticles (NPs) bearing carboxylic acid groups whose p K_a values can be reversibly modulated by electrostatic interactions with counterions in the particles. We found that the deprotonated states of the acids were stabilized by electrostatic interactions with countercations only when the acids and cations were both imprinted in hydrophobic microdomains in the NPs during polymerization. Cationic monomers, like primary amine- and guanidium group-containing monomers, which interacted strongly with growing NPs showed greater p K_a modulation than monomers that did not interact with the NPs, such as quaternary ammonium group-containing monomers. Modulation was enhanced when the guanidium moieties were protected with hydrophobic groups during polymerization, so that the guanidium ions were imprinted in the hydrophobic microdomains; the lowest p K_a of ∼4.0 was achieved as a result. The p K_a modulation of the acids could be reversibly removed by inducing a temperature-dependent volume phase transition of the gel NPs. These design principles are applicable to other stimuli-responsive materials and integral to the development of synthetic materials that can be used to capture, transport, and separate target ions.

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.

Wide-range pKa tuning of proton imprinted nanoparticles for reversible protonation of target molecules via thermal stimuli

pK_a tuning of Brønsted acids in synthetic nano-materials is of great importance for the design of ion exchange and bio-/molecular-separation media and polymer catalysis.

Rational design of thermo-responsive adsorbents: demand-oriented active sites for the adsorption of dyes

A smart adsorbent is fabricated by incorporating a di-block copolymer through the combination of thermo-responsive groups and demand-oriented active sites into mesoporous silica for temperature swing adsorption of dyes.

Cascade DNA logic device programmed ratiometric DNA analysis and logic devices based on a fluorescent dual-signal probe of a G-quadruplex DNAzyme

Two fluorescence sensitive substrates of G4 DNAzyme with inverse responses were simultaneously used to a cascade advanced DNA logic device based DNA analysis for the first time.