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, αCO2/N2 = 25) against post-combustion gases. Acid-containing microgel membranes doped with amines show highly selective CO2 permeation against post-combustion gases (1010 GPU, αCO2/N2 = 216) and direct air capture (1270 GPU, αCO2/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.

Effect of Charged Group Spacer Length on Hydration State in Zwitterionic Poly(sulfobetaine) Brushes.

Effect of alkyl chain spacer length between the charged groups (CSL) in zwitterionic poly(sulfobetaine) (PSB) brushes on the hydration state was investigated. PSB brushes with ethyl (PMAES), propyl (PMAPS), or butyl (PMABS) CSL were prepared by surface-initiated atom transfer radical polymerization on silicon wafers. Hydration states of the PSB brushes in aqueous solutions and/or humid vapor were investigated by contact angle measurement, infrared spectroscopy, AFM observation, and neutron reflectivity. The PSB brushes are swollen in humid air and deionized water due to the hydration of the charged groups leading to the reduction of hydrated PSB brushes/water interfacial free energy. The hydrated PSB brushes exhibit clear interface with low interfacial roughness due to networking of the PSB brush chains through association of the SBs. The hydrated PSB brushes produce diffusive swollen layer in the presence of NaCl because of the charge screening followed by SB dissociation by the bound ions. The ionic strength sensitivity in the hydration got more significant with increasing the CSL in SBs because of the augmentation in partial charge by charged group separation.

Wettability and antifouling behavior on the surfaces of superhydrophilic polymer brushes.

The surface wettabilities of polymer brushes with hydrophobic and hydrophilic functional groups were discussed on the basis of conventional static and dynamic contact angle measurements of water and hexadecane in air and captive bubble measurements in water. Various types of high-density polymer brushes with nonionic and ionic functional groups were prepared on a silicon wafer by surface-initiated atom-transfer radical polymerization. The surface free energies of the brushes were estimated by Owens-Wendt equation using the contact angles of various probe liquids with different polarities. The decrease in the water contact angle corresponded to the polarity of fluoroalkyl, hydroxy, ethylene oxide, amino, carboxylic acid, ammonium salt, sulfonate, carboxybetaine, sulfobetaine, and phosphobetaine functional groups. The poly(2-perfluorooctylethyl acrylate) brush had a low surface free energy of approximately 8.7 mN/m, but the polyelectrolyte brushes revealed much higher surface free energies of 70-74 mN/m, close to the value for water. Polyelectrolyte brushes repelled both air bubbles and hexadecane in water. Even when the silicone oil was spread on the polyelectrolyte brush surfaces in air, once they were immersed in water, the oil quickly rolled up and detached from the brush surface. The oil detachment behavior observed on the superhydrophilic polyelectrolyte brush in water was explained by the low adhesion force between the brush and the oil, which could contribute to its excellent antifouling and self-cleaning properties.

Synthesis of TiO2 nanocoral structures in ever-changing aqueous reaction systems.

A far-from-equilibrium strategy is developed to synthesize coral-like nanostructures of TiO(2) on a variety of surfaces. TiO(2) nanocoral structures consist of anatase base film and rutile nanowire layers, and they are continuously formed on substrates immersed in aqueous TiOSO(4)-H(2)O(2). The sequential deposition of TiO(2) starts with hydrolysis and condensation reactions of titanium peroxocomplexes in the aqueous phase, resulting in deposition of amorphous film. The film serves as adhesive interface on which succeeding growth of rutile nanowires to occur. This initial deposition reaction is accompanied by shift in pH of the reaction media, which is favorable condition for the growth of rutile nanocrystals. During the growth of rutile nanocoral layers, the amorphous base films are transformed to anatase phase. These sequential deposition reactions occur at temperatures as low as 80 °C, and the mild synthetic condition allows the use of a wide range of substrates such as ITO (indium tin oxide), glass, and even organic polymer films. The thickness of nanocoral layer is controllable by repeating the growth reaction of rutile nanocorals. TiO(2) nanocorals show photocatalytic activity as demonstrated by site-specific reduction of Ag(I) ions, which proceeds preferentially on the rutile nanowire layer. The rutile nanowire layer also shows photocatalytic decomposition of acetaldehyde, which is promoted upon increase of the thickness of the nanowire layer. The use of temporally transforming reaction media allows the formation of biphasic TiO(2) nanocoral structures, and the concept of nonequilibrium synthetic approach would be widely applicable to developing structurally graded inorganic nanointerfaces.

Friction behavior of high-density poly(2-methacryloyloxyethyl phosphorylcholine) brush in aqueous media.

Super-hydrophilic polymer brushes were prepared by surface-initiated atom transfer radical polymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) on initiator-immobilized silicon wafers. The graft density was estimated to be 0.22 chains nm based on the linear relationship between and the layer thickness. The contact angle against water was very low, and air bubbles in water hardly attached onto the brush surface, indicating a super-hydrophilic surface. Neutron reflectivity measurements of the poly(MPC) brush showed that the grafting polymer chains extended a fair amount in the vertical direction from the substrate in a good solvent such as water, while they shrunk in a poor solvent. Frictional properties of the poly(MPC) brushes were characterized by sliding a glass ball probe in air and various solvents under a load of 0.49 N at a sliding velocity of 90 mm min. An extremely low friction coefficient of the poly(MPC) brush was observed in humid atmosphere because water molecules adsorbed into the brush layer acted as a lubricant.