Measurement of the Adhesion Force of a Living Sessile Organism on Antifouling Coating Surfaces Prepared with Polysulfobetaine-Grafted Particles.

An antifouling polymer brush-like structure was fabricated by a simple and versatile dip-coating method of sulfobetaine containing copolymer-grafted silica nanoparticles (SiNPs) and alkyl diiodide cross-linkers. Surface-initiated atom transfer radical copolymerization of 3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate (MAPS) and N,N-dimethylaminoethyl methacrylate (DMAEMA) was carried out from initiator-immobilized SiNPs to give poly(MAPS-co-DMAEMA)-grafted SiNPs (MAPS/DMAEMA = 9/1, mol/mol) with diameters of 150-170 nm. The SiNP-g-copolymer/2,2,2-trifluoroethanol solution was dip-coated on silicon and glass substrates. Successive treatment with 1,4-diiodobutane in methanol gave a hydrophilic cross-linked coating film for the SiNP-g-copolymer. The cross-linked particle brushes did not peel off from the substrate even after washing with water in an ultrasonic cleaner despite the simple physical absorption of the SiNP-g-copolymer on the substrate surface. The adhesion force of the tentacle of a living barnacle cyprid on a glass surface covered with the cross-linked SiNP-g-copolymer was directly measured by scanning probe microscopy in seawater. The coating film exhibited extremely low adhesion to the cypris larva in the seawater, expecting this to be an effective antifouling property.

Characterization of Hydration Water Bound to Choline Phosphate-Containing Polymers.

Zwitterionic methacrylate polymers with either choline phosphate (CP) (poly(MCP)) or phosphorylcholine (PC) (poly(MPC)) side groups were analyzed to characterize the bound hydration water molecules as nonfreezing water (NFW), intermediate water (IW), or free water (FW). This characterization was carried out by differential scanning calorimetry (DSC) of polymer/water systems, and the enthalpy changes of cold crystallization and melting were determined. The electron pair orientation of CP is opposite to that of PC, and the former binds the alkyl terminal groups at the phosphate esters. The numbers of NFW and IW molecules per monomer unit of poly(MCP) with an isopropyl terminal group were estimated to be 10.7 and 11.3 mol/mol, respectively, which were slightly greater than those of the poly(MCP) bearing an ethyl terminal group. More NFW and IW molecules hydrated the phosphobetaine polyzwitterions, poly(MCP) and poly(MPC), compared with carboxybetaine and sulfobetaine polymers. Moreover, the hydration states of polyelectrolytes were compared with the zwitterionic polymers. Finally, we discuss the relationship between the amount of hydration water and bio-inert properties.

Spreading Dynamics of a Precursor Film of Ionic Liquid or Water on a Micropatterned Polyelectrolyte Brush Surface.

Time evolution of the microscopic wetting velocity of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI) or water on a micrometer-scale line-patterned surface with a poly(3-sulfopropyl methacrylate) brush and a hydrophobic perfluoroalkyl monolayer was precisely measured by direct observation using optical microscopy and a selective dyeing method over a long period (178 days). When a liquid droplet was placed on the dyed line-patterned brush surface, the liquid penetrated and spread into the polymer brush layer, forming a precursor thin film that extended beyond the macroscopic contact line. The elongation proceeded in two stages by an adiabatic process followed by a diffusive process. The elongation distance X increased with time in proportion to t2.6 for water and t0.81 for EMI-TFSI during the adiabatic process. In a diffusive process, the advancing velocity of the precursor film was markedly reduced to be expressed as X ∝ t0.66 for water and X ∝ t0.21 for EMI-TFSI, indicating that the diffusive process was affected by the energy dissipation of the wetting system. The high viscosity and the strong molecular interaction of EMI-TFSI with the polymer brush gave a large entropy change during the wetting process to result in a slower spreading velocity.

Hydration State Variation of Polyzwitterion Brushes through Interplay with Ions.

Polyzwitterions have emerged as a new class of antifouling materials alternating poly(ethylene glycol). The exemplary biopassivation and lubrication behaviors are often attributed to the particular chemical structure of zwitterions, which involve a large dipole moment of the charged groups and a neutral net charge, while the hydration state and dynamics also associate with these characteristics. Polymer brushes composed of surface-tethered polyzwitterion chains produced by surface-initiated controlled radical polymerization have been developed as thin films which exhibit excellent antifouling and lubrication properties. In past decades, numerous studies have been devoted to examining the structure and dynamics of polyzwitterion brush chains in aqueous solutions. This feature article provides an overview of recent studies exploring the hydration state of polyzwitterion brushes with specular neutron reflectivity, highlights some newly published work on the nonuniform equilibrium structure, ion concentration dependence, ion specificity, and the effects of charge spacer length in the zwitterions, and discusses future perspective in this field.

Intermolecular Interaction of Polymer Brushes Containing Phosphorylcholine and Inverse-Phosphorylcholine.

Choline phosphate (CP) is a phosphobetaine-type zwitterionic functional group, referred to as inverse phosphorylcholine (PC) due to the reverse orientation of a positively charged quaternary amine and anionic phosphate in contrast to PC lipids in nature. The A unique dipole paring between CP and PC groups has attracted much attention in the biointerface research field. Herein, to evaluate the molecular interaction between the CP and PC groups in water, force-distance curve measurements using scanning probe microscopy (SPM) with a PC-group-functionalized cantilever was carried out on the surface of polymer brushes bearing the CP groups. Three types of methacrylate monomers bearing CP with ethyl (Et), methoxyethyl (MOE), and isopropyl (iPr) phosphates were synthesized in 42-71% yields, and polymerized by surface-initiated atom transfer radical polymerization to form polymer brushes on silicon wafers. The surface free energy of CP-polymer brushes with Et, MOE, and iPr was estimated to be 64.0, 61.4, and 57.4 mN m-1, respectively, based on contact angle measurements. Force-distance curve measurements of polymer brushes having a CP group was conducted in water at 25 °C by SPM using a spherical probe produced by attaching a silica particle (SiP; d = 25 µm) covered with PC or CP groups to a tipless cantilever. Adhesion force larger than 14 nN was observed between the CP-polymer brushes and PC-SiP, whereas PC-polymer brushes revealed extremely low adhesion force of less than 0.6 nN with PC-SiP and propylsilane-modified SiP. The specific attractive molecular interaction between CP and PC groups was quantitatively evaluated.

Time Evolution of Precursor Thin Film of Water on Polyelectrolyte Brush.

The microscopic wetting behavior of a water film on the line-patterned surface of a polyelectrolyte brush was directly visualized using an optical microscope by dyeing procedures. Surface line patterns of 5 and 5 µm width or 10 and 5 µm width for the polyelectrolyte brush and hydrophobic monolayer, respectively, were prepared by a photolithography process, chemical vapor adsorption method, and surface-initiated polymerization. A droplet of water containing dye was placed on the line-patterned surface. In front of the contact line, a water film with a nanometer-scale thickness, referred to as a precursor film, elongated along the polymer brush line with time. The elongation velocity at the first stage increased as the brush line width increased. On the other hand, at the second stage after the macroscopic contact line stopped moving, the precursor film continued to elongate in proportion to the 0.6 power of time, independent of the brush thickness, line width, and droplet volume.

Interferometry study of aqueous lubrication on the surface of polyelectrolyte brush.

The water lubrication behavior of a polyelectrolyte brush was investigated by using double-spacer-layer ultra-thin-film interferometry to determine the thickness of the aqueous lubrication layer present at the interface between the brush and a spherical glass lens. A hydrophilic poly{[2-(methacryloyloxy)ethyl]trimethylammonium chloride} brush was prepared on an optical glass disk coated with layers of semireflective chromium and silica. The thickness of the hydrodynamic lubrication layer was estimated interferometrically. On increasing the sliding velocity from 10(-5) to 10(-1) m·s(-1), the gap between the rotating disk and loading sphere glass lens showed a marked increase to 130 nm at 2×10(-2) m·s(-1), and the friction coefficient simultaneously decreased to 0.01-0.02, indicating that the polyelectrolyte brush promoted the formation of a fluid lubrication layer that separates the rubbing surfaces, preventing direct contact and providing a low friction coefficient.

Neutron reflectivity study of the swollen structure of polyzwitterion and polyeletrolyte brushes in aqueous solution.

The swollen brush structures of polycation and zwitterionic polymer brushes, such as poly(2-methacryloyloxyethyltrimethylammonium chloride) (PMTAC), poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), and poly[3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate] (PMAPS), in aqueous solutions of various ionic strengths were characterized by neutron reflectivity (NR) measurements. A series of the polyelectrolyte brushes were prepared by surface-initiated controlled radical polymerization on silicon substrates. A high-graft-density PMTAC brush in salt-free water (D2O) adopted a two-region step-like structure consisting of a shrunk region near the Si substrate surface and a diffuse brush region with a relatively stretched chain structure at the solution interface. The diffuse region of PMTAC was reduced with increase in salt (NaCl) concentration. The PMAPS brush in D2O formed a collapsed structure due to the strong molecular interaction between betaine groups, while significant increase in the swollen thickness was observed in salt aqueous solution. In contrast, no change was observed in the depth profile of the swollen PMPC brush in D2O with various salt concentrations. The unique solution behaviors of zwitterionic polymer brushes were described.

Scaffold for growing dense polymer brushes from a versatile substrate.

We have demonstrated a universal approach to growing polymer brushes from various substrates. Urushiol was mixed with initiator-containing catechol, and it was spin-coated or cast on various substrates. Because urushiol is strongly adhered to various substrates, the thin film can serve as a general scaffold for grafting polymer brushes from various substrates. Note that the film was formed even onto the surface of polyolefins and thermosetting resins that are known as chemically inert materials to functionalize the surface. Moreover, the initiator-immobilized scaffold showed mechanical robustness and chemical inertness because of the flexible long unsaturated hydrocarbon side chain of urushiol. After the grafting of polyelectrolyte PMTAC polymer brushes, the material obtained exhibited hydrophilicity, typical of PMTAC. The concept shown here could provide a general approach for grafting practical polymer brushes from various substrates.

Measurement of the electrostatic interaction between polyelectrolyte brush surfaces by optical tweezers.

We demonstrated an optical tweezers method to measure the electrostatic interaction between the strong polyelectrolyte brushes, poly(2-(methacryloyloxy)ethyltrimethylammonium chloride) (PMTAC), grafted on silica particles in aqueous media. The weak electrostatic interaction was successfully detected with a resolution of less than 0.1 µN m(-1). The apparent Debye length, including the charge distribution in the polymer brush and the surface potential, decreased as the salt concentration in the medium increased. The experimentally obtained surface charge density was much smaller than that estimated from the amount of polyelectrolyte on the surface. Furthermore, the dissociation of ionic groups was enhanced by decreasing the grafting density of the polyelectrolyte brush. The results suggest that the majority of chloride counterions was immobilized in the dense polyelectrolyte brush layer to neutralize the high charge density.

Spreading and structuring of water on superhydrophilic polyelectrolyte brush surfaces.

The wetting behavior of superhydrophilic polyelectrolyte brushes was investigated. Reflection interference contrast microscopy demonstrated that the contact angles of water on the polyelectrolyte brushes were extremely low but remained finite in the range of <3°. The presence of water molecules was evident, even outside the macroscopic water droplet. These water molecules were confined to the thin brush layers and contained a highly ordered hydrogen bond network, which was identified as structural water. The presence of the thin film and the structural water changed the surface energies, which prevented the complete wetting of the surface.

Light-Triggered Adhesion of Water-Soluble Polymers with a Caged Catechol Group.

An acrylamide-type copolymer containing hydroxyl, amino, and ortho-nitrobenzyl protected catechol groups was synthesized as a functional mussel adhesive protein (MAP) mimetic. The resulting copolymer was stable even in the oxidative condition. Light irradiation to aqueous solution of the copolymer induced deprotection of a caged compound to give a native catechol group and triggered an oxidative cross-linking reaction to afford the adhesive gel. Two glass plates were adhered through light-activated gelation of the polymer solution in a humid air atmosphere. A novel type of light-activated adhesive with excellent stability and light controllable activation properties was successfully fabricated by modification of the MAP mimetic copolymer.

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.

Polyelectrolyte brushes: a novel stable lubrication system in aqueous conditions.

Surface-initiated controlled radical copolymerizations of 2-dimethylaminoethyl methacrylate (DMAEMA), 2-(methacryloyloxy)ethyl phosphorylcholine (MPC), 2-(methacryloyloxy)ethyltrimethylammonium chloride) (MTAC), and 3-sulfopropyl methacrylate potassium salt (SPMK) were carried out on a silicon wafer and glass ball to prepare polyelectrolyte brushes with excellent water wettability. The frictional coefficient of the polymer brushes was recorded on a ball-on-plate type tribometer by linear reciprocating motion of the brush specimen at a selected velocity of 1.5 x 10(-3) m s-1 under a normal load of 0.49 N applied to the stationary glass ball (d = 10 mm) at 298 K. The poly(DMAEMA-co-MPC) brush partially cross-linked by bis(2-iodoethoxy)ethane maintained a relatively low friction coefficient around 0.13 under humid air (RH > 75%) even after 200 friction cycles. The poly(SPMK) brush revealed an extremely low friction coefficient around 0.01 even after 450 friction cycles. We supposed that the abrasion of the brush was prevented owing to the good affinity of the poly(SPMK) brush for water forming a water lubrication layer, and electrostatic repulsive interactions among the brushes bearing sulfonic acid groups. Furthermore, the poly(SPMK-co-MTAC) brush with a chemically cross-linked structure showed a stable low friction coefficient in water even after 1400 friction cycles under a normal load of 139 MPa, indicating that the cross-linking structure improved the wear resistance of the brush layer.

Competition between Oxidation and Coordination in Cross-Linking of Polystyrene Copolymer Containing Catechol Groups.

In gelation chemistry, catechol groups are used as cross-linking points. Both oxidation and coordination effects of catechol were investigated for their unique features in chemistry by spectroscopic measurements. Polystyrene copolymers containing catechol groups were synthesized by free radical copolymerization of styrene and N-2-(3',4'-ditriethylsilyloxyphenyl)ethyl methacrylamide, and the successive deprotection reaction was catalyzed by tetra-n-butylammonium fluoride. The copolymer containing catechol units afforded a dual cross-linking system based on completely different coordination and oxidation chemistries, and the competing cross-linking mechanisms are discussed. These findings are useful and important for paving the way for designing a novel bioinspired artificial adhesive surface coating and curing system.

Tribological properties of hydrophilic polymer brushes under wet conditions.

This article demonstrates a water-lubrication system using high-density hydrophilic polymer brushes consisting of 2,3-dehydroxypropyl methacrylate (DHMA), vinyl alcohol, oligo(ethylene glycol)methyl ether methacrylate, 2-(methacryloyloxy)ethyltrimethylammonium chloride (MTAC), 3-sulfopropyl methacrylate potassium salt (SPMK), and 2-methacryloyloxyethyl phosphorylcholine (MPC) prepared by surface-initiated controlled radical polymerization. Macroscopic frictional properties of brush surfaces were characterized by sliding a glass ball probe in water using a ball-on-plate type tribotester under the load of 0.1-0.49 N at the sliding velocity of 10(-5)-10(-1) m s(-1) at 298 K. A poly(DHMA) brush showed a relatively larger friction coefficient in water, whereas the polyelectrolyte brushes, such as poly(SPMK) and poly(MPC), revealed significantly low friction coefficients below 0.02 in water and in humid air conditions. A drastic reduction in the friction coefficient of polyelectrolyte brushes in aqueous solution was observed at around 10(-3)-10(-2) m s(-1) owing to the hydrodynamic lubrication effect, however, an increase in salt concentration in the aqueous solution led to the increase in the friction coefficients of poly(MTAC) and poly(SPMK) brushes. The poly(SPMK) brush showed a stable and low friction coefficient in water even after sliding over 450 friction cycles, indicating a good wear resistance of the brush film.

Macroscopic frictional properties of poly(1-(2-methacryloyloxy)ethyl-3-butyl imidazolium bis(trifluoromethanesulfonyl)-imide) brush surfaces in an ionic liquid.

Poly(1-(2-methacryloyloxy)ethyl-3-butylimidazolium bis(trifluoromethanesulfonyl)imide) (PMIS) and poly(n-hexyl methacrylate) (PHMA) brushes were prepared on initiator-immobilized silicon wafers by surface-initiated atom transfer radical polymerization. The macroscopic frictional properties of the brushes were determined using a ball-on-flat type tribotester under reciprocating motion in a dry nitrogen atmosphere, water, methanol, and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMImTFSI). When the PMIS and PHMA brushes were exposed to EMImTFSI, the friction coefficient of the former was lower than that of the latter. It is thought that the high affinity of the PMIS brush to EMImTFSI led to a reduction in the interaction between the brush and the friction probe, which resulted in a low friction coefficient. The friction force of the PMIS brush in EMImTFSI was proportional to a normal load in the range of 0.2-0.98 N. The friction coefficient gradually decreased to 0.01 with an increase in the sliding velocity from 1 x 10(-4) to 1 x 10(-1) m s(-1). The friction coefficient of the PMIS brush exhibited low magnitude until 800 friction cycles in the dry nitrogen atmosphere, whereas the PHMA brush was abraded away within 150 friction cycles. The XPS spectra of the worn surfaces on the PMIS brush suggested that the brush was gradually abraded by friction.

Contamination-free transmission electron microscopy for high-resolution carbon elemental mapping of polymers.

Specimen contamination induced by electron beam irradiation has long been a serious problem for high-resolution imaging and analysis by a transmission electron microscope (TEM). It creates a deposition of carbonaceous compounds on a region under study, causing the loss of resolution. We developed a method to reduce the beam-induced specimen contamination by cleaning a TEM with activated oxygen radicals. The hydrocarbon contaminants accumulated inside the microscope's chamber can be etched away by gentle chemical oxidation without causing any damage to the microscope. The "contamination-free TEM" can effectively suppress the deposition of carbon-rich products on a specimen and therefore enables us to perform high-resolution carbon elemental mapping by energy-filtering transmission electron microscopy (EFTEM). In this study, we investigated the structure of polymer brushes immobilized on a silica nanoparticle (SiNP), of which molecular weight, length, and density of the brushes had been characterized in detail. The isolated particle showed the stretched formations of the polymer chains growing from the surface, while the densely distributed particles showed the connection of the polymer chains between neighboring particles. Moreover, the polymer brush layer and the surface initiator could be differentiated from each other by the component-specific contrast achieved by electron spectroscopic imaging (ESI). The contamination-free TEM can allow us to perform high-resolution carbon mapping and is expected to provide deep insights of soft materials' nanostructures.

Dimensions of a free linear polymer and polymer immobilized on silica nanoparticles of a zwitterionic polymer in aqueous solutions with various ionic strengths.

The dimensions and intermolecular interactions of a surface-grafted and unbound free polyampholyte, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), were estimated in aqueous solutions with various ionic strengths. Free linear PMPC was synthesized by atom-transfer radical polymerization (ATRP), and static light scattering (SLS) and dynamic light scattering (DLS) were carried out for the PMPC solutions with various concentrations of NaCl, c s. The hydrodynamic radius R H and the second virial coefficient A 2 of PMPC were independent of c s (0-0.5 M), though both R H and A 2 of polyampholytes usually strongly depend on the ionic strength. PMPC-immobilized silica nanoparticles (PMPC-SiNP) were also synthesized by surface-initiated ATRP, and DLS was carried out as for the solutions of linear PMPC to investigate the dependence of the dimensions of PMPC immobilized on a solid surface on the ionic strength. The molecular weight and surface density of PMPC immobilized on SiNP were estimated from the results obtained by GPC, NMR, and thermogravimetric analysis. The independence of R H of PMPC-SiNP was also observed, but its magnitude was larger than that of linear PMPC, although the molecular weight of PMPC immobilized on SiNP was smaller than that of linear PMPC. The larger dimension of PMPC immobilized on SiNP was explained by the excluded volume effect between the immobilized polymer chains.

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.