Norifumi L. Yamada

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.

SAXS, SANS and NSE studies on unbound state in DPPC/water/CaCl2 system

The temperature and CaCl 2 concentration dependence of the lamellar repeat distance of a dipalmitoylphosphatidylcholine (DPPC) aqueous solution was investigated by small-angle X-ray and neutron scattering, and neutron spin echo. At certain CaCl 2 concentrations, the repeat distance in the liquid-crystalline phase tends to increase up to infinity while that in the gel phase has an upper limit. This behavior is attributable to the fact that the steric repulsion of lipid bilayers due to the membrane undulation in the liquid-crystalline phase is larger than that in the gel phase. The free-energy calculation suggested that the cooperation of the short-range electrostatic interaction and the long-range steric interaction is the origin of the increase in the repeat distance in the liquid-crystalline phase, and it can be the origin of an “unbinding transition”.

Effect of local chain dynamics on a bioinert interface.

Although many kinds of synthetic polymers have been investigated to construct blood-compatible materials, only a few have achieved success. To establish molecular designs for blood-compatible polymers, the chain structure and dynamics at the water interface must be understood using solid evidence as the first bench mark. Here we show that polymer dynamics at the water interface impacts on structure of the interfacial water, resulting in a change in protein adsorption and of platelet adhesion. As a particular material, a blend composed of poly(2-methoxyethyl acrylate) (PMEA) and poly(methyl methacrylate) was used. PMEA was segregated to the water interface. While the local conformation of PMEA at the water interface was insensitive to its molecular weight, the local dynamics became faster with decreasing molecular weight, resulting in a disturbance of the network structure of waters at the interface. This leads to the extreme suppression of protein adsorption and platelet adhesion.

Characterization of Swollen States of Polyelectrolyte Brushes in Salt Solution by Neutron Reflectivity

Cationic and zwitterionic polyelectrolyte brushes on quartz substrate were synthesized by surface-initiated atom transfer radical polymerization of 2-(methacryloyloxy)-ethyltrimethylammonium chloride (MTAC) and 2-(methacryloyloxy)ethyl phosphorylcholine (MPC). The effects of ionic strength on brush structure are investigated by neutron reflectivity (NR) in NaCl deuterium oxide (D2O) solutions. We observed that poly(MTAC) chains were drastically shrunk at concentrations above 0.1 M NaCl/D2O, which may be the change in charge-screening effect against ions on poly(MTAC). On the other hand, effect of salt concentration on a swollen state of poly(MPC) brush was negligible, even at the high concentration (5.0 M) close to saturation. The behaviour of poly(MPC) in salt aqueous solution is completely different from that of poly(MTAC), which may arise from the unique interaction properties, neutral nature, and hydrated water structure of phosphorylcholine units.

Unbinding of lipid bilayers induced by osmotic pressure in relation to unilamellar vesicle formation

Small-angle X-ray scattering and phase-contrast microscopy experiments were performed to investigate the effect of osmotic pressure on vesicle formation in a dioleoylphosphatidylcholine (DOPC)/water/NaI system. The multi-lamellar structure of lipid bilayers is unstabilized when a lipid film with a sufficient amount of NaI is hydrated by pure water. It has been confirmed that this phenomenon is due to the effect of osmotic pressure induced by a heterogeneous distribution of NaI molecules. This could be the origin of the unbinding of lipid bilayers to conform large uni-lamellar vesicles.

Small-angle neutron scattering study of droplet density dependence of the water-in-oil droplet structure in a ternary microemulsion

Droplet density, O, dependence of the water-in-oil droplet structure of microemulsion was investigated by means of small-angle neutron scattering (SANS) at room temperature in a ternary system consisting of AOT (Aerosol-OT; dioctyl sulfosuccinate sodium salt), water, and n-decane. SANS profiles from 2 different contrast samples of the bulk contrast (ACT/D 2 O/C 10 H 22 ) and of the film contrast (ACT/D 2 O/C 10 D 22 ) were collected. By introducing a relative form factor as the ratio of scattering functions of the bulk and film contrast samples, we evaluated the O dependence of the shape and size of droplets without the influence of the structure factor, S(Q). The mean radii of droplet core and shell were constant, while the polydispersity index decreased for O ≤ O* ≃ 0.62. The evaluated structure factors S(Q) from the bulk-contrast samples were in good agreement with those from the film contrast. From the O dependence of S(Q), we confirmed that the droplet density fluctuations were dominant at low-O regime and the inter-droplet correlations at high-O regime. The crossover concentration of these two regimes was estimated to be about 0.25. It is demonstrated that this relative form factor method is quite useful for SANS experiments.

Molecular Interdiffusion between Stacked Layers by Solution and Thermal Annealing Processes in Organic Light Emitting Devices

In organic light emitting devices (OLEDs), interfacial structures between multilayers have large impacts on the characteristics of OLEDs. Herein, we succeeded in revealing the interdiffusion in solution processed and thermal annealed OLEDs by neutron reflectometry. We investigated interfaces between a polymer under layer and small molecules upper layer. The small molecules diffused into the swollen polymer layer during the interfacial formation by the solution process, but the polymer did not diffuse into the small molecules layer. At temperatures close to the glass transition temperatures of the materials, asymmetric molecular diffusion was observed. We elucidated the effects of the interdiffusion on the characteristics of OLEDs. Partially mixing the interface improved the current efficiencies due to suppressed triplet-polaron quenching at the interface. Controlling and understanding the interfacial structures of the miultilayers will be more important to improve the OLED characteristics.

Advanced Neutron Reflectometer for Investigation on Dynamic/Static Structures of Soft-Interfaces in J-PARC

A novel neutron reflectometer with horizontal geometry will be established at BL16 in Materials and Life Science Experimental Facility (MLF) of Japan Proton Accelerator Research Complex (J-PARC) as a successor of a reflectometer ARISA-II. ARISA-II corresponding to a single neutron beam downward at 2.22 deg has achieved off-specular and time-resolved reflectivity measurements. The novel reflectometer is designed so as to receive two tilted neutron beams (2.22 & 5.71 deg), which gives us an opportunity in investigation on a free liquid surface. The reflectometer can provide a micro-sized beam by slit collimation and obtain a fair reflectivity with small sample area. Also, T0 chopper and neutron focusing mirror are newly introduced. The T0 chopper can suppress the background due to fast neutrons. The focusing mirror produces further reduction of measurement time not only for specular reflection by focusing neutrons on a sample, but also grazing incidence small-angle neutron scattering (GISANS) measurements by focusing on a detector.

Aggregation states of polystyrene at nonsolvent interfaces.

The aggregation states of polystyrene (PS) thin films at interfaces with nonsolvents such as water, methanol, and hexane were examined by specular neutron reflectivity and sum-frequency generation vibrational spectroscopy. The density profiles of the PS thin films along the direction normal to the interface with water and methanol were comparable to that in air. However, this was not the case for the film in hexane exhibiting a diffuse interfacial layer due to swelling. Also, the local conformation of PS in the outermost region of the films was quite sensitive to the surrounding environment and consequently responded to a change in its environment. This was the case for typical nonsolvents such as water and methanol. The extent of the conformational change might be explained in terms of the interfacial energy.

Bioinspired adhesive polymer coatings for efficient and versatile corrosion resistance

The anticorrosion ability of ultrathin coatings with bio-inspired organic polymers is demonstrated. We prepared a series of catechol-containing poly(alkyl methacrylate)s by free radical polymerization. These copolymers were spin-coated on various corrosion susceptible metal/alloy substrates of magnesium, aluminum, copper and iron without any harsh pretreatment. Several key factors like molecular structure, composition ratio and processing conditions were wisely tailored to afford a transparent, firm and sub-micron polymer coating on those substrates. Corrosion resistance of the polymer-coated substrates was thoroughly investigated by immersion tests in salt-water and acidic solutions, polarization tests, and visual inspection. Formation of an anomalous dense layer of ca. 5 nm thickness adjacent to the metal surface and a remarkable effect of thermal treatment were clearly observed by neutron reflectivity measurements, leading to a highly protective ability against foreign molecules e.g. water or corrosive ions. A key molecular design for anticorrosive polymer coating was revealed to be a combination of strong and versatile binding ability of catechol units and defect free polymer layers formed on the metal substrates in the presence of hydrophobic alkyl chains.