Noboru Osaka

Pressure-induced reentrant micellization of amphiphilic block copolymers in dilute aqueous solutions.

The pressure-induced structural changes of a block copolymer, poly(2-ethoxyethoxyethyl vinyl ether)-block-poly(2-hydroxyethyl vinyl ether) (pEOEOVE-b-pHOVE) in aqueous solutions, were studied by means of small-angle neutron scattering (SANS) and dynamic light scattering (DLS) from atmospheric pressure up to 400 MPa. pEOEOVE-b-pHOVE formed a spherical micellar structure above 40 degrees C due to poor solubility of pEOEOVE. Micellization phase diagram was determined by DLS, and a covex-upward pressure-temperature (P-T) phase diagram was obtained having a peak around (P,T)=(150 MPa,48 degrees C). The SANS curves at 50 degrees C were analyzed as a function of P. The micellar core size decreased by pressurizing at low Ps (P<or=150 MPa) and then increased by further pressurizing (150 MPa<P<400 MPa). It was confirmed that the water volume fraction in the micellar core was larger at high Ps than that at low Ps even when the core sizes are the same at both pressure regions. This means that the temperature dependence of hydration is more selective at low pressures than at high pressures, in good accordance with our previous results on concentrated aqueous solutions of block copolymers [Phys. Rev. Lett. 96, 048303 (2006)].

Quasi-Elastic Neutron Scattering Study on Water and Polymer Dynamics in Thermo/ Pressure Sensitive Polymer Solutions

Dynamics of water and poly(N-isopropylacrylamide) (PNIPA) in concentrated aqueous solutions, where the majority of water molecules are attached to polymer chains, has been investigated with use of incoherent quasi-elastic neutron scattering (QENS) and dynamic light scattering (DLS) measurements as functions of temperature, T, and hydrostatic pressure, P. It was observed by QENS that the self-diffusion coefficient, D(water), of water in PNIPA/H(2)O solutions increased by P at temperatures below the lower critical solution temperature (LCST) of PNIPA aqueous solutions. However, above the LCST, D(water) decreased by P, as is often reported in non-hydrogen bonding solutions. In isobaric heating runs, therefore, the jump in D(water) at LCST decreased with increasing pressure. On the other hand, the mean-square displacement, , of the local vibrational motion of PNIPA in PNIPA/D(2)O solutions, where the incoherent scattering signal of PNIPA was predominantly observed, was reduced due to the aggregation behavior of PNIPA by pressurizing, which was also confirmed by using DLS. The jump in at the LCST became gradual by pressurizing, which was consistent with the changes of the dynamics of water obtained in PNIPA/H(2)O solutions.

In situ small-angle neutron scattering and rheological measurements of shear-induced gelation.

The microscopic structure of shear-induced gels for a mixed solution of 2-hydroxyethyl cellulose and nanometer-size spherical droplets has been investigated by in situ small-angle neutron scattering (SANS) with a Couette geometry as a function of shear rate gamma. With increasing gamma, the viscosity increased rapidly at gamma approximately 4.0 s(-1), followed by a shear thinning. After cessation of shear, the system exhibited an extraordinarily large steady viscosity. This phenomenon was observed as a shear-induced sol-gel transition. Real-time SANS measurements showed an increase in the scattering intensity exclusively at low scattering angle region. However, neither orientation of polymer chains nor droplet deformation was detected and the SANS patterns remained isotropic irrespective of gamma. It took about a few days for the gel to recover its original sol state. A possible mechanism of gelation is proposed from the viewpoint of shear-induced percolation transition.

Structural Characterization of Ionic Gelator Studied by Dynamic Light Scattering and Small-Angle Neutron Scattering

Structural characterization of a hydrogel consisting of an oligomeric electrolyte, poly[pyridinium-1,4-diyliminocarbonyl-1,4-phenylenemethylene chloride] (1-Cl) as an ionic gelator was carried out by static and dynamic light scattering (SLS/DLS) and small-angle neutron scattering (SANS) techniques. All the measurements were performed by changing the concentration and temperature. We have successfully obtained the weight average molecular weight and the degree of polymerization of poly(1-Cl) by SLS. The sol-gel transition was clearly observed as large fluctuations in the scattering intensity of the time-intensity correlation function. Time correlation function of the scattering light intensity entailed a power law behavior at the sol-gel transition. 1-Cl hydrogel showed strong hysteresis and its hysteresis loop was observed both by DLS and theological methods. We have estimated the critical concentration of gelation and the gelation temperature by DLS. The enthalpy change for gelation was estimated to be ca. -10 kJ/mol. SANS experiments revealed that the unit structure of the gel network is responsible for the gelation of 1-Cl hydrogel.

Dynamics and Microstructure Analysis of N-Isopropylacrylamide/Silica Hybrid Gels

The mechanical properties, gelation process, and microstructure of N-isopropylacrylamide (NIPAm)/silica hybrid gels were investigated by using mechanical measurements, dynamic light scattering (DLS), and contrast-variation small-angle neutron scattering (CV-SANS). The three different hybrid gels, containing an equal total volume fraction of silica particles of different sizes (NS_S: small = 20-30 angstroms radius; NS_M: medium = 40-55 angstroms; and NS_L: large = 100-150 angstroms), were employed for comparison. An obvious effect on the compressive properties due to the size of the silica particles was observed, with harder materials produced from particles of smaller size. The gelation process was strongly dependent on the competition between silica aggregation and NIPAm polymerization, as measured by using the DLS methods. In the case of NS_S, the rate of aggregation was larger than NIPAm polymerization, resulting in the formation of an aggregate-dominant microstructure. The aggregation rate for NS_M and NS_L, however, was comparable to the NIPAm polymerization rate, leading to simultaneous aggregation and polymerization. CV-SANS determined that the NIPAm adsorption layer on the silica surface was clearly affected on a microscopic scale by the silica particle sizes.

Detector Area Expansion at iNSE Neutron Spin Echo Spectrometer

A 2D-detector and an analyzer had been installed on iNSE neutron spin echo spectrometer for wide-area data acquisition. Since the areas of the 2nd π/2 spin flipper and the last Fresnel coil were not wide enough for a full area measurement, only the center area of the detector had been used. In 2006, we renewed the spin flipper and the Fresnel coils to expand the detection area. To improve the efficiency of the data acquisition, we proposed a novel method for phase correction of NSE signals. Finally, the efficiency of the NSE measurement increased by approximately one order of magnitude.