Tadanori Koga

Resonant soft x-ray reflectivity of organic thin films

At photon energies close to absorption edges in the soft x-ray range, the complex index of refraction, n=1−δ−iβ, of organic materials varies rapidly as a function of photon energy in a manner that strongly depends on the chemical moieties and functionalities present in the material. The authors present details of how these molecular structure specific variations in the complex index of refraction can be utilized to enhance and tune the contrast in reflectivity experiments of organic films. This near edge contrast enhancement mimics the specific contrast achieved through deuterium labeling in neutron reflectivity (NR). This relatively new x-ray approach, resonant soft x-ray reflectivity (RSoXR), thus combines aspects of NR and conventional x-ray reflectivity (XR), yet does not require special chemical procedures. The capabilities of RSoXR are exemplified using a number of polymeric bi- and multilayers. Furthermore, a direct comparison of RSoXR to conventional x-ray reflectivity and NR for polystyrene and p...

Low-density polymer thin film formation in supercritical carbon dioxide

We report a method for producing stable low-density polymer films by using supercritical carbon dioxide (scCO2). Two different molecular weight polystyrene films with various thicknesses were exposed to scCO2 along the density fluctuation ridge in P–T phase diagram. The swollen structures could be then frozen by flash evaporation of CO2 without forming additional voids. X-ray reflectivity data clearly showed that exposure to scCO2 could be used to produce uniform low-density films of about 2Rg thick or less, where Rg is radius of polymer gyration.

Development of a High-Flux- and High-Temperature-Set-Up Bonse–Hart Ultra-Small-Angle X-ray Scattering (USAXS) Diffractometer

The conventional Bonse-Hart ultra-small-angle X-ray scattering (USAXS) diffract©meter utilizes an asymmetric forecrystal in front of the symmetric grooved crystal in order to increase the flux collected from an X-ray source. To avoid the intensity loss resulting from mixing asymmetric and symmetric Bragg reflections, the forecrystal is carefully aligned with respect to the grooved crystal in such a way that the rocking curves from the asymmetric and symmetric Bragg reflections completely overlap. Our new monochromator for USAXS utilizes both asymmetric and symmetric Bragg reflections in the same rigid monolithic channel-cut 220 germanium crystal. It is therefore stable over a wide temperature range from 258 to 573 K without the use of a special constructing material with a very small thermal-expansion coefficient such as Super Invar.

Ultra-small-angle x-ray scattering studies on order-disorder transition in diblock copolymers

The phase behavior near the order-disorder transition (ODT) in diblock copolymers was characterized by ultra-small-angle x-ray scattering (USAXS) method for two polystyrene-block-polyisoprene (PS-b-PI) copolymers having about equal block volume fraction f=0.45 (f is the volume fraction of PS) but different molecular weights. By using USAXS method, the peak width and peak intensity were found to change by about 2 orders of magnitude across the ODT temperature for both PS-b-PI. Furthermore, in a narrow temperature region very close to the ODT, it is revealed that the coexistence of the ordered and the disordered phases occurs at thermal equilibrium for both PS-b-PI. The systematic results in the present study clearly show the thermal fluctuation effects on the phase diagram in the low molecular weight diblock copolymer systems.

Mixing Symmetric and Oblique Bragg Reflections in Rigid Channel-Cut Crystals

Because the Bragg angle and Darwin range of total reflection in symmetric and oblique Bragg reflections vary with the angle between the Bragg planes and the crystal surface, it is generally believed that the resulting mismatch causes a loss of intensity in monolithic channel-cut crystals that is unacceptable in practice. It has been discovered that in the unique case of the 220 Bragg reflection from germanium no mismatch occurs, so that oblique and symmetric reflections can be mixed in one rigid channel-cut crystal. The results are valid for both X-ray and neutron channel-cut monochromators.

Dynamic light-scattering study of self-assembly of diblock copolymers in supercritical carbon dioxide.

A high-pressure dynamic light-scattering (DLS) technique has been utilized to study the behavior in solution of poly(1, 1-dihydroperfluorooctylacrylate) and poly(vinyl acetate) (PFOA-b-PVAC) in supercritical carbon dioxide. The hydrodynamic-radius distribution for each species, such as unimers, micelles, and large aggregates, were determined under both isobaric and isothermal conditions over a pressure range of 9-55.2 MPa, and a temperature range of 25-75 degrees C, respectively. The DLS results clearly showed both pressure-induced and temperature-induced dissolution and association behavior for the copolymer in supercritical carbon dioxide. Also presented are some preliminary experimental results for the micellar self-assembly of a fluorinated block copolymer, poly(2-tetrahydropyranyl methacrylate)-b-poly(1h, 1h-perfluorooctyl methacrylate) (THPMA-b-F7MA), in supercritical carbon dioxide by use of a new high-pressure cell that allows us to conduct simultaneous small-angle x-ray scattering and DLS measurements.

High-pressure cell for simultaneous small-angle x-ray scattering and laser light scattering measurements

A cell for in situ observation of self-assembled nanostructured materials in high-pressure fluids, including supercritical carbon dioxide, has been constructed and tested. The compact cell can be used for simultaneous small-angle x-ray scattering, laser light scattering, visual observation, and some other types of spectroscopic measurements. The light scattering plane is normal to the x-ray scattering plane with the optical axis and the x-ray axis being perpendicular to each other. The scattering is detected at the intersection of the light and x-ray beams, and thus one effectively probes the same scattering volume. Some preliminary experimental results for the micellar self-assembly of a fluorinated block copolymer in supercritical carbon dioxide are presented.

Domain population in SrTiO3 below the cubic-to-tetragonal phase transition

Abstract High-angle double-crystal X-ray diffractometry (HADOX) has been applied to measure the intensity around three reciprocal lattice points: 400, 040 and 004 of SrTiO3. Each point splits into two in the tetragonal phase according to the appearance of three kinds of domains; the temperature dependence of the intensity of component peaks at these points was measured under identical conditions. The results show that the domain population varies with temperature in a range of about 20 K below the transition. It is found, however, that the total intensities of 400 and 004 of the tetragonal phase are constant; this is observed by summing up the relevant intensity at the three reciprocal lattice points.

Introduction of molecular scale porosity into semicrystalline polymer thin films using supercritical carbon dioxide

We report supercritical carbon dioxide (scCO2) technology used for forming a large degree of molecular scale porosity in semicrystalline polymer thin films. The following three steps were integrated: (i) pre-exposure to an organic solvent which melted crystalline structures but did not cause a decrease in thickness, (ii) scCO2 exposure under the unique conditions where the anomalous absorption of CO2 occurred, and (iii) subsequent quick evaporation of CO2 to preserve the swollen structures. This unified process resulted in homogenous low-density polyphenylene vinylene films (a 15% reduction in density) with the sustained structure for at least 6 months at room temperature.

Directed self-assembly of nanoparticles at the polymer surface by highly compressible supercritical carbon dioxide

We report a versatile route for self-assembly of polymer-soluble nanoparticles at the polymer surface using highly compressible supercritical carbon dioxide (scCO2). Polystyrene and poly(methyl methacrylate)-based nanocomposite thin films with functionalized polyhedral oligomeric silsesquioxane and phenyl C61 butyric acid methyl ester nanoparticles were prepared on Si substrates and exposed to scCO2 at different pressures under the isothermal condition of 36 °C. The resultant structures could be then preserved by the vitrification process of the glassy polymersvia quick pressure quench to atmospheric pressure and subsequently characterized by using various surface sensitive experimental techniques in air. We found that the surface segregation of these nanoparticles is induced in the close vicinity of P = 8.2 MPa where the excess absorption of the fluid into the polymers maximizes. However, when the film thickness becomes less than about 4Rg thick (where Rg is the radius of polymer gyration), the uniform dispersion of the nanoparticles is favorable instead even at the same CO2 conditions. We clarify that the phase transition is correlated with the emergence of a concentration gradient of the fluid at the polymer/CO2 interface and is a general phenomenon for different polymer–nanoparticle interactions.