Published papers

© 2020 Walter de Gruyter GmbH, Berlin/Boston. Various forms of streak patterns are observed in small angle X-ray scattering from Highly Oriented Pyrolytic Graphite (HOPG). The streak patterns are assigned as specular reflection, double Bragg scattering, and diffuse + Bragg double scattering. These scatterings contain various information on the mesoscopic structures of the HOPG sample.

Recently isotactic poly(4-methyl-1-pentene) (P4MP1) has been found to selectively absorb long-chain alkanes from alkane mixed systems. In this article, we introduce the idea of this study from the pressure-temperature phase diagram. We also show that such selective absorption characteristics can be explained from a simple model, namely Asakura-Oosawa theory, and the relationship with molecular recognition is explained.

Small molecules are absorbed selectively in the amorphous regions in semicrystalline polymers, except for some specific polymeric systems forming host-guest cocrystals such as syndiotactic polystyrene. The crystalline density of isotactic poly(4-methyl-1-pentene) (P4MP1) is substantially lower than those of analogs, and it is even slightly lower than the amorphous density. This study is investigated the absorption/desorption behaviors of short alkanes in P4MP1 in situ, and found that the alkanes are mainly absorbed in its amorphous region. Infrared spectroscopy measurements reveal that the P4MP1 film with low crystallinity absorbs higher amount of decane than the high crystallinity film does. Also it is found by small-angle X-ray scattering (SAXS) that, when hexane is dropped onto the P4MP1 as-cast film, a lamellar long-spacing peak appears at 0.16 nm−1 which disappears with the spontaneous desorption of hexane. Because the pristine P4MP1 films give no obvious SAXS peak due to the close density of crystalline and amorphous regions, the appearance of the SAXS peak indicates that an X-ray contrast is obtained by the preferential absorption to amorphous region. The disappearance of the peak is thus reasonably understood by the loss of the contrast due to desorption.

© 2018 American Physical Society. Collective dynamics in liquid GeTe was investigated by inelastic x-ray scattering at 2≤Q≤31nm-1. The dynamic structure factor shows clear inelastic excitations. The excitation energies at low Q disperse with increasing Q, consistent with the behavior of a longitudinal-acoustic excitation. The dispersion curve has a flat-topped region around the pseudo-Brillouin-zone boundary, similar to what is observed in liquid Bi [Inui, Phys. Rev. B 92, 054206 (2015)PRBMDO1098-012110.1103/PhysRevB.92.054206]. The dynamic structure factor shows a low-frequency excitation, and its coupling with the longitudinal-acoustic mode plays an important role for a flat-topped dispersion. From these results, it is inferred that atomic dynamics in liquid GeTe is strongly affected by a Peierls distortion similar to liquid Bi. By comparing the momentum transfer dependence of the excitation energy and quasielastic linewidth to partial structure factors obtained by our own ab initio molecular dynamics simulation for liquid GeTe, the quasielastic and inelastic components were found to be correlated with Te-Te and Ge-(Ge,Te) partial structure factors, respectively.

The dynamic structure factor S(Q, E), where Q and E are momentum and energy transfer, respectively, has been measured for liquid Sb, using inelastic x-ray scattering. A modified damped harmonic oscillator model function was applied to analyse S(Q, E) of liquid Sb and also to that of liquid Bi by Inui et al (2015 Phys. Rev. B 92, 054206). The obtained excitation energy was in fairly good agreement with that predicted by ab initio molecular dynamics simulations on these liquid semi-metals. The excitation energy of the longitudinal acoustic mode in liquid Sb and liquid Bi exhibits flat-topped Q dependence whereas the lower excitation energy below the longitudinal acoustic excitation shows Q-gap behaviour. From the viscosity estimated from the Q-gap experimentally obtained, it is inferred that the lower energy excitation arises from the transverse acoustic excitation in the liquids.

The dynamic structure factor S(Q,E) of liquid Ge2Sb2Te5 has been obtained by inelastic x-ray scattering, where Q and E are momentum and energy transfer, respectively. The dispersion curve of the longitudinal acoustic excitation energy exhibits flat-topped Q dependence similarly to that in liquid GeTe and liquid Bi, where the local structure is modulated by Peierls distortion. The flat-topped energy in liquid Ge2Sb2Te5 is split into low and high energy parts arising from Sb-Te and Ge-Te correlations, respectively. Furthermore, the high energy part depends on Q like an optical mode with decreasing Q to zero, and it approaches the vibrational energy of fourfold coordinated Ge atoms with octahedral configurations. The result indicates that a majority of Ge in the liquid stays in octahedral order as predicted by first-principles molecular dynamics simulations.

© 2019 Elsevier B.V. Inelastic x-ray scattering spectra of fluid S were measured from 403 K near the melting point to 1343 K in the supercritical region at 25 MPa. Although the dynamic structure factor in the supercritical region exhibits a single peak profile, the spectrum includes collective inelastic excitations besides the quasielastic component. The acoustic excitation energy in the supercritical region is close to the one predicted by the sound speed in the fluid consisting of S8 molecules and the observed excitation energies of the stretching modes in S molecules suggest that few fraction of S2 molecules in the supercritical region near the liquid-vapor critical point. This result indicates that the nearest neighbor coordination number in the supercritical fluid S obtained by x-ray diffraction [Inui et al., J. Phys. Conf. Ser. 98 (2008) 012008] was underestimated.

© International Union of Crystallography, 2018 It is observed that radial streak patterns of double Bragg scattering appear in the small-angle X-ray scattering from highly oriented pyrolytic graphite (HOPG). The intensity profile of double Bragg scattering from an HOPG sample is calculated theoretically. Assuming that the c axes of the graphite crystallites in the HOPG sample are distributed around an orientation vector and their distribution function has a Gaussian form, it is found that the intensity profile of double Bragg scattering is expressed by a double Gaussian function of the scattering angle and the azimuthal angle of the streak. The calculated intensity profile is compared with the experimental one. The method developed in this article can be used to estimate the orientational distribution of crystallites in uniaxial polycrystalline materials.

© International Union of Crystallography, 2019 In the small-angle scattering from as-prepared and neutron-irradiated highly oriented pyrolytic graphite samples, a new type of streak pattern is observed near the -type double Bragg scattering. This newly observed scattering is assigned as the double scattering whose first scattering is diffuse scattering near the 00l Bragg scattering and whose second scattering is the Bragg scattering. Specular reflection from the surfaces of the crystallites or microcracks is also observed as a streak. These two types of scattering from the neutron-irradiated sample show oscillatory behavior, unlike that from the non-irradiated samples. The scattering is analyzed using the sharp-boundary model, and is explained by the assumptions that the non-irradiated sample has interfaces with a width of about 1 nm and the neutron-irradiated sample has a slab-like structure with a thickness of about 14 nm.

© 2019 American Chemical Society. The depletion force has been used to explain phase separation phenomena in colloidal systems. Here, we showed that depletion force can explain not only phase separation of large and small colloidal particles but also preferential absorption of larger molecules from a mixture of large and small molecules in a liquid state. When a polyolefin cast film was immersed in a mixture of long and short normal alkanes, the longer molecules were selectively absorbed into the film. This experimental result was explained from the viewpoint of depletion force. The main finding was the use of confined space to emphasize the separation tendency caused by the force. In general, the increase in entropy may serve as a driving force to mix molecules. However, if sufficiently narrow pores are present, large and small molecules are separated naturally by size as the entropy increases. This finding will lead to size exclusion chromatography of low-mass molecules, similar to gel permeation chromatography of macromolecules. In order to demonstrate the effect of depletion force, we selected and experimented with a system based on a polyolefin isotactic poly(4-methyl-1-pentene) (P4MP1) film and a normal alkane mixture and realized high molecular selectivity. The P4MP1 film we used can be prepared simply by evaporating the solvent from the solution and casting the film. On the basis of the Asakura-Oosawa theory, we concluded that spontaneous and high molecular selectivity is attributed to the depletion force provided by the small sub-nanopores with uniform size in the film.