The structure of poly(vinyl alcohol) (PVA) gels formed in a mixture of deuterated dimethyl sulfoxide (DMSO-de) and heavy water at 23 °C has been investigated by wideand small-angle neutron scattering techniques. It was directly confirmed from the wide-angle neutron scattering measurements that cross-linking points or junction points in the gels are crystallites. In the small-angle scattering measurements, it was found that the scattering intensity I(Q) decreases with Q according to the -4th power law (Porod's law) in the Q range above 0.05 A-1 where Q = 4 sin 6/ (26 and are scattering angle and wavelength of neutron, respectively), suggesting that the surface of the crystallites has a clear boundary. On the other hand, the Q dependence of the scattering intensity 7(Q) can be described by 7(0)/[l + £2Q2] in the Q range below 0.035 A-1, where £ is a correlation length. The correlation length £ was assigned to the average distance between the neighboring crystallites. Distance distribution function P(r) which is defined by inverse Fourier transformation of scattering intensity 7(Q) was also calculated in order to see another aspect of the scattering intensity. Two peaks or shoulders at about 70 and 200 A were recognized in the calculated P(r). The former and the latter have been attributed to the intraand intercrystallite correlations, respectively. By separating the two correlations, the distribution of the crystallites is discussed.

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Neutron diffraction experiments for supercritical CO2 have been carried out over a wide range of Q (0.018≤Q≤30 Å−1) at ρ*=ρ/ρc=1.5, 1.2, 0.77, and 0.34 along an isotherm at 310 K (T*=T/Tc=1.02). The measurement enabled us to obtain quantitatively reliable radial distribution functions of the fluid including both short-ranged structure and long-ranged density fluctuation. The structure factor and radial distribution function showed the structural change from the attraction-predominant gaslike structure to the repulsion-predominant liquidlike one with increasing fluid density. With respect to the long-ranged structure, almost linear Ornstein–Zernike–Debye plots were obtained for S(Q) of the fluids at all densities. A plot of correlation length against reduced density seems to have a maximum at the critical density. A ratio, α(r), of the density fluctuation produced by the correlation within r to that to the infinity presented a new aspect of the density fluctuation. Molecular dynamics simulation has also been performed in order to investigate a molecular basis of the experimental radial distribution functions. The calculated GN(r)’s were in good agreement with experimental ones at each density. In laboratory space, a variety of orientational coordination structures are almost equally found in the fluid at all densities examined, while T-shaped structure is preferentially present in polar-angle space.

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