[en] This paper reviews our recent experimental results that address the effects of solvent density inhomogeneities in supercritical carbon dioxide (scCO₂) on polymer thin film processing. The key phenomenon is excess sorption of CO₂ molecules into polymer thin films even when the bulk miscibility with CO₂ is very poor. We have found that the amount of the excess sorption is attributed to the large density fluctuations in scCO₂ near the critical point. Further, taking advantage of the vitrification process of polymer chains through quick evaporation of CO₂, we can preserve the 'expanded' structures as they are. The resultant films have large degree of molecular-level porosity that is very useful in producing coatings with low dielectric constants, enhanced adhesion, and metallization properties. These characteristics can be achieved in an environmentally 'green' manner, without organic solvents, and are not specific to any class of polymers

[en] The horizontal touch and Langmuir-Blodgett techniques have been used to transfer a monolayer of n-octadecanol from the gas-solution interface of a Langmuir trough onto the metal-solution interface of a Au(1 1 1) electrode. Chronocoulometry has been used to determine the charge density at the electrode surface covered by the film of n-octadecanol. The surface pressure of this film was calculated from the charge density data and was found to be controlled by the electrode potential. We have demonstrated that by dialing the potential applied to the electrode via a potentiostat the monolayer adsorbed on the surface can be compressed or decompressed. Two states of the monolayer were observed. The transition between these states took place at a film pressure ∼12 mN m^-1. Neutron reflectometry and polarization modulation Fourier transform infrared reflection absorption spectroscopy have been employed to determine the nature of the two states. The results show that octadecanol molecules form a two-dimensional solid film at all film pressures. At film pressures larger than 12 mN m^-1, the film has low compressibility and n-octadecanol molecules assume a small tilt angle with respect to the surface normal. At film pressures lower than 12 mN m^-1 a compressive film is formed in which the tilt angle progressively increases with decreasing surface pressure. We have demonstrated that the properties of a monolayer of n-octadecanol at the metal-solution interface display many similarities to the properties of that film at the air-solution interface

[en] 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 polymers via 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 4R_g thick (where R_g 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.

[en] Neutron scattering measurements have been made of the soft mode in the uniaxial ferroelectric lead germanate for which the transition temperature is 450 K. The results are consistent with the simple soft-mode picture of the soft mode. A comparison with the results of Raman scattering measurements suggests that the latter measurements do not provide information about the one-phonon spectral function for temperatures greater than (T_c-10) K

[en] The structure of water at aqueous interfaces is of the utmost importance in biology, chemistry, and geology. We use neutron reflectivity and quartz crystal microbalance to probe an interface between hydrophilic quartz and bulk liquid solutions of H₂O/D₂O mixtures. We find that near the interface the neutron scattering length density is larger than in the bulk solution and there is an excess adsorbed mass. We interpret this as showing that there is a region adjacent to the quartz that is enriched in D₂O and extends 5-10 nm into the solution. This suggests caution when interpreting results where D₂O is substituted for H₂O in aqueous interfacial chemistry.

No abstract available

[en] Commensurate monolayers of a mixture of 70% carbon monoxide and 30% argon on graphite are studied by neutron and low-energy-electron diffraction. A 2 x 2 superstructure due to 3:1 compositional ordering is observed below 25 K. The compositional ordering is most likely due to molecular-axis ordering of carbon monoxide molecules into a pinwheel pattern with argon atoms at the central sites of each pinwheel. Such a pinwheel structure has been predicted by Harris, Mouritsen, and Berlinsky for planar rotors with anisotropic interactions on a triangular lattice with vacancies

[en] We synthesized C₁₈-functionalized gold and palladium nanoparticles with average diameter size of 10 and 3 nm, respectively, and carried out a systematic study of the effect of nanoscale metallic fillers on the dewetting dynamics of PS/PMMA bilayer substrates. Optical and atomic force microscopies were used to study the hole growth and determine the viscosity of the films as a function of PS molecular weight, particle radius, and concentration. Neutron reflectivity was used to measure the effects of the nanoparticles on the tracer diffusion coefficient. X-ray reflectivity and TEM microscopy were used to study the distribution of the particles within the film and ensure that no segregation or clustering occurred. The results indicated that the dynamics are a sensitive function of the ratio between the filler radius, R_particle, and the polymer radius of gyration, R_g. The data were found to collapse on a universal curve where the relative velocity of the filled system was faster than that for the unfilled system when R_g/R_particle > 4 and slower when R_g/R_particle < 4. Shear modulation force microscopy method (SMFM) measurements were performed as a function of temperature and indicated that T_g was depressed by 12 C relative to the bulk when R_g/R_particle > 4 and unchanged when R_g/R_particle < 4. The results were interpreted in terms of an increase in the local excluded volume and possible elastic distortions of the polymer matrix

[en] Inelastic neutron scattering spectra around the frequency of the longitudinal-optic branch of the phonon dispersion relation in sodium chloride have been measured at 300 and 600 K. The results are analyzed to indicate distortions of the line shapes from Lorentzian form, and calculations which indicate dependence of this distortion on wave vector are presented

[en] We employed electrochemical methods together with in situ neutron reflectometry to describe the aggregation of organic surfactant molecules at a solid-liquid interface. The neutron reflectometry allowed us to determine the surface coverage, thickness, roughness and the relative positions of the aggregates. We found that the applied electric field may be used to reversibly manipulate the architecture of the organic molecules: from uniform monolayers to adsorbed hemi-micelles. These studies are expected to provide a new insight into the roles played by entropic and electrostatic forces in complex fluids or biomaterials. (orig.)