Intercalated clay nanocomposites: Morphology,mechanics, and fracture behavior

Intercalated nanocomposites of modified montmorillonite clays in a glassy epoxy were prepared by crosslinking with commercially available aliphatic diamine curing agents. These materials are shown to have improved Young's modulus but corresponding reductions in ultimate strength and strain to failure. The results were consistent with most particulate‐filled systems. The macroscopic compressive behavior was unchanged, although the failure mechanisms in compression varied from the unmodified samples. The fracture toughness of these materials was investigated and improvements in toughness values of 100% over unmodified resin were demonstrated. The fracture‐surface topology was examined using scanning electron and tapping‐mode atomic force microscopies and shown to be related to the clay morphology of the system. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1137–1146, 2001     

Diffusion of Steroids in Porous Sol-Gel Glass: Application in Slow Drug Delivery

Samples of porous sol-gel glass, after being stabilized at 750°C, were impregnated with progesterone, estradiol, estrone, and hydrocortisone. Diffusion coefficients for those steroids inside the pores filled with pure ethanol, ethanol-water mixture or physiological solution were determined. It is shown that permeability is favored for steroids having fewer hydroxyl and carbonyl groups. Solvents that readily dissolve steroids increase the diffusion rate and thus the amount of delivered material. The flow of steroids in small pores is less restricted than in larger pores.     

Sol-Gel Porous Glass as Holographic Medium

Experiments on hologram recording in porous structure obtained by the sol-gel technology were performed. It was shown that holograms with a diffraction efficiency close to 100% can be obtained with the exposure of about 0.1 J/cm. A pronounced birefringence was observed in dry samples, but it diminished upon immersion in liquids. To explain this effect, a theoretical model was developed. it was shown that even a low asymmetry in pore structure leads to a pronounced birefringence.     

Diffusion of Solvents and Cations in Porous Sol-Gel Glass

Diffusion coefficients for water, cyclohexane, toluene, chloroform, acetone and acetonitrile in porous sol-gel glass were determined using the diaphragm and radioactive tracer techniques. Polar solvents were found to diffuse faster than nonpolar solvent within porous sol-gel glass. The diffusion coefficients of Nd³⁺ and Er³⁺ inside porous sol-gel glass were determined from concentration profiles within monoliths impregnated by 1.6 M rare earth salts dissolved in either acetone or water. To study the effects of ligands on the diffusion, four different erbium salts were used: nitrate, chloride, bromide, and perchloride. It was found that the diffusion rate increases with decreasing radius of rare earth coordination sphere.     

Properties of Luminescent Si Nanoparticles in Sol-Gel Matrices

In this wor e preparation and properties of silica sol-gels incorporating luminescent Si nanocrystallites extracted from porous Si are described for the first time. These sol-gel/Si nanocrystallite composite materials are characterized by BET isotherm measurements, photoluminescence spectroscopy, and infrared spectroscopy. To stabilize the photoluminescence (PL) of Si crystallites within the silica matrix, a fatty acid (capric (C₁₀), myristic (C₁₄) or arachidic acid (C₂₀)) is added as a passivation agent during the hydrolysis of tetraethoxysilane. The presence of the fatty acid is crucial to the long-term stability of the Si nanocrystallite luminescence, as the Si visible light emission remains essentially unchanged for more than a month when the fatty acid is present in the mixture but degrades quickly (within days) when absent. The thermal stability of the Si luminescence within the sol-gel is also reported. Fluorescence microscopy reveals that the light-emitting Si crystallites aggregate into micron-sized domains somewhat unevenly throughout the silica matrix. This distribution of Si crystallites can be improved by employing a surfactant, dioctyl sodium sulfosuccinate (DSS).     

High-pressure Raman study of vibrational relaxation in N2O

The vibrational widths of the ν₁ and ν₃ Raman bands of N₂O were determined at pressures ranging from 8 bar to 2 kbar and temperatures varied from 25 to 150°C. The different dephasing theories including motional narrowing collisional models and resonant vibrational energy transfer theory were tested. A comparison of the theoretical predictions with the experimental data indicates the resonance VV transfer represents the dominant broadening mechanism. The observed frequency shifts between isotropic and anisotropic components of the bands were interpreted in terms of dipole-dipole interactions in dense N₂O.     

Thermal equation-of-state of osmium: a synchrotron X-ray diffraction study

The pressure-volume-temperature behavior of osmium was studied at pressures and temperatures up to 15 GPa and 1273 K. In situ measurements were conducted using energy-dispersive synchrotron X-ray diffraction in a T-cup 6-8 high pressure apparatus. A fit of room-temperature data by the third-order Birch-Murnaghan equation-of-state yielded isothermal bulk modulus K₀=435(19) GPa and its pressure derivative K′₀=3.5(0.8) GPa. High-temperature data were analyzed using Birch-Murnaghan equation of state and thermal pressure approach. The temperature derivative of bulk modulus was found to be −0.061(9) GPa K⁻¹. Significant anisotropy of osmium compressibility was observed.     

Pressure and temperature raman study of the rotational motion of propyne

The Raman line shapes in liquid propyne have been measured to obtain the rotational correlation times and rotational second moments. The correlation times are compared to the predictions of hydrodynamic model. The second moments aie shown to be density dependent. This effect is attributed to collision-induced scattering.     

Vibrational relaxation of liquid dichloromethane at high pressure

The Raman spectra of two symmetric bands ν₁ and ν₃ of CH₂Cl₂ have been measured as a function of pressure to 300 MPa (3 kbar) and over the temperature range 303–363 K. For all bands the isotropic width increases with inreasing pressure and temperature. The experimental vibrational relaxation times are compared with the predictions of different combination of mass factors using the Fischer—Laubereau vibrational dephasing model.