Measurements of transient membrane potential after current switch-off as a tool to study the electrochemical properties of supported thin nanoporous layers

We have studied the potential of chronopotentiometry after current switch-off as a tool for electrochemical characterization of thin supported nanoporous layers. Within the scope of this technique, a thin supported electrochemically active layer is polarized by direct electric current until a steady state is reached. After that, the current is switched-off in a stepwise manner, and the reading of transient membrane potential begins. A linear non-steady-state theory of the method has been developed in terms of a model-independent approach of network thermodynamics. The measurements of transient membrane potential after current switch-off have been carried out in KCl solutions of various concentrations for a commercially available nanofiltration membrane (Desal5 DK). Such membranes consist of micron-thick active (or barrier) nanoporous layers and much thicker (100-200 microm) and coarse-porous supports (the pore size usually is 0.1-5 microm). The reproducibility of the method has been found to be quite reasonable especially in not too dilute electrolyte solutions and at not too short times (> or = 10 ms). The relaxation measurements have been complemented by the measurements of the steady-state membrane potential and by sample measurements of salt rejection in the pressure-driven mode, which enabled us to carry out a self-consistent interpretation of the experimental data. This has revealed, in particular, that the ion rejection mechanism related to the fixed electric charges is not the dominant one in the case of the Desal5 DK nanofiltration membrane. Proceeding from a quantitative interpretation of relaxation patterns, we could also determine some properties of membrane support, namely, the porosity and the salt diffusivity. They have been found to have reasonable values remarkably independent of salt concentration, which confirms the self-consistency of our interpretations.     

Electrooptic properties of liquid crystals filled with silica nanoparticles of different sorts

In this work we investigate electrooptic properties of nematic liquid crystal (LC) E7 from Merck filled with two types of silica nanoparticles (NPs). The particles of these types are of similar size and shape, however, they are made by different methods. The NPs of the first type, aerosil (A), are prepared by a high-temperature hydrolysis of chlorosilanes. The second type of NPs, based on hydrolysis and controlled precipitation of tetraethylorthosilicate in alcohol, yields monodispersed NP (MNPs) of silica. Transmittance versus applied voltage curves of the suspension layers are measured and analyzed. Our results show a strong dependence of the electrooptic properties on the particle preparation method. The contrast ratio (CR) 61:1 of LC-A suspension is achieved at relatively low A content (17 wt %). Unlike it, the value of the CR of LC-MNP suspensions is low (about 2:1) even at 30 wt % of MNPs in the mixture. Both types of suspensions show electrooptic memory. The maximum of the memory efficiency of LC-A suspensions is detected at 4 wt % of A, while for LC-MNP suspensions the maximum is achieved at 15 wt % of MNPs. The difference in the results for the LC-A and LC-MNP series of composites is caused by much stronger aggregation of MNPs compared with A in the LC matrix. Moreover, reactive particle fusion and formation of silica microcrystallites substantially enhances the effect caused by MNP aggregation. These processes deteriorating electrooptic performance may be stabilized by introduction of a polymer network in LC-MNP suspensions.     

Statistical Modeling in the Problem of Excitation of Fields by Random Sources on a Surface

We develop a method for statistical modeling of the emission of noise fields by sources located on a surface and consider the problem of sound-wave emission by the ocean surface. We also construct a model of distribution of the pressure and vibrational velocity of the sound field in a medium on the basis of randomization of the spectral density of pressure fluctuations on the surface. The obtained samples are exact solutions of the wave equation. The behavior of some statistical moments of the simulated field is studied.     

Photoluminescence of pentyl-cyanobiphenyl in liquid-crystal and solid-crystal states

The stationary and time-resolved photoluminescence spectra of n-pentyl-n′-cyanobiphenyl (5CB) are studied in a mesophase and solid-crystal state. The photoluminescence spectra of 5CB are determined by the molecular configuration and the intramolecular charge transfer in the excited state. It is shown that in the mesophase and solid-crystal state, 5CB exhibits along with monomer radiation at least two types of excimer radiation from different pre-dimer states.     

Anisotropic surface morphology of azopolymer films generated by polarized UV light irradiation

X-ray reflectivity measurements reveal anisotropy of the vertical surface roughness caused by exposure to linearly polarized UV (LPUV) light in the films of two azopolymers. The irradiated surface is found to have higher roughness in the direction parallel to the direction of polarization than in the orthogonal direction. The photo-modification of the surface morphology is caused by spatial changes induced in polymer films by LPUV irradiation. The important role of surface roughness anisotropy in determining the alignment of liquid crystals is discussed.