Investigations of twist elastic constant K<Subscript>22</Subscript> of new nematic liquid crystal materials using threshold IPS method

The liquid crystal technology is one of the most popular at the present time. To the most important parameters of liquid crystals belong optical anisotropy, viscosity, and twist elastic constants. One of the most difficult and problematic to measure parameters is a twist elastic constant K₂₂. For new liquid crystalline materials, we have to characterize their physical properties, also a twist elastic constant K₂₂.     

Surface functionalization of titanium dioxide nanoparticles with alkanephosphonic acids for transparent nanocomposites

The surface functionalization of rutile titanium dioxide nanoparticles with 1-decylphosphonic acid and diethyl undec-10-enyl phosphonate in a two-stage process, involving a change in reaction medium, is described. Similarly, 1-decylphosphonic acid and diethyl 1-decylphosphonate were employed as surface modifiers. The nanoparticles coated in two successive steps formed stable, transparent dispersions in toluene. Surface functionalization was monitored using thermogravimetric analysis (TGA), which showed enhanced surface coverage after the second capping step. Incorporation of C=C-terminal surface coupling molecules in the second stage was directly proved using FTIR. Dynamic light scattering measurements showed that the dual-functionalized particles possessed a uniform size of around 13 nm. Particle dimensions were further analyzed using atomic force microscopy (AFM) and transmission electron microscopy (TEM). Transparent nanocomposites were formed by introducing the functionalized nanoparticles into a poly(benzyl acrylate) matrix. The refractive index of poly(benzyl acrylate) composites, measured by spectroscopic ellipsometry, increased from 1.57 for the pure polymer to 1.63 for 14.0 vol.% TiO₂ at λ = 586 nm. Nanocomposite films with particle weight percentages of up to 30% (9.5 vol.%) showed a high light transmittance of around 90% at wavelengths above λ = 400 nm.     

Ab initio determination of atomic structure and energy of surface states of bare and hydrogen covered GaN (0001) surface — Existence of the Surface States Stark Effect (SSSE)

Density Functional Theory (DFT) calculations indicate that energetically stable structure of clean GaN(0001) surface posses (2 × 1) reconstruction, having every second row of Ga located near plane of N atoms, that gives rise to Ga-related dispersionless surface electronic state, already identified by angle resolved photoelectron spectroscopy (ARPES) measurements [S.S. Dhesi et al. Phys. Rev. B 56 (1997) 10271, L. Plucinski et al. Surf. Sci 507-10 (2002) 223, S. M. Widstrand et al. Surf. Sci. 584 (2005) 169]. The energy reduction in reconstruction proceeds via change of the hybridization of the occupied Ga surface states from sp³ to sp², transforming the empty states to p_z type. It is also shown that the electric subsurface field, modeled in new slab model which allows to simulate electric fields at the semiconductor surfaces [P. Kempisty et al., J. Appl. Phys. 106 (2009) 054901], strongly affects the energy of electronic states of GaN(0001) surfaces. The change of the field may shift the energy of surface states of bare and hydrogen covered GaN(0001) surface, by several eV with respect to the band states. The phenomenon, denoted as Surface States Stark Effect (SSSE), explains various band bending values, measured at differently doped n-type GaN(0001) surfaces. It is shown also that, for the adsorbate density up to one H atom for each Ga surface atom i.e. 1 monolayer coverage (1 ML), the hydrogen adatoms are located at the on-top positions, i.e. directly above Ga atoms. For these adsorbate densities, the H-related quantum surface state is located slightly below the valence band maximum (VBM) in the case of p-type GaN surface. For n-type GaN, the H-related surface state is located deeply in the valence band, about 2 eV below VBM. For higher, 1.25 ML hydrogen coverage, the two H adatoms create either surface attached H₂ ad-molecule (energetically stable) or triple bridge configuration is created (metastable). The H₂ ad-molecule is weekly attached to the surface, having the desorption energy barrier equal to 0.16 eV. For 1.25 ML coverage the DFT results were obtained for p-type GaN only. They show that in the ad-molecule case, a new surface electronic state arises which is located about 6.7 eV below VBM. In the case of the bridge configuration, the bridge related surface state is located closely to the conduction band minimum (CBM).     

Reprint of: Application of micro-thin-layer chromatography as a simple fractionation tool for fast screening of raw extracts derived from complex biological,pharmaceutical and environmental samples

The main goal of present paper is to demonstrate the separation and detection capability of micro-TLC technique involving simple one step liquid extraction protocols of complex materials without multi-steps sample pre-purification. In the present studies target components (cyanobacteria pigments, lipids and fullerenes) were isolated from heavy loading complex matrices including spirulina dried cells, birds’ feathers and fatty oils as well as soot samples derived from biomass fuel and fossils-fired home heating systems. In each case isocratic separation protocol involving less that 1 mL of one component or binary mixture mobile phases can be completed within time of 5–8 min. Sensitive detection of components of interest was performed via fluorescence or staining techniques using iodine or phosphomolybdic acid. Described methodology can be applied for fast fractionation or screening of whole range of target substances as well as chemo-taxonomic studies and fingerprinting of complex mixtures, which are present in raw biological or environmental samples.     

Wavelength dependence of light-induced fading and free radical formation in azoreactive dyes

Light-induced stable free radicals (SFRs) were detected in azoreactive dyed cotton fabrics. Extremely slow photofading occurred under exposure to light of relatively long wavelength. The rate of photofading and SFR formation depend on the structure of the dye. Some of them show little drop in the rate of fading with the increasing wavelength, whereas others show nearly negligible changes under incident light of relatively long wavelength. In general a high rate of photofading was found when the SFR concentration was low.     

Microwave-assisted Cannizzaro reaction—Optimisation of reaction conditions

The microwave-assisted Cannizzaro reaction was studied in order to develop fully reproducible synthetic protocols for transformation of aldehydes to carboxylic acid and alcohols. Optimised were the following process parameters: power, temperature, and time. Aromatic, heteroaromatic and aliphatic aldehydes were used in the studies. It was found that furfural, thiophene-2-carbaldehyde, pyridinecarboxaldehyde and aromatic aldehydes react under mild conditions, while 1-methyl-pyrrole-2-carboxaldehyde derivatives and aliphatic aldehydes require more drastic reaction conditions and a longer exposure time to microwave radiation.     

Synthesis and proton conductivity studies of polystyrene‐based triazole functional polymer membranes

In this study, poly(vinylbenzylchloride) (PVBC) was produced by free‐radical polymerization of 4‐vinylbenzylchloride, and then it was functionalized with 3‐amino‐1,2,4‐triazole (ATri) and 1H‐1,2,4‐triazole (Tri). The composition of the polymers was verified by elemental analysis, and the structure was characterized by Fourier transform infrared and ¹³C‐nuclear magnetic resonance spectra. PVBC was modified by ATri with 68% and Tri with 50% yield. The polymers were doped with trifluoromethanesulfonic acid (TA) at various molar ratios, X = 0.5, 1, 2, and 3 with respect to aminotriazole and triazole units. Proton transfer from TA to the triazole rings was proved with Fourier transform infrared spectroscopy. Thermogravimetric analysis showed that the samples are thermally stable up to approximately 200 °C. Differential scanning calorimetry results illustrated the homogeneity of the materials. Under anhydrous conditions, PVBCATri3TA and PVBCTri3TA showed highest proton conductivity of 0.086 and 0.042 S/cm, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Fast graphs for the random walker

 Consider the time T _oz when the random walk on a weighted graph started at the vertex o first hits the vertex set z. We present lower bounds for T _oz in terms of the volume of z and the graph distance between o and z. The bounds are for expected value and large deviations, and are asymptotically sharp. We deduce rate of escape results for random walks on infinite graphs of exponential or polynomial growth, and resolve a conjecture of Benjamini and Peres. Received: 31 October 2000 / Revised version: 5 January 2002 / Published online: 22 August 2002