Ionic relaxation in nematic liquid crystal cells

We investigate the dependence of the relaxation time of the current flowing in a nematic cell submitted to an external dc voltage on the physical properties of the substrate. We show that previously presented analyses of the same problem are not very useful for practical applications. We compare our theoretical predictions with experimental data, and show that the agreement is rather good. The influence of the adsorption-desorption phenomenon on the relaxation time is also discussed.     

Optimization of a New Extraction Technique for Analysis of Verbenone and cis-Verbenol in Pine Seeds

Results from a systematic study of the factors affecting extraction of cis-verbenol and verbenone from pine seeds are presented. Five extraction conditions were investigated: extraction solvent, method of extraction, extraction temperature, volume of solvent, and the ratio of the mass of sample to the amount of extraction solvent. The resulting optimized method uses magnetic-stirring-assisted extraction of pine seeds (5 g) with ethyl acetate (75 mL) for 20 min, at room temperature. RSDs were less than 5% for both compounds. GC–FID was used for quantification of cis-verbenol and verbenone in the extracts.     

Microgravimetric study of electrochemically controlled nucleophilic addition of sulfite to polyaniline

The sulfonation of polyaniline (PANI) films by nucleophilic addition of sulfite ion has been controlled through the polymer oxidation state under electrochemical control. The process was monitored by in situ electrochemical quartz crystal microbalance (EQCM), and the polymer oxidation was accomplished by electrode potential steps in sulfite aqueous solutions. The nucleophilic addition of sulfite to PANI only takes place on the oxidized polymer. From the ratio of added mass to the injected charge, the degree of sulfonation has been obtained with a yield as high as 50%. It has been observed that the ion-exchange mechanism during the oxidation-reduction process in the resulting sulfonated polymer is analogous to the polymer produced by electrophilic sulfonation of polyaniline or by copolymerization of aniline with aminosulfonic acids, unlike the ionic exchange observed for unmodified PANI.     

Formation of Tertiary 3-Nitro Allylic Alcohol by Condensation of 2-Chloroiso- butyrophenone with Nitromethanide Anion: Estimation of Gibbs Free Enthalpies of Reaction in Solution for the Reaction Mechanism [1]

Summary. The reaction of 2-chloroisobutyrophenones and nitromethanide anion gives stereoselectively (E)-3-nitro allylic alcohols. The Gibbs free enthalpies of reaction in DMSO for carbanion addition, epoxide formation, and rearrangement to 3-nitro allylic alcohol, as elementary steps for the reaction, were estimated from corresponding neutral gas reactions and using a thermodynamical approach to the transfer of gaseous compounds to DMSO. A criterion for assigning the sign of affinity of liquid compounds to DMSO was developed on the basis of the Gibbs enthalpies of liquefaction. The information obtained on reaction rate and thermodynamic viability of the steps indicates that carbanion addition is the rate-determining step.In memory of Prof. Dr. M. Ballester, deceased on April 6, 2005     

The contrast gain in a guest-host nematic display due to a periodic boundary condition

The mean square tilt angle of a nematic slab with finite anchoring energy and periodic boundary conditions has been theoretically investigated, as a function of the slab geometry and of the reduced extrapolation length. If the anchoring strength is free-surfacelike, the contrast is affected by a loss 10% at room temperature if the ratio between the anchoring pitch and the cell thickness is 0.5.Glossary anchoring pitch - h cell thickness - /h - ( = x/, = y/h) reduced coordinates - (, ) local tilt angle - elastic constant - w_a anchoring energy anisotropy - b=/w _a de Gennes-Kleman extrapolation length - B=b/h reduced extrapolation length - T _NI nematic-isotropic transition temperature - :=(T/T _NI ) – 1 reduced temperature - easy axis direction - _MAX - _± ² mean square tilt angle along the boundary - () absorbance coefficients of the p-dye - r /: dichroic ratio - c contrast - G contrast gain - S order parameter     

Influence of the surface topography on the surface transitions in nematics

Abstract

In quasi-homeotropic oriented liquid crystal cells, obtained by oblique SiO deposition on glass and surfactant added to the nematic, a continuous tilt angle versus temperature change followed by an abrupt surface transition was observed. At room temperature, the sign of the tilt angle measured from the normal to the surface depends on the SiO deposition angle. A model for the temperature dependence of the tilt angle is proposed.     

Evidence for the flexo-electric effect in a nematic liquid crystal cell

Abstract

We present evidence for the flexo-electric effect in a hybrid aligned nematic cell containing 4-methoxybenzylidene-4'-n-butylaniline (MBBA) subject to d.c. excitation, by comparing the theoretical curve derived by including the flexo-electric term in the free energy with the experimental points. The agreement appears to be very good for MBBA doped with a small amount of an additive which shows reversible electrochemical reactions at very low redox potential; this allows us to determine the sum of the flexo-electric coefficients and the anchoring energy on the planar wall. For undoped MBBA the experimental results can be justified only by assuming a low voltage screening of the electric field, due to the presence of ionic impurities in the material.