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L Komitov

Publications and source records attributed to L Komitov.

7 recordsLinked to original sources
PubMed2002

Photomanipulation of the anchoring strength of a photochromic nematic liquid crystal.

The behavior of the Fréedericksz threshold voltage of a planarly oriented sample filled with liquid crystalline azobenzene material possessing a nematic phase is investigated under UV illumination. It is found that the anchoring strength strongly decreases with the UV-exposure time due to the photoisomerization process that takes place in the sample. The changes in the anchoring strength are due to a selective adsorption of cis isomers at the surface. We show that the experimental data can be explained by a model that takes into account the changes in the molecular shape and in the net molecular dipole moment due to the cis-trans isomerization. We obtain a good agreement between the predictions of the model and the experimental data.

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PubMed2001

Sensitive methods for estimating the anchoring strength of nematic liquid crystals on Langmuir-Blodgett monolayers of fatty acids.

The anchoring of the nematic liquid crystal N-(p-methoxybenzylidene)-p-butylaniline (MBBA) on Langmuir-Blodgett monolayers of fatty acids (COOHC(n)H(2n+1)) was studied as a function of the length of the fatty acid alkyl chain n (n=15,17,19,21). The monolayers were deposited onto glass plates coated with indium tin oxide, which were used to assemble sandwich cells of various thicknesses that were filled with MBBA in the nematic phase. The mechanism of relaxation from the flow-induced quasiplanar to the surface-induced homeotropic alignment was studied for the four aligning monolayers. It was found that the speed of the relaxation decreases linearly with increasing length of the alkyl chain n, which suggests that the Langmuir-Blodgett film plays a role in the phenomenon. This fact was confirmed by a sensitive estimation of the anchoring strength of MBBA on the fatty acid monolayers after anchoring breaking, which takes place at the transition between two electric-field-induced turbulent states, denoted as DSM1 and DSM2 (where DSM indicates dynamic scattering mode). It was found that the threshold electric field for the anchoring breaking, which can be considered as a measure of the anchoring strength, also decreases linearly as n increases. Both methods thus possess a high sensitivity in resolving small differences in anchoring strength. In cells coated with mixed Langmuir-Blodgett monolayers of two fatty acids (n=15 and n=17) a maximum of the relaxation speed was observed when the two acids were present in equal amounts. This observation suggests an efficient method for controlling the anchoring strength in homeotropic cells by changing the ratio between the components of the surfactant film.

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PubMed2000

Intermolecular and intramolecular reorientations in nonchiral smectic liquid-crystalline phases studied by broadband dielectric spectroscopy

Molecular dynamics has been studied by broadband dielectric relaxation spectroscopy in the Sm-A, Sm-B, and Sm-E phases (Sm denotes smectic) of a homologous series of nonchiral stilbenes. An assignment of modes is presented based on their dependence on temperature and molecular length, and, as far as they obey the Arrhenius law, their activation energy has been determined. In general, reorientations of entire molecules around their short axis are active, whereas reorientations of entire molecules around their long axis are locked out in the Sm-E phase of shorter homologs, yet intramolecular reorientations of polar sites have been established. Strong evidence is presented for an interdependence of reorientations of entire molecules around the short and long axes within the biaxial Sm-E phase of longer homologs.

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PubMed2000

Effect of photoisomerization of azobenzene dopants on the flexoelectric properties of short-pitch cholesteric liquid crystals

The flexoelectric properties of short-pitch cholesteric mixtures doped with three different azobenzenes, 4,4'-dihexyloxyazobenzene (4,4'-azo), 3,3'-dihexyloxy-2,2'-dimethylazobenzene (3,3'-azol), and 3,3'-dihexanoyloxy-2,2'-dimethylazobenzene (3,3'-azo2), respectively, were studied upon illumination with uv light. Their effective flexoelectric coefficients were derived from the flexoelectro-optic response of the mixtures aligned in uniform lying helix texture. Considering the fact that the pitch of the mixtures became shorter upon uv illumination, an increase of their effective flexoelectric coefficients was found to take place due to the photoisomerization of the dyes. The largest change was found for the coefficient of the guest-host mixture containing 4,4'-azo dye, most probably due to the bent shape of the dye cis-isomer. This observation is in good agreement with our previous studies on the influence of the molecular shape on the liquid-crystal flexoelectric properties and it suggests a possible way for enhancement of the amplitude of flexoelectro-optic response in cholesterics by using liquid-crystal materials with pronounced molecular shape dissymmetry.

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