H. Naradikian

Electroconvective dendrites in nematic phases having short range smectic order exhibited by the 4,n-alkoxybenzoic acids

Electroconvective (EC) dendrites (parabolic type) in nematic (N) liquid crystals with short range smectic order shown by 4,n-octyloxybenzoic acid (8-OBA) were observed. The driving of the dendrites by lateral d.c. and a.c. electric field parameters in the nematic temperature range was carried out. The dynamics of dendrite growth were studied and analysed by the Ivantsov two-dimensional solution for the diffusion equation. Using various combinations of blocked and unblocked electrodes, we studied dependences between the dimensionless electric field parameter and product of the dendrite parabolic radius and the velocity of the dendrite growth and found that the type of electrode significantly modifies these basic dependences. We attribute this effect possibly to the opposing ion propagation, i.e. gradient of electric field in direction opposite to that of the charge injection. A possible mechanism of EC dendrite growth is suggested. A comparison with thermal dendrites in liquid crystals is presented.

Liquid crystal nanocomposites produced by mixtures of hydrogen bonded achiral liquid crystals and functionalized carbon nanotubes

The liquid crystalline (LC) nature of alkyloxybenzoic acids is preserved after adding of any mesogenic or non-mesogenic compound through hydrogen bonding. However, this noncovalent interaction provokes a sizable effect on the physical properties as, e. g. melting point and mesomorphic states. In the present work we investigate nanocomposites, prepared by mixture of the eighth homologue of p-n-alkyloxybenzoic acids (8OBA) with single-walled carbon nanotubes (SWCNT) with the purpose to modify the optical properties of the liquid crystal. We exercise optical control on the LC system by inserting SWCNT specially functionalized by carboxylic groups. Since the liquid crystalline state combines order and mobility at the molecular (nanoscale) level, molecular modification can lead to different macroscopical nanocomposite symmetry. The thermal properties of the functionalized nanocomposite are confirmed by DSC analyses. The mechanism of the interaction between surface-treated nanoparticles (functionalized nanotubes) and the liquid crystal 8OBA bent- dimer molecules is briefly discussed.

Electroconvection in dimeric nematic liquid crystals with short-range smectic C order: dynamical characteristics

We describe the specific features of electroconvection (EC) in the nematic liquid crystal (NLC) phase with short-range smectic C order (the seventh homologue of the p,n-alkyloxybenzoic acid, 7OBA, which is dimeric). These features are revealed below a definite temperature (T*) within this liquid crystal phase. By polarisation analysis of the azimuthal in-plane director deflection in the EC regime we suggest that this instability has a twist-type character. We indicate that the specificity of the temperature variation of the electroconductivity and the EC in 7OBA is determined by the smectic tilt angle magnitude and the dimers compactness within the supramolecular complexes. We find that response times below T* are about two times shorter than those in classical NLCs, as an equalisation of both response and decay times, which is preferable in LC display techniques. A fourth frequency harmonic is detected for the first time in the N state.

Optical properties of dimeric liquid crystals doped with single-walled carbon nanotubes

Well oriented distinct local single crystals were grown in smectic C liquid crystal (LC) (heptyloxybenzoic acid – 7OBA) using predefined orientation of the single-walled carbon nanotubes (SWCNT). The SWCNT/SiOx/ITO/glass surface is found to cause a substantial enlargement of the smectic C distinct local single crystals and to reinforce both surface memory effect and surface anchoring. We synthesized and examined a set of nanocomposites, built of LC with hydrogen-bonded in dimers molecules (7OBA) and SWCNT. We found that the surface orientation strength and the interaction of the dimeric molecules with the carbon nanotubes play a decisive role for the type (symmetry), thermal stability and chirality of the LC states induced in the nanocomposites. As a result, a set of phase transitions and new phases (chiral smectic C, reentrant nematic, reentrant chiral nematic and a unique low-temperature chiral biaxial fluid smectic phase CG) not typical for pristine 7OBA were found.

The electroconvection in nematic liquid crystals with short range smectic C order and negative electroconductivity anisotropy

The electroconvective instability (ECI) in nematic liquid crystals (NLCs), appearing beyond smectic C at cooling known as N with short range smecic C order, presents an unique electrooptical phenomenon due to generation and frustration of the smectic layering, which manifests inside the N temperature range. We found that the response time in this material is about an order shorter than those in the classical nematics. For the first time in NLCs were detected harmonics higher than fourth order. A twist type instability is suggested for this material.

Ferroelectric state induced in mixture of dimer liquid crystal and perfluorooctanoic acid

ABSTRACT Phase transitions and electro-optical effects are studied in a nanocomposites, grown by mixing heptyloxybenzoic acid (7OBA), displaying hydrogen bonded dimeric liquid crystal (LC) state, with perfluorooctanoic acid. Due to the interaction with the dopant structural units the dimer rings of the LC matrix bent. As a result, transitions from achiral to chiral states, including ferroelectric smectic CG with the lowest C1 symmetry take place.