Longin N. Lisetski

Suppressed ionic effect and low-frequency texture transitions in a cholesteric liquid crystal doped with graphene nanoplatelets

We focus on investigating the dielectric behaviors and the low-frequency texture transitions in a cholesteric liquid crystal (CLC) doped with graphene nanoplatelets (GNPs) by means of dielectric spectroscopy and measurements of electro-optical responses. The experimental results indicate that incorporating GNPs at a content of 0.5 wt% into the CLC leads to significant suppression of ionic behaviors, as manifested by the reduction in ionic density, diffusivity, and relaxation frequency. In addition, the electro-optical properties of the GNP-doped CLC cell show the lowered operation voltage for the switching from the planar to focal conic state and the absence of the low-frequency focal-conic-to-uniform-lying-helix texture transition. Such results are attributable to the effects of GNPs as nuclei in the CLC medium, giving rise to the repression of the ionic and electrohydrodynamic effects.

Carbon Nanotubes in Liquid Crystals: Fundamental Properties and Applications

The structure and properties of liquid crystalline suspensions filled by carbon nanotubes (CNTs) are critically reviewed. Special attention is paid to interactions between CNTs and molecules of the liquid crystals (LC), which lead to formation of ordered supramolecular structures . These structures, in turn, determine unique physical properties of LC + CNT suspensions, including electrical conductivity, dielectric permittivity , phase transitions , optical transmission , memory effects that can be used in electrooptic and optoelectronic devices, etc. Great variety of LC phases are considered as a host media, such as nematics, cholesterics, smectics of different types (including ferroelectrics ), lyotropic , chromonic , ionic and hydrogen-bonded liquid crystals. Alongside multi- and single-walled carbon nanotubes, the suspensions can also contain the platelets of organoclays used for facilitation of CNT dispersing. Recent practical applications of LC + CNT suspensions and nanomaterials based thereon are also outlined.

Dispersions of Carbon Nanotubes in Cholesteric Liquid Crystals

Dispersions were studied of multiwall carbon nanotubes in cholesteric liquid crystals of three types: nematics with chiral dopant, mixtures of cholesterol derivatives, and similar mixtures close to smectic-A transition. In the first two cases, no appreciable effects of nanotubes upon selective reflection spectra were noted, while optical transmission increased significantly at the isotropic phase transition. Aromatic and steroid matrices differed significantly in this respect, though in both cases the nanotubes were apparently oriented along the directors of quasi-nematic layers. In the third case, nanotubes slightly but clearly affected the measured selective reflection peaks, suggesting suppression of the smectic-A phase formation.

LC nanocomposites: induced optical singularities, managed nano/micro structure, and electrical conductivity

Microstructure, phase transitions, electrical conductivity, and optical and electrooptical properties of multiwalled carbon nanotubes (NTs), dispersed in the cholesteric liquid crystal (cholesteryl oleyl carbonate, COC), nematic 5CB and their mixtures, were studied in the temperature range between 255 K and 363 K. The relative concentration X=СОС/(СОС+5CB) was varied within 0.0-1.0. The concentration C of NTs was varied within 0.01-5% wt. The value of X affected agglomeration and stability of NTs inside СОС+5CB. High-quality dispersion, exfoliation, and stabilization of the NTs were observed in COC solvent (“good” solvent). From the other side, the aggregation of NTs was very pronounced in nematic 5CB solvent (“bad” solvent). The dispersing quality of solvent influenced the percolation concentration Cp, corresponding to transition between the low conductive and high conductive states: e.g., percolation was observed at Cp≈1% and Cp≈0.1% for pure COC and 5CB, respectively. The effects of thermal pre-history on the heating-cooling hysteretic behavior of electrical conductivity were studied. The mechanism of dispersion of NTs in COC+5CB mixtures is discussed. Utilization of the mixtures of “good” and “bad” solvents allowed fine regulation of the dispersion, stability and electrical conductivity of LC+NTs composites. The mixtures of COC and 5CB were found to be promising for application as functional media with controllable useful chiral and electrophysical properties.

Lyotropic Mesophase of Hydrated Phospholipids as Model Medium for Studies of Antimicrobial Agents Activity

The lyotropic L α-phase of hydrated dipalmitoylphosphatidylcholine has been proposed as a solvent medium for modeling the effects of membranotropic agents upon cell membranes, including interactions between different agents inside the membrane. For quasibinary systems containing bisquaternary ammonium compounds decamethoxinum or aethonium and 2,5-dihydroxybenzoic acid, calorimetry data allowed us to establish the stoichiometry of the intermolecular complexes incorporated in the membrane. The interaction mechanisms have been clarified using additional data obtained with tetramethylammonium and l-menthol, which model the moieties of the decamethoxinum molecule separated by the alkyl chain. Possible implications of the obtained results for pharmacology are discussed.

Rotation of Single Crystals of Chiral Dopants at The Top of a Nematic Droplet: Factors Affecting the Angular Velocity

In the Lehmann-type effect of rotation of small single crystals of chiral substances during their dissolutionat the top of a nematic droplet the angular velocity of rotation of the chiral dopant crystals is experimentally shown to be dependent on the crystal dimensions, helical twisting power of the dopant (determined in independent measurements) and viscosity of the nematic solvent, whereas no rotation was observed for non-chiral dopants or in isotropic solvents under the same conditions. For systematic series of dopants (e.g., cholesterol esters), linear dependencies between the angular velocity and helical twisting power were found. It is argued that the obtained results (a) suggest a novel method for evaluation of helical twisting power, especially useful for dopants of weak chirality, and (b) are a direct evidence of transformation of the energy of chiral interactions into the energy of molecular movement.

Temperature Dependences of the Elastic Constants in the p-Alkoxybenzylidene-p′-n-Butylanilines Homologous Series

Abstract Temperature dependences of the three elastic constants in the title compounds are reported and discussed.

The combined effects of nitrates on multibilayer lipid membranes: Thermodynamic effects

Model multibilayer membranes based on L-α-dimyristoylphosphatidylcholine containing nitrates of silver, sodium, potassium, and copper as AgNO3–NaNO3, AgNO3–KNO3, and AgNO3–Cu(NO3)2 pairs were investigated. In each system studied the molar fraction of nitrates relative to the lipid was kept unchanged at 0.35, whereas the molar fraction of silver nitrate (xAg) was varied from 0.0 tо 1.0 within the pair. Thermodynamic parameters of the main phase transition and pre-transition of the model membranes were determined using differential scanning calorimetry. Positive deviations from additivity as a function of xAg for a number of these parameters were detected, including changes in the main phase transition and the pre-transition temperatures of up to 0.5 and 2.7°C, respectively; the deviation for hysteresis and half-width of the main phase transition reached up to 30%. The physicochemical mechanisms of competitive interactions between cations in membranes composed of L-α-dimyristoylphosphatidylcholine are discussed.

Mixtures of Thermotropic Mesogens as Components of Model DPPC Membranes: Effects of Intermolecular Interactions on Phase Transitions

Thermotropic mesogens of different chemical nature - 4-pentyl-4′-cyanobiphenyl (5CB), azoxy nematic ZhK-440, and cholesteryl oleyl carbonate (COC) were introduced as dopants to hydrated DPPC, with their effects on phase transitions recorded by DSC. When mesogen pairs (5CB + ZhK-440 and 5CB + COC) were added at constant total concentration, dependence of transition temperature to Lα phase on 5CB concentration was linear for COC and nonlinear for ZhK-440, with effects of cyanobiphenyl-azoxybenzene charge transfer complexes manifested in both types of mesophases. Such dual doping of hydrated DPPC allows modeling of drug-membrane interactions in the case of joint application of different drugs.

Complex light with optical singularities induced by nanocomposites

The nanocomposites on the base of long (5-10μm, o-MWCNTs) and short (~ 2μm, m-MWCNTs) multi-walled carbon nanotubes (MWCNTs) hosted by nematic 5CB were investigated in details by means of polarizing microscopy, studies of electrical conductivity and electro-optical behaviour. The spontaneous self-organization of MWCNTs was observed and investigated both theoretically and experimentally. The efficiency of MWCNT aggregation in these composites is controlled by strong, long ranged and highly anisotropic van der Waals interactions and Brownian motion of individual nanotubes and their aggregates. The simple Smoluchowski approach was used for estimation of the half-time of aggregation. It was shown that aggregation process includes two different stages: fast, resulting in formation of loose aggregates (L-aggregates) and slow, resulting in formation of compacted aggregates (C-aggregates). Both L- and C- aggregates possess extremely ramified fractal borders. Formation of the percolation structures was observed for o-MWCNTs at C=Cp≈0.025-0.05 % wt and for m-MWCNTs at C=Cp≈0.1- 0.25 % wt. A physical model describing formation of C-aggregates with captured 5CB molecules inside was proposed. It shows good agreement with experimentally measured characteristics. It was shown that MWCNTs strongly affect the structural organization of LC molecules captured inside the MWCNT skeleton and of interfacial LC layers in the vicinity of aggregate borders. Moreover, the structure of the interfacial layer, as well as its birefringence, drastically changed when the applied electric voltage exceeded the Freedericksz threshold. Finally, formation of the inversion walls between branches of the neighbouring MWCNT aggregates was observed and discussed for the first time.