Natalia Boiko - Academia.edu (original) (raw)

Papers by Natalia Boiko

Optical Materials Express, 2021

Hybrid fluorescent cholesteric liquid crystalline (CLC) materials are representatives of “smart” ... more Hybrid fluorescent cholesteric liquid crystalline (CLC) materials are representatives of “smart” soft matter, and are characterized by light emission that can be flexibly controlled by various external stimuli. This fact is due to the many possibilities for potential applications in the fields of photonics and optics stimulating design, and study of this type of hybrid materials. Here, we report on the optical and fluorescence properties of the hybrid CLC material based on a low-molecular-weight CLC matrix and CdSe/ZnS quantum dots (QDs) stabilized by LC diblock copolymers. The hybrid CLC material is characterized by the cholesteric phase in a wide temperature range, the high loading of QDs, and no QD aggregation. We demonstrate that the cholesteric stop band alters characteristics of the QD emission due to the resonance effect. This makes the polarization state and wavelength of the QD emission thermo- and angle-dependent. This work provides a way for the design of a wide range of ...

Russian Journal of Physical Chemistry A, 2003

The dielectric properties of fourth-generation mesogenic carbosilane dendrimers with alkyloxycyan... more The dielectric properties of fourth-generation mesogenic carbosilane dendrimers with alkyloxycyanobiphenyl terminal groups were studied. Permittivity and dielectric loss measurements were performed in the mesophase and isotropic phase in the range of electric field frequencies I kHz-100 MHz. A dispersion of permittivities was observed in this frequency range. It was shown that dispersion in the isotropic phase could be caused by two Debye-type relaxation processes. Relaxation times and activation energies were determined. The activation energy of the low-frequency relaxation process decreased as the length of the alkyl chain of terminal dendrimer mesogen groups increased.

RSC Advances, 2020

Spatial distribution of QDs within hybrid composite films was programed by varying the molecular ... more Spatial distribution of QDs within hybrid composite films was programed by varying the molecular architecture of the host LC block copolymers.

Polymers, 2020

Dye-doped nematic side-chain liquid-crystalline polymers possess extraordinary large optical nonl... more Dye-doped nematic side-chain liquid-crystalline polymers possess extraordinary large optical nonlinearity and ability to store the induced orientational deformations in a glassy state, which makes them a very promising material for photonic applications. In this study, the phase structures were generated and recorded in the bulk of a 50-μm layer of a nematic liquid-crystalline side-chain polymer, containing polyacrylate backbone, spacer having five methylene groups, and phenyl benzoate mesogenic fragment. The polymer was doped with KD-1 azodye. The director field deformations induced by the light beam close to the TEM01 mode were studied for different geometries of light–polymer interaction. The phase modulation depth of 2π was obtained for the 18-μm spacing between intensity peaks. The experimental data were analyzed based on the elastic continuum theory of nematics. The possibility to induce and record positive and negative microlenses in the polymer bulk was shown experimentally.

Journal of Materials Chemistry C, 2020

Using liquid crystalline triblock copolymer complexes with hydrogen-bonded azobenzene moieties, p... more Using liquid crystalline triblock copolymer complexes with hydrogen-bonded azobenzene moieties, photopatterned colorless birefringent films were prepared by irradiation with linearly polarized light and subsequent extracting azobenzene moieties.

Macromolecular Rapid Communications, 2019

Photosensitive liquid crystalline (LC) block copolymers are a universal platform for designing fu... more Photosensitive liquid crystalline (LC) block copolymers are a universal platform for designing functional materials for optics and photonics. Here, azobenzene LC ABA triblock copolymer containing mesogenic groups in each block is studied as a medium for the recording of polarization holographic gratings. The fast recording of polarization gratings is successfully performed with two orthogonally circularly polarized light beams of a 532‐nm laser. It is shown that annealing of the recorded polarization grating increases diffraction efficiency considerably, by a factor of 70, and provokes simultaneous erasing of the surface relief grating if it has been recorded. Holographic behavior of the triblock copolymer is compared with that of an azobenzene homopolymer and a random copolymer, which has the same monomer ratio as the triblock copolymer. The obtained results are expected to allow designing new materials for the polarization of optical elements and holographic data storage.

Bulletin of the Lebedev Physics Institute, 2019

In this study, the photoinduced orientational transitions at the variation of boundary conditions... more In this study, the photoinduced orientational transitions at the variation of boundary conditions in the droplets of cholesteric (chiral nematic) liquid crystal are obtained and investigated. The effect is caused by photoisomerization of the azobenzene dendrimer additive dissolved in the liquid crystal and adsorbed on its interface. A sequence of orientational transitions under lowintensity (∼1 mW/cm 2) ultraviolet irradiation is revealed.

Beilstein Journal of Nanotechnology, 2018

Two sequential transformations of the orientational structure in nematic liquid crystal droplets ... more Two sequential transformations of the orientational structure in nematic liquid crystal droplets containing a dendrimer additive (nanosized macromolecules with light-absorbing azobenzene terminal moieties) under light irradiation in the UV–blue spectral range were investigated. The origin of these transitions is in the change of the boundary conditions due to photoisomerization of the dendrimer adsorbed onto the liquid crystal–glycerol interface. It was shown that the photoisomerization processes of dendrimer molecules in a liquid crystal are accompanied by a spatial rearrangement of their azobenzene moieties, which is the key point in the explanation of the observed effects.

Optical Materials Express, 2019

We propose a simple and effective method of liquid crystal photoalignment that does not require a... more We propose a simple and effective method of liquid crystal photoalignment that does not require any preliminary treatment of the cell substrates. To this aim, a small amount (0.1 wt %) of azobenzene carboxylic acid is added into the nematic liquid crystal. After filling the liquid crystal cell, a part of the dopant molecules is spontaneously adsorbed and attached to the glass surface by hydrogen bonds. This allows one to switch the boundary conditions of liquid crystal between homeotropic and planar due to the reversible trans-cis photoisomerization.

Liquid Crystals, 2017

ABSTRACT High sensitivity of liquid crystals to the electric field makes them highly demanded and... more ABSTRACT High sensitivity of liquid crystals to the electric field makes them highly demanded and widely used in different applications. Despite the large number of the electro-optical research on the low-molar-mass liquid crystals electro-optics of the liquid crystalline (LC) polymers is much less studied. Herein, the comparative electro-optical behaviour of two nematic comb-shaped polyacrylates with phenylbenzoate mesogenic side groups was studied in detail. These two polyacrylates have completely the same structure of polymer backbone and spacer length but different in the direction of the ester group in the phenylbenzoate fragments. It was found that this difference predetermines their completely opposite electro-optical properties. The influence of the electric field of different strength and frequency on the orientation of the mesogenic groups of these polymers is studied. It is shown that application of the electric field at temperatures above the glass transition temperature (~25°C) induces reorientation of the mesogenic groups along or perpendicular to the electric field direction depending in its turn on the ester group direction. For one of the polyacrylates an unusual textural transition is found; during cooling of the polymer sample under applied field at definite temperature a sharp change in the mesogen’s orientation from homeotropic to planar one is found. This electro-optical phenomenon is observed for the first time and probably associated with sharp change in sign of anisotropy of dielectric permittivity from positive (at high temperatures) to negative one (at lower temperatures). Kinetics of the electro-optical switching at different temperatures, influence of the molar masses of the polymer and frequency of the applied AC field on electro-optical behaviour of the polymers are studied. The possibility of the fixation of the electroinduced homeotropic alignment of the mesogenic groups by photopolymerisation of the diacrylate dissolved in the polymer is demonstrated. Graphical Abstract

Physical Review E, 2017

Light-induced director orientation of polymeric liquid-crystalline systems was investigated. The ... more Light-induced director orientation of polymeric liquid-crystalline systems was investigated. The materials under study were composed of a nematic liquid-crystalline polymer (NLCP) and a small amount (0.05-0.5 wt.%) of conformationally active (azobenzene) or stable (anthraquinone) dye impurity. Light action on the homogeneously aligned polymer films above glass transition temperature leads to the director reorientation and, consequently, to a change in the extraordinary refractive index. The effect is associated with the dye molecule excitation and related change of intermolecular forces. In the case of NLCP with conformationally active dye dopant, an extremely high orientational optical response was detected (nonlinear coefficient is n 2 ∼ 0.1 cm 2 /W). In contrast, the efficiency of orientational light action on NLCP with conformationally stable dye dopant is of the same order of magnitude as that of dye-doped low-molar-mass liquid crystals. At the normal light incidence on the NLCP doped with azo-dye, the threshold director orientation is observed which is similar to the Fréedericksz transition under the action of magnetic and electric fields. The obtained high-orientational optical response of NLCP caused by azo-dye dopant in combination with the possibility of the recording of deformed structure in the glassy state, typical for polymer compounds, reveals new opportunities in photonics applications.

Molecular Crystals and Liquid Crystals, 2017

The dynamics of the light-induced director rotation has been studied for a comb-shaped polymers i... more The dynamics of the light-induced director rotation has been studied for a comb-shaped polymers in nematic phase with a small amount (0.05 wt %) of azobenzene-dye additive. The effect of temperature on the rotational viscosity and nonlinear optical response of the liquid crystalline polymers was investigated. The orientational processes occured at the beam intensity ∼0.5 W/cm 2. Cooling the samples below the glass temperature results in the memory effect.

Journal of Optical Technology, 1999

Thermochimica Acta, 2015

In the present work temperature dependences of heat capacity of liquid-crystalline carbosilane de... more In the present work temperature dependences of heat capacity of liquid-crystalline carbosilane dendrimers with terminal methoxyphenylbenzoate groups of the second and the fourth generations have been determined in the range from 6 to 370 K by the precision adiabatic vacuum calorimetry. In the above temperature range the phase transformations have been detected and their thermodynamic characteristics have been determined and analyzed. The experimental data were used to calculate standard thermodynamic functions, namely the heat capacity C o p ðTÞ, enthalpy H o ðTÞ À H o ð0Þ, entropy S o ðTÞ À S o ð0Þ and Gibbs energy G o ðTÞ À H o ð0Þ, for the range from T ! 0 to 370 K. The standard entropies of formation of dendrimers in liquid-crystalline state at T = 298.15 K and the standard entropies of hypothetic synthesis at the same temperature were estimated. It was shown that thermodynamic properties dendrimers under study do not depend on generation number and are defined by nature of terminal groups. 2015 Elsevier B.V. All rights reserved.

Russian Journal of Physical Chemistry A, 2016

The temperature dependence of the heat capacity of a first-generation liquid crystal carbosilane ... more The temperature dependence of the heat capacity of a first-generation liquid crystal carbosilane dendrimer with methoxyphenyl benzoate end groups is studied for the first time in the region of 6-370 K by means of precision adiabatic vacuum calorimetry. Physical transformations are observed in this interval of temperatures, and their standard thermodynamic characteristics are determined and discussed. Standard thermodynamic functions , , , and are calculated from the obtained experimental data for the region of Т → 0 to 370 K. The standard entropy of formation of the dendrimer in the partially crystalline state at Т = 298.15 K is calculated, and the standard entropy of the hypothetic reaction of its synthesis at this temperature is estimated. The thermodynamic properties of the studied dendrimer are compared to those of second-and fourth-generation liquid crystal carbosilane dendrimers with the same end groups studied earlier.

The Journal of Chemical Physics, 2017

Reversible orientational transitions in the droplets of a nematic liquid crystal (NLC) caused by ... more Reversible orientational transitions in the droplets of a nematic liquid crystal (NLC) caused by the change of boundary conditions under the low intensity diode illumination are investigated. Photosensitivity of NLC is achieved by the addition of the dendrimer compound with azobenzene terminal groups. Two types of NLC droplets in glycerol are considered: the spherical droplets in the bulk of glycerol and the droplets laid-down onto the solid substrate. In the second case, the first order phase transition is revealed. The effects described can be useful for the development of highly sensitive chemical detectors and microsized photo-tunable optical devices.

Polymer Engineering and Science, 1997

Scientific reports, Jan 7, 2016

We have studied copolymer dendrimer structure: carbosilane dendrimers with terminal phenylbenzoat... more We have studied copolymer dendrimer structure: carbosilane dendrimers with terminal phenylbenzoate mesogenic groups attached by poly(ethylene) glycol (PEG) spacers. In this system PEG spacers are additional tuning to usual copolymer structure: dendrimer with terminal mesogenic groups. The dendrimer macromolecules were investigated in a dilute chloroform solution by (1)H NMR methods (spectra and relaxations). It was found that the PEG layer in G = 5 generations dendrimer is "frozen" at high temperatures (above 260 K), but it unexpectedly becomes "unfrozen" at temperatures below 250 K (i.e., melting when cooling). The transition between these two states occurs within a small temperature range (~10 K). Such a behavior is not observed for smaller dendrimer generations (G = 1 and 3). This effect is likely related to the low critical solution temperature (LCST) of PEG and is caused by dendrimer conformations, in which the PEG group concentration in the layer increases ...

Journal of Materials Chemistry, 2000

Advances in Silicon Science