Sergei Kostromin

Polymer-Dispersed Liquid Crystals: Progress in Preparation, Investigation, and Application

Polymer-dispersed liquid crystals (PDLCs), composed of micro-sized liquid crystal domains embedded in a continuous polymer matrix, are an important class of materials that hold promise for many opto-electronic applications. In this review, based on an analysis of research publications appearing between 2006 and 2012, preparation of conventional PDLCs and nanoparticles-doped PDLCs, their properties, and their application for opto-electronic devices are described.

Polyazomethine with m-tolylazo side groups: thermal, dielectric and conductive behaviour

Using solution polycondensation, a new polyazomethine with m-tolylazo side groups (PAz) exhibiting thermotropic liquid crystalline phase was synthesised and its chemical structure was characterised with generally accepted methods. Its phase transition temperatures were detected with both polarising optical microscopy and differential scanning calorimetry. Using dielectric spectroscopy method, both real and imaginary parts of the permittivity were investigated in wide regions of temperature (from −100°C to 170°C) and frequency (from 1 Hz to 1 MHz). Analysis of frequency dependent permittivity allowed finding three relaxations (α, β1 and β2) in PAz. β-relaxations were described with the Arrhenius equation, whereas α-relaxation was described with the Vogel–Fulcher–Tammann equation. The alternating current conductivity (ACC) of PAz was studied in the same regions of temperature and frequency. The frequency dependent ACC was described with an exponent power equation. Presentation of ACC as a function of inverse temperature allowed us to describe ACC with the Arrhenius equation.

Kinetics of the isotropic–nematic phase transition in melted multi-component liquid crystal mixtures upon cooling

The kinetics of the phase separation and the nematic phase growth in two melted commercial multi-component liquid crystal mixtures upon cooling was studied using polarising optical microscopy and IR spectrometry. The droplets of the nematic phase revealed in the optical images across the phase transition were segmented and treated statistically. In the resulting size histograms of mixture B, two overlapping statistical ensembles related to two co-existing nematic phases were recognised; these phases were shown to be different in their chemical structures. In mixture A, any separation within the nematic phase was not found. The statistical ensembles of the nematic droplets were successfully described using the principles of irreversible thermodynamics. Analysis of the mean droplet diameter as a function of time allowed recognition of two regimes of the nematic phase evolution: (1) nucleation and rapid nucleus growth and (2) nucleus coalescence. Both the regimes were quantitatively described with the universal law for the cluster growth.

Thermally Induced Isotropic-Nematic Phase Separation in Mixtures of Low-Molecular Weight and Polymer Liquid Crystals

Using a polarizing optical microscope, kinetics of the thermally induced phase separation in the mixtures of a nematic low-molecular weight (LMW) liquid crystal (LC) and a LC polymer was examined. As a LC polymer, either a nematic side-chain polymer (P1) or a smectic main-chainpolymer (P2) was chosen. Depending on structure of the LC polymer (P1 or P2), kinetics of the LMW LC phase growth was analytically described either by the universal law for the cluster growth or by the Avrami equation, respectively.

Kinetics of the growth of the ordered phase at isotropic liquid-nematic phase transition in mesomorphic dimers

The kinetics of the growth of ordered phase drops at isotropic liquid-nematic phase transition in melts of mesomorphic dimers on cooling was studied by polarization optical microscopy. The drop size on microscopic images was analyzed statistically, and the histograms obtained were described by an equation deduced within the framework of equilibrium irreversible thermodynamics. Analysis of the equation parameters revealed three steps of the kinetics of drop growth and allowed their analytical description by a universal scaling function. The correlation between the kinetics of phase transitions and the molecular structure and dipole architecture of the mesomorphic dimers was demonstrated.

Nature of the liquid-crystalline phase in supramolecular linear polymers

The development of the liquid crystal (LC) state in the melt of a supramolecular linear LC polymer based on a diacid and bipyridine base has been studied by IR spectroscopy and polarizing optical microscopy. Relying on experiments on the hydrogen-bonded complex formation and statistical analysis of the formation kinetics of the nematic phase, it is inferred that the system under study behaves like a binary mixture of weakly interacting components rather than a polydisperse polymer.

A heterojunction photovoltaic cell based on a mixture of silane copolymer with C70 fullerene derivative

Heterojunction photovoltaic cells with a photoactive layer composed of a mixture of a silane copolymer with a fullerene derivative PC70BM have been fabricated, and their working characteristics have been measured. Data obtained by cyclic voltammetry were used to construct the energy level diagram for the cell components. Analysis of the diagram and photoelectric characteristics of the photovoltaic cell made it possible to find ways to improve its efficiency.

Dielectric properties of liquid-crystal azomethine polymer with alkyl-substituted side chain

Relaxation processes in side-chain liquid-crystal azomethine polymer have been studied using dielectric spectroscopy in wide ranges of temperatures and frequencies. Two low-temperature relaxation processes (β1 and β2) caused by low-amplitude noncooperative motion of kinetic units in side groups and a high-temperature α-process caused by the large-scale cooperative motion of kinetic units in the main polymer chain have been revealed.

The Photovoltaic Effect and Charge Carrier Mobility in Layered Compositions of Bithiophene or Related Rotaxane Copolymer with C70 Fullerene Derivative

Organic photovoltaic cells with a bulk heterojunction have been manufactured in which the photoactive layer consists of a mixture of bithiophene copolymer or related rotaxane with a fullerene derivative (PC70BM). The mobility of charge carriers in photoactive layers has been determined, the current–voltage characteristics of photovoltaic cells have been measured, and the energy level diagram of cell components has been constructed. It is established that the polyrotaxane component (macrocycle) insulates a part of thiophene fragments of the macromolecule, thus hindering the transport of carriers and leading to large energy losses for exciton dissociation, which results in a decreasing photovoltaic effect.

Dielectric properties of liquid-crystal azomethine polymer with a side alkyl-substituted chain, doped with fullerene C60

We studied the actual and imaginary components of the dielectric constant of liquid-crystal azomethine polymer with a side chain, doped with 0.5 wt % of fullerene C60, over a wide range of temperatures and frequencies; measurements were made by means of dielectric spectroscopy. By analyzing the frequency dependence of the dielectric constant, we detected the relaxation processes (α, β1, and β2) in the nanocomposite, corresponding to certain modes of molecular motion and described them by the Arrhenius equations (β1- and β2-processes) and the Vogel–Fulcher–Tamman equation (α-process). An antiplasticization effect is discovered after doping the polymer with fullerene C60, which manifests itself in increasing the glass transition temperature of the nanocomposite compared to this parameter typical of pure polymer.