Sergei Bronnikov

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.

Kinetics of the nematic ordered phase growth during a temperature quench of an isotropic siloxane‐azomethine polymer

Kinetics of the nucleus growth during a deep temperature quench across the isotropic to nematic phase transition was experimentally investigated for a siloxane‐azomethine polyether at cooling rates of 10 and 20°C min−1. Nematic droplets revealed in the optical images during the phase separation were treated statistically and the resulting statistical size distributions were described using the model of reversible aggregation. Analysis of the time‐dependent distribution parameters allowed two processes involved in liquid crystal phase ordering to be identified: nucleus growth and nucleus coarsening. Both regimes are quantitatively described using the universal growth law.

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 nematic phase growth across the isotropic-nematic phase transition in polymer-dispersed liquid crystals

Polymer-dispersed liquid crystals (PDLCs) composed of poly(dimethyl siloxane), cured poly(dimethyl siloxane) and polysulfone (as matrices), and an azomethine compound (as an embedded mesogen varying in weight from 5 to 80%) were prepared via solvent-induced phase separation. After preparation, they were heated to the melt and then cooled; phase transitions upon both heating and cooling were detected with a differential scanning calorimeter and a polarising optical microscope (POM). The nematic droplets appearing in the POM images across the isotropic-nematic phase transition were treated statistically and described with principles of irreversible thermodynamics. Furthermore, kinetics of the nematic phase growth across this phase transition was studied and described analytically with the universal law for cluster growth. Both flexibility of the polymer matrix and the mesogen content in PDLCs were shown to influence the processes studied.

Self‐assembling in a living supramolecular linear nematic polymer‐like system

Self‐assembling in a linear supramolecular liquid crystalline (LC) polymer based on 4,4′‐bipyridine and bis‐(4‐carboxyphenyloxycarbonyl)‐heptanoate was studied by IR spectroscopy and optical microscopy. Although just a simple single hydrogen bond is required for the formation of a polymer chain, the reversibility of the hydrogen bonding forces the supramolecular macromolecule to behave under specific conditions similar to a mixture composed of two partly immiscible low molecular weight materials rather than as a polymer. An analytical description indicates that nematic droplets form two overlapping thermodynamically optimised statistical ensembles all across the phase transition in the linear supramolecular LC polymer studied. The kinetics of the nematic nucleus growth in the melted polymer was also studied. The number of generated nematic droplets oscillates with time. Two regimes of the growth kinetics were recognised: (i) nucleation and nucleus growth and (ii) nucleus coarsening, i.e. Ostwald ripening.

Statistical analysis of micro-domain ensembles at the surface of polyamic acid films during their conversion to polyimide

The surface morphology of polyamic acid films evolving to polyimide films is investigated using transmission electron microscopy. Two methods of film conversion, thermal and chemical, are studied. The micro-domains at the surface of the films are shown to form thermodynamically optimised statistical ensemble(s) during polymer conversion. The statistical distribution of the micro-domain diameters is described utilising the model of reversible aggregation. The distribution parameters, aggregation energy and mean diameter of the micro-domains, are shown to depend on the chemical structure of the polymer, the method of conversion and, in the case of thermal conversion, the temperature at which the processing occurs.

Stationary statistical size distribution of nematic droplets in the course of the isotropic liquid-nematic phase transition

The statistical size distribution of nematic droplets in the course of the phase transition isotropic liquid-nematic has been analysed using polarizing optical microscopy and described by the model of reversible aggregation. The materials under investigation are low molecular mass nematics, polymeric nematics and polymer dispersed liquid crystals observed at their clearing temperatures in the stationary time regime.

Kinetics of the Isotropic‐Ordered Phase Transition in Binary Mixtures of Mesogenic Monomers and Polymers

Kinetics of the nematic phase growth in the melted liquid crystalline azomethine monomer, siloxane‐azomethine polymer, and their mixtures was studied under a deep cooling using polarizing optical microscopy. The nematic droplets revealed in the optical images across the phase transition were treated statistically and the resulting statistical size distributions were described using principles of irreversible thermodynamics. Analysis of the mean droplet diameter as a function of time allowed recognition of two regimes involved to the nematic phase evolution: (1) nucleation and rapid nucleus growth and (2) nucleus coarsening. Both regimes were quantitatively described with the universal law of the cluster growth.

Synthesis and thermotropic properties of polyazomethines–containing side chain azobenzene moieties

Aromatic polyazomethines–containing azobenzene side groups have been synthesized by solution polycondensation reaction of aromatic dialdehydes bearing flexible spacers between the aromatic rings with aromatic diamines having preformed azobenzene side groups. Thermogravimetric analysis, differential scanning calorimetry, polarized optical microscopy, and wide angle X-ray diffraction have been used to evaluate their thermal and mesomorphic properties. The target polyazomethines showed nematic liquid crystalline phase, large mesophase range, and good thermal stability.

Synthesis and characterization of polyurethane-urea microcapsules containing galangal essential oil: statistical analysis of encapsulation

Galangal essential oil (GEO) is known to possess antimicrobial activity (e.g. against Staphylococcus aureus). A way to increase oil lifetime in plants is encapsulation in polyurethane–urea (PUU) microcapsules. In this study, PUU microcapsules with GEO were synthesized by interfacial polymerization at oil–water interface in oil–water emulsion. A statistical analysis of the microcapsule size was successfully applied for characterization of the encapsulation process. Using the model of reversible aggregation, it was shown that the process of encapsulation takes place in the conditions of thermodynamic control. The polymerization conditions (agitation rate in the range 2000–10 000 rpm/min) are the key factors that affect the mean microcapsule size of primary capsules formed during encapsulation. Two complementary processes were determined the mean capsule size during a transformation of these primary microcapsules: break-up and coalescence of oil droplets in the oil-in-water emulsion. The agitation rate does not influence the coalescence of the oil droplets, but the threshold value of agitation speed (in this system 4000 rpm/min) does exist and that is what strongly increases break-up of oil droplets. The higher agitation rate resulted in smaller size of microcapsules (mean diameter decreasing from 5.6 to 4.9 µm for primary capsules and from 13.8 to 9.8 µm for secondary capsules) and with a narrower size distribution. The last mode of encapsulation allows the more effective use the shell material for encapsulating of larger amount of oil.