Oleg Yaroshchuk

Photoalignment of liquid crystals: basics and current trends

The review describes the status of the studies and the recent achievements in the field of photoalignment of liquid crystals. An update classification of photoaligning materials and exposure schemes, and analyzes of the relationship between the molecular structure of the materials and characteristics of LC alignment are provided. In addition, bulk mediated photoalignment and combination of photoalignment with other alignment methods are discussed. Along with traditional, recently proposed applications of the photoalignment technique are considered.

Planar and Tilted Uniform Alignment of Liquid Crystals by Plasma Treated Substrates

We describe a new aligning technique that yields uniform planar or tilted orientation of a nematic liquid crystal at different organic and inorganic substrates. The method is based on the oblique irradiation of an aligning substrate with a partially collimated flux of accelerated plasma. The sheet-like plasma flux is produced by an anode layer thruster with a race track geometry of the discharge channel. For liquid crystals with a positive dielectric anisotropy the technique produces two modes of uniform alignment: (1) with the easy axis in the incident plane of the plasma beam; (2) with the easy axis perpendicular to the plane of incidence. Mode 1 transforms into mode 2 as the irradiation dose increases. In mode 1, the pretilt angle can be controlled by changing the parameters of irradiation such as incidence angle, current density and particle energy. In mode 2, the pretilt angle is zero (planar alignment). The azimuthal anchoring coefficient is relatively weak (W a∼10−6 J m−2) for the first type of alignment and strong (W a≥10−4 J m−2, comparable to rubbed polymer substrates) for the second type. The two-mode alignment feature can be used to control alignment properties and to create alignment patterns. The method is free from the usual shortcomings of the traditional rubbing technique. We discuss possible mechanisms of the two-mode alignment and show that plasma-induced modifications of the substrate topography might be an important factor in liquid crystal alignment.

Stabilization of liquid crystal photoaligning layers by reactive mesogens

Via passivation of liquid crystal (LC) photoaligning layers by thin layers of reactive mesogens, strong enhancement in LC alignment stability is achieved. Simultaneously, the passivation layers improve alignment uniformity and chemical stability of photoaligning layers and widely change pretilt angle of liquid crystal.

High-Resolution and High-Throughput Plasmonic Photopatterning of Complex Molecular Orientations in Liquid Crystals.

A plasmonic photopatterning technique is proposed and demonstrated for aligning the molecular orientation in liquid crystals (LCs) in patterns with designer complexity. Using plasmonic metamasks in which target molecular directors are encoded, LC alignments of arbitrary planar patterns can be achieved in a repeatable and scalable fashion withunprecedentedly high spatial resolution and high throughput.

Effect of Electrooptical Memory in Suspensions of Carbon Nanotubes in Liquid Crystals

Electrooptical response and microstructure of dispersions of multiwall carbon nanotubes in N-(4-ethoxybenzilidene)-4-n-butyl aniline nematic liquid crystal (LC) are studied. Irreversible response on the applied electric field (electrooptical memory) was revealed in oriented layers of such suspensions. The essence of this effect consists in the fact that, after the switch-on and subsequent switch-off of the field, the optical transmittance of suspension layer placed between two crossed polarizers substantially increases compared to the initial value which is typical for homeotropic orientation of LC. The efficiency of electrooptical memory nonmonotonically depends on the concentration of nanotubes in suspension, cCNT, reaching its maximum at cCNT = 0.02–0.05 wt %. It is shown that the memory of suspensions is caused by the incomplete relaxation of LC molecules from planar to initial homeotropic state after the electric field switch-off. The model is proposed and substantiated, according to which the planar state of LC is stabilized by the network of nanotubes formed upon the disintegration of aggregates under the action of electrohydrodynamic flows. The disclosed memory effect is rather common; it is brought about in the suspensions of carbon nanotubes based on other LCs in which electrohydrodynamic instabilities are developed.

Photosensitive Materials on a Base of Polysiloxane for the Alignment of Nematic Liquid Crystals

The light-induced alignment of a number of commercial liquid crystals (LC) on the surface of photosensitive polysiloxane based polymers was studied. It was shown that the materials can provide thermostable homeotropic, planar and tilted alignments of good quality. The value of tilt angle depends upon the LC material, irradiation intensity and exposure as well as aligning film thickness and temperature treatment.

Cholesteric liquid crystal–carbon nanotube composites with photo-settable reversible and memory electro-optic modes

The photoresponsive electro-optical composites based on cholesteric liquid crystal (CLC) with optically controlled chirality and a minute amount of carbon nanotubes (CNTs) are studied. In cells with homeotropic anchoring, these composites exhibit a transition from fingerprint texture to homeotropic nematic texture in the course of photoinduced unwinding of the cholesteric helix. Compared with the CLC counterpart, this transition is much delayed, because of the stabilization of cholesteric filamentary domains by CNTs. The CLC-CNT composites demonstrate dual-mode operation with optical switching between reversible and memory mode. It is found that the memory response is associated with the elastic network of filamentary cholesteric domains that stabilizes the planar CLC texture reached in an electric field. In turn, the reversible mode corresponds to the unwound cholesteric state. Potential applications of this effect are discussed.

Plasma beam alignment for the large-area substrates: Equipment and process

— We have developed an effective method for liquid-crystal alignment of the large-area substrates. This method is based on the oblique treatment of the alignment substrates with a “sheet” of accelerated plasma generated by the anode layer source of the “race track” geometry. During this treatment, the substrate or source is cyclically translated in the direction perpendicular to the plasma “sheet.” This method provides planar, tilted, and vertical liquid-crystal alignment with excellent uniformity and reproducibility and easy axis control in the azimuthal and polar planes.

Azodyes as Photoalignment Materials for Polymerizable Liquid Crystals

We demonstrate the excellent photoalignment capability of sulfuric azodyes for polymerizable liquid crystals (PLC) whose anisotropic films are extensively studied as basic elements of many passive optical devices (e.g., retardation films, polarizers, and color filters). These dyes exhibit a high affinity to various substrates and insolubility in PLC and organic solvents commonly used for their dilution. An extremely low exposure dose (less than 50 mJ) is needed to induce an excellent planar alignment of PLC. This alignment can be easily patterned using minimal numbers of masks and exposure steps. No sign of alignment deterioration is observed after photopolymerization. These imply that new photoalignment materials can be effectively used for the industrial production of optical films based on PLC. Particularly, they suit the continuous manufacturing process for such films.

SEM Investigations of the Polymer Morphology in the Liquid Crystal-Polymer Composites with Different Polymer Contents

Scanning electron microscopy was employed to study the evolution of the morphology of liquid crystal-polymer composites with increasing contents of a polymer. The composites were formed by a nematic mixture E7 from Merck and a photocurable adhesive NOA65 from Norland by means of photoseparation. The polymer phase reveals a smooth transition from a polymer network to a sponge-like structure and then to the ‘Swiss cheese’ morphology with the increasing fraction of a polymer. It is established that the best electro-optic performance is demonstrated by samples with the well-developed sponge-like structure of the polymer phase.