Ferroelectric polymer nanocomposite alignment layer in twisted nematic liquid crystal devices for reducing switching voltage

Twisted nematic liquid crystal device (TNLCD) was fabricated using a ferroelectric zinc oxide (ZnO)-doped polyimide alignment layer. The ferroelectric nanoparticle can produce a local electric field, which can trigger the orientation of liquid crystal molecule and reduces the switching voltage. The uniform dispersion of ferroelectric ZnO nanoparticles in the alignment layer was studied using field emission scanning electron microscopy and atomic force microscopy. The ferroelectric property of ZnO-doped polyimide was investigated using dynamic contact electrostatic force microscopy. An increased local electric field due to the presence of nano ZnO was confirmed with the help of scanning tunnelling microscopy. An augmentation of capacitance was observed with an increase in concentration, which substantiates the reduction of switching voltage of TNLCD with the modification of ferroelectric nanoparticle-doped alignment layer.     

Self-activating liquid crystal devices for smart laser protection

ABSTRACT

Lasers are now extensively used in a multitude of optical devices and photonic systems spanning from sensing, communication, entertainment, medical surgery to military applications. Direct accidental or intentional exposures to high power lasers may lead to severe temporary or even permanent harm of human eye, skin, or optical sensors. The effective laser protection and shielding are currently not only a subject of scientific research but also a potential public safety issue, therefore there is an urgent need to develop the intelligent laser protection devices for keeping human eyes, optical sensors, and other sensitive components from these unintended or intended damages by laser radiations without warning. Self-activating liquid crystal devices undoubtedly represent such an elegant example because they could be autonomously activated to block or attenuate the lasers when the laser intensity is higher than a maximum permissible exposure value. This review is devoted to summarising the up-to-date significant advances of self-activating liquid crystal devices for potential smart laser protection, including twist-aligned nematic liquid crystal devices, liquid crystal cored waveguide fibre arrays, and photovoltaic/pyroelectric-hybridised liquid crystal devices. Finally, the review concludes with the perspectives and challenges for the future development of self-activating liquid crystal devices. It is anticipated that this glimpse and further endeavours in the emerging field will help the researchers from different backgrounds towards the fabrication of highly efficient laser protection devices, their real-world widespread applications and beyond.     

Distinctive dielectric properties of nematic liquid crystal dimers

ABSTRACT

We provide an overview of the effect of the molecular structure on the dielectric properties of dimers exhibiting nematic and twist-bend nematic phases with special focus on how the conformational distribution changes are reflected by the dielectric behaviour. Nematic dimers show distinctive dielectric properties which differ from those of archetypical nematic liquid crystals, as for example, unusual temperature dependence of the static permittivity or dielectric spectra characterised by two low-frequency relaxation processes with correlated strengths. The interpretation of such characteristic behaviour requires that account is taken of the effect of molecular flexibility on the energetically favoured molecular shapes. The anisotropic nematic interactions greatly influence the conformational distribution. Dielectric behaviour can be used to track those conformational changes due to dependence of the averaged molecular dipole moment on the averaged molecular shape. Results for a number of dimers are compared and analysed on the basis of the influence of details of the molecular structure, using a recently developed theory for the dielectric properties of dimers.     

Reliability of liquid crystal cell and immiscibility between dual-curable adhesives and liquid crystal

The dual-curable adhesive used to attach thin film transistors (TFTs) to color filters in the construction process of liquid crystal display (LCD) panels requires fast curing by UV irradiation and strong bond strength after thermal-curing. In addition, it is necessary to consider the immiscibility of the dual-curable adhesives with the liquid crystal, because they come directly into contact with the liquid crystal without curing process in the large LCD panel production. In this study, dual-curable adhesives based on partially acrylated epoxy acrylate oligomers were prepared and investigated with nematic liquid crystals using gas chromatography (GC), polarized optical microscopy and the measurement of the transmittance of the liquid crystal.As the concentration of CC bonds was increased, the immiscibility was enhanced due to the fast curing rate of the partially acrylated epoxy acrylate oligomers and reduced visual contamination was observed in the polarized optical microscope images. Moreover, the transmittance of the liquid crystal cells was not changed before and after the dual-curing of the adhesives and was maintained for 100 h.     

Ionic effects on electro-optics and residual direct current voltages of twisted nematic liquid crystal cells

By using electro-optical and dielectric absorption [1–4] measurements, we report our investigations of ionic effects on electro-optics and residual direct current voltages (VrDCs) of two polyimide (PI)-aligned twisted nematic (TN) cells with same liquid crystal mixture but different PI-alignment materials. We have also carried out new experimental methods to find out that the observed VrDCs were caused by LC-PI-interfacial trapped ions generated and transported from the LC medium for one TN cell, and from the PI layers for the other TN cell. Our measured VrDCs indicated that the former had two different exponential-decay rates similar to the published results by M. Mizusaki et al. [2] but the latter had only a single exponential-decay rate.     

Refractive index and absorption coefficient of blue phase liquid crystal in terahertz band

The refractive index and absorption coefficient of blue phase liquid crystals are investigated by terahertz time-domain spectroscopy system. Four phases, including isotropic phase, BPII, BPI, and cholesteric phase are investigated. The refractive index of liquid crystal sample is dependent on the temperature, which is related to the different phases. In our experiment, in the frequency range of 0.2–1.2 THz, the refractive index of BPII and BPI is about 1.620 and 1.625, respectively. For BPII and BPI, the measured absorption coefficients increase with the increase of frequency, with absorption peak appearing at 1.11 and 1.05 THz, respectively. These properties of BPLC provide useful information for BPLC-based terahertz components such as phase shifter, wave-plates and so on.     

Accurate modelling of nematic liquid crystal under the combination of microwave signal and applied voltage

ABSTRACT

In this paper, a three-dimensional modelling of nematic liquid crystal (NLC) under the combined action of applied voltage and microwave signal is presented. The analytical method applied in the modelling is detailed. In previous research, the modelling of liquid crystal usually uses a small signal wave, and neglected its effect on the orientation of liquid crystal. In this paper, we take the microwave signal into consideration in the calculation of liquid crystal orientation, and get the influence of the power of microwave signal on the orientation. The variations of the relative permittivity of liquid crystal E7 with the power of microwave signals at 30 GHz are obtained. This method is applicable for the modelling of NLC under high power signals excitation.     

Synthesis of azobenzene-containing liquid crystalline gelator for use in liquid crystal gels

A liquid crystalline gelator containing the azobenzene chromophore was synthesized for the first time; it was used to form self- assembled network in nematic liquid crystals resulting in liquid crystal gels with distinct features.     

gem-Difluorocyclopropane as core molecule candidate for liquid crystal compounds

The synthesis of a novel chiral gem-difluorocyclopropane building block has been accomplished using chemo-enzymatic reaction protocol; the prochiral diol of 1,4-bis(2,2-difluoro-3-(hydroxymethyl)cyclopropyl)benzene (5) was converted to the corresponding chiral diacetate by Pseudomonas lipase (lipase SL-25, Meito)-catalyzed transesterification with vinyl acetate as acyl donor with >99% enantiomeric excess. Various types of diesters or dialkyl ether were prepared from the diol and their helical twisting power (HTP) was evaluated by addition of 1.0 wt% to a non-chiral nematic liquid crystal host; the HTP was significantly dependent on the structure of ester or ether moieties and diester of diol 5 with isopropylfumalic acid showed the largest HTP.     

Investigation of alignment direction in wide view film and rubbing angle of twisted nematic liquid crystal display mode

Wide view (WV) film is an important material in the polariser. It is a hybrid-aligned, discotic liquid crystal (DLC) on an alignment layer on the tri-acetyl cellulose film, which is used as compensation film in twisted nematic (TN) mode liquid crystal display (LCD). The relation between the alignment direction of the DLC in the WV film and the rubbing direction of the polyimide on the glass substrate in the TN mode LCD was investigated. The results indicated that the contrast ratio (CR) of the TN mode LCD can be increased by adjusting this angle . When an 88°?rubbing angle was used in this work, the CR of the TN mode LCD could reach 1000:1 and the horizontal and vertical viewing angles were 170 and 160°, respectively.     

High-transmittance liquid crystal cell fabricated using nanoparticle-doped polyimide

We found that we can increase the transmittance of a liquid crystal (LC) cell by doping the alignment material with nanoparticles. We fabricated an LC cell using indium–tin–oxide glass substrates coated with SiO₂-doped polyimide. We confirmed that the transmittance of LC cells fabricated using SiO₂-doped polyimide was higher than that of LC cells fabricated using pure polyimide. To account for the increase of transmittance, we measured the order parameter of the fabricated LC cells and investigated the surface morphology of the alignment layer using atomic force microscopy.     

Low-birefringence liquid crystal mixtures for photonic liquid crystal fibres application

ABSTRACT

Series of low-birefringence liquid crystal mixtures composed of alkyl alkylbicyclohexyl carbonates and modified mixtures with other compounds have been formulated, their refractive indices and electric permittivity measured upon temperature. They exhibit the ordinary refraction index n_o lower than the refractive index of silica glass in a different range of temperature. This enables to observe in photonic liquid crystal fibres (PLCFs) a change in the light propagation mechanism from photonic band gap guiding to modified total internal reflection at different temperatures. Selected applications of PLCFs are also discussed.     

Fabrication of vertical alignment liquid crystal cell without forming a conventional polyimide-alignment layer

Two series of asymmetric banana-shaped compounds have been synthesized and studied. In the 1,3-phenylene bis[4-(4′-alkoxybenzoyloxy)]benzoate series the lack of symmetry was derived solely from the difference in length of the two terminal alkoxy chains. In the 3,4′-biphenylene bis[4-(4′-alkoxybenzoyloxy)]benzoate series the asymmetric nature originates from the 3,4′-substitution of the central biphenyl group and from the difference in length of the two terminal chains. All the melting points of the asymmetrical compounds in the series with the central phenyl unit are lower than those of the symmetrical compounds. The liquid crystalline B₁ or B₂ phase was retained in all cases. In the series with the central biphenyl unit the compounds with the shortest chain attached to the para-position of the central biphenyl unit have the lowest melting points. A significant lowering of the melting points in comparison with the symmetrically substituted compounds, however, could not be achieved. All the compounds of both series show a layer spacing which is comparable to those of the symmetrically substituted parent compounds. The observed switching behaviour of both the symmetric and asymmetric compounds with a B₂ phase was antiferroelectric.     

A liquid crystal cell with double-side protrusion electrodes for fast response and low-voltage operation

We propose a homogeneously aligned liquid crystal (LC) cell with double-side protrusion electrodes for fast response and low-voltage operation. In the proposed device, both the bottom and top substrates have pixel electrodes to generate the fringe electric field. Because the penetration depth of the electric field is increased owing to the protrusion electrodes, the operating voltage is very low and the turn-on time is dramatically reduced compared with the conventional in-plane switching (IPS) mode. Moreover, LC molecules anchored strongly to the penetrated protrusion electrodes on both substrates exert a strong restoring force, resulting in a fast turn-off time. We found that the total response time of the LC cell with the proposed structure is three times faster than that of the conventional IPS mode.     

Polyfluorinated cycloalkoxyphenyl ether systems as dopants for liquid crystal display applications

The effects on response times of representative commercially available dielectrically negative (−LC) and positive (+LC) type liquid crystal mixtures doped with small quantities of various tetrafluorinated cycloalkoxyphenyl ether derivatives were investigated. The synthesis of various polyfluoroaryl systems by nucleophilic aromatic substitution processes is described and electro-optical measurements concerning the rise and decay times and voltage holding ratios of each doped LC formulation are reported. The response time of the dielectrically negative LC host was improved upon doping with each of the tetrafluorinated cycloalkoxyphenyl ether derivatives, although no improvement in the performance of the dielectrically positive LC host was achieved.     

Alternative synthetic strategies for Schiff base derived liquid crystal polymers: A comparative study

Semiflexible liquid crystalline polymers based on Schiff bases as mesogenic cores have been synthesized through different synthetic strategies. The mesogenic cores are derived from 1,5-naphthalenediamine linked to a polymethylenic spacer by means of ether or ester groups. The synthetic strategies differ in the disconnection of the molecules to design the synthesis of the repeating units. Polymers were synthesized either as polyazomethines, where imine bonds are formed in the polymer synthesis, or as polyethers or polyesters, where the ether or ester groups are formed at the polymer-forming stage. The influence of the synthetic method and the linking groups between mesogenic core and flexible spacer on the thermal and mesogenic properties has been analyzed by DSC, thermogravimetry, and optical microscopy. The possibility of obtaining hydroxy-functionalized polymers by different synthetic strategies has been studied as well as the influence of these groups on the thermal and mesogenic properties. © 1996 John Wiley & Sons, Inc.     

The alignment of nematic liquid crystal by the Ti layer processed by nonlinear laser lithography

It is well known that the alignment of liquid crystals (LCs) can be realised by rubbing or photoalignment technologies. Recently, nonlinear laser lithography (NLL) was introduced as a fast, relatively low-cost method for large area nano-grating fabrication based on laser-induced periodic surface structuring. In this letter for the first time, the usage of the NLL as a perspective method of the alignment of nematics was presented. By NLL, nanogrooves with about 0.92 μm period were formed on Ti layer. The nanostructured Ti layer (NSTL) was coated with oxidianiline-polyimide film with annealing of the polymer followed without any further processing. Aligning properties of NSTLs were examined with combined twist LC cell. The dependencies of the twist angle of LC cells and azimuthal anchoring energy (AE) of layers on scanning speed and power of laser beam during processing of the Ti layer were the focus of our studies as well. The maximum azimuthal AE, obtained for pure NSTL, is comparable with photoalignment technology. It was found that the deposition of polyimide film on NSTL leads to the gain effect of the azimuthal AE. Also, atomic force microscopy (AFM) study of aligning surfaces was carried out.     

Enlargement of blue-phase stability for rod-like low-molecular-weight chiral nematic liquid crystal mixtures

In this study, we investigated the enlargement of liquid crystal (LC) blue-phase (BP) temperature range using the rod-like low-molecular-weight cyano phenyl-type chiral nematic LC with various core group and chiral dopant concentrations. Also, the electro-optic response time was investigated for them. We found that the BP temperature range was strongly dependent upon the core structure and the chiral dopant concentration for the chiral nematic LC mixtures having the same terminal group. Also, we found a stable BP with a wide temperature range (more than 6 K), including a BP-isotropic coexistence state over 13.5 K upon heating and cooling processes and very fast response time (less than 1 ms), by using the cyano phenyl-type chiral nematic LC mixture with a high molecular aspect ratio and a high chiral dopant concentration.     

Step-shaped electrode for low-voltage and high-optical-efficiency blue-phase transflective liquid crystal displays

A step-shaped electrode design is proposed for Polymer Stabilised Blue Phase (PSBP) Transflective LCD (TR-LCD). This new electrode design is capable of delivering very high optical efficiency (both T and R) of >95% with operation voltage of ~13 V (or >~90% with ~9 V). The step-shaped electrode helps generate strong horizontal electric fields across the whole cell and suppress the formation of dead zone.     

Strategies for the synthesis of fluorinated liquid crystal derivatives from perbromofluoroaromatic systems

The use of perbromofluorobenzene derivatives as starting materials for the synthesis of a variety of model liquid crystal systems by a combination of nucleophilic aromatic substitution, debromolithiation/trapping, dehydration and reduction processes is described.