Three‐dimensional structure in holographic polymer‐dispersed liquid crystals

Polymer‐dispersed liquid crystals, heterogeneous structures consisting of a liquid crystal and a polymer, are promising materials for generating holograms with switchable diffraction efficiency. The experiments presented here reveal that the generation of a face‐centered cubic structure can be achieved by exposing a liquid crystal/reactive monomer mixture to the interference pattern of four coherent laser beams under appropriate geometrical conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Self-aligning liquid crystals in polymer composite systems

Soft composite materials combined with a holographic photopolymerization process are used for realizing an innovative switchable periodic structure made of slices of almost pure polymer alternated with films of well aligned nematic liquid crystals named POLICRYPS. It exhibits negligible scattering losses, while the effect of the spatial modulation of the refractive index (from polymer to nematic liquid crystal) can be switched on and off by applying a low (few V/μm) electric field. The diffractive properties of the POLICRYPS structure are characterized in terms of cell thickness, impinging probe angle and wavelength revealing a strong correlation between the diffraction efficiency and all the above mentioned parameters. These results are very attractive for many applications such as switchable Bragg gratings for telecom devices, phase modulators, and displays. Other advantages of the technology include absence of an alignment layer, absence of haze, robust structure, and inexpensive manufacturing. In addition, no special alignment layers are required. This is a unique opportunity and a big advantage compared to conventional liquid crystal devices. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 158–162     

A switchable optical diffuser based on a polymer/nematic liquid crystal composite film with transient polymer balls-networks microstructure

ABSTRACT

With the increasing development of multifunction liquid crystal displays (LCDs), multifunction optical diffusers have attracted more attention. In this work, switchable optical diffusers based on the transient polymer balls-networks in polymer/nematic liquid crystal composite films have been prepared and investigated, and the effects of thiol content, curing time, LC content and photo-initiator content on the polymer microstructures and optical properties of optical diffusers are systematically studied. An optical diffuser with the transient polymer balls-networks microstructure has achieved the high transmission (96.42%), high haze (90.49%), excellent optical diffusing ability and wide viewing angle of 150° (from ?75° to 75°) on its off state. By the application of electric field of 40.0 voltage, the viewing angle of the optical diffuser turns to be narrower (from ?60° to 60°), which gives great applications in tunable viewing angle devices and other electric optical devices.     

Reverse-mode polymer dispersed liquid crystal films prepared by patterned polymer walls

This article proposes a methodology to prepare polymer dispersed liquid crystal (PDLC) films working in the reverse-mode operation, where the ion-doped nematic liquid crystals (NLCs) with negative dielectric anisotropy (Δε) were locked by polymer walls. On-state and off-state of films were controlled by an electric field. In the absence of an electric field, it appears to be transparent. In the field, the homogeneous alignment NLCs form dynamic scattering, giving rise to opaque. The effect of the cylindrical holes with different diameters of photo masks and liquid crystal Δε on the electro-optical properties and transmittance wavelength range of 400–3000 nm light of samples were investigated. It was found that it exhibited very good electro-optical characteristics, high contrast ratio and excellent infrared energy-efficient of films used as switchable windows.     

Switchable anti-peeping film for liquid crystal displays from polymer dispersed liquid crystals

Liquid crystals displays (LCDs) currently dominate the display market, wherein a wide viewing angle is considered as one of the most important characteristics. However, for LCDs with wide viewing angles, some private information inevitably becomes more visible; thus, an LCD with a switchable viewing angle has attracted greater interest. Here, we report a novel switchable viewing angle film that can make the viewing angle of an LCD electrically switchable between ±30° and ±60°, i.e. between an anti-peeping mode and a share mode, by 5.0 V is turned on and off, respectively. The response time necessary to change between the modes is in milliseconds. It is believed that it has potential applications in LCDs with high quality.     

Molecular relaxations in dual-frequency nematic liquid crystals

The preparation of nematic liquid crystals mixtures results in changing of molecular relaxations in comparison to pure substances. Typical example is the creation of dual-frequency nematic liquid crystals using a base mixture and functional admixtures. In this paper, we present how dielectric properties of starting compounds change at mixture preparation. Three dual-frequency nematic mixtures of different composition were prepared and examined by means of dielectric spectroscopy in a wide frequency (100 Hz to 10 MHz) and temperature range (170°C to ?60°C). Parameters of detected modes for pure compounds and final mixtures were calculated and their relationships with crossover frequency are discussed.     

Planar and twisted lyotropic chromonic liquid crystal cells as optical compensators for twisted nematic displays

We have explored the use of lyotropic chromonic liquid crystals (LCLCs) as promising materials for optical compensators for liquid crystal displays. We used aqueous solutions of disodium cromoglycate in the range of concentrations that yield a chromonic N phase. The N phase is aligned by rubbed polyimide layers giving planar orientation. Two different types of LCLC-based elements are described: (a) a uniformly aligned planar N phase that forms an optically uniaxial plate with a negative birefringence; (b) twisted cells of LCLC obtained by doping the N phase with chiral amino acids. Both planar and twisted N* phase cells were used for compensation of the positive birefringence of twisted nematic (TN) displays; the TN devices were appreciably improved. We achieved achromatic dark state and contrast ratio up to 50:1 at all directions within a 40° cone of viewing angle.     

Continuous viewing angle controllable patterned vertical alignment liquid crystal display

A continuous viewing angle switchable patterned vertical alignment liquid crystal display (PVA-LCD) with circle polarisers is proposed. In wide viewing angle (WVA) mode, the viewing cone with contrast ratio larger than 1000:1 is extended to 50° and the contrast ratio larger than 100:1 is extended to 80°. Meanwhile, the viewing cone with contrast ratio more than 10:1 is limited to 40° in all direction for narrow viewing angle (NVA) mode. The viewing angle of the display can be controlled by applying a bias voltage on the common electrode. With the increasing bias voltage, the viewing angle cone with contrast ratio more than 10:1 can be changed from 40° to 20°. The proposed PVA-LCD has high transmittance (~93%) under the circle polarisers because the dead zone in the normal PVA-LCD with linear polarisers is eliminated.     

Switchable Fabry–Perot filter for mid-infrared radiation

ABSTRACT

In this work, switchable liquid crystal infrared filter is presented. The tailored superfluorinated liquid crystal material exhibiting low parasitic absorption at the operating range has been used for the construction of the filter. The switching range as high as 300 nm is achieved by applying 18 V. The filter can operate continuously from 3.45 up to 3.75 μm.     

First observation of electromechanical effects in a chiral ferroelectric columnar liquid crystal

Electromechanical (converse piezoelectric) responses of an electrically switchable chiral ferroelectric columnar liquid crystal 1,2,5,6,8,9,12,13-octakis\[(S)-2-heptyloxy] dibenzo \[ e , l] pyrene, were studied under a.c. electric fields. The liquid crystal phase has a C2 rotational symmetry, the same as that of SmC* liquid crystals or of Rochelle Salt, but the responses are orders of magnitude weaker. The possible physical reasons for the observed weak mechanical responses and, in view of the results, the switching mechanism are discussed.     

Synthesis of cyclopentyloxy terphenyl liquid crystals with negative dielectric anisotropy

Liquid crystalline materials possessing negative dielectric anisotropy (Δε) have attracted considerable attention because they can be used to formulate eutectic mixture for several display applications. The negative Δε can be achieved by introducing a lateral polar substituent onto the mesogen core of a liquid crystalline material, and fluorine atoms are usually used because of the small size and high electronegativity. 2,3-ortho-Difluoro substituent liquid crystals exhibit stable and profound physical properties such as the low viscosity, strong negative Δε, and high solubility. To avoid the decrease of the mesophase range, 2,3-ortho-difluorophenyl is often placed next to the terminal chain. In this paper, we have synthesised a new series of 2,3-ortho-difluoroterphenyl liquid crystals with a negative dielectric anisotropy. Ether oxygen is introduced to the structure because it can further increase the lateral dipole moment. 3-Alkylcyclopentane end group decreases both the melting point and the viscosity, which makes it an appropriate substituent for negative Δε 2,3-ortho-difluoroterphenyl liquid crystals. The mesomorphic properties as well as the optical anisotropies of the synthesised liquid crystals are discussed in this paper. All compounds in the series are found to possess highly negative dielectric anisotropy, wide mesophase ranges; therefore, they have a great potential to formulate eutectic mixture for liquid crystal displays.     

Planar and twisted lyotropic chromonic liquid crystal cells as optical compensators for twisted nematic displays

We have explored the use of lyotropic chromonic liquid crystals (LCLCs) as promising materials for optical compensators for liquid crystal displays. We used aqueous solutions of disodium cromoglycate in the range of concentrations that yield a chromonic N phase. The N phase is aligned by rubbed polyimide layers giving planar orientation. Two different types of LCLC-based elements are described: (a) a uniformly aligned planar N phase that forms an optically uniaxial plate with a negative birefringence; (b) twisted cells of LCLC obtained by doping the N phase with chiral amino acids. Both planar and twisted N* phase cells were used for compensation of the positive birefringence of twisted nematic (TN) displays; the TN devices were appreciably improved. We achieved achromatic dark state and contrast ratio up to 50:1 at all directions within a 40° cone of viewing angle.     

Dependence of viewing angle characteristics on pretilt angle in the in-plane switching mode

Viewing angle characteristics were systematically obtained when using in-plane switching (IPS) of liquid crystals. Although the IPS mode originally shows stable electro-optical performance regardless of viewing directions, the viewing angle characteristics are found to be strongly dependent on pretilt angle (slant angle of the liquid crystals from the substrate). Experimentally, the smaller the pretilt angle of the liquid crystal, the much wider the viewing angle characteristics, while larger pretilt angles of liquid crystals cause the characteristics to deteriorate. This deterioration occurs in a particular viewing direction, i.e. at right angles to the initial orientation direction of the liquid crystal when there is no in-plane electric field. The experimentally observed behaviour of the viewing angle dependence on the pretilt angle was also confirmed by computer simulations. Calculated iso-contrast contour lines, as a function of the pretilt angles, nearly coincide with the experimentally obtained viewing angle characteristics of the contrast ratio.     

Suppressing pretilt angle for enhancing black quality in diagonal viewing angle of homogenously aligned liquid crystal display

The diagonal viewing angle light leakage in a black state of in-plane switching (IPS) liquid crystal display (LCD) associated with pretilt angle has been investigated. The mechanical rubbing process with a cloth causes relatively high pretilt angle in the homogenously aligned liquid crystals (LCs) so that the tilted LC director results in increase of a light leakage in a black state at diagonal viewing angles. In this study, we theoretically estimated using classical optic theory how the light leakage in a black state at diagonal viewing angle is associated with the pretilt angle and also proposed an effective method to reduce the pretilt angle from 1.5° to 0° in rubbed IPS LCD by utilising polymer stabilisation. With this approach, we could successfully acquire a better black quality in all viewing angles as compared with normal IPS LCD.     

Experimental investigation of the compressible continuum theory of a homeotropically aligned ferroelectric liquid crystal

The optical tensor configuration in a homeotropically aligned ferroelectric liquid crystal (FLC), SCE13 cell, is investigated by means of optical excitation of half leaky guided modes. A thin slab of FLC is confined between a high index pyramid and a low index substrate whose indices bound those of the liquid crystal. In this geometry there exists a small angle range over which a series of sharp resonant modes may propagate in the liquid crystal. Detecting the angular dependent reflectivity for plane polarized radiation and subsequently fitting the data by iteratively modelling from multilayer Fresnel theory, a full characterization of the tilt and twist profile throughout the cell is achieved. The temperature dependence of the tilt of the principal director, which is related to the smectic cone angle, and of the optical permittivity, as well as the pitch have been obtained. The tilt director profile across the cell is interpreted using a compressible continuum theory for SmC* liquid crystals which includes the possibility of variable cone angle and layer spacing.     

Haze and opacity control in polymer dispersed liquid crystal (PDLC) films with phase separation method

Polymer dispersed liquid crystals (PDLCs) are materials composed of liquid crystal microdroplets dispersed in a polymer matrix. Their electro-optic properties make them useful for applications as large-area electrically switchable architectural windows (smart windows). For these applications, the key parameters of performance are the haze (both normal and offaxis) and the opacity. In the present work we show how it is possible to prepare a high performance smart window by controlling the haze and opacity of PDLC films using the polymer induced phase separation (PIPS) method.     

Self-organization of disc-like molecules: chemical aspects

The hierarchical self-assembly of disc-shaped molecules leads to the formation of discotic liquid crystals. These materials are of fundamental importance not only as models for the study of energy and charge migration in self-organized systems but also as functional materials for device applications such as, one-dimensional conductors, photoconductors, light emitting diodes, photovoltaic solar cells, field-effect transistors and gas sensors. The negative birefringence films formed by polymerized nematic discotic liquid crystals have been commercialized as compensation foils to enlarge the viewing angle of commonly used twisted nematic liquid crystal displays. To date the number of discotic liquid crystals derived from more than 50 different cores comes to about 3000. This critical review describes, after an in-depth introduction, recent advances in basic design principles and synthetic approaches towards the preparation of most frequently encountered discotic liquid crystals.     

Reverse mode operation polymer dispersed liquid crystal with a positive dielectric anisotropy liquid crystal

The development of electrically activated chromogenic materials is important for their potential applications in smart windows. Several previous works have reported on reverse mode operation polymer dispersed liquid crystals (PDLCs) based on negative dielectric anisotropy liquid crystals. They have a transparent OFF state, which turns opaque after the application of a suitable external electric field. Nevertheless, these devices have some limitations such as the use of large amount of expensive liquid crystals with peculiar physical‐chemical properties. In addition, a good matching between the refractive index of liquid crystal and the polymer matrix one is required. The main result of this work is the achievement of reverse mode operation devices prepared with a positive dielectric anisotropy liquid crystal and characterized by a high OFF state transmittance obtained by the onset of high intensity built‐in DC electric fields in a direct mode operation PDLC, which allows the OFF state homeotropic alignment of liquid crystal directors. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Wide-viewing-angle switchable image splitter obtained using varied-line-spacing H-PDLC Bragg grating

We propose the use of a varied-line-spacing (VLS) holographic polymer-dispersed liquid crystal (H-PDLC) Bragg grating as a switchable image splitter to generate a compatible three-dimensional (3D) stereogram and to increase the viewing angle of the observer. To fabricate the VLS grating, a cylindrical lens is adopted to form a cylindrical wave, which interferes with a plane wave, generating an H-PDLC grating with a continuously varying period. The proposed holographic optical element (HOE)-based image splitter comprises two VLS H-PDLC gratings. It can be attached on a designed pattern, with two rectangles taking the place of conventional liquid crystal display panel pixels, and can separate them into right and left viewing fields with a wider range. Experimental results show that the movement of the eyes of the observer can reach 37.6 mm. A theoretical simulation indicates that a shorter focal length of the cylindrical lens can yield a larger movement range. Switching between two-dimensional and 3D modes can be performed by applying an external alternating-current voltage at ~80 V. The contrast ratio of the diffracted images induced by crosstalk is greater than 60%, which indicates the feasibility of the proposed HOE for 3D image splitting.     

Ferroelectric liquid crystal composites based on the porous stretched polyethylene films

Electro-optically active polymer–liquid crystal composites based on ferroelectric liquid crystals and stretched porous polyethylene films were developed. The alignment of ferroelectric liquid crystals incorporated into the porous polyethylene films with average porous diameter of around 200 nm was observed and studied. It was shown experimentally that these samples containing ferroelectric liquid crystals are flexible electro-optical films exhibiting a saturation electric field near 2·10⁷ Vm^?1 and a response time of about 30 μs under the action of the saturation field. A simple theoretical model of ferroelectric liquid crystal molecules' complete reorientation in electric fields inside pores of the films has been proposed and confirmed experimentally.