Polymer Dispersed Liquid Crystal Research Articles

14.3: See‐through 2D/3D Compatible Display Based on Integral Imaging

A see‐through 2D/3D compatible display based on integral imaging is proposed. It consists of a lens‐array holographic optical element (LAHOE) film, a polymer dispersed liquid crystal (PDLC) film and two projectors. The see‐through projection screen is composed of the LAHOE film and the PDLC film. The PDLC film is tightly behind the LAHOE film. With applied voltage, the projection screen works in the transparent display mode. 3D image for Bragg matched image can be presented, and 2D image can also be presented.

26.6: Spectroscopic Response of Polymer Network Liquid Crystal Cell

The Polymer Network Liquid Crystal (PNLC) cell we fabricated is operating in a reverse mode of voltage‐off transparent state. It is found that when we apply voltage to the PNLC cells, the transmitted light color changed gradually and finally became a bit blue. This effect is not found in Polymer Dispersed LC (PDLC) window. The wavelength dependent transmittance voltage curves are reported and we will try to explain this effect by applying the anomalous diffraction theory.

Switchable pupil expansion propagation using orthogonal superposition varied-line-spacing H-PDLC gratings in a holographic waveguide system.

To solve the problem of small field of view in traditional holographic waveguides, this paper proposes a waveguide-coupling technique using orthogonal superposition varied-line-spacing (VLS) gratings. These gratings expand the x and y directions and orthogonally superimpose to achieve transmission in a waveguide and enlarge the pupil field of view. At the optical coupling input end and the coupling output end of a waveguide, one-dimensional VLS gratings are used to realize horizontal expansion of the image. Then a vertical VLS grating is orthogonally superimposed at the exit end to realize vertical expansion of the image. Finally, the waveguide transmission and expansion of the image are completed. In the experiment, holographic polymer dispersed liquid crystal one-dimensional VLS gratings in the x and y directions are fabricated and coupled with a waveguide. An image source with a diameter of 0.5cm is waveguide-transferred and coupled out, and then passed through the vertical grating. Amplification is performed to obtain an expanded image of a diameter of 2.28cm. In this study, the diffraction characteristics of the grating used to realize pupil expansion in the holographic waveguide system are analyzed and simulated. It is calculated that the diffraction efficiency of the VLS gratings can reach 80% or more in the 532nm band. Additionally, the characteristics of an electronically controlled switch are studied. Experimental results show that the method can be used for expanding the field of view and can be applied to waveguide systems for image transmission and expansion.

The effect of thickness on electro-optical properties of plastic UV-curable polymer dispersed liquid crystal (PDLC)

We investigated the effect of polymer dispersed liquid crystal (PDLC) layer thickness on morphology and electro-optical properties of plastic UV-curable PDLC films with two nematic liquid crystal mixtures TNO623 and TNO403. The PDLC films were prepared on flexible Indium Tin Oxide (ITO) coated Poly-Ethylene-Terephthalide (PET) supports by UV-polymerization induced phase separation, and the uniform film thickness was insured by plastic micro-spacers. We found that the PDLC layer thickness at the same morphology effects on electro-optical properties of off-state and on-state transmissions, switching voltages and response times.

Fast Switchable Micro‐Lenticular Lens Arrays Using Highly Transparent Nano‐Polymer Dispersed Liquid Crystals

AbstractA polarization‐dependent micro‐lenticular lens array (MLA) based on phase modulation of the nanostructured polymer dispersed liquid crystal (nano‐PDLC) is proposed. The transparent and optically isotropic composite is formed by photo‐polymerization induced phase separation of the LC and pre‐polymer mixture. The gradient field is generated by interdigitated electrodes and the LCs in nanosized droplets in nano‐PDLC orient along the field direction, resulting in field‐induced birefringence from the optically isotropic phase. Consequently, the MLAs are formed at the focal plane, in which its measured focal length is 22 µm at 50 Vrms. Besides, the position of focused beam in MLA device can be tuned depending on the polarization direction of an incident beam. The MLA device can realize a 3D image when the polarized 2D image arrays encounter the microlens. The proposed MLA device can pave the way toward upcoming fast switchable 2D/3D display device applications with a very fast switching time of 3.8 ms.

Comparative studies of polymer‐dispersed liquid crystal films via a thiol‐ene click reaction

In this work, the thiol‐ene click reaction is employed to fabricate polymer‐dispersed liquid crystal (PDLC) films by photoinitiated polymerization. The PDLC films are prepared by systematic variation of key conditions: variety and content of ‐ene monomer, liquid crystal (LC) content, curing time, and curing light intensity. We find that both the morphologies and electro‐optic properties of these films are adjustable. When increasing the length of alkyl main chain of ‐ene monomers, the driving voltages reduce, but in turn, the contrast ratio decreases. Increasing ‐ene monomer content raises the driving voltages as well as the response time, and the increase of LC content lowers the driving voltages but has a negative effect on the contrast ratio. The changes to the curing conditions (both curing time and UV light intensity) can be used to modify the driving voltages, response time, and contrast ratios of PDLC films. These comparative studies will elucidate new insights in commercial applications of intelligent PDLC films.

Electro-Optical Effects of a Color Polymer-Dispersed Liquid Crystal Device by Micro-Encapsulation with a Pigment-Doped Shell

Polymer-dispersed liquid crystals (PDLCs) refer to nematic liquid crystals, which are embedded in a polymer matrix. A conventional PDLC device is fabricated by phase separation. However, this method leads to non-uniform electro-optical characteristics of the device due to the non-uniform size distribution of the liquid crystal droplets. Moreover, the PDLC device is switched between the transparent state and the scattering state so that a full color scheme is intrinsically impossible without a color filter. In this paper, a fabrication method for a color PDLC device with uniform size and shape for liquid crystal droplets is proposed. Droplets of a fairly uniform size in large quantities can be obtained by means of membrane emulsification. Microcapsules are fabricated by complex coacervation with gelatin and gum arabic. By adding red, green, and blue pigments, color microcapsules are obtained. The electro-optical effects of the fabricated color PDLC devices are also demonstrated. The driving voltage of the device is 90 V, and the switching time is 8.3 ms. In the turn-on state, the measured hazes of red, green, and blue PDLC devices are 16.89%, 15.82%, and 18.55%, respectively, while in the turn-off state, the measured hazes of those devices are 65.21%, 67.32%, and 70.76%, respectively.

Controlling morphological and electro-optical properties via the phase separation in polymer/liquid-crystal composite materials

ABSTRACT The polymer/liquid-crystal composite materials have been extensively studied for their potential applications. Various optical devices based on this composite material have been proposed and realised. The device performance is highly dependent on the phase separation of this composite material. Here, we investigate the photopolymerisation-induced phase separation in this composite material. Depending on the mass ratios between the polymer and the liquid crystal, the phase separation can be well controlled and subsequently affect the morphological and electro-optical properties. At a fixed ratio, we can realise either phase-separated composite films or conventional polymer-dispersed liquid crystal films with completely different optical properties. By carefully controlling the exposure conditions, the morphologies and electro-optical properties have been studied and optimised in details. With in-depth studies and optimisation, the photopolymerisation-induced phase separation technique could be utilised to realise many different optical functions based on the polymer/liquid-crystal composite materials.

Electrically tunable dual-wavelength organic laser based on holographic polymer dispersed liquid crystal grating

Abstract An organic dual-wavelength surface-emitting distributed feedback (DFB) laser with low threshold and high conversion efficiency based on holographic polymer dispersed liquid crystal (HPDLC) grating is reported. In such a laser configuration, the dye blend 4-(dicyanomethylene)-2-methyl-6-( p -dimethylaminostyryl)-4H-pyran (DCM)/1,3,5,7,8-pen- tamethyl-2,6-diethylpyrromethene-difluoroborate (PM567) -doped HPDLC grating is formed on the top of an organic semiconductor poly(2-methoxy-5-(20 -ethyl-hexyloxy)-p- phenylenevinylene) (MEH-PPV) layer. The dual-wavelength laser located at 605.2 nm and 629.8 nm are obtained in a single beam. The dual-wavelength laser performance under an external electric field are investigated and illustrated for the first time. This study provides some new ideas for the improvement of tunable dual-wavelength laser and extends more applications in laser displays and integrated photonic circuits.

Simultaneous achievement of high visible transmission and near-infrared heat shielding in flexible liquid crystal-based smart windows via electrode design

Light and heat management are the two vital requirements for smart window towards ideal solar control. However, as a representative polymer dispersed liquid crystal (PDLC) device, little attention has been paid to heat management. Herein, a TiO2/Ag(Cu)/TiO2 (TCAT) electrode was designed and optimized to achieve the tunability of transmission in visible range and near-infrared heat shielding simultaneously. The continuous growth of ultrathin Ag films with low percolation threshold was achieved on TiO2 underlayer by doping a certain amount of Cu. Then the trade-off of PDLC devices’ optoelectrical properties was realized. The average ON state specular transmittance of TCAT-based PDLC devices reached 81.9%, 84.9%, 81.9% and 79.9% for Ag(Cu) thicknesses of 7 nm, 9 nm, 11 nm and 13 nm, respectively. In addition, TCAT electrodes exhibited satisfactory ratio of light to solar gain by taking visible transmittance and solar heat gain coefficient into comprehensive consideration. Compared with ITO-based PDLC counterpart, a notable temperature-decreases of 4.6 °C was observed at ON state of TCAT-based device, indicating an excellent heat shielding performance. The proposed TCAT electrode design is promising in smart window application especially for building energy conservation in hot areas or the summer season.

Recent trends in studies on polymer — dispersed liquid crystal composites

The paper presents a review of results of studies in the field of PDLC material science and physics obtained during last few years and shows the main fields of interest in that subject. It covers an application of new polymers and liquid crystalline materials used to prepare those composites, modification of their properties by different inorganic and organic dopants as well as new optical properties. The evolution of scientific interest regarding PDLC composites in recent years is shown. Keywords: material science, composites, polymer-dispersed liquid crystals, optics, electrooptics.

Evaluation of Polymer Dispersed Liquid Crystal (PDLC) for Passive Rear Projection Screen Application

In the past few years, there have been many attempts to commercialize Polymer Dispersed Liquid Crystal (PDLC) for Rear Projection Screen (RPS) in electrically switchable films and glass products. This application is based on the exceptional image quality of PDLC at a wide viewing angle. However, due to the high price of conventional switchable PDLC privacy glass, the integration of RPS in PDLC failed to attract large multimedia and entertainment markets. An obvious solution to this problem is the independent development and evaluation of PDLC for Passive Rear Projection Screen (PRPS) application. However, in order to approve the application of PDLC for this purpose, further optical and image (visual) analyses are required. This work is the first part of our research project on evaluation of PDLC for RPS application, where we studied the effect of material and process parameters on optical properties of passive PDLC screens. The results indicate a strong dependence of the performances of passive PDLC screen on the material and process parameters, though it offers great advantages over conventional commercial RPS products. The second part of this study will focus on image evaluation of PDLC that will be the subject of a future study.

An electrically light-transmittance-switchable film with a low driving voltage based on liquid crystal/polymer composites

ABSTRACTAs a typical class of electrically light-transmittance-switchable (ELTS) composites materials, polymer dispersed liquid crystal (PDLC) films have been widely used in displays, smart windows, light shutters, etc. However, the commercialised PDLC film still requires a comparatively high voltage to maintain its transparent state, leading to huge power consumption and even a potential safety risk. In this regard, we proposed a ‘heat followed UV’ stepwise polymerisation strategy for preparing a kind of ELTS film with a low driving voltage (~20.7 V) through constructing a coexistent system of polymer dispersed and polymer stabilised liquid crystal (PD&SLC). In this new PD&SLC system, vertically orientated polymer networks were formed within LC domains to induce the vertical alignment of LC, thereby reducing the driving voltage. Also, the as-made PD&SLC film exhibited good flexibility due to the high content of polymer. Moreover, the effects of the liquid crystalline polymerisable monomers content on the polymer morphologies as well as the electro-optical properties of the as-made PD&SLC films were elaborately investigated.

Effects of the chain length of crosslinking agent and dye-doped amount on the electro-optical properties of polymer-dispersed liquid crystal films prepared by nucleophile-initiated thiol-ene click reaction

ABSTRACTA series of crosslinking agents with different chain lengths were used to explore their effect on electro-optical properties of dye-doped polymer dispersed liquid crystals (PDLCs) film prepared by nucleophile-initiated thiol-ene click reaction. The influences of dye contents and temperatures on electro-optical properties were also investigated. It was found that the increase in chain length of crosslinking agent leads to the increase in driving voltage and the decrease in memory effect. The decrease in reaction temperature causes a decrease in driving voltage and an increase in transmittance. Particularly, the dye content could be optimised to obtain promising materials with minimum driving voltage and high contrast ratio for display applications.

Fabrication of a controllable anti-peeping device with a laminated structure of microlouver and polymer dispersed liquid crystals film

ABSTRACTDisplay devices with the narrow viewing angle (NVA) and the wide viewing angle (WVA) characteristics can simultaneously meet the demands for privacy protection and information sharing. Herein, we propose a novel method for fabricating a controllable anti-peeping device with a laminated structure of microlouver and polymer dispersed liquid crystals (PDLCs) film, and this device can be reversibly switched between a wider viewing angle range (VAR) for the WVA mode and a narrower viewing angle range (VAR) for the NVA mode at 0 V and 8 V, respectively. Also, the VARs of the controllable anti-peeping device can be dynamically tuned by varying dimensions of the microlouver structures and applying various voltages across the PDLCs film from 0 V to 8 V. The as-fabricated device provides display devices with controllable viewing angle characteristics, which can be applied in the display field with fascinating and practical functions.

Flexible light valves using polymer-dispersed liquid crystals and TiO2/Ag/TiO2 multilayers.

The era of flexible optoelectronics demands development of wearable and bendable structures, foldable touch screens, paper-like displays, and curved and flexible solid-state lighting devices. Here, we demonstrate the fabrication of highly flexible light valves using polymer-dispersed liquid crystal (PDLC) and TiO2/Ag/TiO2 transparent conductive films. TiO2/Ag/TiO2 multilayers were prepared by magnetron sputtering technique on polyethylene terephthalate (PET) substrates at room temperature. By keeping the equivalent TiO2 layers and varying the deposition time of the Ag layer, proper metal nanograins on TiO2 planar plane were formed, providing the best tradeoff between the transmittance, sheet resistance and bending ability. The results are validated by numerical simulations that suggest the best match between the deposition time and individual layer thickness. Based on the performed characteristics of TiO2/Ag/TiO2/PET structures, several flexible light valves are fabricated and characterized. The sheet resistance values of TiO2/Ag/TiO2/PET remain unchanged over 1000 bending cycles. The measured driving voltage and response time values open great potential of TiO2/Ag/TiO2/PET for integration into next-generation ITO-free flexible and stretchable devices.

Electro-Optical Properties of a Polymer Dispersed and Stabilized Cholesteric Liquid Crystals System Constructed by a Stepwise UV-Initiated Radical/Cationic Polymerization

Polymer-dispersed liquid crystal (PDLC) and polymer-stabilized liquid crystal (PSLC) are two typical liquid crystal (LC)/polymer composites. PDLCs are usually prepared by dispersing LC droplets in a polymer matrix, while PSLC is a system in which the alignment of LC molecules is stabilized by interactions between the polymer network and the LC molecules. In this study, a new material system is promoted to construct a coexistence system of PDLC and PSLC, namely PD&SChLC. In this new material system, a liquid-crystalline vinyl-ether monomer (LVM) was introduced to a mixture containing cholesteric liquid crystal (ChLC) and isotropic acrylate monomer (IAM). Based on the different reaction rates between LVM and IAM, the PD&SChLC architecture was built using a stepwise UV-initiated polymerization. During the preparation of the PDS&ChLC films, first, the mixture was irradiated with UV light for a short period of time to induce the free radical polymerization of IAMs, forming a phase-separated microstructure, PDLC. Subsequently, an electric filed was applied to the sample for long enough to induce the cationic polymerization of LVMs, forming the homeotropically-aligned polymer fibers within the ChLC domains, which are similar to those in a PSLC. Based on this stepwise UV-initiated radical/cationic polymerization, a PD&SChLC film with the advantages of a relatively low driving voltage, a fast response time, and a large-area processability is successful prepared. The film can be widely used in flexible displays, smart windows, and other optical devices.

35‐2: 3D/2D Switchable Display System Based on Integral Imaging

A 3D/2D switchable display system based on integral imaging is proposed by using a projection screen and a projector. The projection screen consists of a lens array holographic optical element (LAHOE) and a polymer dispersed liquid crystal (PDLC) film. The experimental results prove the feasibility of the proposed system.

Formation of Optical Vortices by Controllable Holographic Diffraction Structures in Liquid Crystal – Photopolymer Compositions

Formation of optical vortices by Gaussian light beams diffraction on holographically formed diffractive optical elements (DOE) in polymer-dispersed liquid crystals (PDLC) is theoretically described. The feature of investigated DOEs is in their special structure designed to convert the incident light fields into Bessel-like. Numerical simulations for the external electric field’s interaction on these DOEs are obtained. Its influence on the light beams conversion efficiency is studied. The possibility to create electrically controllable transformation elements in PDLCs is theoretically justified.

Thermoplasmonic Activated Reverse-Mode Liquid Crystal Gratings

A new generation of reconfigurable optical components is conceived by bridging the photothermal properties of gold nanoparticles and the thermosensitivity of liquid crystalline materials. As such, gold nanorods (GNRs) heated using light are used to activate efficient hidden diffraction gratings realized in a blend made of a room temperature polymerizable liquid crystal (PLC) and nematic liquid crystal (NLC). Holographic liquid crystal polymer dispersed liquid crystal (HLCPDLC) gratings containing a small percentage of GNRs are fabricated with periodicity of 2.6 μm by means of a conventional UV holographic recording setup. The HLCPDLC structures are characterized using morphological, optical, and thermooptical techniques. Because of the initial refractive index match between the polymer-rich and LC-rich regions, the “grating” is hidden and the films appear transparent and nondiffractive. Illumination of these GNR-containing structures with a suitable light source (808 nm) induces a local heating due to the...