Polymer-stabilized Liquid Crystal Research Articles

The comparison of self-assembling behaviour of phenyl biphenylcarboxylate and biphenyl benzoate compounds with the different length and shape of chiral terminal chain

Our recent advances on polymer-stabilised orthoconic antiferroelectric liquid crystals (PSOAFLCs) eliminated the asymmetry of switching times in electro-optical effect based on surface stabilized OAFLCs and paved the way for their effective use in many photonic and optoelectronic devices requiring very fast switching. To fabricate new modifications of PSOAFLCs, besides bi- and tetrafunctional monomers, multicomponent AFLCs mixtures with a very low meting point, high stability of the antiferroelectric phase at lower temperatures and higher helical pitch length are desirable. Herein, a very convenient synthetic method of new AFLC compounds with a phenyl biphenylcarboxylate structure of the rigid core with different length and shape of terminal chains is reported. The resulting mesomorphic properties and helical parameters are compared with those of analogues with a biphenyl benzoate structure of the rigid core. The designed AFLC compounds clearly possess definite advantages. Specifically, they exhibit lower meting points, higher clearing points, broader temperature range of the antiferroelectric phase (SmCA*) and much longer helical pitch length, especially for analogues with the shortest achiral terminal chain, but unexpectedly lower tendency to give the direct transition between the antiferroelectric and isotropic phase. The introduction of the cycloalkyl ring at the end of the chiral terminal chain increases the melting points, flattens the temperature dependence of the helical pitch and changes the sense of the helical twist, but also enhances the ability of the direct SmCA*-Iso phase transition, regardless the type of the rigid core. The obtained results highlight a possibility of utilizing compounds with the phenyl biphenylcarboxylate structure of the rigid core, designed under this work, together with previously prepared compounds with the biphenyl benzoate rigid core to formulate multicomponent OALFC mixtures with a very low melting point and a very high helical pitch length.

Over 200 °C Broad-Temperature Lasers Reconstructed from a Blue-Phase Polymer Scaffold.

Blue-phase liquid crystal (BPLC) lasers have received extensive attention and have potential applications in sensors, displays, and anti-counterfeiting, owing to their unique 3D photonic bandgap. However, the working temperature range of such BPLC lasers is insufficient, and investigations are required to elucidate the underlying mechanism. Herein, a broad-temperature reconstructed laser is successfully achieved in dye-doped polymer-stabilized blue-phase liquid crystals (DD-PSBPLCs) with an unprecedented working temperature range of 25-230 °C based on a robust polymer scaffold, which combines the thermal stability and the tunability from the system. The broad-temperature lasing stems from the high thermal stability of the robust polymerized system used, which affords enough reflected and matched fluorescence signals. The temperature-tunable lasing behavior of the DD-PSBPLCs is associated with the phase transition of the unpolymerized content (≈60 wt%) in the system, which endows with a reconstructed characteristic of BP lasers including a U-shaped lasing threshold, a reversible lasing wavelength, and an obvious lasing enhancement at about 70 °C. This work not only provides a new idea for the design of broad-temperature BPLC lasers, but also sets out important insight in innovative microstructure changes for novel multifunctional organic optic devices.

Liquid crystal-polymer composites switchable windows for radiant energy flow and privacy control

Global warming is becoming a more and more severe crisis for humans. One way to resolve the concern is to reduce energy consumption. Smart switchable windows for office and residential buildings and vehicles can help reduce energy consumption. An ideal smart window should be able to control radiant energy flow and privacy. We investigated the capability of switchable windows based on liquid crystal/polymer composites, such as polymer dispersed liquid crystal (PDLC), polymer stabilized liquid crystal (PSLC), and polymer stabilized cholesteric texture (PSCT), to control the privacy and radiant energy flow. Through a systematic study, we identified methods to improve their capabilities. We demonstrated that PDLC and PSCT windows of sufficient thick film thickness can control both privacy and energy flow.

Preparation of polymer stabilised cholesteric broadband reflection films based on ZIF-8 assisted bidirectional diffusion method

ABSTRACT The polymer-stabilised cholesteric liquid crystal (PSCLC) broadband reflective films are prepared by zeolite imidazole framework-8 (ZIF-8)-assisted bidirectional diffusion combined with light curing, altering the single pitch of cholesteric liquid crystals (CLCs). By changing temperature, chiral compounds and ultraviolet (UV) absorbing dyes adsorbed on porous ZIF-8 are able to diffuse bidirectionally into the CLCs’ system and produce concentration gradients, thereby inducing a pitch gradient, and the structure of broad reflection bandwidth can be fixed through UV irradiation. In addition, the effects of various factors, such as UV irradiation intensity, UV irradiation time, mass fraction of polymerisable monomers and curing temperature on the pitch gradient of the films are systematically investigated, and the maximum reflection bandwidth of the prepared films is 1274 nm. There is no doubt that new and modified processes and techniques provide significant possibilities for preparing and applying PSCLC films.

Development and Prospect of Viewing Angle Switchable Liquid Crystal Devices

As we move from the industrial age to the information age, nowadays, the opportunity to access personal information in public increases as personal computers (PCs), cell phones, automated teller machines (ATM) and other portable display devices have come into wider use, so it is well suited for these liquid crystal displays (LCDs) to switch between wide viewing angle (WVA) (share mode) and narrow viewing angle (NVA) (privacy mode). In this review, we have summarized structures, principles and characteristics of several devices that show great potential application in controllable anti-peeping displays in the eyesight of materials consist of pure liquid crystals (LCs), polymer dispersed LCs (PDLCs), polymer stabilized LCs (PSLCs) or polymer network LCs (PNLCs) and non-LCs, which provides systematic information for next-generation viewing angle-controllable LCDs with lower operating voltage, higher transmittance and good viewing angle controllable characteristics. Because LCs/polymer composite films have the advantages of long life, low power consumption and energy saving, they are regarded as the mainstream technology of next-generation viewing angle controllable displays.

Electrically induced bistable switching of stop band in chiral nematic photonic crystal

Chiral nematic liquid crystals (CLCs) are highly promising tunable photonic materials that own various states and exhibit different optical properties in each state. The natural bistability between planar and focal conic states is widely exploited in power-saving applications. This work proposes another bistable electrical switching mode that involves two planar states that are realized by pitch jumps in polymer-stabilized chiral nematic liquid crystals (PSCLCs) with negative anisotropy. Under an appropriate polymerization condition, a polymer network can be formed throughout the CLC cell; it not only eliminates the original pitch jump behavior that is associated with thermal hysteresis in CLCs but also provides a balance between two planar states in the neighborhood of a pitch jump. The helical pitch of the PSCLC can be reversibly switched from the long pitch to the short pitch by applying pulse direct current (DC) voltages. The switching mechanism involves field-induced distortion in the polymer network, which generates the mechanical force that causes the pitch jump transition. Both planar states with different pitches can shift the wavelength of Bragg reflection, which depends strongly on the thickness of the PSCLC cells. A tunable laser that is based on a dye-doped PSCLC cell is fabricated and its wavelength-tuning ability is demonstrated.

Light Intensity-Selective Photopolymerization and Photoisomerization for Creating Colorful Polymer-Stabilized Cholesteric Liquid Crystal Patterns.

Polymer-stabilized cholesteric liquid crystal (PSCLC) films have been widely studied for their application as sensors, polarizers, and reflective windows. However, the preparation of programmable and colorful patterns based on the structural color is still challenging. Herein, the photochromic CLC mixtures were prepared by adding a photoisomerizable chiral additive (CA) and a photoinitiator in the nematic liquid crystal LC242. Under UV irradiation with weak intensity, photoisomerization of the CA was carried out and photopolymerization was suppressed by oxygen inhibition. With extending the irradiation time, the helical pitch of the CLC film increased and the selective Bragg reflection band tended to redshift. Under strong UV irradiation, oxygen inhibition was overcome and photopolymerization dominates the reaction. Therefore, the colorful-patterned PSCLC films were able to be prepared using masks. The results shown here not only give us a better understanding of the effect of oxygen inhibition but also lay the foundations for practical applications such as decoration and optical devices.

Pseudopeptidic polymers for modulating electro-optic switching, dielectric and electrical properties of liquid crystal

Herein, we report the stabilization of a nematic liquid crystal using chiral amino acid-based polymers. For the first time, the effect of molecular weight of structurally similar polymers on the physical parameters of polymer-stabilized liquid crystals (PSLCs) is investigated. To obtain PSLCs, three pseudopeptidic bottlebrush polymers of different molecular weights are synthesized from the same monomer using a ring-opening metathesis polymerization (ROMP) reaction. The bottlebrush polymers form self-assembly networks in the LC matrix. The size of the self-assembled network is readily tunable by the systematic variation in the molecular weight of the constituent polymer. The polymer network assembly is characterized by using field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and polarizing optical microscopy (POM). The ion density and diffusion coefficient of two different types of mobile ions are calculated by utilizing experimental dielectric data. The constituent polymer determines the structure of self-assembly network in PSLC and hence, the dielectric, electrical, electro-optic, and optical properties. The PSLCs show significant improvement in the response time (∼21%), dielectric anisotropy (∼40%), conductivity (∼11%), current (5-fold increment at 20 V), and phase transition temperature (PTT) upon variation in the molecular weight of the doped polymer units. This simple method of tuning response time, dielectric anisotropy, conductivity, current, and PTT using polymers of different molecular weights presents several possibilities of fabricating optoelectronic devices.

Effect of different monomers on the electro-optical properties of reverse-mode polymer stabilized liquid crystal

Reverse-mode polymer-stabilized liquid crystal (RPSLC) plays an important role in optical display field and becomes one of the research hotspots because of its normal transparent state without driving voltage. The concentration and type of polymer monomer has a great influence on the electro-optical properties of the RPSLC, so the optimization of the polymer monomer can reduce the driving voltage and hysteresis, and improve the contrast ratio of the RPSLC. In this paper, two kinds of V-type and two kinds of linear type monomers with bi-functional group, and a mesogenic monomer were used, the effect of the concentration and type of non-mesogenic monomers on electro-optical properties of RPSLC were studied with the fixed concentration of mesogenic monomer in experiment. The results show that the electro-optical properties of the RPSLC can be improved more when the suitable ratio of a V-type monomer is added. The maximum and minimum transmittances are 85.9% and 3.1%, the contrast ratio is 27.6, the threshold and saturation voltages are 6.4 V and 11.6 V, and the hysteresis is less than 1 V. Combined with the scanning electron microscopy morphology of polymer network, the mechanism of the influence of non-mesogenic monomer on the electro-optical properties of RPSLC is revealed. These experimental results will be of great significance for the research of RPSLC.

Reflectance-enhanced liquid crystal displays and thermochromic multi-color patterning

This paper presents a reflective liquid crystal display (LCD) combined with thermochromic liquid crystal microcapsules to enhance the electro-optical performance of the LCD and to empower new driving modes. The prepared reflectance-enhanced liquid crystal display (Re-LCD) possesses a bilayer structure, with the upper layer being a polymer-stabilized liquid crystal layer prepared by UV-induced phase separation and the lower layer being a thermochromic layer prepared by coating with thermochromic liquid crystal microcapsules. The effect of chiral compound content on the reflection wavelength of liquid crystal center was investigated. After that, the effects of monomer ratio, polymerization time and spin coating speed on the electro-optical properties and morphology of Re-LCD were investigated comprehensively by three sets of experiments. The results show that the combination of LCD and thermochromic materials has a synergistic effect in display, and the prepared Re-LCD shows more than 51% reflectance compared with typical LCD. The multi-field driving function allows the Re-LCD to rapidly switch optical states by force, electric, and temperature fields and obtain multi-color patterns during the driving process. This provides new application possibilities for LCD.

Effect of polymer concentration on the electro-optical, dielectric and photoluminescence properties of confined ferroelectric liquid crystals composites

ABSTRACT In this work, the effect of polymer concentration (1%, 3% and 5% wt./wt.) on the physical properties of polymer-stabilised ferroelectric liquid crystal composite is presented. The applied voltage and temperature effects have been investigated on electro-optical and dielectric parameters. The concentration of polymer reduces the spontaneous polarisation up to 34.04% at 5% polymer-stabilised ferroelectric liquid crystal composites. At higher concentration, the rotational viscosity also increased by 26.26% but reduced the dielectric strength by 62.8%. The relaxation frequency shifted towards the higher frequency side from 248.183 to 323.52 Hz. A fluorescence study reveals that the emission peak is observed at a wavelength of 424 nm and concentration quenching is observed with 3% and 5% polymer-stabilised ferroelectric liquid crystal composites. The present work also provides information about the interaction mechanism between polymer and ferroelectric liquid crystal molecules.

Broad-wave reflection of doping double-layered nanofibers in polymer-stabilized cholesterol-based liquid crystal films

In this study, polymer-stabilized cholesterol-based liquid crystal (PSCLC) films with broadband reflection were prepared using bilayer sodium alginate (SA)/polyvinyl alcohol (PVA) composite nanofiber films as a carrier for loaded photoinitiators as well as chiral compounds. The loaded photoinitiators and chiral compounds in the bilayer nanofibers could load a superposition of pitch inhomogeneous distribution to achieve broadband reflection. The effects of the concentration of chiral compounds in the films, the concentration of polymerization monomers, polymerization time, light intensity and temperature on the wave width were investigated. The morphology and diameter of the nanofibers were observed by SEM, and the weaving of the liquid crystals (LCs) was characterized by POM. The reflectance wavelengths were mainly measured by a UV–Vis spectrophotometer. This method broadens the wavelength of cholesteric phase liquid crystals from 256 nm to 436 nm, and lays the foundation for the study of the combination of wide wave reflection through nanofiber films.

Electrically induced coloration of polymer-stabilized cholesteric liquid crystal films with broadband reflection capability for smart windows

In this work, a novel approach to realizing the electrically induced coloration of polymer-stabilized cholesteric liquid crystal (LC) films with broadband reflection capability for smart windows was reported. Only starting common materials and an established preparation method were used. The polymer-stabilized cholesteric LC films with broadband reflection capability were prepared via the method of polymerization-induced pitch gradient. After doping with a commercial ionic liquid which served as the supporting electrolyte in the system, the initial broadband reflection capability of the polymer-stabilized cholesteric LCs film was not damaged, and the film can change into a colored state from a colorless state under a low direct current (DC) voltage. The color of the polymer-stabilized cholesteric LCs also can gradually disappear after the removal of the field despite the slow response rate. The as-prepared polymer-stabilized cholesteric LCs in this work not only can be applied in the energy-saving smart windows and thermal management for buildings, but also presented the performance of electrically induced coloration under a low voltage. This work is expected to provide a creative approach for the preparation of electrochromic energy-saving smart windows.

Principles of preparing broad-wave reflective films supported by nanofiber networks

ABSTRACT In this study, polymer-stabilised cholesterol-based liquid crystal (PSCLC) films with broadband reflection were prepared using nanofiber networks loaded with nanoparticles as the network supports throughout the liquid crystal cassette. The nanofiber prepared by electrostatic spinning technique ensures that the nanoparticles can be relatively uniformly distributed in the films, in addition to the formation of a fibre network that can replace the role of monomers and form network support. A series of studies on the concentration of nanoparticles and polymerisation conditions were carried out to obtain the maximum inhomogeneous distribution of spacing. The distribution of nanoparticles in the film was characterised by SEM and EDS; the stability of liquid crystals (LCs) and fibres with temperature was demonstrated by POM. The prepared broad-wave reflective films are of great value in intelligent windows and infrared shielding devices.

Tunable Circularly Polarized Luminescence with a High Dissymmetry Factor Emitted from Luminogen-Bonded and Electrically Controlled Polymer-Stabilized Cholesteric Liquid Crystals

Circularly polarized luminescence (CPL)-active materials with high dissymmetry factor (glum) values show great potential in photonic devices. In this study, electric-field-driven systems with tunable CPL signals are successfully fabricated based on polymer-stabilized cholesteric liquid crystal (PSChLC) doping by fluorescent molecules. By constructing a gradient helical superstructure of PSChLCs, distinctive CPL emission from two sides of a single sample is realized, in which the |glum| values were measured to be 0.6 and 1.5, respectively. Herein, we discussed the possible mechanism of this phenomenon. In addition, an applied electric field could broaden the reflection bandwidth of PSChLCs from 150 to 500 nm, covering the whole visible light region. Furthermore, this electric field-induced behavior leads to the variation of CPL signals and corresponding glum values, indicating the potential of the novel materials in the design and preparation of CPL-emitting devices with electrically tunable CPL intensity and glum.

Image quality enhancement of transparent waveguide display using a twisted nematic mode polymer-stabilized liquid crystal.

In this study, a twisted nematic mode polymer-stabilized liquid crystal (TN mode PSLC) integrated with a crossed polarizer was used to create a transparent waveguide display. When a voltage was applied, the PSLC scattered the waveguide light with a high polarization selectivity such that no substantial loss of the outgoing light intensity was observed after integrating the polarizer. However, with a crossed polarizer, in the ON state, the background light was not only scattered but also absorbed by the analyzer. Using this device configuration, with a 12 µm cell gap and 7% monomer concentration, we successfully realized a normally transparent waveguide display. The contrast ratio of the waveguide outgoing light was 26 and that of the undesired background reached 90. This device can display images due to waveguide edge-lit light scattering and simultaneously block the background information to improve the image quality.

Electrically controllable reflection bandwidth polymer-stabilized cholesteric liquid crystals with low operating voltage

ABSTRACT An electrically controllable reflection bandwidth polymer-stabilised cholesteric liquid crystals (PSCLCs) with low operating voltage is reported. This work focuses on the contribution of the crosslink density of the polymer network to broaden the bandwidth and the ability of the polymer network to trap ionic impurities under direct-current (DC) electric fields. Furthermore, it is shown that the appearance of nonideal optical properties in some samples, such as scatter and haze, is correlated to the polymer/LC compatibility and the soft segments motion under DC electric field. Based on these results, a model relating the motion of the polymer network with the broadening of the reflection bandwidth and the baseline optical properties is developed. This study provides useful guidelines for future PSCLCs production and performance optimisation.

Preparation of transmittance-switchable composites from a coexistent system of polymer-dispersed and polymer-stabilized cholesteric liquid crystals.

Polymer-dispersed and polymer-stabilized liquid crystals (PD&PSLCs) are the coexistent systems of PDLCs and PSLCs, in which a mesogenic polymer network forms in the LC droplets to construct the PSLC system and the combination of the LC domains and the isotropic polymer matrix makes up the PDLC system. In this work, a PD&PS cholesteric LC (PD&PSCLC) system, where the isotropic polymer walls make up the PDCLC microstructure and the liquid crystalline polymer network in the CLC droplets constructs the PSCLC system, is fabricated successfully. The CLCs are composed of a negative dielectric anisotropy nematic LC and a chiral dopant. Owing to the stabilizing effect of the liquid crystalline polymer network, the planar texture of the CLCs can be retained after the preparation process, and the PD&PSCLC sample is in a large-transmittance state. The action of a low-frequency electric field induces the disordered texture of the CLCs and the light-scattering (low-transmittance) state because of the electro-hydrodynamic effect and dynamic scattering effects. After the removal of the low-frequency electric field, the stabilizing effect of the liquid crystalline polymer network induces the CLC molecules to return to the homogeneous orientation state, and the sample also re-obtains the large transmittance. This work may provide a facile approach to fabricating polymer/CLC composites with switchable transmittance for reverse-mode smart windows.

Low-voltage and fast-response polymer-stabilized blue-phase liquid crystals achieved using a new organosilicone monomer.

In this paper, two types of polymer-stabilized blue-phase liquid crystals (PS-BPLCs) with different monomers were designed and prepared. The morphology, temperature range and electro-optical properties of the blue phases were studied and discussed. The temperature range of both types of PS-BPLC is greater than 110 °C, and both samples can be stabilized well at room temperature. The organosilicone monomer 3-methacryloxypropyltrimethoxysilane (KH570), which contains double bonds, was introduced to a blue-phase system for the first time. Regarding the electro-optical performance, the on-state voltage of the PS-BPLCs with the KH570 monomer is reduced to 30 V compared with traditional C12A monomer systems in which the on-state voltage is 75 V at 458 nm. Meanwhile, a fast response and suppressed hysteresis are obtained. These results are helpful to the application of displays and photonic devices.

Reflection band gap broadening of polymer-stabilized cholesteric liquid crystal by low voltage

Reflection band gap broadening of polymer-stabilized cholesteric liquid crystal by low voltage