Polymer-stabilized Liquid Crystal Research Articles

Enhanced Laser Emission by Incorporating Helically Twisted Silver Nanoflakes within Polymer-Stabilized Cholesteric Liquid Crystals

Enhanced Laser Emission by Incorporating Helically Twisted Silver Nanoflakes within Polymer-Stabilized Cholesteric Liquid Crystals

Polymer-stabilized cholesteric liquid crystals with multi-state reversibility for applications in advanced smart adjustable anti-counterfeiting and energy-saving displays

The creation of an anti-counterfeiting material with reflection color, fluorescence, infrared imaging, and electrical property information will increase the level of anti-counterfeiting. However, the creation of such a material has proved to be extremely challenging. In this study, twin anti-counterfeiting labels with intelligently tunable patterns were obtained through a strategy of PMMA loaded chiral molecules via diffusing on the micrometer scale. In order to enhance anti-counterfeiting ability, the multi-mode intelligently modulated advanced anti-counterfeiting material was obtained through the strategy of introducing photo-isomerizing dithienylethene and infrared-shielding Cs0.33WO3 nanoparticles. This material can be reversibly converted into fluorescent, reflected colors, infrared, and electrical characteristic signals. The encrypted information is displayed with red fluorescence under UV light irradiation. Furthermore, the encrypted information can be reversibly displayed and erased under repeated stimulation by voltage and pressure, heating and cooling, UV, visible and near infrared (NIR) light. The aim of our work is to create a simple method to prepare complex patterns of structural and fluorescent color anti-counterfeiting films with the improved anti-counterfeiting capability. Based on this, the strategy of co-doping fluorescent molecules and photochromic molecules in liquid crystals combined with etching technology has realized a multiple anti-counterfeiting mode with a higher level of security. The advanced multiple anti-counterfeiting mode can output encrypted information of multiple states which will greatly improve the level of anti-counterfeiting. In terms of display, we obtained billboards with complex patterns, price intelligently tunable and full-color display by diffusion S5011 in liquid crystals, thus extending their application in the field of electronic shelf labels.

Epoxy resin-based polymer-wall stabilized liquid crystal films with low driving voltages and improved mechanical performance for smart window applications

Polymer stabilized liquid crystals (PSLCs) have wide applications in smart windows and light shutters. However, for most applications, it requires low driving voltages and high mechanical performance, which is arduous to achieve both. Herein, a novel epoxy resin-based PSLC device is prepared showing improved electro-optical and mechanical properties simultaneously. This is benefited from the construction of a period polymer wall structure in liquid crystal matrix through cationic polymerization of epoxy monomers and preparation of multi-functional surfaces through self-assembly of silane mixtures. The multi-functional surfaces could ensure homeotropic alignment of liquid crystals and photo-polymerize with the polymer networks. As a result, the polymer-wall stabilized liqduid crystal (PWSLC) film with multi-functional surfaces shows lower (24%) saturation voltage, higher (20%) contrast ratio and improved (2.4 times) peel strength than the PSLC film with only vertical alignment surfaces. This research provides important reference to experiment and manufacture of reverse-mode dimming films for smart windows applications.

Fast-response and high figure of merit phase shifter based on polymer- stabilised liquid crystals

ABSTRACT This article presents a novel fast-response and high figure of merit (FoM) phase shifter based on polymer-stabilised liquid crystal (LC) for Ka-band. The developed LC phase shifter (LCPS), utilising a coplanar differential line loading 19 parallel-plate variable capacitors with 4.6 μm LC-cell, adopts polymer-stabilised LC technology in which the parallelly orientated polymer networks facilitate the LC molecules’ return to their initial state upon removal of the bias voltage, thus accelerating response time. The proposed LCPS has been fabricated to evaluate its superior capabilities. The experimental results closely match the simulation results, confirming its good congruity. This design has measured S 11 < −10 dB in 29–31 GHz. LCs with different HCM002 concentrations are injected to LCPS, and the experimental outcomes showed that LCs with 9 wt% HCM002 concentration significantly improve the response time of LCPS to 2.8 ms with a high FoM of 91.2°/dB at 30 GHz.

Reverse Mode Polymer Stabilized Cholesteric Liquid Crystal Flexible Films with Excellent Bending Resistance.

The reverse-mode smart windows, which usually fabricated by polymer stabilized liquid crystal (PSLC), are more practical for scenarios where high transparency is a priority for most of the time. However, the polymer stabilized cholesteric liquid crystal (PSCLC) film exhibits poor spacing stability due to the mobility of CLC molecules during the bending deformation. In this work, a reverse-mode PSCLC flexible film with excellent bending resistance was fabricated by the construction of polymer spacer columns. The effect of the concentration of the polymerizable monomer C6M and chiral dopant R811 on the electro-optical properties and polymer microstructure of the film were studied. The sample B2 containing 3 wt% of C6M and 3 wt% R811 presented the best electro-optical performance. The electrical switch between transparent and opaque state of the flexible PSCLC film after bending not only indicated the excellent electro-optical switching performance, but also demonstrated the outstanding bending resistance of the sample with polymer spacer columns, which makes the PSCLC film containing polymer spacer columns have a great potential to be applied in the field of flexible devices.

Study on the Polymer Morphology and Electro-Optical Performance of Acrylate/Epoxy Resin-Based Polymer-Stabilized Liquid Crystals Based on Stepwise Photopolymerization.

Stepwise photopolymerization is a miraculous strategy modulating the polymer skeleton and electro-optical properties of light modulators based on liquid crystal/polymer composites. However, owing to the indistinct polymerization mechanism and curing condition discrepancy, the required polymer structures and electro-optical properties are hard to be controlled precisely. Herein, a novel polymer-stabilized liquid crystal film based on acrylate/epoxy resin is proposed, fabricated and the relationships between preparation process, polymer content, polymer morphology and electro-optical properties are studied. The in-situ photopolymerization of acrylate/epoxy resin liquid crystalline polymer is fulfilled using cation photo-initiator UV 6976. The distinct photopolymerization speed between acrylate and epoxy resin benefits the polymer morphology control, and with accurate containment of the polymerization process and polymer composition, the superior electro-optical properties at a higher polymer content are acquired. The polymer morphology and electro-optical properties are influenced by the polymer content and mass ratio between acrylate and epoxy resin. The best electro-optical properties among samples are attained by controlling the mass ratio between acrylate and epoxy resin to 1:1, integrating higher densities of scattering centers and lower anchoring effect. With higher polymer content, the strategy of increasing the mass ratio of E6M benefits the improvement of E-O properties for alleviating polymer density. This work provides insights to stepwise polymerization of liquid crystalline monomers and offers a fancy strategy for the preparation of novel liquid crystal dimming films.

Electro-Optic Response of Polymer-Stabilized Cholesteric Liquid Crystals with Different Polymer Concentrations.

Polymer-stabilized cholesteric liquid crystals (PSCLCs) have emerged as promising candidates for one-dimensional photonic lattices that enable precise tuning of the photonic band gap (PBG). This work systematically investigates the effect of polymer concentrations on the AC electric field-induced tuning of the PBG in PSCLCs, in so doing it explores a range of concentrations and provides new insights into how polymer concentration affects both the stabilization of cholesteric textures and the electro-optic response. We demonstrate that low polymer concentrations (≈3 wt. %) cause a blue shift in the short wavelength band edge, while high concentrations (≈10 wt. %) lead to a contraction and deterioration of the reflection band. Polarization optical microscopy was conducted to confirm the phase transition induced by the application of an electric field. The observations confirm that increased polymer concentration stabilizes the cholesteric texture. Particularly, the highly desired fingerprint texture was stabilized in a sample with 10 wt. % of the polymer, whereas it was unstable for lower polymer concentrations. Additionally, higher polymer concentrations also improved the dissymmetry factor and stability of the lasing emission, with the dissymmetry factor reaching the value of around 2 for samples with 10 wt. % of polymer additive. Our results provide valuable comprehension into the design of advanced PSCLC structures with tunable optical properties, enhancing device performance and paving the way for innovative photonic applications.

5CB-doped polymer-stabilised sphere-phase liquid crystal smart window with fast response and high transmittance

ABSTRACT Smart windows have become increasingly popular due to their ability to provide privacy protection, energy efficiency, and user control. Therefore, this technology has a wide range of applications in various fields such as construction, automotive, aviation, and renewable energy. However, commercialisation faces two significant challenges due to the slow response speed and low transmittance. This work demonstrates a 5CB-doped polymer-stabilised sphere-phase liquid crystal (PS-SPLC) smart window with sub-millisecond response time, which can be switched between transparent and scattering states by an electric field. The transmittance exceeds 90% when the voltage is on in the visible light range. This 5CB-doped PS-SPLC smart window has the potential to usher in a new era of smart window research.

Electrically reflectance-tunable circular polarization reflector based on thin polymer-stabilized cholesteric liquid crystals

This paper presents a reflectance-tunable circular polarization reflector that utilizes thin polymer-stabilized cholesteric nematic liquid crystals (PSCLCs) with negative anisotropy. The switching mechanism involves direct-current (DC) field-induced movement in the polymer network, which generates the mechanical force that causes a pitch-gradient helical structure and thus a distorted Bragg reflection. The pitch-gradient helical structure of the 5 μm-thick cells only allows for a limited number of pitch lengths, resulting in a decrease in reflectance at the designed Bragg reflectance and a reflectance tunable range determined by applying DC voltages. The PSCLCs reflecting red, green, and blue colors were measured for their reflectance tuning ability and circular polarization selectivity. A tunable polarization volume grating was fabricated using a PSCLC cell, and the ability of the grating to diffract light intensity was demonstrated.

Reverse-Mode Polymer-Stabilized Liquid Crystal Films with Enhanced Peel Strength and Electro-Optical Performance Based on Photoreactive Self-Assembly Alignment Layers and Patterned Polymer Walls.

Polymer-stabilized liquid crystal (PSLC) is a promising material toward the practical application of serving as energy-saving reverse-mode smart windows owing to its superior electro-optical (E-O) properties, simple and efficient processability, and compatibility to most practical circumstances. However, its feeble peel strength originated from low polymer content and poor adhesion between polymer networks and substrates inhibited its large-scale flexible film production. It is still a challenging task to derive good mechanical properties and superior E-O performance for PSLCs at the same time. In this study, a highly durable liquid crystal/polymer composite film showing enhanced peel strength and excellent E-O properties was attained by simultaneously building photoreactive self-assemble alignment layers through an efficient one-step method and the sculpture of a patterned polymer wall structure. This film has comprehensive ascendant E-O properties of lower driving voltages, faster response times, and higher contrast ratio, together with an over 30 times lift of the peel strength. The effectuation mechanisms of the alignment, E-O properties, peel-strength, microstructures, and cyclic durability of the films have been systematically studied. This novel liquid crystal/polymer composite film demonstrates advantages in every aspect of performance compared to traditional PSLC devices, which hoards promising applications in smart windows for cars and buildings.

Influence of polymerisation parameters on the electro-optical properties of polymer-stabilised liquid crystals for smart glass application

ABSTRACT Smart windows are an important application of liquid crystal/polymer nanocomposites. Recently, an original design has been proposed for a window that switches from a scattering to a transparent state under an electric field. By polymerisation, the topological defects of the smectic A phase under hybrid anchoring conditions are stabilised in the nematic phase, making them addressable by the field. Here, we report the impact of the polymerisation parameters on the electro-optical properties and microstructure of the nanocomposites. We have first estimated the polymerisation kinetics and then investigated the influence of the UV-light intensity, monomer concentration, and photoinitiator concentration. UV-light intensity has little effect on the electro-optical properties of the device, although microstructural changes were observed. The monomer concentration has a strong impact on the scattering power in both the on- and off-states. A model is proposed to describe the dependence of the response times on the monomer concentration. Finally, no influence of the photoinitiator concentration on the electro-optical properties was observed. Polymerisation could occur even in the absence of photoinitiator, possibly due to the presence of impurities that could generate radicals under irradiation. Our study should help optimising the formulation of polymer-stabilised liquid crystal nanocomposites for industrial applications.

Study of polymer-stabilized cholesteric liquid crystals for reflective displays in active matrix driving

This paper attempts to explore the feasibility of using cholesteric liquid crystals (CLCs) for reflective displays in active matrix (AM) driving. The study focuses on reverse mode polymer-stabilized cholesteric liquid crystals (PSCLCs) to overcome the inherent bistability of CLCs and achieve faster response times for switching between bright and dark states. Key parameters such as cell gap, monomer concentration, polymerization temperature, and different monomer types and liquid crystal hosts are systematically investigated to optimize the electro-optical properties of the liquid crystals for active matrix driving. The research demonstrates that PSCLCs can operate within a 15 V driving voltage range while maintaining a good bright-dark state contrast ratio (>10) and a response time of less than 42 ms. This suggests significant potential for their application in dynamic reflective display technologies.

Evaluation of cutoff characteristics in oscillating liquid crystal molecules under sinusoidal electric fields

Sinusoidal electric fields are applied to liquid crystals (LCs) across various frequencies ranging from 0.1 Hz to 10 kHz to investigate the oscillatory behavior of LC directors. In a 1.5 μm thick 5CB cell, a significant decline in refractive index change occurs in the frequency range greater than 10 Hz, in which the LC director cannot sufficiently follow the rapid fluctuations in field intensity. To evaluate the response speed of the LCs under sinusoidal electric fields, the cutoff frequency is introduced as a response indicator. Enhancement of the cutoff frequency can be achieved through conventional fast response techniques such as polymer-stabilized LCs.

Ultrasensitive Biomimetic Skin with Multimodal and Photoelectric Dual-Signal Sensing.

Mimicking biological skin enabling direct, intelligent interaction between users and devices, multimodal sensing with optical/electrical (OE) output signals is urgently required. Owing to this, this work aims to logically design a stretchable OE biomimetic skin (OE skin), which can sensitively sense complex external stimuli of pressure, strain, temperature, and localization. The OE skin consists of elastic thin polymer-stabilized cholesteric liquid crystal films, an ion-conductive hydrogel layer, and an elastic protective membrane formed with thin polydimethylsiloxane. The as-designed OE skin exhibits customizable structural color on demand, good thermochromism, and excellent mechanochromism, with the ability to extend the full visible spectrum, a good linearity of over 0.99, fast response speed of 93 ms, and wide temperature range of 119 °C. In addition, the conduction resistance variation of ion-conductive hydrogel exhibits excellent sensing capabilities under pressure, stretch, and temperature, endowing a good linearity of 0.99998 (stretching from 0 to 150%) and high thermal sensitivity of 0.86% per °C. Such an outstanding OE skin provides design concepts for the development of multifunctional biomimetic skin used in human-machine interaction and can find wide applications in intelligent wearable devices and human-machine interactions.

Doped nanofibers achieve tunable broadband in cholesteric phase liquid crystals

ABSTRACT In this study, polymer-stabilised cholesterol-based liquid crystal (PSLC) broadband reflective films were prepared by doping UV absorbers and chiral compounds in bilayer poly (methyl methacrylate) (PMMA) nanofiber films. Broadband reflection was achieved by diffusion of the UV absorber and chiral compounds in the bilayer nanofibers into the liquid crystal layer, which resulted in the formation of an inhomogeneous pitch distribution in the liquid crystal layer. To attain an optimal reflective effect, various factors such as polymer monomer concentration, concentration of chiral compounds in the spun film, diffusion temperature, diffusion time, and intensity of polymerised light were investigated. The effects of these factors on the reflective bandwidth were explored. The method significantly broadened the reflective bandwidth of the cholesteric phase liquid crystal layer, establishing the foundation for studying the combination of nanofiber films with cholesteric phase liquid crystals to form an adjustable pitch.

Preparation of a double-layer liquid crystal optical shutter with fluorescent properties and its ultraviolet shielding screening properties

ABSTRACT Polymer dispersed liquid crystal (PDLC) films and polymer-stabilised cholesteric liquid crystals (PSCLC) films have been widely applied in smart optoelectronics due to electrical switching properties. In this study, fluorescent dyes were introduced into PDLC films and combined with PSCLC films to invent a new type of optical shutter, and the optimal formulations of each of liquid crystal structural layers were investigated. Then, PDLC anti-counterfeit optical device doped with fluorescent materials were fabricated by UV curing method using polymerisation-induced phase separation. The novel optical shutter could be driven layer by layer at room temperature according to different voltages to realise the switching of three states, namely, off state, PDLC layer-driven, and full-driven (open state). Samples under optimal conditions showed that the new optical shutter could achieve 96.5% UV shielding in the off state and had a long lifetime. The results demonstrate the attractive potential of the new optical shutter for optoelectronic devices and smart curtain applications.

A novel double-layer liquid crystal electro-optic switch and its infrared shielding properties investigation

ABSTRACT In this study, a novel light switch was developed by integrating polymer dispersion liquid crystals (PDLC) and polymer-stabilised cholesteric liquid crystals (PSCLC), leveraging the infrared shielding properties of these two liquid crystal films. The optimal formulation for each layer of the liquid crystal structure was investigated. Subsequently, the effects of light intensity, high temperature, and humidity on the performance of the light switch were examined. This new light switch can be selectively driven layer by layer at room temperature, achieving three states: the off state, PDLC layer-driven state, and full-driven (on state). This functionality serves the purpose of electrogenerated temperature control. Samples under optimal conditions exhibited a 31% enhancement in infrared shielding compared to a single-layer PDLC film. Moreover, the IR shielding rate could be adjusted according to the voltage applied, realising an intelligent temperature-control effect. The results of this study suggest that such bilayer liquid crystal structures hold significant potential for application in electro-optical devices and smart curtain systems.

Low-voltage electrochromic behaviour of polymer-stabilised liquid crystal with considerable broadband reflection

ABSTRACT In this work, a novel method was proposed to achieve electric-induced colouration of polymer-stabilised liquid crystal (PSLC) with considerable broadband reflection. The formation of broadband reflection was due to the introduction of the UV absorber that induced a light intensity gradient within the liquid crystal system and promoted the non-uniform distribution of the pitch. Experiments were performed to optimise the components and polymerisation conditions, allowing the bandwidth to be broadened from 252 nm to 906 nm. The texture of PSLC was characterised using POM, revealing that the planar texture was not disrupted. Thereafter, the addition of ionic liquid as supporting electrolyte enabled the PSLC to be switched from colourless to coloured at DC voltage as low as 1.7 V and the colouration behaviour can be independently modulated by changing the voltage level and the duration. Meaningfully, after the DC electric field was removed, the colour of the PSLC can also disappear gradually. In addition, the as-prepared electrochromic PSLC presented enhanced UV shielding, IR thermal control efficacy and good colouration performance on flexible substrates, which can be applied to low-energy smart glass and thermal management of buildings. This study provided a promising approach for the development of smart optical devices.

Colorful multi-plane augmented reality display with dynamically tunable reflective Pancharatnam-Berry phase lens.

Reflective cholesteric liquid crystal (CLC) Pancharatnam-Berry phase lens (PBL) devices have attracted significant attention in augmented reality (AR) display due to their wide spectral and angular response bandwidths, high diffraction efficiency, and polarization selectivity. However, currently reported CLC reflective PBLs are either limited by monochrome display or suffers from complicated design for colorful display. Herein, we demonstrate a colorful multi-plane AR display system with dynamically tunable reflective PBL. The reflective PBL is fabricated by polymer-stabilized cholesteric liquid crystal (PSCLC) that provides dynamical and continuous tunability of color and focal length by direct current (DC) voltage. A proof-of-concept colorful multi-plane AR device is demonstrated, where over 90% diffraction efficiency at desired wavelength has been obtained. The proposed simple, compact, and light AR display system capable of color-imaging with multi-depth shows great application potential in the vehicle-mounted head-up display (HUD).

Surface induced effects on the dielectric and optical properties of polymer-stabilized cholesteric liquid crystal composites

Polymer-stabilized cholesteric liquid crystal is one of the most coveted systems in the ever-expanding field of liquid crystal due to its wider potential application in smart windows, optical shutters, tunable lenses, etc. The dispersion of polymer into liquid crystal by introducing a small amount of chiral dopant prepared polymer-stabilized cholesteric liquid crystal. In this work, two samples of polymer- cholesteric liquid crystal were prepared. Surface treatment is applied on one composite while another sample is prepared without any treatment on the indium tin-oxide coated glass substrates. Frequency and temperature-dependent dielectric parameters are observed for both samples. The dielectric studies revealed that after surface treatment, the permittivity of the sample significantly increased. AC conductivity of the sample also increased. The photoluminescence spectra also show improvement in the intensity with the surface-treated sample.