Polymer-stabilized Cholesteric Liquid Crystal Research Articles

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.

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.

Low driving voltage reverse-mode polymer-stabilised cholesteric liquid crystal devices using small phenylacetylene molecule

ABSTRACT Polymer-stabilised liquid crystal devices have found widespread use in intelligent displays and dimming glass applications, but their high driving voltage induces high power consumption and needs to be reduced. In this paper, we use several small phenylacetylene molecule materials with linear structures to reduce the driving voltage and obtain a high contrast ratio in reverse-mode polymer-stabilised cholesteric liquid crystals (RPSCLC). In particular, the drive voltage of the 8-micron-thick sample was reduced by nearly 40%, and the 20-micron-thick sample was reduced by nearly 45%. Additionally, we explore the physical mechanism through which various small phenylacetylene molecule materials improve the electro-optical properties, based on polymer network morphology effects. The results will be of great value for enhancing the use of RPSCLC devices.

The association of structural chirality and liquid crystal anchoring in polymer stabilized cholesteric liquid crystals.

Polymer stabilized cholesteric liquid crystals (PSCLCs) are electrically reconfigurable reflective elements. Prior studies have hypothesized and indirectly confirmed that the electro-optic response of these composites is associated with the electrically mediated distortion of the stabilizing polymer network. The proposed mechanism is based on the retention of structural chirality in the polymer stabilizing network, which upon deformation is spatially distorted, which accordingly affects the pitch of the surrounding low molar-mass liquid crystal host. Here, we utilize fluorescent confocal polarized microscopy to directly assess the electro-optic response of PSCLCs. By utilizing dual fluorescent probes, sequential imaging experiments confirm that the periodicity of the polymer stabilizing network matches that of the low molar-mass liquid crystal host. Further, we isolate distinct ion-polymer interactions that manifest in certain photopolymerization conditions.

Smart beam via electrically addressing the dye-doped and polymer-stabilized cholesteric liquid crystals

Smart beam via electrically addressing the dye-doped and polymer-stabilized cholesteric liquid crystals

Control the structure of the polyacrylate/epoxy resin film through photopolymerisation

ABSTRACT Double-layered polymer-stabilised cholesteric liquid crystal (PSCLC) films constructed from epoxy resin and polyacrylate were prepared based on photopolymerisation-induced phase separation. Moreover, a PSCLC film with an interpenetrating network was prepared through a one-step photopolymerisation approach. A colourful pattern was prepared based on these two approaches, which was suitably applied for decoration.

Programmable Dynamic Information Storage Composite Film with Highly Sensitive Thermochromism and Gradually Adjustable Fluorescence.

The development of an integrated material system capable of effectively organizing and combining multisource information, such as dynamic pigmentary, structural, and fluorescent colors, is significant and challenging. Achieving such programmable dynamic information storage can considerably enhance the diversity and security of information deliveries. Here, a polymer-stabilized cholesteric liquid crystal system with highly temperature-sensitive structural color and light-sensitive pigmentary and fluorescence colors is presented. The prepared cholesteric liquid crystals (clcs) can reversibly change their structural color from red to blue within variational 3°C near room temperature, and exhibit a gradually adjustable fluorescence which can transform from blue to pink and finally to bright red. All this dynamic information is programmable and tailored, hundreds of thousands of (>540000) pattern combinations can easily be achieved by optical writing with a "bagua" pattern photomask. Therefore, if the corresponding code combinations to the pattern are assigned particular meanings, encrypted transmission of information with very high security can be achieved by utilizing applicable information encoding tables and decryption rules.

Preparation of the colorful retroreflective film based on the polymer-stabilized cholesteric liquid crystal

A cholesteric liquid crystal (CLC) mixture was prepared using a photoisomerizable chiral dopant and acrylates. With extending the irradiation time of a 365-nm UV light, the Bragg reflection band of the CLC mixture shifted to long wavelength. After photopolymerization, a polymer-stabilized CLC (PSCLC) film was obtained. The colorful PSCLC patterns with glass microspheres on the surface were prepared by controlling the competition between the photoisomerization of the chiral dopant and the photopolymerization of the acrylates. Due to the retroreflective property, the patterned PSCLC films can be applied as the traffic signs and for advertisement.

Multilayer, Broadband Infrared Reflectors Based on the Photoinduced Preparation of Cholesteric Liquid Crystal Polymers.

This paper focuses on preparing broadband reflective liquid crystal films through the diffusion of monofunctional and bifunctional monomers in a photoinduced trilayer system. By combining the hydrophilic and hydrophobic liquid crystal glass surface treatment technologies, the polymer network of polymer-stabilized cholesteric liquid crystal (PSCLC) itself serves as a diffusion channel to form a trilayer cholesteric liquid crystal composite system containing bifunctional monomers, a nematic liquid crystal composite system, and a cholesteric liquid crystal composite system containing monofunctional monomers. Utilizing the difference in the polymerization rates of monofunctional and difunctional polymerizable monomers, the monomers and chiral compounds diffuse relative to each other, so that the liquid crystal pitch exhibits a gradient distribution, and the broadened reflective width can reach up to 1570 nm. There is no doubt that new and improved processes and technologies offer important possibilities for preparing and applying PSCLC films.

Polymer-Stabilized Cholesteric Liquid Crystal Films with Double Reflection Bands Prepared Based on the Competition between Photopolymerization and Photoisomerization.

Composite colors have been widely found in nature. Herein, polymer-stabilized cholesteric liquid crystal (PSCLC) films with composite structural colors were prepared through a one-step photopolymerization approach. The CLC mixtures were prepared using a mixture of acrylates and a mixture of two chiral dopants. One of the chiral dopants is polymerizable, and the other one is photoisomerizable. After photopolymerization, the PSCLC films with double Bragg reflection bands were obtained on the surface of a substrate. The competition between the photopolymerization of the acrylates and the photoisomerization of the chiral dopant was proposed to drive the formation of the double reflection bands. Without oxygen inhibition, the polymerization of the acrylates near the substrate surface was carried out. However, due to oxygen inhibition, the polymerization of the acrylates near the air was retarded. Then, the photoisomerization of the chiral dopant was carried out prior to the polymerization of the acrylates. The wavelengths of the double reflection bands were tunable by changing the concentrations of the acrylates and chiral dopants and the polymerization temperature. Colorful patterns with composite structural colors were prepared, which were suitable for decoration and anti-counterfeiting.

A PSCLC Pattern Prepared Based on Handedness Inversion for Anti-counterfeiting.

Handedness inversion has been widely studied in supramolecular chemistry and material sciences. Herein, a photoisomerizable chiral dopant was synthesized, which could induce the formation of a cholesteric phase with right-handedness. The Bragg reflection band of the cholesteric liquid crystal (CLC) mixture shifted to the long wavelength with extending 365 nm UV light irradiation time. Based on this photochromic property, a colourful polymer-stabilized CLC (PSCLC) film was prepared using a grayscale mask. A handedness reversible CLC mixture was prepared using a mixture of this chiral dopant and S5011. With extending the UV light irradiation time, the handedness of the CLC mixture changed from right- to left-handedness. A patterned PSCLC film was prepared using this CLC mixture. Complementary images were observed under right- and left-handedness circularly polarized lights. The results shown here not only give us a better understanding the competition between photopolymerization and photoisomerization, but also lay the foundations for decoration and anti-counterfeiting.

Preparation and application of broadband reflective polymer-stabilised cholesteric liquid with nanoscale fibres

ABSTRACT The bandwidth broadening of polymer-stabilised cholesteric liquid crystal (PSLC) has the potential to enhance its optical performance. In this study, a novel approach was presented for preparing broadband reflective films by using nanofibers loaded with zinc oxide nanoparticles as network supports in chiral nematic liquid crystals. The introduction of nanofiber played a pivotal role in replacing some monomers, resulting in cost-efficiency. Experiments were conducted to investigate the influence of different compositions and conditions on the broadband reflection performance. Specially, the effect of the concentration of nanoparticles loaded in nanofibers, direct-current (DC) voltage, monomer content, ultraviolet (UV) light intensity, polymerisation temperature, and UV irradiation time were examined. The microstructure of nanofiber and optical texture of PSLC were characterised using SEM and POM, respectively. With the above optimisations, the broadening from 246 nm to 412 nm was achieved and the films embodied brilliant UV shielding and infrared (IR) heat control effectiveness. This study provides a potential method for the preparation of smart liquid crystal devices in the future.

Polymer-stabilized fast-relaxation dye-doped cholesteric liquid crystals with a wide temperature range by thiol-Michael addition polymerization

The transient planar (TP) state during the relaxation process, from homeotropic (H) to planar (P) state, of dye-doped cholesteric liquid crystals (DDCLCs) can be eliminated by adjusting the elastic constants, reducing the time and haze of the relaxation process. However, the elastic constants are easily influenced by temperature. Therefore, the operating temperature range for fast-relaxation DDCLCs is relatively narrow. This work demonstrates that a polymer helical template can be formed in the P state of DDCLCs by thiol-Michael addition polymerization. It can significantly expand the temperature range from 20 °C to 60 °C for fast-relaxation DDCLCs. More importantly, polymer-stabilized dye-doped cholesteric liquid crystals (PS-DDCLCs) did not show any reduction in the absorption of the dye or discoloration after polymerization. PS-DDCLCs could still achieve tri-state switching and could be used for fast-relaxation smart windows capable of adapting to increasing environmental temperatures. The proposed smart window can be used in vehicles and aircraft to prevent glare and protect privacy.

A Review of Developments in Polymer Stabilized Liquid Crystals.

Polymer-stabilized liquid crystals (PSLCs) are multi-functional materials consisting of polymer networks in a continuous phase of liquid crystals (LCs), of which polymer networks provide anchoring energy to align the LCs. A number of improvements are detailed, including polymer-stabilized nematic liquid crystals (PSNLCs), polymer-stabilized cholesteric liquid crystals (PSCLCs), polymer-stabilized blue phase liquid crystals (PSBPLCs), polymer-stabilized smectic liquid crystals (PSSLCs), polymer-stabilized ferroelectric liquid crystals (PSFLCs), and polymer-stabilized antiferroelectric liquid crystals (PSAFLCs) in this review. Polymer stabilization has achieved multiple functionalities for LCs; in smart windows, a sufficiently strong electric field allows the LCs to reorient and enables switching from a scattering (transparent) state to a transparent (scattering) state. For broadband reflectors, the reflection bandwidth of LCs is manually tuned by electric fields, light, magnetic fields, or temperature. PSBPLCs open a new way for next-generation displays, spatial light modulators, sensors, lasers, lenses, and photonics applications. Polymer networks in PSFLCs or PSAFLCs enhance their grayscale memories utilized in flexible displays and energy-saving smart cards. At the end, the remaining challenges and research opportunities of PSLCs are discussed.

Preparation and applications of the epoxy resin-based polymer- stabilised cholesteric liquid crystal films with microcracks at the surface

ABSTRACT Polymer-stabilised cholesteric liquid crystal (PSCLC) films attracted much attention in decoration and anti-counterfeiting. Herein, the PSCLC films were prepared using the mixtures of epoxy monomers through cationic photopolymerisation. The structural colour was tunable by changing the concentration of chiral dopant in the reaction mixture and polymerisation temperature. Due to polymerisation shrinkage, microcracks were formed at the surfaces of the PSCLC films, which originated from the oily streak defects of the CLC mixtures. Colourful patterns were prepared by controlling the polymerisation temperature. In addition, since light can leak out through the microcracks, the tetraphenylethylene-doped patterns can be applied in anti-counterfeiting.

Colourful patterns prepared using a cholesteric liquid crystal mixture with both thermochromic and photochromic properties

ABSTRACT Colourful polymer-stabilised cholesteric liquid crystal (PSCLC) film attracted much attention for their applications in decoration and anti-counterfeiting. Herein, a chiral dopant was synthesised using trans-p-hexoxycinnamic acid. With the addition of this chiral dopant in a reactive LC mixture, both thermochromic and photochromic phenomena were found. The Bragg reflection band of the CLC mixture shifted to long wavelength with extending 365-nm UV light irradiation time or increasing temperature. The structural colour can cover the visible range. The photochromic property of the CLC mixture was driven by the photoisomerisation of the chiral dopant. Colourful PSCLC patterns were prepared based on the thermochromic and/or photochromic properties and using masks. The results shown here not only give us a better understanding the competition between photopolymerisation and photoisomerisation, but also lay a foundation for the preparation of the decorations.

Preparation of Cholesteric Polymer Networks with Narrow-Bandwidth Reflection and Memory Effect

A polymer network with a memory effect based on a polymer-stabilized narrow-bandwidth cholesteric liquid crystal (CLC) was prepared using the washing-out/refill method. The effects of different polymerization conditions on the reflection properties of CLC films were investigated. Meanwhile, the selective reflection property and narrow-bandwidth reflection memory effect of the polymer network were proved, and the response mechanism was provided. Furthermore, different materials from liquid crystals, with an anisotropic refractive index, to toluene, with an isotropic refractive index, were refilled to polymer scaffolds with helical structures, which originated from the periodic arrangement of CLCs. It was confirmed that the reflection bandwidth of these films can be dramatically narrowed by the reduced birefringence (Δn) of the refilled materials. The narrowest bandwidth of 22.5 nm refilling toluene with an isotropic refractive index (Δn = 0) was obtained. These results may provide a novel idea for flexible reflective displays, color filters, printing, and colored cladding of a variety of objects.

Investigating the Electro-Optic Response of Steroid Doped Liquid Crystal Devices

Nature is highly efficient at producing chiral compounds that are enantiomerically pure. The inherent chirality of naturally occurring biomolecules means that many have the potential to be used as chiral dopants for cholesteric liquid crystal (CLC) systems. Though many biomolecules have been identified as chiral dopants, many remain yet to be probed for their ability to function as chiral dopants. Here, 10 naturally occurring biomolecules comprised of steroids and bile acids were tested as chiral dopants for CLCs. Progesterone was identified as having high miscibility with nematic liquid crystals and was used in responsive liquid crystal devices. Progesterone-doped CLC devices were fabricated to exhibit either normal mode or reverse mode switchable behavior. Polymer stabilized CLCs (PSCLC) devices exhibiting dynamic electro-optic red- and blue-tuning behaviors were also fabricated. Furthermore, immiscible lithocholic acid was synthetically modified to afford two derivatives that were miscible at 10 wt. % in nematic liquid crystals. The two lithocholic acid derivatives were used as chiral dopants and incorporated into polymer stabilized CLCs which exhibited blue tuning behavior.

Fluorescent Dye-Doped Brightening Polymer-Stabilized Bistable Cholesteric Liquid Crystal Films.

Brightening polymer-stabilized bistable cholesteric liquid crystal (PSBCLC) films with doped fluorescent dyes were prepared using the polymerization-induced phase separation (PIPS) method. The transmittance performance behavior of these films in both states (focal conic and planar) and absorbance change in multiple dye concentrations were studied using a UV/VIS/NIR spectrophotometer. The change occurring in dye dispersion morphology with different concentrations was obtained by means of the polarizing optical microscope. The maximum fluorescence intensity of different dye-doped PSBCLC films was measured using a fluorescence spectrophotometer. Moreover, the contrast ratios and driving voltages of these films were calculated and recorded to demonstrate film performance. Finally, the optimal concentration of dye-doped PSBCLC films with a high contrast ratio and a relatively low drive voltage was found. This is expected to have great potential applications in cholesteric liquid crystal reflective displays.