Cholesteric Liquid Crystal Films 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.

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.

Near-Infrared Reflective Polymer Films Based on UV-327-Doped Zinc Oxide Nanoparticles.

We prepared cholesteric liquid crystal (CLC) films with broadband reflective properties by admixing organic dye UV-327 into inorganic zinc oxide nanoparticles (ZnO NPs), utilizing the principle of pitch distribution from a large to a small gradient along the film thickness direction, leading to broadband reflection. ZnO NPs are poorly dispersed and easy to gather, but they do not decompose easily. The addition of UV-327 makes up for the above shortcomings. UV-327 is an organic compound with good compatibility and dispersion with liquid crystal systems. Therefore, we used the method of mixing two UV-absorbing dyes (UV-327 and ZnO NPs) to obtain CLC films. UV-absorbing dyes (UV-327 and ZnO NPs) made the liquid crystal films form a UV intensity gradient in the direction of thickness, prompting the polymerizable monomers to polymerize faster on the stronger side of the light, leading to the relative diffusion of chiral molecules and polymerizable monomers, forming the concentration gradient of chiral molecules in the direction of thickness. The pitch has a gradient distribution as the chiral concentration varies. Then, anchored by the polymer network, the pitch gradient distribution no longer changes. Broadened reflective bandwidth can reach up to 881 nm. Furthermore, the film covers the near-infrared wavelength band well, which can be applied to future smart windows or laser shielding for medical and military applications. It is also believed that this achievement will optimize the preparation technology of broadband reflective CLC films in the future.

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.

Extending the capabilities of vectorial ptychography to circular-polarizing materials such as cholesteric liquid crystals.

The problem of imaging materials with circular-polarization properties is discussed within the framework of vectorial ptychography. We demonstrate, both theoretically and numerically, that using linear polarizations to investigate such materials compromises the unicity of the solution provided by this computational method. To overcome this limitation, an improved measurement approach is proposed, which involves specific combinations of elliptical polarizations. The effectiveness of this strategy is demonstrated by numerical simulations and experimental measurements on cholesteric liquid crystal films, which possess unique polarization properties. With the help of Pauli matrices algebra, our results highlight the technique's ability to discern between the different types of circular polarizers, uniform vs. non-uniform, and determine their handedness.

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.

Preparation of polymer‐stabilized cholesteric liquid crystal films capable of broadband reflection using a three‐layer system of self‐diffusion

AbstractThrough a simple and effective method for preparing broadband reflective liquid crystal films, the advantages of the three‐layer system are expanded based on the two‐layer system. The cholesteric liquid crystals form a prepolymerized layer after semi‐curing. In the design of the top prepolymerization layer, there is less content of the chiral compound R5011, which results in the formation of cholesteric liquid crystal molecules with a larger pitch. Mainly by controlling the content of polymerizable monomers and UV‐absorbing dyes in cholesteric liquid crystals (CLCs), a polymer network with density gradient distribution suitable for substance diffusion is formed, and the oleophobic substrate on one side of the low‐density polymer network is more likely to be peeled off. In the design of the lowest prepolymerization layer, the content of R5011 is higher, which results in the formation of cholesteric liquid crystals with a larger pitch. One side substrate of two cholesteric liquid crystal cassettes is peeled off and merged into a new liquid crystal cassette whose left and right sides are stacked with septum pads (20 μm), and the nematic liquid crystals are poured into the middle layer to form a three‐layer integrated film. The polymer network of the polymer‐stabilized cholesteric liquid crystals (PSCLCs) itself in the bilayer was used as a diffusion channel. Chiral compounds and UV‐absorbing dyes were diffused from the cholesteric liquid crystal layer to the intermediate nematic liquid crystal layer by diffusion gradient method, resulting in liquid crystal films with arbitrary wavelength bands and ultrawide broadband reflection.

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 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.

64‐2: Distinguished Paper: Novel Cholesteric Liquid Crystal Films Creating Angle Insensitive Reflective Colors

We have developed novel cholesteric liquid crystal (Ch‐LC) films creating angle insensitive reflective colors. The wavy alignment formed by rapid change of the helical twisting power (HTP) enables the suppression of color shift with different incident angles. With this approach various decorative films for innovative applications can be created.

Tuning the Reflection Bandwidth of Polarization Volume Gratings by Guest Material Penetration

Polarization volume gratings (PVGs) based on polymerized cholesteric liquid crystal (CLC) films can potentially be applied as optical combiners in augmented reality (AR) smart glasses. The reflection bandwidth of PVGs is determined by the helical pitch and birefringence of the CLC, which must be tuned to match the desired color for reflection, but tuning the material properties is challenging as the materials must also be polymerizable. Here, we report a postprocessing method based on guest material penetration for the tuning of the reflection bandwidth of CLC polymer films. Solution coating of a high-birefringence guest liquid crystal on a CLC film modifies the birefringence as well as elongates the helical pitch. Penetration of a high-birefringence 5CT liquid crystal was found to cause a birefringence increase of the CLC film by approximately 18%.

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.

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.

Novel cholesteric liquid crystal films creating angle insensitive reflective colors

AbstractWe have developed novel cholesteric liquid crystal (Ch‐LC) films creating angle insensitive reflective colors. The wavy alignment formed by rapid change of the helical twisting power (HTP) enables the suppression of color shift with different incident angles of light. These films open the door to create various decorative films for innovative applications.

Enhanced reflection band broadening in PSCLC with CeO2 NPs for building energy-saving windows

The polymer-stabilized cholesteric liquid crystal (PSCLC) films that exhibited broadband reflective in the near-infrared region were prepared by spin-coating CeO2 dispersion. And then, the influence of dispersion concentration, polymerizable monomer concentration, light intensity, and polymerization temperature on the reflection wavelength of PSCLC films were studied. The results showed that reflection bandwidth of PSCLC film was broadened from 237 nm to 426 nm. The experimental data confirmed that the diffusion of NP forms a light intensity gradient in the system, which was beneficial to forming the non-uniform pitch distribution. Base on Based on these properties of PSCLC film, the film was used in the small building energy-saving experiment mentioned. It was found that the sample containing the PSCLC film can lower the temperature of the temperature sensor by about 8 °C than the sample without the PSCLC film. Therefore, the prepared PSCLC film is expected to be used in building energy-saving and other aspects.

Colorful patterned polyacrylate films prepared using cholesteric liquid-crystalline mixtures with a smectic order

ABSTRACT Polymer-stabilised cholesteric liquid crystal (PSCLC) films with selective circularly polarised light reflection have attracted much attention for their applications as polarisers, energy-saving windows and for displays. Herein, CLC mixtures were prepared using a nematic LC LC242 and a chiral compound S-6 with enantiotropic SmA and SmC* phases, which exhibited a cholesteric phase with a smectic order. After photopolymerisation, the PSCLC films with fingerprint structure at the surfaces and supramolecular helical structure inside were obtained. Due to the existence of short-range smectic order in the cholesteric structure, the Bragg reflection bands were broadened. For a CLC mixture, with increasing temperature, the short-range smectic order was suppressed, and the selective Bragg reflection band shifted to the short wavelength. Based on this thermochromic behaviour, colourful PSCLC patterns and gratings were prepared, which were able to be applied in decoration and anti-counterfeiting. Since the PSCLC films can be obtained within several seconds under air, large-area films can be prepared on coating lines at low-cost.

Hybrid Encoding Strategy of Nanostructures for High‐Performance Security Information Encryption

AbstractPrograming nanostructure of soft photonic crystal materials to try to diversify has great potential for application in the field of information security, lasing, and display. However, complex programming nanostructures for regulating and controlling their structural color and stimuli‐response behavior remain a technical challenge. Here, a strategy is demonstrated for hybrid encoding nanostructures of soft photonic crystals using cholesteric liquid crystal films (CLCFs). The public and secure information can be easily edited through the independent tuning of Bragg center‐wavelength and thermal sensitivity of the CLCF through the alternating current electric field and Michael addition reaction. A photomask is used when ultraviolet irradiates to record the edited information on one CLCF. The encrypted information can be protected by a triple mechanism of spectral analysis, algorithm encryption, and temperature response encryption, which all can be defined as needed. High encrypted composite coding security and visual patterns are fabricated using this coordination strategy. This hybrid encoding provides a platform for a complex program of nanostructure of the photonic crystals and shows great potential in secure information writing and encryption enhancement.

Research of nanofibres loaded with ultraviolet absorber to increase the wavewidth of cholesteric liquid crystals

In this study, polymer-stabilized cholesteric liquid crystal (PSCLC) films with high broad-wave reflection were prepared by using nanofibre films loaded with UV absorbers as carriers. The nanofibre films were prepared by electrostatic spinning technique and the uniform distribution of the UV absorber on the nanofibre films was ensured by adjusting the spinning parameters. In order to obtain the maximum pitch inhomogeneous distribution, a series of studies were carried out on the thickness of the nanofibre films, the content of the loaded UV absorber in the nanofibre films and the UV light intensity. By screening different experimental conditions, the optimum conditions for broadening the reflection band were obtained, widening the reflection bandwidth from 236 nm to 700 nm and performing well in the application experiments. The prepared cholesteric liquid crystal films with broadband reflection properties have potential applications in many fields such as light-enhancing films, smart windows, infrared shielding devices and laser protective coatings.

Molecular Engineering for Chiroptical Smart Paints: Photo‐Tunable and Polymerizable Helical Nanostructures by Cyanostilbene‐Based Reactive Mesogens

AbstractMany studies on the basis of cholesteric liquid crystal (CLC) have focused on dynamic optical changes utilizing photochemical reactions raising helical pitch variations. However, the conventional dyes doped in CLC system often show reversible isomerization such as trans‐cis, E‐Z, and open‐close forms that hampers the fixation of desired color cues. Here, a cyanostilbene‐based reactive mesogen (CSRM) for phototunable and subsequently polymerizable smart CLC paints is newly designed and synthesized. The structural color of the CLC paint can be precisely manipulated in remote controllable manner by inducing a photochemical reaction of CSRM. Due to the irreversible isomerization behaviors of CSRM, the constructed periodic structures with different helical pitches are intactly fixed via in situ photopolymerization under 460 nm light. In addition, the combination of helical nanostructures and cyanostilbene luminophore produces a circularly polarized light emission. The dissymmetry factor of circularly polarized luminescence is dependent on the photoregulated helical nanostructures in CLC films. The decoupled system of triple photofunctionality in a single component of chirophotonic crystalline material is applied to develop advanced optical information storages.