Cholesteric Liquid Crystal Films Research Articles

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.

Mechanochromic Displays Based on Photoswitchable Cholesteric Liquid Crystal Elastomers

Stimuli responsive optical materials are attractive for many areas, from healthcare to art design. However, creating intricate color‐changing patterns for visual information is still a challenge. This work describes the preparation of mechanochromic structural colored intricate pictures imprinted in cholesteric liquid crystal elastomers by using a chiral isosorbide molecular photo‐switch. The photo‐switch contains a photoisomerizable cinnamate moiety and was incorporated in a main chain liquid crystal oligomer with photopolymerizable acrylate end groups. After coating, the structural colored film was irradiated with ultraviolet (UV) light in air causing E/Z isomerization of the cinnamate units leading to a redshift of the structural color of the film. A grayscale photomask was used to spatially control the photoisomerization reaction and imprint colorful pictures such as portraits and landscapes, in the cholesteric liquid crystal films with high resolution. Photopolymerization in a nitrogen atmosphere led to a mechanochromic cholesteric liquid crystal elastomer with striking structural colors that blueshift upon strain independently. The sharp details of the patterns were preserved even under deformation and the system returned to the initial state upon strain removal. Our work offers a simple photo‐switch approach to prepare stimuli responsive optical polymers imprinted with color‐changing pictures of unprecedented complexity.

Mechanochromic Displays Based on Photoswitchable Cholesteric Liquid Crystal Elastomers.

Stimuli responsive optical materials are attractive for many areas, from healthcare to art design. However, creating intricate color-changing patterns for visual information is still a challenge. This work describes the preparation of mechanochromic structural colored intricate pictures imprinted in cholesteric liquid crystal elastomers by using a chiral isosorbide molecular photo-switch. The photo-switch contains a photoisomerizable cinnamate moiety and was incorporated in a main chain liquid crystal oligomer with photopolymerizable acrylate end groups. After coating, the structural colored film was irradiated with ultraviolet (UV) light in air causing E/Z isomerization of the cinnamate units leading to a redshift of the structural color of the film. A grayscale photomask was used to spatially control the photoisomerization reaction and imprint colorful pictures such as portraits and landscapes, in the cholesteric liquid crystal films with high resolution. Photopolymerization in a nitrogen atmosphere led to a mechanochromic cholesteric liquid crystal elastomer with striking structural colors that blueshift upon strain independently. The sharp details of the patterns were preserved even under deformation and the system returned to the initial state upon strain removal. Our work offers a simple photo-switch approach to prepare stimuli responsive optical polymers imprinted with color-changing pictures of unprecedented complexity.

Narrow linewidth optical feedback vertical cavity surface emitting lasers based on cholesteric liquid crystal

Abstract In this paper, we present a narrow linewidth vertical cavity surface emitting laser (VCSEL) based on cholesteric liquid crystal (CLC). We investigated the impact of incorporating optical feedback into the external cavity on the linewidth of VCSELs, and successfully utilized CLC film as the reflector for the external cavity. This enabled us to achieve mode control and linewidth compression in VCSELs through optical feedback. The linewidth of the optically feedbacked VCSEL was measured at different driving currents and oxidation apertures. Under the same oxidation aperture, when the driving current is 1.2 mA, the linewidth is compressed from 120.9 MHz to 55.97 MHz; when the driving current is 0.8 mA, the linewidth is compressed from 22.87 MHz to 12.68 MHz. When the driving current remains constant, the linewidth is compressed from 169.31 MHz to 72 MHz when the oxidation aperture is 7 μm; and from 39.13 MHz to 10 MHz, when the oxidation aperture is 3 μm. By validating the feasibility of employing CLC film as an external cavity reflector mirror, our findings offer new insights towards achieving narrow-linewidth external cavity systems, thereby laying an experimental foundation for subsequent high-quality and highly integrated LC optical feedback VCSELs.

Ultrastrong Circularly Polarized Luminescence Triggered by the Synergistic Effect of Chiral Coassembly and Selective Reflective Cholesteric Liquid Crystal Film

Ultrastrong Circularly Polarized Luminescence Triggered by the Synergistic Effect of Chiral Coassembly and Selective Reflective Cholesteric Liquid Crystal Film

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.