Polymer Dispersed Cholesteric Liquid Crystals Research Articles

Mechanosensitive polymer-dispersed cholesteric liquid crystal composites based on various polymer matrices

• Elastic PDLCs based on cross-linked PU, PDMS and plasticized PVA were prepared. • Their stretching causes the changes in color and intensity of selective light reflection. • These effects is explained by a compression of cholesteric droplets during stretching. • Planar orientation of LC molecules is preferable for mechano-optical response. • PU composites are the most promising for the applications.

Mechano-Optical Response Behavior of Polymer-Dispersed Cholesteric Liquid Crystals for Reversible and Highly Sensitive Force Recorders

Force recording (mode, intensity, and orientation) is of great importance in medical rehabilitation, military reconnaissance, space exploration, etc. However, sensors with both reversibility and memorability are still challenging. Here, a reversible sensor based on polymer-dispersed cholesteric liquid crystals (CLC) is developed as a force recorder. Based on the microarea mechano-optical response and finite element analysis, it is confirmed that the mechanochromic response is mediated by the shear deformation of the polymer network and neighboring CLC. There is an obvious quantitative relationship between force intensity, mode, orientation, and the microarea optical response. Moreover, the sensing layer with a lower modulus or thickness is advantageous for a more sensitive device with lower starting pressure. Additionally, the excellent sensitivity and accuracy also highlight the potential applications in force analysis, path tracking, or pattern detection.

Tuning of Thermal and Electrical Properties of Polymer Dispersed Cholesteric Liquid Crystal

When a pure liquid crystal is dispersed into a suitable polymer to form micron-sized droplet, then it is called Polymer-dispersed liquid crystal (PDLC). In the present study, PDLC of different concentrations were prepared by dispersing a conducting polymer poly (3, 4–ethylenedioxy thiophene): poly (styrene sulfonate) into a cholestryl palmitate. The differential scanning calorimetry and fabry perot scattering studies were employed to study thermal and optical properties. It was found that the phase transition for PDLC occurs at a temperature different than those exhibited by pure liquid crystal. The behaviour of PDLC for parallel and perpendicular electric field has been investigated and the dielectric constant is determined. The value of dielectric constant and conductivity were found to increase with increasing concentration of polymer. The bistability and reflective properties of pure cholesteric liquid crystal can be minimized by dispersing polymer which makes material suitable for high contrast at large viewing angles.

Polymer Dispersed Cholesteric Liquid Crystal Mixtures for Optical Time–Temperature Integrators

AbstractLow molecular weight cholesteric liquid crystals (CLCs) can be trapped as droplets or particles inside a polymer binder, forming polymer dispersed cholesteric liquid crystal (PDCLC) systems, which are typically used for smart windows, displays, and optical sensors. While dispersing a single CLC mixture via emulsification, rendering reflective PDCLC films upon drying with a stable optical response, is well studied, the incorporation of distinct microdroplets inside a PDCLC coating is barely explored. Here, structural colored PDCLC coatings are prepared by mixing distinctive thermosensitive photonic microdroplets, featuring an optical time–temperature response that is visualized as a reflective color loss or shift dependent on the composition of the CLC mixtures. The presented optical time–temperature response is induced by diffusion of CLCs between adjacent liquid crystal domains via interconnecting passageways, causing an irreversible optical response until homogeneous mixing is attained. Hence, blending distinct CLC microdroplets in a polymer matrix provides a facile and scalable method for fabricating optical time–temperature integrators; moreover, alternative optical time–temperature responses can be realized when adjusting the composition of the photonic mixtures, demonstrating the potential of this method to acquire novel photonic applications based on low molecular weight CLCs.

Polymer-Dispersed Cholesteric Liquid Crystal under Homeotropic Anchoring: Electrically Induced Structures with λ1/2-Disclination.

Orientational structures of polymer-dispersed cholesteric liquid crystal under homeotropic anchoring and their transformations under the action of an electric field are studied. The switching of cholesteric droplets between different topological states are experimentally and theoretically demonstrated. Structures with -disclination are found and considered. These structures are formed during the transformation of a twisted toroidal configuration induced by a decrease in the electric field when a relative chiral parameter . The transformation of the initial structure with a bipolar distribution of the helix axis into a twisted toroidal configuration and then into a structure with -disclination is investigated in detail. The behavior of these structures under the influence of an external electric field, as well as the appearance of structures with -disclination, are studied. Obtained results are promising for the development of optical materials with programmable properties.

Chiral, Thermally Irreversible and Quasi-Stealth Photochromic Dopant to Control Selective Reflection Wavelength of Cholesteric Liquid Crystal.

A chiral and thermally irreversible photochromic fulgide derivative incorporating an (R)-binaphthol unit in its acid anhydride moiety was used for the photoswitching of the pitch length of cholesteric liquid crystals. Since the absorption maximum wavelengths of both thermally stable photoisomers are nearly in the UV region (quasi-stealth photochromism), it can be exposed to visible light without inducing photochromic reactions. Therefore, when the photoswitching molecule is added to a permanent cholesteric liquid crystal whose reflection light wavelength is in the visible region, the UV light-induced photochromic reaction of the photoswitching molecule changes the wavelength of the reflection light in the visible light region. We have succeeded in regulating the color of cholesteric liquid crystalline cells between red and blue upon UV light irradiation. Attempts to introduce this system in polymer dispersed cholesteric liquid crystals are also described.

Strong CPL of achiral liquid crystal fluorescent polymer via the regulation of AIE-active chiral dopant.

Ternary polymer dispersed cholesteric liquid crystals (PD-CLCs) prepared by doping AIE-active binaphthyl-based molecules (R/S-AD, chiral dopant) and the achiral liquid crystal fluorescent polymer, poly(p-phenylene ethynylene) (LC-PPE, CPL emitter) into 5CB, can emit strong circularly polarized luminescence (CPL) with gem up to 0.97. This work develops a new strategy for designing strong CPL materials.

Spiropyran doped rewritable cholesteric liquid crystal polymer film for the generation of quick response codes

With the rapid development of modern information technology, massive amounts of data are stored that require an information security. Here, we introduce a spiropyran doped polymer dispersed cholesteric liquid crystal film based rewritable medium that can produce high quality visual two-dimensional barcodes for anti-counterfeiting, authentication and security purposes. The prepared film exhibits excellent photochromic properties, improved photosensitivity, high spatial resolution, and numerous rapid write/erase capabilities. Such rewritable material can be efficiently printed over 70 cycles without compromising its readability. Furthermore, the fabricated film is mechanically resistant to small stresses and, hence, it can be directly handled.

Liquid-crystalline composites for point temperature measurements

The application of cholesteric liquid crystals and composites containing them for visualization of thermal field and temperature measurement is very well known. Despite all disadvantages as limited precision and adhesion to the studied surface, this method is still of interest. In this work, we present the results of preliminary studies on polymer-dispersed cholesteric liquid crystal (PDCLC) composites designed for visualization and measurement of point temperature changes which can be used in medical diagnostics. Basing on the results of former studies, the perspective PDCLC preparation method as well as high-performance liquid-crystalline chiral nematics were obtained and applied. The microencapsulation in poly(vinyl, alcohol) has been chosen as the method of PDCLC preparation while as liquid-crystalline materials the chiral nematics were adopted. Keywords: material science, liquid-crystalline composites, medical thermography

A Study on the Electro-Optical Properties of Thiol-Ene Polymer Dispersed Cholesteric Liquid Crystal (PDChLC) Films.

In this study, a polymer dispersed cholesteric liquid crystal (PDChLC) film obtained via a one-step fabrication technique based on photopolymerization of a thiol-acrylate reaction system was prepared and characterized for the first time. The effects of the chiral dopant, the influence of thiol monomer functionality and content on the morphology and subsequent performance of the PDChLC films were systematically investigated. It was demonstrated that the addition of a small amount of chiral dopant slightly increased the driving voltage, but decreased the off-state transmittance significantly. Furthermore, scanning electron micrographs (SEM) shown that the liquid crystal (LC) droplet size decreased at first and then increased with the increasing amount of thiol monomer functionality, while increasing the thiol content increased the LC droplet size. Correspondingly, the electro-optical switching behavior was directly dependent on LC droplet size. By tuning the raw material composition, PDChLC film with optimized electro-optical performance was prepared.

Primary converters for optical sensors of physical values based on polymer dispersed cholesteric liquid crystal

ABSTRACTThe investigations of system based on cholesteric liquid crystal (CLC) sensors of physical quantities with an information transfer optical channel are carried out in the paper. The results of experimental investigations of spectral characteristics of polymer dispersed cholesteric liquid crystals (PDCLC) are represented. The application such material as primary transducer of a temperature sensor is proposed. The structure of an optical fiber temperature sensor with successive allocations of primary transducers and shifted areas of selective reflection band has been designed.

Study of laser action in dye-doped polymer dispersed cholesteric liquid crystal film

In this work, laser dye DCM, nematic liquid crystal TEB30A, chiral dopant S-811 and PVA are used to prepare polymer dispersed cholesteric liquid crystal (LC) films by the method of microcapsule. Lasing mechanism and temperature-tunable characteristics of the films are investigated. The diameters of large droplets in the films are about 80 μm and LC molecules are planar-textured in the droplets when viewed between crossed polarisers through a polarization microscope. Pumped by the 532 nm Nd:YAG laser, narrow linewidths in laser emissions are obtained at 634.5 nm and 680.2 nm, and the linewidths are of 0.25 nm and 0.29 nm, respectively. The device is then compared with a dye-doped cholesteric liquid crystal laser. The wavelength of output laser will blueshift when the temperature of the device is increased, and a 22.8 nm tunable lasing, from 666.7 nm to 643.9 nm, is obtained. Analysis concludes that the lasing mechanism of the film is band-edge lasing, and the emission wavelengths are corresponding to the two edges of the photonic band gap.

Bistable polymer-dispersed cholesteric liquid crystal thin film enabled by a stepwise polymerization

By using a proper polymer network and an ion-doped cholesteric liquid crystal with negative dielectric anisotropy, bistable polymer-dispersed liquid crystal thin films were prepared by first photo-curing, and then thermally curing.

用偏光显微术研究聚合物分散胆甾相液晶微滴形貌

Polymer dispersed cholesteric liquid crystal samples with large droplets were made,and the droplets had strange"gem"texture by polarizing microscope.The new vertical helix and parallel helix configuration in the polymer disperse cholesteric liquid crystal droplets were proposed.Considering the bipolar and radiation configuration of polymer disperse nematic liquid crystal,the"gem"texture may correspond with degeneracy coexistence of multi domain texture of vertical helix and parallel helix configuration.This experiment had basic research significance for liquid crystal optics and reference value to design the new liquid crystal device.

Electrically Induced Multicolored Hyper-Reflection and Bistable Switching from a Polymer-Dispersed Cholesteric Liquid Crystal and a Templated Helical Polymer

In this study, a special bistable polymer-dispersed cholesteric liquid crystal (PDCLC) with two different reflection modes was fabricated by refilling a PDCLCs composites into a template helical polymer with the same or opposite helical structure, in which the templated helical polymer with a half of the cell thickness was fabricated by nonuniform photopolymerization of monomers induced by ultraviolet light absorbing dye in the cholesteric liquid crystal (Ch-LCs) mixture. Modulation of a hyper-reflective state (>50% reflection) and normal-reflective state (≤50% reflection) at a fixed wavelength, even switching between bicolor reflection with different handedness can be achieved. Additionally, a patterned multicolored switchable Ch-LCs film can be achieved by infiltrating the above PDCLCs into a patterned templated helical polymer, and the switching behavior of the patterned film was exhibited. These special optical properties make the novel Ch-LCs composites an interest for great potential application in many fields such as flexible display, optical components, LCs lasing, etc.

P.94: A High Sensitivity PDCLC‐based Electro‐optic Modulator for TFT Array Inspector

AbstractWe proposed a high sensitive electro‐optic modulator based on a polymer‐dispersed cholesteric liquid crystal (PDCLC) for the TFT array inspector. Since the PDCLC is operated in a reflective mode, the PDCLC‐based inspection system is very simple since a dielectric mirror, which is generally used in a conventional TFT array inspector, is not required.

Polymer network microstructures and electro-optical properties of a pressure-sensitive cholesteric liquid crystal device

We present a pressure-sensitive polymer dispersed cholesteric liquid crystal (PDCLC) device, in which a cholesteric liquid crystal (ChLC) is encapsulated by a polymer network via an ultraviolet photopolymerization induced phase separation method. The effects of the acrylate monomer concentration and functionality on the network morphology, and the electro-optic behaviour of the PDCLC device were studied. The results demonstrated that the electro-optic properties, including reflectivity and the contrast ratio of the pressure-sensitive device, were improved as the ChLC droplet size increased. An appropriate network density in the system limited the flow of the ChLC and allowed high-resolution lines to be drawn on the device. Additionally, the critical pressure switching of the device from a focal conic state to a planar state was investigated. The device developed in this study has promising applications in many fields such as flexible display devices and optical sensors.

Multistability in polymer-dispersed cholesteric liquid crystal film doped with ionic surfactant

A memory effect has been discovered in composite films based on a polymer containing dispersed cholesteric liquid crystal, which is related to the modification of the surface anchoring by an ionic surfactant under the action of an applied electric field. Parameters of electric signals are selected that ensure the switching of the composite film between some stable states with different levels of optical transmittance.

聚合物分散胆甾相液晶相形态调控与光电性能

Polymer dispersed cholesteric liquid crystal(PDCLC) films are prepared by means of ultraviolet(UV) induced phase separation.The effects of UV cure power and UV cure temperature on morphology of PDCLC have been studied in detail,the relationship between morphology of PDCLC and reflectivity has also been studied.The results show that low UV cure power allows complete phase separation,large LC domains size and smooth interface between polymer and LC.Strong UV cure power results in a rough interface which also decreases the size of LC domains and reflectivity of PDCLC films.But the relationship of PDCLC morphology and UV cure temperature is complex.At low UV cure temperature,LC dispersed as circular droplets in polymer matrix.The size of LC domains become bigger which yield adjacent and uniform layers of LC and polymer when the UV cure temperature is up to 40 ℃.Higher UV cure temperature causes a type of three-dimensional polymer network structure in which LC is dispersed.Reflectivity of PDCLC films is up first and down later with UV cure temperature increasing.In this paper,the PDCLC films prepared with UV cure power 0.01 mW/cm2 and UV cure temperature 40 ℃ exhibit max reflectivity of 32.2%.

Monitoring spatial distribution of ethanol in microfluidic channels by using a thin layer of cholesteric liquid crystal

Monitoring spatial distribution of chemicals in microfluidic devices by using traditional sensors is a challenging task. In this paper, we report utilization of a thin layer of cholesteric liquid crystal for monitoring ethanol inside microfluidic channels. This thin layer can be either a polymer dispersed cholesteric liquid crystal (PDCLC) layer or a free cholesteric liquid crystal (CLC) layer separated from the microfluidic device by using a thin film of PDMS. They both show visible colorimetric responses to 4% of ethanol solution inside the microfluidic channels. Moreover, the spatial distribution of ethanol inside the microfluidic channel can be reflected as a color map on the CLC sensing layers. By using this device, we successfully detected ethanol produced from fermentation taking place inside the microfluidic channel. These microfluidic channels with embedded PDCLC or embedded CLC offer a new sensing solution for monitoring volatile organic compounds in microfluidic devices.