Cholesteric Liquid Crystal Polymer Research Articles

The role of conductive dopants in polymer cholesteric liquid crystals

<p align="center"> </p><p><strong>Abstract</strong></p><p>Variety of conductive dopants were used to modify the electric properties of polymer cholesteric liquid crystals. Different types of carbon blacks, carbon nanotubes, as well as metallic particles were used as conductive dopants in this work. All of them affected the conductivity of the polymer matrix, but at different dopant concentrations. The percolation threshold behaviour of used dopants in the polymer host showed pronounced dependence on the dopants’ particle size and particle shape anisotropy. For instance, the higher the aspect ratio of carbon-based dopant particles (aspect ratio = length : diametar), the lower the percolation threshold; while increase in metallic dopants' diameter yielded higher percolation threshold of polymer composition.</p><p align="center"><strong><br /></strong></p>

Infrared liquid crystal polymer micropolarizer.

The ability to create arbitrary patterned linear and circular infrared (IR) liquid crystal polymer (LCP) polarizers is demonstrated. The operating wavelength of the thin-film polarizer ranges from 700 to 4200nm. The linear micropolarizer is fabricated using IR dichroic dye as a guest in LCP host with feature size as small as 4μm. The circular micropolarizer is fabricated using cholesteric LCPs with feature size as small as 6.2μm.

Application of broadband infrared reflector based on cholesteric liquid crystal polymer bilayer film to windows and its impact on reducing the energy consumption in buildings

An infrared (IR) polymer reflector based on chiral nematic (cholesteric) liquid crystals has been fabricated which can reflect more than 60% of solar IR energy without interfering with the visible solar radiation. Simulations show that the polymer bilayer film applied to a window of a typical building can have a significant impact on the interior temperature in living and working spaces.

An Optical Sensor for Volatile Amines Based on an Inkjet‐Printed, Hydrogen‐Bonded, Cholesteric Liquid Crystalline Film

A printable hydrogen‐bonded cholesteric liquid crystal (CLC) polymer film is described, which can be used as a sensor for detection of gaseous trimethylamine (TMA). In this optical sensor the virgin CLC polymer network reflects green light. When anhydrous TMA gas penetrates the film, disruption of the hydrogen bonds occurs, with the simultaneous formation of carboxylate salts. The consequent reduction of the molecular order causes the green reflecting CLC film to become colorless. However, exposure to TMA in water‐saturated nitrogen gas results in a red reflecting film. Due to the hygroscopic nature of the polymer salt that is formed by TMA, water vapor which is present in the environment is absorbed by the films. This leads to swelling of the film, resulting in an increase in pitch size and therefore a red shift of the reflection band. Interestingly, after exposure to ambient conditions, restoration of the green reflecting film takes place, showing that the sensor can be used multiple times. In a proof of principle experiment, it was shown that these CLC films can be used as optical sensors to detect volatile amines, that are produced by decaying fish.

Patterned cholesteric liquid crystal polymer film

Herein, the ability to create arbitrarily patterned circular polarized optical devices is demonstrated by using cholesteric liquid crystal polymer. Photoalignment with polarized ultraviolet light is utilized to create aligned cholesteric liquid crystal films. Two different methods, thermal annealing and solvent rinse, are utilized for patterning cholesteric liquid crystal films over large areas. The patterned cholesteric liquid crystal films are measured using a Mueller matrix imaging polarimeter, and the polarization properties, including depolarization index, circular diattenuation (CD), and circular retardance are derived. Patterned nonlinearly polarized optical devices can be fabricated with feature sizes as small as 20μm with a CD of 0.812±0.015. Circular polarizing filters based on polymer cholesteric liquid crystal films have applications in three-dimensional displays, medical imaging, polarimetry, and interferometry.

Discrimination of Alcohol Molecules Using Hydrogen-Bridged Cholesteric Polymer Networks

This paper presents a novel hydrogen-bridged cholesteric liquid crystal (CLC) polymer networks with a porosity. This kind of material is adopted to distinguish methanol from ethanol. Because of the diversity of molecular affinities in ethanol and methanol with the hydrogen-bridged cholesteric polymer networks, the reflected colors of the cholesteric polymer networks are obviously different upon the uptake of the alcohol solutions with different ratios of methanol/ethanol. Furthermore, the composition of the monomers was altered to obtain the different sensitivities of the different alcohol concentrations and ratios of methanol/ethanol. The reduction of cross-link density and increasing amount of alkyloxybenzoic acid in polymer networks can enhance the distinguishing ability between methanol and ethanol. The function of change in transmission-valley wavelength of CLC versus the alcohol concentration and the ratio of methanol/ethanol was also obtained from the experimental results.

Printable Optical Sensors Based on H-Bonded Supramolecular Cholesteric Liquid Crystal Networks

A printable H-bonded cholesteric liquid crystal (CLC) polymer film has been fabricated that, after conversion to a hygroscopic polymer salt film, responds to temperature and humidity by changing its reflection color. Fast-responding humidity sensors have been made in which the reflection color changes between green and yellow depending on the relative humidity. The change in reflection band is a result of a change in helix pitch in the film due to absorption and desorption of water, resulting in swelling/deswelling of the film material. When the polymer salt was saturated with water, a red-reflecting film was obtained that can potentially act as a time/temperature integrator. Finally, the films were printed on a foil, showing the potential application of supramolecular CLC materials as low-cost, printable, battery-free optical sensors.

Alcohol‐Responsive, Hydrogen‐Bonded, Cholesteric Liquid‐Crystal Networks

AbstractHydrogen‐bridged, cholesteric liquid‐crystal (CLC) polymer networks are adopted as an optical sensor material to distinguish between ethanol and methanol. Fast uptake of the alcohols is facilitated by an incorporated porosity created by breaking the hydrogen bridges and by a previously removed non‐reactive liquid‐crystal agent. The discrimination between the alcohols is based on the diversity in molecular affinity of ethanol and methanol with the hydrogen‐bridged CLC polymer networks. The CLC networks are molecular‐helix‐based, one‐dimensional bandgap materials with a discrete reflection band in the visible part of the spectrum that depends on the pitch of the molecular helix. The changes in positions of the reflection bands of the CLC network accurately discriminate between the alcohol types and provide information on their ratio in case they are blended.

Analysis of Cavity‐Mode Lasing Characteristics from a Resonator with Broadband Cholesteric Liquid‐Crystal Bragg Reflectors

AbstractDetailed optical lasing characteristics in liquid crystal (LC) microlasers consisting of multiple polymer cholesteric LC (PCLC) layers are presented as broadband resonators sandwiching a layer of thick gain media, dye‐containing nematic LC (NLC) or isotropic liquid, in between. Multiple lasing emission peaks due to Fabry‐Perot cavity modes are observed for both gain media, and their polarization characteristics investigated. To analyze lasing characteristics, specified eigen modes are defined, the polarization states of which are maintained before and after passing through the broadband resonator, and obtained for the present full system by using the Berreman 4 × 4 matrix method. Using these specified eigen modes, the optical density for each mode is calculated and compared with the experimental results, and shows good agreement. Finally, lasing characteristics between the resonators with NLC and isotropic gain media are compared, and the advantages of adopting dye‐doped NLC gain medium are shown for tunable red, green, blue lasing in a microlaser system with a broadband resonator.

Search for Hermite-Gauss mode rotation in cholesteric liquid crystals

In theory, there are analogous transformations of light's spin and orbital angular momentum [Allen and Padgett, J. Mod. Opt. 54, 487 (2007)]; however, none have been observed experimentally yet. In particular, it is unknown if there exists for the orbital angular momentum of light an effect analogous to the spin angular momentum-based optical rotation; this would manifest itself as a rotation of the corresponding Hermite-Gauss mode. Here we report an experimental search for this effect in a cholesteric liquid crystal polymer, using strongly focussed, spin-orbit coupled light. We find that the relative phase velocities of the orbital modes constituting the Hermite-Gauss mode agree to within 10(-5).

Multi-wavelength laser from dye-doped cholesteric polymer films

A multi-wavelength laser is demonstrated using a dye-doped cholesteric polymer film whose reflection bandwidth is broadened with several oscillations. Due to the abrupt change of the density of state between oscillation peak and valley, each oscillation functions as a photonic band gap for generating a laser wavelength under the excitation of a pumping laser. As a result, a multiple wavelength laser is generated. Results indicate that the dye-doped cholesteric liquid crystal polymer film is a good candidate for fabricating broadband lasers such as white light lasers. Potential applications include experimental testing of laser materials, identification markers, information displays, and inertial confinement laser fusion.

Color-temperature tunable white reflector using bichiral liquid crystal films

We have demonstrated multicolored reflections showing various whitish colors from a bichiral liquid crystal (LC) film. The bichiral LC film was fabricated by using left-handed and right-handed polymeric cholesteric liquid crystal (CLC) films with two different helical pitches and an isotropic polymer film in between. Color temperatures of the multiple reflections are controlled from ~4000 K to ~10000 K by changing linear polarization directions of normally incident and reflected lights. This characteristic can extend practical applications of CLCs to illuminant devices.

Permeation flows in cholesteric liquid crystal polymers under oscillatory shear

We investigate the permeation flowof cholesteric liquid crystal polymers (CLCPs) subject to a small amplitude oscillatory shear using a tensor theory developed by the authors [8]. We model the material system by the Stokes hydrodynamic equations coupled with the orientational dynamics. At low frequencies, the steady permeationmodes are recovered and the director rotates in phase with theapplied shear. At high frequencies, the out of phase componentdominates the dynamics. The asymptotic formulas for the loss modulus($G''$) and storage modulus ($G^{'}$) are obtained at both low andhigh frequencies. In the low frequency limit, both the loss modulus and thestorage modulus are shown to exhibit a classical frequency $\omega$ dependence($G^{''} \propto \omega$, $G^{'} \propto \omega^2$ ) with the proportionality of order $O(Er)$ and $O(q)$, respectively, where$\frac{2\pi}{q}$ defines the pitch of the chiral liquid crystal and$Er$ is the Ericksen number of the liquid crystal polymer system. The magnitudes of dimensionless complexflow rate and complex viscosity are calculated. They are shown to have two Newtonian plateaus at low and high frequencieswhile a power-law response at intermediate frequencies.

Broadband Cavity‐Mode Lasing from Dye‐Doped Nematic Liquid Crystals Sandwiched by Broadband Cholesteric Liquid Crystal Bragg Reflectors

Multiple polymer cholesteric liquid crystal (PCLC) layers with different pitches and a sandwich structure of nematic liquid crystal by these PCLC films are presented. The former serves as broadband-distributed Bragg reflectors and the latter as multiple-band electrotunable filters. Using the latter dye-doped hybrid film red–green–blue simultaneous lasing due to Fabry–Perot cavity modes is achieved (see figure).

Polarization-tunable chiral nematic liquid crystal lasing

A chiral nematic liquid crystal laser with arbitrarily controllable polarization was demonstrated by combining polymeric cholesteric and nematic liquid crystal films. The lasing polarization states can be continuously varied by applying voltages from 1.25 to 2 V. Right-handed circular polarization is switched to left-handed circularly polarization through −45° linearly polarized states. The trajectories of the Stokes parameters on the Poincaré sphere are used to analyze the variation in lasing polarization. The change in polarization is simulated and the simulation agrees closely with the experimental data.

Low threshold lasing from dye-doped cholesteric liquid crystal multi-layered structures

We fabricated novel hybrid structures composed of a dye-doped low-molecular-weight cholesteric liquid crystal sandwiched by multi-layered polymer cholesteric liquid crystal films and evaluated their lasing characteristics. Lasing was observed with an extremely reduced threshold (12 nJ/pulse) by a factor of 10 compared with that in a simple dye-doped low-molecular-weight cholesteric liquid crystal cell. Lasing characteristics experimentally obtained were discussed by comparing them with the simulated photonic density of states spectra.

Polarization-independent multiple selective reflections from bichiral liquid crystal films

We have demonstrated polarization-independent multiple reflections covering overall visible region from a bichiral liquid crystal (LC) film with both chiral structures. The bichiral LC film was fabricated by an all-solution process using left-handed and right-handed polymeric cholesteric liquid crystal (CLC) films with a single pitch and an isotropic polymer film. The multiple reflections exceeding 50% reflectance were clearly observed at various polarizations of normally incident light. This characteristic can extend practical applications of CLCs to polarization-independent devices over broad-band regions.

Small amplitude oscillatory shear permeation flow of cholesteric liquid crystal polymers

We investigate the small amplitude oscillatory shear permeation flow (SAOSPF) of cholesteric liquid crystal polymers (CLCPs) using a mesoscopic model obtained from the Doi kinetic theory for flows of CLCPs. We model the system by Stokes hydrodynamic equations coupled with orientational dynamics and study the frequency-locked solutions of the system by employing a coarse- grained approach. The leading order solutions exhibit boundary layers of thickness of the order of the cholesteric pitch in low frequency regimes but in the order of 1 √ ! in high frequency regimes, where ! is the plate driven frequency. The response of CLCPs has been calculated. Viscoelastic behavior is only observed at intermediate frequencies and the material is essentially viscous at low and high frequency regimes. At both low and high frequencies, the real component of complex viscosity controls its magnitude which is of the order of the reciprocal of the cholesteric pitch at low frequencies while the order of 1 at high frequencies.

Modeling the effects of microencapsulation on the electro-optic behavior of polymer cholesteric liquid crystal flakes

A method for modeling the effect of microencapsulation on the electro-optical behavior of polymer cholesteric liquid crystal (PCLC) flakes suspended in a host fluid is introduced. Several microencapsulation configurations in an applied ac electric field are investigated using COMSOL MULTIPHYSICS software in combination with an analytical model. The field acting on the flakes is significantly altered as various encapsulant materials and boundary conditions are explored. The modeling predicts that test cells with multiple materials in the electric field path can have a wide range of electro-optic responses in ac electric fields. Both theoretical predictions and experimental evidence show that for PCLC flake reorientation to occur due to Maxwell–Wagner polarization, a reasonably strong electric field must be present along with at least moderately dissimilar PCLC flake and host fluid material dielectric constants and conductivities. For materials with low dielectric constants, electrophoretic behavior is observed under dc drive conditions at high field strengths for all evaluated microencapsulation configurations. This modeling method is shown to be a useful predictive tool for developing switchable particle devices that use microencapsulated dielectric particles in a host fluid medium.

Synthesis and characterization of polyurethane-based side-chain cholesteric liquid crystal polymers

Polyurethane-based side-chain cholesteric liquid crystalline polymers (ChLCPs) with variable clearing temperatures were synthesized in a two-step reaction. The chemical structures of ChLCPs were confirmed by FT-IR and 1H-NMR spectroscopy. The mesogenic properties and phase transition behavior were investigated by means of differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction measurements. The DSC studies show that the melting temperature and isotropic transition temperature of the ChLCPs increased with the weight percentage of cholesterol in the polymer. POM shows that the ChLCPs had a distinct spherulite structure that melted at about 140 °C, and these results are consistent with those of the DSC studies. The thermogravimetric studies show that the ChLCPs were stable up to 200 °C, though there was a reduction in the thermal stability as the weight proportion of cholesterol and glycerol in the polymer increased.