Nematic Liquid Crystal Polymer Research Articles

Programmable multi-wavelength distributed feedback laser array integrated in a liquid crystal polymer waveguide.

Liquid crystal (LC) distributed feedback (DFB) lasers hold significant potential for integrated photonics applications. However, limitations in wavelength spacing for wavelength switching, device size, and compatibility with other technologies have impeded advancements of the LC DFB laser in integration and responsiveness. Herein, we propose a thin-film multi-wavelength DFB laser array utilizing high-resolution patterned programmable nematic LC polymers, enabling rapid switching with high-resolution wavelength spacing between wavelength division multiplexing channels while maintaining a stable single longitudinal mode (SLM) for each laser. The underlying physical mechanism involves modulating the effective refractive index of the DFB laser by varying the LC molecules' orientation angles between adjacent regions of the LC grating to achieve wavelength modulation. Additionally, a specialized LC waveguide design connects the DFB lasers, facilitating wavelength modulation as well as straight-line and bending propagation of the laser. Furthermore, the laser array demonstrates a relatively low energy threshold, facilitating its applications in high-integration scenarios.

Impact of Quantum Dots on Refractive Index of Nematic Liquid Crystal Polymer System

In the present study the effect of two quantum dots ZnS and CdSe on the Refractive Index of Nematic Liquid Crystal Polymer System (NLCPS) is investigated and compared. The measurement was performed for both the systems separately, using an Abbe Refractometer, within a temperature range of 20°C to 80°C with wavelengths in the visible spectrum. The refractive index with respect to temperature for various wavelengths was studied. We observed that the response of ZnS quantum dots dispersed in NLCPS was found to be better as compared to the CdSe system. This study may be useful for display and sensor applications.

Achieving a wide color gamut based on polarization interference filters in a liquid crystal display.

We propose two polarization interference filters (PIF1 and PIF2) used in the backlight unit of a liquid crystal display (LCD) to achieve a wide color gamut. Both PIF1 and PIF2 consist of two polarizers and two 720° super twisted nematic liquid crystal polymer (LCP) layers, where two polarizers are crossed in PIF1, and two polarizers are parallel in PIF2. The PIFs can eliminate unwanted cyan and yellow parts in the output spectrum, which can improve the color gamut of LCD. In our calculation, when the PIF1 is employed in the LCD with normal color filter and QD-LED backlight, the color gamut increases from 107.3% NTSC to 124.6% NTSC, which is 13.7% NTSC larger than that of the LCD with high-performance color filter. When the PIF2 is employed in the LCD with normal color filter and QD-LED backlight, the color gamut of LCD with a normal color filter is improved by 6.8% NTSC larger than that of LCD with high-performance color film, and the transfer efficiency is close to that of the LCD with high-performance color film. We define the color gamut enhancement ratio (CGER) to compare the influence of PIFs and the high-performance color filter on the color gamut enhancement performance of LCD. Compared with the high-performance color filter, the two kinds of PIFs have a higher CGER. The PIFs have a great potential for achieving a wide color gamut.

Effective Chiral Interactions between Nonchiral Rigid Macromolecules in a Chiral Solvent and the Induced Cholesteric Liquid Crystal Phase

It has been shown that a nonchiral anisotropic macromolecule embedded in a chiral dielectric solvent possesses an effective optical activity proportional to the optical activity of the solvent. As a result, there exists an effective chiral interaction between the macromolecules, which creates a torque acting on the primary axes of the two interacting molecules. A general expression for the effective chiral interaction potential has been derived in terms of the effective polarizability and the effective gyration tensor of the macromolecule in the chiral solvent. Explicit expressions for the components of the effective polarizability and the gyration have been obtained using the model of a hard rod filled with anisotropic dielectric and embedded into the isotropic chiral dielectric medium. The theory predicts the formation of the cholesteric helical structure in the nematic polymer liquid crystal phase induced by a chiral solvent.

Fabrication of a spatially tunable band reject filter with a very wide tunability range based on a chiral nematic liquid crystal polymer.

A thin, waterproof, and stable spatially tunable band reject filter is fabricated based on a chiral nematic liquid crystal polymer. The fabrication method for this filter is new, to the best of our knowledge, and straightforward. The photonic bandgap (PBG) of the proposed filter can be tuned from 350 nm to 760 nm by a mechanical movement of 6.5 mm. The filter reflects almost 50% of unpolarized incident light in the PBG and remains practically transparent for other wavelengths. The filter remains stable for four years and has acceptable resistance to polar protic solvents and thermal stability up to 90°C. The filter can be detached from the glass substrates, to be used as a thin 8-µm free-standing film or to be attached to a flexible substrate. This spatial tunable band reject filter may be used in displays, optical devices, and optical communication.

Generalized elliptical retarder design and construction using nematic and cholesteric phase liquid crystal polymers.

Elliptical retarders have important applications in interferometry and polarimetry, as well as imaging and display technologies. In this work, we discuss the traditional elliptical retarder decomposition using Pauli matrices as basis sets and then introduce a solution to the inverse problem: how an arbitrary elliptical retarder with desired eigenpolarizations and retardance can be constructed using a combination of linear and circular retarders. We present a simple design process, based on eigen-decomposition, with a solution determined by the intrinsic properties of each individual retarder layer. Additionally, a novel use of cholesteric liquid crystal polymer as a circular retarder is presented. Through simulation and experimental validation, we show cholesteric phase liquid crystal has an achromatic region of circular retardance at shorter wavelengths, outside of the Bragg regime. Finally, we verify our design process by fabricating and testing an elliptical retarder using both nematic and cholesteric phase liquid crystal polymers. The performance of the elliptical retarders shows excellent agreement with theory.

Reduction of solar infrared heating by using highly transparent thin films based on organic chiral nematic liquid crystal polymer.

This paper demonstrates a thin and transparent reflector film for the near infrared, based on chiral nematic liquid crystal (CLC) polymers. Two films reflect almost 50% of unpolarized incident light from 730 to 820 nm and from 880 to 1030 nm, while remaining completely transparent in the visible region with transmittance >90%. An efficient window uses the combination of two reflectors. After exposing two window-cubes for 2 h to direct sunlight, the temperature inside the cube with reflector windows was 4°C lower than in cube with plain windows. This reveals that the infrared (IR) reflectors can effectively control the indoor temperature. These films, which are 8 µm in thickness, can be detached from the glass substrates and used as a free-standing film, or be attached to a flexible optical foil or a solid window. The foils can be applied in buildings, offices, and automobiles to statically reduce the energy consumption required for air conditioning or lighting. The free-standing foils show acceptable resistance to polar protic solvents and are thermally stable up to 100°C.

The effect of a compatible liquid crystal polymer on homogeneous reinforcement of mesogenic and nonmesogenic rigid-rod monomers

Discrete amount of main-chain Liquid Crystal Polymers (LCP) could mechanically reinforce the mesophase of rigid-rod monomers to form a supramolecular homogeneous composite for Liquid Crystal Display (LCD) materials. Moreover, a homogeneously dispersed LCP may induce an enantiotropic transition in monotropic or non-mesogenic rigid-rod compounds. In this work, we studied the blends of a flexible main-chain nematic LCP with an enantiotropic nematic, a monotropic nematic, and two non-mesogenic rigid-rod compounds as model systems. The results indicate that homogenous reinforcement of thermotropic LCP in these monomers is a valid concept and could lead to improvement of mesogenic stability, orienational and mechanical properties of rigid-rod materials for various applications.

Patterned achromatic elliptical polarizer for short-wave infrared imaging polarimetry

Short-wave infrared (SWIR) imaging polarimetry has widespread applications in telecommunication, medical imaging, surveillance, remote-sensing, and industrial metrology. In this work, we design, fabricate, and test an achromatic SWIR elliptical polarizer, which is a key component of SWIR imaging polarimetry. The elliptical polarizer is made of a patterned linear polarizer and a patterned optical elliptical retarder. The linear polarizer is a wire grid polarizer. The elliptical retarder is constructed with three layers of nematic phase A-plate liquid crystal polymer (LCP) films with different fast axis orientations and physical film thicknesses. For each LCP layer, four arrays of hexagonal patterns with individual fast-axis orientations are realized utilizing selective linearly polarized ultraviolet (UV) irradiation on a photo-alignment polymer film. The Mueller matrices of the optical filters were measured in the wavelength range 1000 nm to 1600 nm and compared with theory. Our results demonstrate the functionality and quality of the patterned retarders with normalized analyzer vector parameter deviation below 7% over this wavelength range. To the best of our knowledge, this work represents the first polymer-based patterned elliptical polarizer for SWIR polarimetry imaging applications.

Twisted nematic liquid crystal polymer-based multi-layer composite polarizer with low azimuthal transmittance variation

The polarizer is an indispensable element in modern displays, but the azimuthal transmittance variation of the polarizer also affects the display quality, and it is hard to eliminate the azimuthal transmittance variation because of the polarizer’s structure. In this paper, we propose a multi-layer composite polarizer with polarizing layer – twisted nematic liquid crystal polymer layer – polarizing layer structure. An iodide molecule distribution model is introduced to explain the azimuthal transmittance variation of the normal polarizer and guide the design of the multi-layer composite polarizer. The azimuthal transmittance variation of the multi-layer composite polarizer and normal polarizer is measured and calculated, and the results show that the azimuthal transmittance variation of the multi-layer composite polarizer is only about one-fifth of that of the normal polarizer. Except for the most uniform transmittance distribution, the transmittance distribution of the multi-layer composite polarizer can be adjusted to meet different requirements. The potential concerns about low transmittance and color shift are also discussed, and they can be solved by reducing the polarization degree of the polarizing layers and increasing the thickness of the liquid crystal polymer layer. The results show that the multi-layer composite polarizer can replace the normal polarizer to reduce the viewing angle defects of the emissive displays.

Inner helical waveplate with angle-insensitive retardation

The angle-sensitivity is a serious defect of birefringent waveplate, and it draws lots of interest from researchers. In this paper, an angle-insensitive waveplate with inner helix structure is proposed. The super-twisted nematic liquid crystal polymer is used to fabricate the inner helical waveplate, and the retardation-shift is measured and analyzed. In the measured result, the retardation-shift is in the range from -0.311% to 0.194% at 45° polar angle, which is 2 orders of magnitude lower than that of the conventional waveplate. Theoretically, the retardation-shift is less than 0.5% in the polar angle range from 0° to 60°, and it will be less if the material with high refractive index is used. The analysis of the self-compensation effect explains the principle of angle-insensitivity, and the influence of tilt angle on angle-sensitivity is also investigated to guide the better design. At last, the application of the inner helical waveplate in wide color gamut display is taken as an example to show the broad application prospects.

Evidence of Chemical Reaction in Binary Blends of Polycarbonate and a Semi-Flexible Liquid Crystal Polymer

In this study, we provide the experimental results of the binary blends of a semi-flexible nematic liquid crystal polymer (LCP1) and polycarbonate (PC) within their phase diagram. The LCP1/PC blends were investigated by Differential Scanning Calorimetry (DSC), Optical Microscopy (OM), Wide-angle X-Ray Diffraction (WXRD), Fourier Transfer Infrared (FTIR), Gel Permeation Chromatography (GPC), and Nuclear Magnetic Resonance (NMR) techniques and observed the evidence of chemical reaction between LCP1 and PC in their heat-treated blends possibly by transesterification mechanism.

Homogeneous reinforcement of liquid crystal monomers by compatible mesogenic polymers

The thermal and mechanical properties of Liquid Crystal Monomers (LCM) could be reinforced by a discrete amount of Liquid Crystal Polymers (LCP) with compatible mesogenic moieties to establish a supramolecular homogeneous composite for improvement of LCM materials in display devices. Moreover, a homogeneously dispersed LCP may induce an enantiotropic transition in monotropic LCM compounds. In this work, we studied the blends of a flexible nematic LCP with two LCM compounds as model systems. The results indicate that homogenous reinforcement of LCM with LCP is a valid concept and could lead to improvements in mesogenic, thermal, orientational and mechanical properties of LCM materials for application in LCD devices.

'Smart' light‐reflective windows based on temperature responsive twisted nematic liquid crystal polymers

Abstract'Smart' windows which reversibly increase their reflectivity upon heating are attracting considerable attention as devices for maintaining comfortable indoor environmental conditions. In this work, twisted nematic semi‐interpenetrating liquid crystal networks which lose their order upon heating are sandwiched between reflective linear polarizers. This 'smart' window reversibly decreases its transmission from about 50–10% over a wavelength range between 400 and 1100 nm upon heating, resulting in the window becoming darker and reflecting more light. This 'smart' window is potentially interesting for energy saving window applications where variation between privacy and visible light transparency states is required.

Oblique wrinkling patterns on liquid crystal polymer core–shell cylinders under thermal load

Smart soft materials, which can flexibly respond to external multi-physics stimuli, have attracted considerable attention over the past few years. Here, we present tunable wrinkling patterns in cylindrical core–shell systems under thermal load via the orientation of director in nematic liquid crystal polymer (LCP). To quantitatively analyze mechanical behavior and morphological evolution of LCP core–shell cylinders, we develop a core–shell model that accounts for director-induced anisotropic spontaneous strains. By tuning the alignment of liquid crystal director, we explore the effects of anisotropy of spontaneous strains on instability pattern formation and evolution. When the director is along the circumferential direction, wrinkling pattern can be axisymmetric or diamond-like, determined by a single dimensionless parameter Cs which characterizes the stiffness ratio and curvature of the system. When the director is parallel to the axial direction, leading to circumferential expansion upon heating, the core–shell cylinder usually buckles into churro-like mode. For general spatial director alignment, the ultimate wrinkling patterns depending on the anisotropy state of spontaneous strains, can be diamond-like, parallel bead-chain or oblique stripe mode. We draw director-affected phase diagrams to provide an overall view of pattern selection affected by liquid crystal director orientation, which could be used to quantitatively guide the effective design of wrinkling morphology-related smart surfaces.

Thermal wrinkling of liquid crystal polymer shell/core spheres

Smart soft materials that can flexibly respond to external multi-physics stimuli, have shown intriguing applications in shape-morphing and morphology control. Here, we present tunable wrinkling patterns in core–shell spheres under thermal load via controlling the orientation of director in nematic liquid crystal polymer (LCP). To analyze nonlinear instability and morphological evolution of LCP shell/core spheres, we develop a shallow core–shell model that accounts for director-induced anisotropic spontaneous strains. By tuning the alignment of liquid crystal director, we explore effects of anisotropy of spontaneous strains on wrinkling pattern formation and evolution. When the director is along the out-of-plane direction, the surface shell undergoes equi-biaxial expansion, resulting in hexagonal, checkerboard, herringbone or labyrinth pattern, determined by a single dimensionless parameter Cs which characterizes the stiffness ratio and curvature of the system. Moreover, stability analysis yields an analytical solution of the critical wrinkling strain and wavelength for equi-biaxially stressed shell/core spheres. When the director is in-plane aligned, leading to primary uniaxial expansion upon heating, the core–shell usually buckles into stripes. For general spatial director alignment, the ultimate wrinkling patterns depending on the anisotropy state of spontaneous strains, can be hexagonal, parallel bead-chain, stripe, herringbone, labyrinth or hybrid. Lastly, we provide director-affected phase diagrams on pattern selection, which could be used to quantitatively guide the effective design of morphology-related smart surfaces such as anticounterfeiting system.

Photoalignment at the nematic liquid crystal–polymer interface: The importance of the liquid crystalline molecular structure

The influence of the molecular structure of the nematic liquid crystal (NLC) on the photoalignment process at the NLC–polymer interface has been investigated experimentally. NLCs having exclusively phenyl-, or cyclohexane rings in the rigid core, as well as NLCs containing both phenyl and cyclohexane groups have been considered. Substantial differences have been found in the photoalignment process depending on the molecular structure of the NLC, supporting the assumption that the polymer–NLC interface should be regarded as a coupled system, where the two components mutually influence each other. A phenomenological explanation is given for the observed differences.

Design and Synthesis of Polysiloxane Based Side Chain Liquid Crystal Polymer for Improving the Processability and Toughness of Magnesium Hydrate/Linear Low-Density Polyethylene Composites.

In this study, a polysiloxane grafted by thermotropic liquid crystal polymer (PSCTLCP) is designed and synthesized to effectively improve the processability and toughness of magnesium hydroxide (MH)/linear low-density polyethylene (LLDPE) composites. The obtained PSCTLCP is a nematic liquid crystal polymer; the liquid crystal phase exists in a temperature range of 170 to 275 °C, and its initial thermal decomposition temperature is as high as 279.6 °C, which matches the processing temperature of MH/LLDPE composites. With the increase of PSCTLCP loading, the balance melt torque of MH/LLDPE/PSCTLCP composites is gradually decreased by 42% at 5 wt % PSCTLCP loading. Moreover, the power law index of MH/LLDPE/PSCTLCP composite melt is smaller than 1, but gradually increased with PSCTLCP, the flowing activation energy of PSCTLCP-1.0 is lower than that of MH/LLDPE at the same shear rate, indicating that the sensitivity of apparent melt viscosity of the composites to shear rate and to temperature is decreased with the increase of PSCTLCP, and the processing window is broadened by the addition of PSCTLCP. Besides, the elongation at break of MH/LLDPE/PSCTLCP composites increases from 6.85% of the baseline MH/LLDPE to 17.66% at 3 wt % PSCTLCP loading. All the results indicate that PSCTLCP can significantly improve the processability and toughness of MH/LLDPE composites.

Preparation and electrooptic study of reverse mode polymer dispersed liquid crystal: Performance augmentation with the doping of nanoparticles and dichroic dye

ABSTRACTReported herein, the preparation, morphological, and electrooptic (EO) characterization of reverse mode polymer dispersed liquid crystals (PDLCs) with nematic liquid crystal (LC) and UV curable polymer optical adhesive using polymerization induced phase separation method. The PDLCs are switchable between transparent and opaque states due to the homeotropic and planar alignment of LC in their OFF and ON states of applied voltage, respectively. Further, effect of doping of a fixed concentration of azo dye and silica nanoparticles (NPs) on morphological, EO and response characteristics of same PDLC sample was also analyzed. Experimental results showed that doped reverse mode PDLCs have the higher OFF state optical transmission and boost up in the scattering ON state compared with pristine reverse mode PDLC. The phenomenon is also supported by extinction coefficient and absorption study based on Beer's law. The threshold and operating voltages were found reduced ~1.56 and ~1.73 times for NPs and (NPs + dye) doped reverse mode PDLCs, respectively, along with better contrasts to the pristine reverse mode PDLC. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48745.

Polarizing beam splitter cube for circularly and elliptically polarized light.

A method of designing an arbitrary polarizing beam splitter (PBS) cube using multiple layers of thin-film liquid crystal polymer is demonstrated. This methodology utilizes cholesteric phase liquid crystal polymer (Ch-LCP) to transmit one handedness of elliptically polarized light and reflect the orthogonal state when unpolarized light is incident. Using additional nematic liquid crystal polymer layers, the polarization state for the transmitted and reflected light can be controlled and output to any two orthogonal states represented on the Poincaré sphere. Two cubes are designed, fabricated, tested, and compared with theory. One cube is constructed with a single layer of Ch-LCP, and another cube is constructed with a layer of Ch-LCP and an additional nematic liquid crystal polymer layer.