Polymer Dispersed Liquid Crystal Films Research Articles

A Smart Window with Passive Radiative Cooling and Switchable Near-Infrared Light Transmittance via Molecular Engineering.

Smart windows with synergetic light modulation have heightened demands for applications in smart cars and novel buildings. However, improving the on-demand energy-saving efficiency is quite challenging due to the difficulty of modulating sunlight with a broad bandwidth in an energy-saving way. Herein, a smart window with switchable near-infrared light transmittance and passive radiative cooling is prepared via a monomer design strategy and photoinduced polymerization. The effects of hydrogen bonds and fluorine groups in acrylate monomers on the electro-optical properties as well as microstructures of polymer-dispersed liquid crystal films have been systematically studied. Some films show a high contrast ratio of 90.4 or a low threshold voltage (Vth) of 2.0 V, which can be roll-to-roll processed in a large area. Besides, the film has a superior indoor temperature regulation ability due to its passive radiative cooling and controllable near-infrared light transmittance properties. Its radiative cooling efficiency is calculated to be 142.69 W/m2 and NIR transmittance could be switched to below 10%. The introduction of a carboxylic monomer and fluorinated monomer into the system endows the film with a highly efficient temperature management capability. The film has great potential for applications in fields such as flexible smart windows, camouflage materials, and so on.

Use of polymer dispersed liquid crystal (PDLC) film as a greenhouse cover material

Use of polymer dispersed liquid crystal (PDLC) film as a greenhouse cover material

Annual Energy-Saving Smart Windows with Actively Controllable Passive Radiative Cooling and Multimode Heating Regulation.

Smart windows with radiative heat management capability using the sun and outer space as zero-energy thermodynamic resources have gained prominence, demonstrating a minimum carbon footprint. However, realizing on-demand thermal management throughout all seasons while reducing fossil energy consumption remains a formidable challenge. Herein, an energy-efficient smart window that enables actively tunable passive radiative cooling (PRC) and multimode heating regulation is demonstrated by integrating the emission-enhanced polymer-dispersed liquid crystal (SiO2@PRC PDLC) film and a low-emission layer deposited with carbon nanotubes. Specifically, this device can achieve a temperature close to the chamber interior ambient under solar irradiance of 700W m-2, as well as a temperature drop of 2.3°C at sunlight of 500W m-2, whose multistage PRC efficiency can be rapidly adjusted by a moderate voltage. Meanwhile, synchronous cooperation of passive radiative heating, solar heating, and electric heating endows this smart window with the capability to handle complicated heating situations during cold weather. Energy simulation reveals the substantial superiority of this device in energy savings compared with single-layer SiO2@PRC PDLC, normal glass, and commercial low-E glass when applied in different climate zones. This work provides a feasible pathway for year-round thermal management, presenting a huge potential in energy-saving applications. This article is protected by copyright. All rights reserved.

Optimization of transparent conductive properties of magnetron sputtered Sb-doped SnO2 films by RF power: Towards application in polymer-dispersed liquid crystal (PDLC) films

Sb-doped SnO2 (ATO) films were prepared with different RF powers (PRF) under two O2 flow rates (FO2) by magnetron sputtering, and then subjected to rapid thermal annealing (RTA) treatment. The effects of PRF, FO2, and RTA on the crystallinity, stress, surface morphology, conductive properties, transmittance in visible-light range (TVis), and energy gap (Eg) of the films were investigated. As the PRF increases, the film crystallinity improves or first improves and then slightly degrades, and the compressive stress decreases. With increasing PRF, the surface changes from textured to granular morphology and from granular to honeycomb-like morphology under two FO2, respectively. Meanwhile, the conductive properties first enhance and then degrade, and the TVis and Eg tend to decrease, especially at higher PRF. Basically, the films prepared with higher FO2 have lower compressive stress, inferior conductive properties, and larger TVis and Eg compared with the films prepared with lower FO2. After RTA, the film crystallinity obviously improves, the compressive stress transforms into tensile stress, and the surface morphology has not undergone significant changes. Meanwhile, the conductive properties significantly enhance and Eg obviously increases. The TVis of the films prepared with lower FO2 can be increased by RTA. The influence mechanisms of the above factors on the structure and optoelectronic properties of the ATO films were explored, and ultimately, the ATO films have the TVis of 76.35–80.86%, sheet resistance of 51.7–91.9 Ω/sq., and quality factor of 1.03–1.67 × 10−3 Ω−1 within the optimized PRF range. Polymer-dispersed liquid crystal (PDLC) films were fabricated using optimized performance ATO films as electrodes, and their on-state transmittance, off-state transmittance, threshold electric field, and saturation electric field were 2.08–4.61%, 61.69–68.56%, 0.656–0.796 V/μm, and 1.68–2.108 V/μm, respectively. The current results indicate the potential application of ATO films in PDLC films as electrode.

Study on polymer dispersed liquid crystal films with wide viewing angle

ABSTRACT Polymer Dispersed Liquid Crystal (PDLC) films have been widely used as electronically controlled dimming technology, and solving the problem of narrow angle of PDLC is a cutting-edge challenge for their application. In this paper, a theoretical model of the viewing angle characteristics of PDLC is constructed, and the corresponding samples are experimentally fabricated. It is found that the simulation results of transmittance changing with the viewing angle are in good agreement with the experimental results. Furthermore, the technical approaches to improve PDLC viewing angle characteristics and achieve a wide viewing angle are discussed theoretically. The results show that the PDLC films for wide viewing angle can be obtained by using liquid crystal materials with smaller anisotropy of refractive index and polymer materials with refractive index slightly higher than the ordinary refractive index of liquid crystal.

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.

Preparation and Characterization of Bilayer Polymer-Dispersed Liquid Crystals Doped with Gd2O3 Nanoparticles and Rhodamine B Base Fluorescent Dye.

Using the polymerization-induced phase separation (PIPS) method, bilayer polymer-dispersed liquid crystal (PDLC) films with a PDLC-PVA-PDLC structure were prepared in this work. It was found that all PDLC performance indexes were affected by polymer mesh size after comparing the microscopic morphology and electro-optical properties of samples with different monomer ratios. Gd2O3 nanoparticles and rhodamine B base fluorescent dyes introduced into the bilayer PDLC optimized the samples' electro-optical properties and developed new functionalities. In addition, the bilayer PDLC doped with Gd2O3 and rhodamine B base held excellent progressive driving functions as well as stable durability properties. Samples doped with Gd2O3 nanoparticles and rhodamine B base also produced excellent anti-counterfeiting effects under UV irradiation at different angles, further exploiting the application potential of PDLC.

Preparation and Application of Polymer-Dispersed Liquid Crystal Film with Step-Driven Display Capability.

The realization of multifunctional advanced displays with better electro-optical properties is especially crucial at present. However, conventional integral full drive-based transparent display is increasingly failing to meet the demands of the day. Herein, partitioned polymerization as a novel preparation method was introduced innovatively into polymer-dispersed liquid crystals (PDLC) for realizing a step-driven display in agreement with fluorescent dye to solve the above drawback. At first, the utilization of fluorescent dye to endow the PDLC film with fluorescent properties resulted in a reduction in the saturation voltage of the PDLC from 39.7 V to 25.5 V and an increase in the contrast ratio from 58.4 to 96.6. Meanwhile, the experimental observations and theoretical considerations have elucidated that variation in microscopic pore size can significantly influence the electro-optical behavior of PDLC. Then, the step-driven PDLC film was fabricated through the exposure of different regions of the LC cell to different UV-light intensities, resulting in stepwise voltage-transmittance (V-T) responses of the PDLC film for the corresponding regions. Consequently, under appropriate driving voltages, the PDLC can realize three different states of total scattering, semi-transparent and total transparent, respectively. In addition, the PDLC film also embodied an outstanding anti-aging property and UV-shielding performance, which makes it fascinating for multifunctional advanced display applications.

The regulation of electro-optical properties and polymer morphology of polymer-dispersed liquid crystal films with silicon nanostructure

ABSTRACT In this work, the thiol modified silica nanostructures (SiO2-SH) were doped into polymer-dispersed liquid crystal (PDLC) films. The SEM was used to observe the morphology of the polymer microstructure. The electro-optical (E-O) property test was determined by a liquid crystal parameter tester. The SEM images suggested that the SiO2-SH nanoparticles can react with acrylate groups to form the polymer matrix. The saturation voltage decreased by half in the sample when the dosage of the SiO2-SH nanoparticles was 7.5 wt%, which may be due to the reduced anchoring strength of the polymer matrix incorporated with silicon nanoparticles resulting from the lower surface energy and the enhanced steric repulsions of LC droplets and polymer matrix. In addition, the preparation conditions like polymerisation temperatures and UV light intensity also effectively regulated the E-O properties and polymer microstructure of the PDLC film with silica nanostructure. Thus, the results showed that the E-O properties and the polymer morphology of PDLC films with silicon nanostructures can be effectively regulated by doping SiO2-SH nanoparticles and regulating the preparation conditions such as the polymerisation temperature and UV light intensity. This work can provide practical guidance for modulating the properties of PDLC films.

Effects of hydroxylated acrylates on electro-optical performance and adhesion strength of polymer dispersed liquid crystal films

In this study, effects of hydroxylated acrylate monomers that includes both monoacrylate and diacrylate on properties of the as-made polymer dispersed liquid crystal (PDLC) films are explored. The electro-optical response, microstructure and adhesion strength are characterized to compare the influences in detail. It is revealed that hydroxylated acrylate monomers can significantly enhance the adhesion strength in comparison with the non-hydroxylated monomer. Moreover, the hydroxyl group can alter the microstructure of the polymer matrix by affecting the polymerization and phase separation process. Thereby, the electro-optical response of the PDLC film is affected. Additionally, the influences are closely related to the position and concentration of hydroxyl group in acrylate monomers. When the hydroxyl group is located in the α-position or is distant from the reactive site, the improvement is limited. Increasing the concentration of hydroxyl groups by introducing monomers containing multiple hydroxyl groups in one molecule or combining hydroxylated monoacrylate and hydroxylated diacrylate monomers can change the microstructure type and deteriorate the electro-optical performance. By carefully balancing these factors, an optimized PDLC film can be obtained, whose adhesion strength is elevated to about 2.3 times that of the non-hydroxylated film and contrast ratio is improved from 135 to 262. This work is anticipated to provide deeper insights into the role of hydroxylated monomers and offer more possibilities to improve the overall performance of the PDLC films.

Effects of polarizing film arrangement on the viewing angle of polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) film is an electrically controlled dimming film in which the orientation of the liquid crystal molecules can be changed by applying an electric field. When the liquid crystals are perpendicular to the PDLC film, the film is transparent. Birefringence occurs after light passes through the liquid crystal molecules, resulting in two beams of light with perpendicular vibration directions. Therefore, PDLC film appears hazy when viewed from the side. In this study, a polarizing film was introduced to reduce the side-view haze, and its influence on the haze at different angles and positions was investigated. Experiment results revealed that aligning the PDLC film along the polarizing film’s absorption axis reduced the haze at all angles. When the polarizing film was attached to each side of the PDLC film and the absorption axis angle of the films was 0∘, a wide-viewing-angle PDLC film was obtained; at a 45∘ tilt, the haze only increased by 0.33%, and at 60∘ it maintained a clear field of view. This may be because only light in the same direction as the absorption axis passed through the film, reducing the amount of light that did not match the polymer refractive index.

Rheological Properties Study of Polymer Dispersed Liquid Crystals Doped by SiO2 Nanoparticles

The present report discusses the development of a novel nano-composite using a polymer dispersed liquid crystal (PDLC) and inorganic silicon dioxide (SiO2) nanoparticles. The aim of this work was to investigate the effect of the SiO2 nanoparticles on the rheological properties of the PDLC films. In our research described in this paper SiO2 nanoparticle-doped polymer dispersed liquid crystal films were made from a mixture of ethylene glycol phenyl ether methacrylate (EGPEMA) pre-polymer, 4-Cyano-4′-pentylbiphenyl (5CB) liquid crystals (LC) and SiO2 nanoparticles by the polymerization induced phase separation (PIPS) process. Doping the PDLC systems with SiO2 improved the rheological properties of these materials; the percolation (network formation) observed reflected the good interfacial adhesion between the nanoparticles, the polymer matrix and the LC. On the other hand, a decrease in the storage modulus (G′) and loss modulus (G″) of the PDLC systems after addition of the SiO2 nanoparticles confirmed their high flexibility and low crystallinity. A slight increase in G′ at 0.05 wt.% SiO2 resulted in the good mechanical strength of the doped PDLC. The viscoelastic behavior of the doped and undoped PDLC systems was confirmed at high frequencies. Thermal characterization by differential scanning calorimetry (DSC) showed a decrease in the glass transition temperature (Tg) of the PDLC with 0.05 wt.% of SiO2 nanoparticles, which showed that we had a more flexible material.

Liquid crystal/polymer composites for energy-efficient smart windows with a wide working temperature range and low off-axis haze

As a type of intelligent dimming film, polymer dispersed liquid crystal (PDLC) films show attractive application prospects in field of smart windows. However, their current performance cannot meet the requirement of outdoor window applications, especially in severe conditions, such as poor shielding effect at high temperatures, delayed response at low temperatures and high off-axis haze. To simultaneously solve these problems, a PDLC film was prepared by optimizing the structure of acrylate crosslinking agent and birefringence of LC, which exhibits good shielding effects and contrast ratio (10) at high temperature of 100 °C, short rise time (0.16 s) and decay time (2.4 s) at low temperature of −30 °C, and a satisfying low off-axis haze (on-state haze <10 % at ±45°, on-state transmittance >65 % at ±60°). This study provides meaningful supplements for the application of PDLC film in outdoor smart windows, especially in automotive smart windows under extreme weather conditions.

Study on the Effect of α-Substituted Acrylate Monomers on the Electro-Optical Properties of Polymer-Dispersed Liquid Crystal Films.

Polymer-dispersed liquid crystals (PDLCs) show great application potential in the areas of displays and smart windows. However, their electro-optical (E-O) properties such as contrast ratio and threshold voltage still need further improvement. In this study, the effects of α-substituted acrylate monomers on the morphology and E-O properties of PDLC composite films were systematically studied. It was found that the large substituent tended to increase the void size of the polymer matrix, while the small fluorine substitution led to a microsphere-type polymer morphology, which deteriorated the E-O performance. Finally, a largely improved E-O performance of low threshold voltage (0.437 V/μm), low saturation voltage (1.012 V/μm), and high contrast ratio (27) was achieved in an 8 μm-thick film by the addition of a chlorine-substituted monomer. This study provides a new approach for optimizing PDLCs from a material perspective.

A novel low-voltage fast-response electrically controlled dimming film based on fluorinated PDLC for smart window applications

A fluorinated polymer dispersed liquid crystal (PDLC) film with low operating voltage and high contrast ratio was successfully fabricated via the ultraviolet-initiated polymerization induced phase separation method. Effects of chemical structure and weight ratio of fluorinated monomers on the properties of PDLC films were systematically investigated through characterization of electro-optical response, micro morphology, optical spectrum and thermal analysis. Compared with the primitive sample, the addition of fluorinated monomers effectively improves the electro-optical properties of the film. The threshold voltage (Vth) is decreased by 39.7 % from 6.3 V to 3.8 V and the saturation voltage is reduced by 30 % from 15.8 V to 11.8 V for a 20 μm-thick film. Besides, the contrast ratio is improved to 110. Meanwhile, its thermal stability and hydrophobic property are also improved. Additionally, due to the abundance of C-O and C-F bonds in these compounds and the porous structure, the prepared film allows for transmittance modulation of sunlight. This research is anticipated to provide an innovative approach to optimize the performance of PDLC films for practical applications.

Enhanced thermo-electro-optical and dielectric properties of carbon nanoparticle-doped polymer dispersed liquid crystal based switchable windows

The effect of temperature as well as voltage on switchable smart windows was studied. These smart windows are composites of a polymer and a liquid crystal (LC) and collectively known as polymer dispersed liquid crystal (PDLC) films. In order to enhance the efficiency of the smart windows/PDLC films, the LC material was doped with carbon nano particles (CNP) in various concentrations ranging from 0 to 0.1%. To examine the efficacy and the temperature dependent performance of the CNP doped PDLC (CPDLC) film, firstly morphological and thermo-morphological studies were performed using polarizing optical microscopy (POM). POM in combination with temperature changes allows to record texture changes and the nematic-isotropic transition of the composite films for applied voltage. The complete electro-optical (EO) study comprising of voltage and temperature dependent transmittance, threshold voltage (Vth), saturation voltage (Vsat), contrast ratio (CR) and response time measurement was performed through an assembled EO set-up. To infer intermolecular cooperative processes between polymer and the dipoles of the LC, the relaxation mechanism of the composite films for various temperatures was studied by employing Debye and Cole-Cole models in dielectric spectroscopy. Parameters like dielectric strength and dissipation factor were determined to allow optimization of the CPDLC films.

Dielectric characterization of polymer dispersed liquid crystal film with chitosan biopolymer

Polymer Dispersed Liquid Crystals (PDLC) systems were obtained using chitosan as a biopolymer matrix and the nematic liquid crystal (LC) mixture E7 in various concentrations (30, 40 and 50% E7 in chitosan). Several techniques were used to analyze the obtained films: polarizing optical microscopy (POM), IR spectroscopy (FTIR), differential scanning calorimetry (DSC) and dielectric spectroscopy (DS). FTIR spectroscopy revealed minor changes for all PDLC films, suggesting weak interactions between LC and chitosan matrix. For each sample a clear segregation of the nematic droplets could be observed by POM. DSC measurements show that the phase transitions have shifted to lower temperature values for PDLC samples with approximately 2.7 °C compare to pure E7. The values of α (fraction of LC in the droplets) for compositions of 30, 40 and 50 wt% E7 in Chitosan were 0.48, 0.55 and 0.61 respectively, that are lower than for E7/PMMA system.The qualitative and quantitative analysis of the permittivity spectra obtained by DS reveals the fact that there are no significant changes in the molecular dynamics attributed to pure LC E7 compared to the PDLC composites, leading to the conclusion of the existence of a weak interaction between the two components, fact confirmed by the findings from the FTIR results.

Effect of doped SiO2 microspheres on the high temperature properties of PDLC films

ABSTRACT Polymer-dispersed liquid crystal (PDLC) films are electrically controlled dimming films. By applying an electric field to the PDLC film, it becomes transparent, whereas in the absence of an electric field, the PDLC film is opaque. In recent years, with the development of new-energy vehicles, the utilisation of glass curtains instead of the conventional skylight or sunshade has become a trend, because the combination of PDLC films and glass can realise random switching between shading and transparency; thus, it has great application prospects in the automotive industry. However, the low operating temperature of PDLC films limits their application. In this study, a characterisation method for PDLC film performance at high temperatures is discussed, and the effect of SiO2 microspheres on the high-temperature properties of PDLC films is investigated. When the proportion of doped SiO2 was 1.5%, the maximum operating temperature reached 95°C, and the light transmittance and hazeon (power on) did not change compared with the PDLC film without doped SiO2. The utilisation of the power-off haze at high temperature as a performance index of PDLC films has universal significance for the evaluation of the high-temperature resistance of these materials.

Preparation of triple-layer polymer dispersed liquid crystal films with progressive driving functionality

ABSTRACT In this paper, three single-layer polymer dispersed liquid crystal (PDLC) films with different electro-optical properties were prepared by the polymerisation-induced phase separation (PIPS) method firstly. The triple-layer PDLC film with progressive drive function was obtained by integrating three single-layer PDLC films. In contrast to the single-layer PDLC film, it is possible to make the triple-layer PDLC film enter the intermediate state between the on-state and off-state, while its transmittance can be controlled in low voltage by adjusting the applied voltage. It was found that the polymerisation temperature, time and UV intensity had significant effects on the electro-optical properties of the triple-layer PDLC films by comparing the electro-optical curves at various temperatures, times and UV intensities. The sample obtained under the optimal polymerisation condition showed a 31% reduction in threshold voltage, a 56% reduction in saturation voltage, a 20% reduction in off-state response time, and a 21% increase in contrast ratio than the original condition. The possibility to broaden the application area of PDLC is offered.

Effect of Liquid Crystalline Acrylates on the Electro-Optical Properties and Micro-Structures of Polymer-Dispersed Liquid Crystal Films

Polymer-dispersed liquid-crystal (PDLC) films have wide applications in light shutters, smart windows for cars and buildings, dimming glasses, and smart peep-proof films due to their switchable optical states under electrical fields as well as large area processibility. They are usually prepared with liquid crystals (LCs) and non-liquid crystalline monomers (NLCMs). Introduction of liquid crystalline monomers (LCMs) into PDLCs might spark new functionality or high-performance devices such as polymer-dispersed and -stabilized liquid crystals. In this study, the effect of molecular structures and doping concentrations of acrylate LCMs on the electro-optical properties and micro-structures of PDLC films is systematically studied. The pore size of polymer networks and the driving voltage of the PDLC films are affected by the molecular polarity and degree of functionality of the LCMs. The electro-optical properties of the PDLC films are affected by the synergistic influence of molecular structure of LCMs and micro-structures of PDLCs. These results might provide the experimental and theoretical basis for constructing the relationship between the molecular structure of LCM, micro-structure and electro-optical response of PDLC films.