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A study on the polymer structures and electro-optical properties of epoxy-mercaptan-based polymer dispersed liquid crystal films
Abstract
In this paper, polymer dispersed liquid crystal (PDLC) films based on epoxy-mercaptan system were prepared by thermal-initiated polymerization. The effects of the liquid crystal (LC) content, the proportion and the functionality of epoxy monomers on the polymer structures and electro-optical properties of the as-made PDLC films were investigated systematically. It was found that the morphologies of the polymer matrix can be altered from polymer meshes to polymer balls by increasing the LC content as well as the functionality of epoxy monomers. Accordingly, the electro-optical properties could be regulated by the morphologies of polymer networks. Especially, the as-made PDLC films with homogeneous porous structures exhibited the optimal electro-optical properties. Consequently, this work offers a meaningful approach to control the microstructures and optimize the electro-optical properties of PDLC films, which indeed can form a wonderful footstone for the wide application of PDLC.
... With increasing attention to environmental protection and energy conservation, the research and application of environmentally friendly and smart devices have been a research hotspot [1,2]. For example, smart windows based on electro-, thermo-, gaso-, and photochromogenic materials can change transmittance for blocking intense visible light [3][4][5][6][7]. Benefiting from the large-scale processability, long-term stability, good flexibility, and high mechanical strength, a polymer-dispersed liquid-crystal (PDLC) system has become a mainstream solution [8][9][10][11]. ...
... PDLC films can be driven by an electric field for optical-state transition; accordingly, there are some parameters that can be used to characterize the electro-optical properties of the films, such as threshold voltage (Vth) and saturation voltage (Vsat). Typically, they are inversely proportional to the LC droplet radius (R), as shown by the following equations [3]: PDLC films can be driven by an electric field for optical-state transition; accordingly, there are some parameters that can be used to characterize the electro-optical properties of the films, such as threshold voltage (V th ) and saturation voltage (V sat ). Typically, they are inversely proportional to the LC droplet radius (R), as shown by the following equations [3]: ...
... Typically, they are inversely proportional to the LC droplet radius (R), as shown by the following equations [3]: PDLC films can be driven by an electric field for optical-state transition; accordingly, there are some parameters that can be used to characterize the electro-optical properties of the films, such as threshold voltage (V th ) and saturation voltage (V sat ). Typically, they are inversely proportional to the LC droplet radius (R), as shown by the following equations [3]: ...
With the switchability between transparent and light-scattering states, polymer-dispersed liquid crystals (PDLC) are widely used as smart windows, flexible display devices, projectors, and other devices. In outdoor applications, in addition to excellent electro-optical properties, there is also a high demand for film stability. In this work, a PDLC film with high mechanical strength and structural stability is prepared that can maintain stability at 80 °C for 2000 h. By choosing liquid crystals with a wide temperature range, adopting acrylate polymer monomers containing hydroxyl groups, and adjusting the polymer content, the PDLC film can work well from -20 °C to 80 °C. On this basis, the effects of the introduction of rigid monomers on the mechanical properties and electro-optical properties of PDLC films are investigated.
... Several key aspects used to evaluate the electro-optical performance of LC/polymer materials were the driving performance (threshold voltage V th and saturation voltage V sat ), the response time (rise time τ on and decay time τ off ) and the optical contrast (ratio of transmittance in the on-state and off-state of the film). Theoretically, the driving performance of PDLC films and the response time can be predicted according to Eqs. 3-6 [132][133][134][135][136] . ...
... d Scattering properties of CLC. e Dynamic LC scattering properties the polymer morphology affects these properties. For example, Li et al. 136 modified polymer microstructure and altered the electro-optical properties of films by adjusting the content of epoxy resin monomer, as shown in Fig. 10a. ...
... Therefore, these materials are potential candidates for windshield light protection. However, the reported switching speeds of commercial P-LC materials were typically within hundreds of milliseconds 128,130,131,136,[142][143][144][155][156][157][158][159][160][161][162] , and for important windows such as windshields, faster switching speeds are often sought. ...
With the development of optical technologies, transparent materials that provide protection from light have received considerable attention from scholars. As important channels for external light, windows play a vital role in the regulation of light in buildings, vehicles, and aircrafts. There is a need for windows with switchable optical properties to prevent or attenuate damage or interference to the human eye and light-sensitive instruments by inappropriate optical radiation. In this context, liquid crystals (LCs), owing to their rich responsiveness and unique optical properties, have been considered among the best candidates for advanced light protection materials. In this review, we provide an overview of advances in research on LC-based methods for protection against light. First, we introduce the characteristics of different light sources and their protection requirements. Second, we introduce several classes of light modulation principles based on liquid crystal materials and demonstrate the feasibility of using them for light protection. In addition, we discuss current light protection strategies based on liquid crystal materials for different applications. Finally, we discuss the problems and shortcomings of current strategies. We propose several suggestions for the development of liquid crystal materials in the field of light protection. With the development of optical technology, LCs with intelligent light protection function become more and more important. This paper discusses the research progress and challenges of LCs for light protection.
... Thermotropic liquid crystals are known to have high interfacial surface energy and low surface anchoring energy. [35][36][37] Furthermore, several investigations have shown that the longer the τ R and the shorter the τ D are, the smaller the LC droplet size. The outcomes relate to the management of the LC droplet's reorientation and orientation under the influence of an electrical field by the boundary contact between the polymer matrix and LC droplet. ...
... The difference in the optical states of the digital devices is caused by photo-induced changes in the molecular arrangement of LC materials. 37 Figure 4 shows that T off declined sequentially for all samples A1-A4. A highly birefringent LC, a high droplet density, and thick films can all be used to improve the strong scattering in smart glass. ...
The goal of the current study was to develop a method for producing polymer-dispersed liquid crystal (PDLC) films for smart glasses that are thin and use a nematic liquid crystal system, epoxy resin monomers, hardeners, and epoxy resin. The performance of optical and electro-optical display panels is directly impacted by the techniques employed to refine smart PDLC films to increase their stability and characteristics. In this investigation, we set up the polymerization-induced phase separation heat curing method to use thermosetting resin 3-(diethylamino)propylamine as a primary functional heat curing agent and hardener, cross-linking agent poly(propylene glycol) diglycidyl ether, tri-functional trimethylolpropane triglycidyl ether, and tetra-functional pentaerythritol tetraglycidyl ether epoxy resin. Investigations were carried out using a liquid crystal device parameter tester, scanning electron microscope, UV–Vis–NIR spectrophotometer, and the generated smart glass panel sheets. The interaction between the monomer and the 3-(diethylamino)propylamine, a primary functional hardener, was confirmed using the liquid crystal device method and Scanning electron microscopy. Experiments were conducted on epoxy resins cured with 3-(diethylamino)propylamine to determine the feed ratio of epoxy monomers, the amount of curing agent, active diluents, and curing temperature. Moreover, the methyl groups of the trimethylolpropane triglycidyl ether, pentaerythritol tetraglycidyl ether, and 3-(diethylamino)propylamine were shown to play the main role in reducing the domain size of PDLC thin films in all samples. This smart glass technology will serve to create a barrier between the patient with COVID-19 and the doctor, as well as in healthcare center buildings, affected patient isolation rooms, hospitals, and smart helmets, enabling social distancing and eco-friendly environments for flexible electronics.
... Regarding a subject that is especially important in the case of LCERs, Carfagna and his team, the great pioneers in this field, found that both the level of curing and the physical properties of the hardener are strongly affected by the nature of the hardener [10], so it is extremely important to investigate a broad selection of curing agents and compare their effects on the network. The importance of this matter has become even greater since LCERs became the foundation for manufacturing high-end materials for 5G antennas [11], tunable actuators with reprocessibility [12], microrobotics and micromachinery [13], highly conductive polymers [14], vitrimers [15], optical devices [16,17], and high-impact-strength polymers [18], as well as for providing a property-enhancing factor in powder coatings [19]. ...
Modern science and technology demand a low glass transition temperature, yet one tailored to specific thermoset needs and specific to individual hardener applications. Two novel, nonterminal liquid crystalline epoxy resins (LCER) were synthesised, with their structures characterized via nuclear magnetic resonance (NMR), mass spectrometry (MS), and elemental analysis. Their liquid crystalline nature and thermal properties were determined using polarized optical microscopy (POM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). A set of seven aromatic amines serving as curing agents was used to perform curing in fourteen different systems in order to assess the glass transition temperature (Tg) of the obtained polymer networks using DSC. The liquid crystalline elastomers were obtained with vitrification occurring in a low temperature range (−10-40 °C), with a more predictable outcome for amines with two aromatic rings in the structure than with one. Moreover, the resin with a core consisting of four aromatic rings produces networks with higher Tg than the three-aromatic resin. The use of nonterminal LCER allowed the lowering of the glass transition temperature of the polymers to more than 70 °C compared to a terminal analogue. This brings new possibilities of designing highly elastic yet cured polymers with potential for use in smart applications due to the LC nature of the resin.
... Basically, the mesogenic optical axis in the microdroplet is randomly oriented resulting in a strong light scattering without any electric field (off state) while mesogens are forced to rearrange depending on the direction and strength of the electric field to induce an optical change in the transmittance with an AC electric field applied (on state). Electro-modulation of the refractive index between the mesogens and the polymer matrix realizes the transmittance variation of PDLCs [18,19]. At present, PDLCs demonstrate tremendous potential for a variety of applications including physical and chemical sensors [20][21][22], displays [23,24], time temperature indicators [25], smart windows [26], etc. ...
In this work, we provide a fabrication method for dual-responsive electrochromic (EC) polymer dispersed liquid crystal (PDLC) devices. The EC PDLC device was developed by combing the PDLC technique and a colored complex formed via a redox reaction without a specific EC molecule in a simple preparation method. The mesogen played dual roles in the device for scattering in the form of microdroplets and participating in the redox reactions. Orthogonal experiments were performed with the acrylate monomer concentration, the ionic salt concentration, and the cell thickness as factors to investigate the electro-optical performance for the achievement of optimized fabrication conditions. The optimized device presented four switchable states modulated by external electric fields. The light transmittance of the device was changed by an alternative current (AC) electric field while the color change was realized by a direct current (DC) electric field. Variations of mesogen and ionic salt species can modulate the color and hue of devices, which solves the disadvantage of a single color for traditional EC devices. This work lays the foundation for realizing patterned multi-colored patterned displays and anti-counterfeiting via screen printing and inkjet printing techniques.
... [9][10][11] Owing to large-scale processability, long-term stability, good flexibility and high mechanical strength, PDLC-based smart windows show significant promise for building application. [12][13][14][15][16][17] However, most PDLC systems can only work in visible wavelengths and have no effective modulation ability in other wavelengths, such as infrared and ultraviolet (UV) wavelengths. ...
Recently, smart windows based on polymer‐dispersed liquid crystals (PDLCs) composites have garnered enormous interest owing to their electrically switchable properties. However, the low UV‐shielding characteristics of these films limit their widespread application. Here, we reported novel PDLC composite films doped with functionalized TiO2 nanoparticles (NPs) to enhance the UV‐shielding efficiency. The TiO2 NPs were chemically functionalized by 3‐aminopropyltriethoxysilane (APTS) and 3‐methacryloxypropyltrimethoxysilane (MPTS), respectively. Scanning electron microscopy (SEM) studies revealed that APTS functionalized TiO2 NPs tended to cluster in the polymer network instead of liquid crystal droplets, and MPTS functionalized TiO2 NPs were independently and uniformly dispersed in the polymer network. Particularly, optical studies showed that the TiO2(MPTS)/PDLC composite film can effectively shield 95.9% UV light at on‐state, and 99.16% at off‐state. Furthermore, compared to traditional PDLC film (without TiO2 NPs), the total response time of the TiO2(MPTS)/PDLC film is reduced from 60.4 to 13.4 ms, and the contrast ratio (CR) was increased by more than twofold from 35.6 to 97.26.
... Fig. 4(b) depicts that the V th and V sat of sample A3 are relatively higher than those of samples A1 and A2. These outcomes are consistent with the morphologies, as shown in Fig. 3. Generally, the small LC droplets are firmly anchored by polymer matrix; therefore, the high applied electric field would enable the orientation of LC droplets [43]. Fig. 4(c) illustrates the CR value of films A1-A3. ...
Keywords: Oxygen heterocyclic acrylate Chain length High contrast ratio Low driving voltage Polymer dispersed liquid crystal A B S T R A C T The current work aimed to fabricate polymer dispersed liquid crystal (PDLC) films using het-erocyclic acrylate monomers containing oxygen atoms. The incorporation of oxygen heterocycles into the host polymer matrix significantly improved the electro-optical properties of PDLC films. Particularly, the high contrast ratio 330.0 was realized for the monomer Trimethylolpropane formal acrylate (TFA). Further, the excellent electro-optical properties, including low driving voltage (10.0 V), moderately high contrast ratio (64.0) and short response time (<8.0 ms) were obtained when the liquid crystals/polymer composite film thickness decreased from 20 μm to 8 μm. This work has provided a novel materials system to promote the application of PDLC films in the thin-layer liquid crystal display.
... In general, these films exhibit a milky white scattering state due to the random orientation of LC droplets in the polymer matrix [1][2][3]. Once an electric field of sufficient strength is applied, the film becomes transparent and this phenomenon is attributed to the alignment of LC droplets along the direction of the electric field (Figure 1), if the ordinary refractive index n o of LC matches with the refractive index n p of the polymer matrix [4]. Spurred by the employment of PDLC in display technologies, research on these materials has rapidly grown in recent decades and is now extending into areas beyond smart windows [5] and displays [6], including diffuse film [7], antipeeping film, quantum dots (QDs) film [8] and components of organic light emitting diode (OLEDs) [9], field effect transistors (FETs) [10], energy storage [11] and solar-energy harvesting [12]. ...
Polymer dispersed liquid crystals (PDLCs) have kindled a spark of interest because of their unique characteristic of electrically controlled switching. However, some issues including high operating voltage, low contrast ratio and poor mechanical properties are hindering their practical applications. To overcome these drawbacks, some measures were taken such as molecular structure optimization of the monomers and liquid crystals, modification of PDLC and doping of nanoparticles and dyes. This review aims at detailing the recent advances in the process, preparations and applications of PDLCs over the past six years.
... It should be noted that low V dr can be realized for PDLC systems with large LC domain size (pores) since large domain size lowers the surface anchoring forces from polymer network because of the weak polymer-LC interfaces. Accordingly, the LC molecules would be much easily aligned under the low applied electric field [44][45][46]. On the other hand, except A1, V th and V sat of acrylate containing monomers are not realized (Figure 3(a-d)). ...
This contribution investigates the preparation of polymer-dispersed liquid crystal (PDLC) films based on meth(acrylate) monomers containing rigid structures. The current study specifically focuses on comparison of the monomers on the basis of meth(acrylate) side group, flexible/rigid structures and rigidity. We find that methacrylate monomers exhibit more preferable electro-optic properties than that of the acrylate monomers. Furthermore, through the systematic variation of rigidity, composition of the monomers and chain length of crosslinking agents, both the morphologies and electro-optic properties of these films are found to be adjustable. A composite film is demonstrated by employing moderately rigid monomer (tetrahydrofurfuryl methacrylate) with low driving voltage (21.0 V) and high contrast ratio (87.5). Therefore, the studies here provide a new approach to optimize the electro-optical properties of the PDLC films by introducing monomers with rigid structures.
... The formation of liquid crystal droplets depends on the degree of supersaturation and the diffusion of liquid crystal molecules in the polymer network. When the content of liquid crystal remains constant, the size regulation of liquid crystal droplets will be related to the formation of polymer network driving from UV-cured acrylate [9][10][11][12] or thermally cured epoxy resin [13,14]. The former has low tunability because the viscosity of system suddenly increases and the diffusion of liquid crystal molecules is difficult resulting from its fast radical polymerisation, while the latter has higher tunability because the reaction rate is relatively slow and the viscosity gradually increases [15]. ...
To improve the electro-optical properties of polymer-dispersed liquid crystal (PDLC) film, the size and number of liquid crystal droplets were regulated by changing component, curing method, curing temperature and time, and so on, but the regulation reported in the literatures was constrained within the curing process. In this work, the size of liquid crystal droplets is regulated by quenching around the glass transition temperature (Tg) at which the mobility of polymer chains changes dramatically. At the same time, the degree of supersaturation of liquid crystal molecules changes due to the decrease of temperature, thereby regulating the number of nucleation of liquid crystal droplets. Through the study of the above process, the effects of the size and number of liquid crystal droplets and the content of liquid crystal dissolved in the polymer network on the electro-optical properties of PDLC film are discussed. The research provides a new method to regulate the structure and properties of PDLC film aiming at the cured PDLC film, effectively overcoming the disadvantage of the regulation aiming at the curing process.
... In our previous works, LCs/polymer composite films have been found greater advantages over particle-diffusing type optical diffusers due to the adjustable polymer microstructures in the composite films [22][23][24][25]. Generally, high haze accompanies with low transmission, according to Sajad's work, in the definition of haze, H λ ð Þ¼ 1À direct transmissionðλÞ total transmissionðλÞ 100%, we only consider the transmission and haze of 560 nm wavelength light. ...
Optical diffusers are promising diffusing materials in the optical devices such as monitors, projectors, fibre optics, light-emitting diode (LED) systems and liquid crystal displays (LCDs). We report optical diffusers comprising uniformly distributed nano-sized polymer balls/nematic liquid crystals (LCs) by ultraviolet (UV) click reaction of ene monomer and thiol monomer. By optimising the mass ratio 1:1 of ene and thiol, of which the average diameter of the corresponding nano-sized polymer balls is about 900 nm, relatively high optical transmission and haze with 88.99% and 94.49% are yielded, respectively. Furthermore, by controlling the curing time, the average diameter of nano-sized polymer balls can be reduced to 810 nm, and the developed film exhibits high transmission (98.49%) without sacrificing the high haze (91.77%). This paper demonstrates that UV click reaction is an economical approach to fabricate optical diffusers in a controllable manner.
Windows are used everywhere, from buildings to vehicles. Smart switchable windows are highly sought‐after in order to reduce energy consumption. An ideal smart window should have two functions. First, it should be able to control privacy. Second, it should be able to control radiant energy flow. Here, a smart window is reported that possesses the two functions. The smart window is based on a dichroic dye‐doped cholesteric liquid crystal. It exhibits two stable states in the absence of applied voltage. One of them is the planar state that is optically absorbing without haze and can control radiant energy flow. The other state is the focal conic state that is optically scattering and can control privacy. Furthermore, when voltage is applied, the liquid crystal is switched to a heliconical state, where the transmittance can be tuned continuously. Thanks to its superior performance, this smart window has a big potential for applications in architectural and vehicle windows.
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 electro-optical properties of polymer dispersed liquid crystal films vary with liquid crystal contents and applied electric field, but the analysis of the film morphology cannot directly reflect the mechanism of electro-optical properties. Therefore, the polymer dispersed liquid crystal film prepared by blending liquid crystal material E7 and photopolymer NOA65 was applied. Herein, the dielectric polarization regulated electro-optical properties and their related mechanism under different liquid crystal contents and electric fields are revealed. The results show that in the frequency range of 10⁻¹~10⁶ Hz, the film exhibits three relaxation processes in low frequency, medium frequency and high frequency, separately, which are generated by thermionic polarization, interfacial polarization and orientation polarization, respectively. According to the Arrhenius equation, the activation energy of such polarization processes were calculated. It was found that with the increase of liquid crystal content, the activation energy of orientation polarization decreased from 0.88 eV to 0.83 eV, resulting in the decrease of threshold field strength and saturation field strength of the diversion of liquid crystal molecule. Thermionic polarization under DC electric field forms an internal electric field, which results in a large increase in threshold field strength and saturation field strength, as compared with the situations under AC electric field. Such thermionic polarization also leads to the increase of the polarization relaxation time, which results in the extension of response time. This study shows guiding significance for further analysis and improvement of the electro-optical properties of polymer dispersed liquid crystal films.
In this article, polymer-dispersed liquid crystal (PDLC) films were prepared from a mixture of prepolymer, liquid crystal and reticular nanofibre films containing multi-walled carbon nanotubes (MWCNT) by the polymerisation-induced phase separation (PIPS) method. By comparing the microstructure and electro-optical profiles of different contents of liquid crystals as well as monomer types, it was demonstrated that the pore size pairs of the polymer network can modify the electro-optical properties of PDLC. Different types of loaded multi-walled carbon nanotube reticular nanofibre films were introduced into the liquid crystal mixture specimens and the altered polymer network and electro-optical properties of these specimens were analysed using SEM, POM, TEM, and electro-optical curves. It was found that the electro-optical properties of PDLC were greatly improved and the resistance to ageing was also greatly enhanced by mesh nanofibre films doped with a certain amount of multi-walled carbon nanotubes. Therefore, the development of PDLC doped with loaded MWCNT reticular nanofibre films with faster response times and better electro-optical properties will greatly improve the promising technological applications of PDLC-based devices for optical windows, displays, energy storage and flexible devices.
Polymer dispersed liquid crystal (PDLC) films have potential applications in many aspects due to unique electrically switching properties, and thus the films with excellent comprehensive properties are highly demanded at present. In this study, aromatic polyurethane acrylate (PUA) was used as polymer-matrix, and effect of diluents [lauryl methacrylate (LMA) and isobornyl acrylate (IBOA)] content on the electrical-optical properties of PDLC films was investigated. The results indicate that increase of PUA/LMA or decrease of LMA/IBOA weight ratio increases threshold and saturation voltages (Vth and Vsat), decreases on- and off-state transmittances (Ton and Toff) and increases contrast ratio. Under optimised PUA/LMA weight ratio of 31/19 or PUA/LMA/IBOA weight ratio of 31/15/4, the Vth and Vsat of the films first increase and then decrease with increasing Ag nanowires (NWs) content, and better overall properties are achieved at 0.05 or 0.075 wt.% Ag NWs, respectively. In order to explore essential reasons for the influence of diluents and Ag NWs on the properties of PDLC films, the relationships between film properties (Vth or Vsat, Ton, Toff, and response time) and liquid crystal (LC) droplets size were analysed emphatically, and the changes of system viscosity and anchoring force between polymer-matrix and LC droplets were also discussed.
Polymer-dispersed liquid crystals (PDLCs) are very attractive due to their electrically switchable properties. However, current PDLC films still have problems such as high driving voltages, low contrast ratio (CR), and poor bending resistance and spacing stability. To solve these problems, a PDLC film with a system of coexisting polymer spacer columns and polymer network was proposed. First, based on the adhesive systems of IBMA and UV6301, the effects of IBMA concentration and LC content on the morphology of the polymer network and the electro-optical properties of PDLC were investigated, respectively. Then, the effects of the process conditions of mask polymerization such as temperature, time, and UV light intensity on the morphology and electro-optical properties of the polymer spacer columns were systematically investigated. It was found that PDLC films with the coexistence system exhibit both excellent electro-optical properties and outstanding bending resistance and spacing stability. Thus, it provides new practical possibilities for the preparation of high-performance PDLC films used in flexible devices.
Polymer-dispersed liquid crystals (PDLCs), comprised of submicron-sized LC droplets embedded in a continuous polymer matrix, have been the subject of interest for scientists and manufacturers owing to their promise for optoelectronic applications. Recently, many research works have made significant progress in improving performance, developing new operation modes, and expanding the application fields for PDLCs. In this review, recent advancements in the research work for performance optimization, novel operation modes, and application developments of PDLCs are overviewed. The working principle and the general fabrication procedure of PDLCs with different modes are first briefly explored. Then, the recent research developments performed in the preparation, experimental investigation, and applications of PDLCs are summarized systematically. Eventually, the challenge and future improvement are also discussed.
In this work, the thiol‐ene click reaction was used to prepare polymer-dispersed liquid crystal films by a photo-initiated polymerization reaction. Different content of DET was introduced into the polymer matrix to enhance the electro-optical performance of PDLC. Our results showed that the introducing of DET significantly improved the electro-optical properties of PDLC. The SEM results showed that larger and uniform pores were observed with increasing DET content. And it was found that an increase in UV intensity resulted in a continuous decrease in Toff and an increase in CR for weakening the anchoring effect of the polymer matrix.
Polymer dispersed liquid crystals (PDLCs) have lit a flash of interest due to the distinctive property of electrically controlled switching. However, too high‐driving voltage associated with porous polymer networks always limit their wider range of applications. Herein, we reported a PDLC system containing LCs, 2,2′‐(Ethylenedioxy)diethanethiol (DET) with thiol groups, cage‐like nanostructure acrylic polyhedral oligomeric silsesquioxane (KH570‐POSS) and KH570‐SiO2 nanoparticle modified by acrylic groups. The cage‐like KH570‐POSS microstructure was injected to the polymer matrix when KH570‐POSS reacted with DET via thiol‐ene click reaction. The morphological results demonstrated that the droplet size increased with the higher content of DET due to the decrease of the crosslink between the acrylic groups in KH570‐POSS, which results in a less dense of polymer network and thus make the LC droplets easier to be driven in the electric filed. Then, a silica‐based nanoparticle KH570‐SiO2 modified by acrylic groups was introduced into the system. The results indicated that KH570‐SiO2 could replace partial KH570‐POSS to form the polymer network via thiol‐ene click reaction, which increased the compatible ability of SiO2 nanoparticles in the as‐made film. The contrast ratio was increased to 165 when there was nearly 5 wt% content of KH570‐SiO2. Besides, the driving voltage was reduced by almost 60% and the sample could be fully driven by 30 V which is lower than the safe voltage (36 V). This study opens a route for the preparation of commercial PDLC films by thiol‐ene click reaction, enabling the creation of low‐voltage‐driven smart windows.
In this paper, the viewing-angle-switching (VAS) films based on polymer dispersed liquid crystals (PDLCs) were fabricated and the optical properties of the VAS films were studied. The VAS films based on PDLC films can exhibit electrically switchable viewing-angle properties, and the films showed a narrow viewing angle under no electric field and presented a wide viewing angle under electric field (24 V). Furthermore, the smart anti-peeping liquid crystal display (LCD) containing the VAS film based on PDLC film also displayed the electrically switchable viewing-angle performance and the corresponding mechanism was discussed. The results demonstrated both the anti-peeping effect and electrically switchable view-angle effect of the as-fabricated VAS film, promising potential applications in intelligent optical films and novel information displays.
Accelerators are usually used to promote the thermal curing of epoxy/thiol mixtures and help to improve the preparation efficiency of epoxy-based polymer-dispersed liquid crystals (PDLCs). However, some accelerators may cause serious yellowing of the resultant products and their catalytic effect is also limited. In this work, a commercial compound with multi-amine structures is employed as the catalyst to prepare epoxy/thiol polymer-based PDLC films. The experimental results indicate that this catalyst not only does not lead to the serious yellowing problem of the PDLC films, but also can obviously improve the thermal curing rate between epoxy and thiol monomers in the PDLC system. The effects of curing times and curing temperatures on the electric-optical performance and the polymer morphology of the epoxy-based PDLCs are studied systematically. A flexible epoxy-based PDLC film, which can be completely driven by a voltage below the safety voltage (36 V), is also realized successfully.
This study demonstrates a simple and effective method for the fabrication of PDLC films with low driving voltage by applying a pre-orientation voltage during the photoinitiated polymerisation process. The results indicated that the electro-optical performances of PDLC films have been regulated successfully with the pre-orientation voltage method, in which the voltage could pre-orient the LC molecules and change the interface between the LC molecules and the polymer matrix. By manipulating the proportion of liquid crystal and polymer monomers and introducing ZnO whiskers as the dopant, the driving voltage was dramatically reduced and the sample could be fully driven by an electric field with a voltage of 32 V below the safe voltage. The results indicated that the polymer meshes were broadened by the addition of ZnO whiskers due to their excellent UV-absorbing properties that result in a slower polymerisation rate, while the contrast ratio could maintain a good value of 31. It has been found that both application of pre-orientation voltage and doped ZnO whiskers can optimise the electro-optical performance of the PDLC and reduce the driving voltage. Therefore, this experiment is expected to provide certain research value to fabricate the low-driven PDLC films for practical applications.
Polymer dispersed liquid crystal (PDLC) with polymer spacer columns were prepared by ultraviolet (UV) stepwise polymerization induced phase separation (PIPS) process. The influences of the formulation composition, such as the internal proportion of monomers and the liquid crystal (LC) content, on the morphologies of polymer spacer columns and electro-optical properties of PDLC films, were researched. Due to the difference in cross-linking density and polymerization rate, PDLC films exhibit different morphologies of polymer spacer columns and electro-optic properties. Based on the above, the CeO2 nanoparticles were doped into PDLC films. The influences of the doping content of CeO2 nanoparticles on the polymer network and electro-optical properties of PDLC were studied, and the EDS spectra were used to characterize the extent of dispersion of CeO2 nanoparticles in the polymer matrix. The results of the study showed that the contrast ratio (CR) of PDLC film increases from 10.9 to 27.3 at 0.8% CeO2 nanoparticles doping, while the saturation voltage (Vsat) decreases from 71.3 V to 47.9 V. Therefore, the work offers a new approach to the preparation of PDLC films with excellent comprehensive performance.
The electro-optical properties of the nanofibre/nanoparticle/polymer dispersed liquid crystals (PDLC) composite system were investigated. The nanofiber network was prepared by electrostatic spinning and introduced into the PDLC system as a carrier of nanoparticles. The effects of nanofibers and the doping of CeO2 nanoparticles on the PDLC system were investigated. It was found that the nanofibers had a significant impact on the response time of PDLC, while the nanoparticles could improve the contrast of PDLC very well. The fiber morphology, and the polymer mesh morphology, were characterised by SEM; the distribution of nanoparticles on the fiber was characterized by EDS. The experimental results show that nanofiber/nanoparticle doping can optimise the performance of PDLC and achieve low response time as well as high contrast in display applications.
Polymer dispersed liquid crystal (PDLC) films with polymer spacer columns were prepared by photomask stepwise polymerisation method. The effects of crosslinker content and liquid crystal (LC) content on the morphologies of polymer spacer columns and electro-optical properties of PDLC films were investigated. Due to the difference of crosslink density and system viscosity, the polymer spacer columns exhibit different morphologies and the electro-optical properties of PDLC films are also altered. Based on the above, the PVA coatings loaded with rare earth phosphor were introduced into PDLC films. The influences of rare earth phosphor concentration on the fluorescent and electro-optical properties of PDLC films were studied. Also, it was found that the UVaging resistance of PDLC films was enhanced because of the loading of rare earth phosphor absorbing UV light. Therefore, the successful preparation of PDLC films with polymer spacer columns and reversible fluorescence properties opens new possibilities for expanding the flexible display applications of PDLC.
In this paper, multi-field driven thermochromic films (TCF) were successfully prepared firstly. The upper layer is polymer-dispersed liquid crystal (PDLC) film prepared by polymerisation-induced phase separation (PIPS), and the lower layer is thermochromic liquid crystal microcapsule film prepared by centrifugal spin-coating method. The effects of diluent content, liquid crystal content and spin coating speed on the electro-optical properties and morphology of the thermochromic films were investigated by three sets of experiments secondly. Finally, multi-field driven multi-colour patterns were successfully prepared by film formation on conductive glass and flexible conductive films. Thermal ageing tests proved that the thermochromic films possess better service life, which provides a possibility for the wide application of liquid crystal films.
In this paper, polymer-dispersed liquid crystal (PDLC) films of acrylate polymer systems were fabricated by polymerization-induced phase separation. It was a complex process involving simultaneous curing and phase separation to form nematic microdomains. We also have examined the morphology of PDLC films, which show great potential for electro-optic applications. Here we used the adsorbability of white carbon black and loaded the photoinitiator (IRG651) in it to prepare PDLC films. Electrooptic devices based on the PDLC films were constructed by using indium-tin-oxide transparent electrodes, and the electro-optic responsivity was also explored.
A wavelength-selective, two-stage polymerization of acrylate-thiol for the formation of polymer dispersed liquid crystal (PDLC) films is developed. Via this method, the PDLC with microsphere polymer morphology was successfully prepared, which greatly improved the contrast ratio (CR) and adhesion without causing other performance degradation. The change of morphology from the porous to the microsphere structure could be controlled by adjusting the reaction time of the Michael addition reaction. Furthermore, the size and uniformity of the microsphere structure could be controlled by tuning the content of the free radical initiator. This work is expected to provide an effective method to control the microstructure of acrylate-thiol polymers in solution polymerization, and can possibly allow the fabrication of PDLCs with high CR and adhesion, which is of great significance in large, flexible displays, switchable windows, and light valves.
In this work, polymer dispersed liquid crystal (PDLC) films, with good electro-optical and adhesion properties, were obtained by phase separation induced through thiol-ene click chemistry. The effects of liquid crystal content, thiol content and thiol functionality on the electro-optical and adhesion properties of the PDLC films were systematically investigated. It is found that adding an appropriate proportion of high-molecular-weight polymer thiol, Capcure 3–800 as the curing agent can effectively decrease the saturation voltage of the PDLC films. And when the content of Capcure 3–800 is 5.5 wt.%, the PDLC films can show the optimised electro-optical performance. Moreover, the adhesion properties between the two pieces of substrates and the PDLC films are related to the density of the polymer network.
In this study, a method to regulate the visible transmittance in off-state (Toff) and the contrast ratio (CR) for polymer-dispersed liquid crystals (PDLCs) via electrospinning polyvinyl alcohol (PVA) nanofibers on ITO surfaces in liquid crystal cells was reported. The PDLC samples were prepared in the conventional liquid crystal cells and in the nanofiber cells. The results indicated that the sample in the nanofiber cell presented a lower Toff and a larger CR in comparison with the sample in the conventional cell if the samples were prepared under the same UV light.
In this paper, we report a simple and effective technique to improve the electro-optical performance of polymer-dispersed liquid crystal (PDLC) films via the implantation of polyhedral oligomeric silsesquioxane (POSS) microstructure into the polymer matrix. The measurement results of the electro-optical properties indicated that the electro-optical performances of PDLC films have been regulated successfully via the implantation of POSS microstructure into the polymer matrix. SEM results suggested that the size of polymer meshes was broadened with the increase in the content of POSS microstructure, while the static water contact angles of the polymers increased after the implantation of POSS microstructure. Because of the lower surface energy as well as the increased steric repulsions between liquid crystal molecules and polymer surfaces, the anchoring effect of the polymer matrix containing POSS microstructure was concluded to be weakened, which was the dominant factor in achieving the dramatic regulation effect of the electro-optical properties of the PDLC films. This work could provide a creative approach for the fabrication of low voltage-driven PDLC films for their practical applications.
Terpenes are a large class of hydrocarbon compounds derived from isoprene, which can be produced by plants or emitted by insects. Terpene alcohols contain double bonds that may affect the addition polymerization, and hydroxyl groups that can promote the network crosslinking density during the formation of polymer-dispersed liquid-crystal (PDLC) composite films. In this work, effects of different terpene alcohol dopant on the morphology and electro-optical properties of PDLC films are systematically investigated. It is found that the appropriate dopant amount of geraniol (GOL) has the superior effect to improve the electro-optical performance. The optimum addition amount is at 3 wt%, where the transmittance vs. voltage curve gets steeper, the driving voltage of PDLC film reduces from 74 to 32 V and meanwhile the contrast ratio maintains at a relatively high level (123). Thus, this work not only broadens the application scope of the natural spices but also provides a new strategy to reduce the driving voltage of PDLC films for practical applications.
In this paper, rare earth oxide CeO2 nanoparticle-doped polymer dispersed liquid crystal (PDLC) films were prepared by the polymerisation induced phase separation (PIPS) process. The effects of the crosslinking agent TMPTA content, liquid crystal (LC) content and the CeO2 content on the polymer structures and electro-optical properties of the as-made PDLC films were investigated systematically. It has been found that by changing the TMPTA content, LC content and doping a small amount of CeO2 nanoparticles, the electro-optical properties of PDLC films can be optimised. After adding a low concentration of CeO2 nanoparticles, the electro-optical properties of the PDLC film have been well optimised: after adding 0.8% CeO2, Vth decreased from 26.6 V to 16.8 V, and CR increased from 89.5 to 137.6. Consequently, this work offers a meaningful approach to optimise the electro-optical properties of PDLC films, which indeed can form a wonderful footstone for the wide application of PDLC.
Polymer dispersed liquid crystal (PDLC) is a striking material and has an enormous role in electro-optic display devices. In this paper, nanoparticles (NPs) doped PDLCs were prepared from a mixture of UV curable liquid polymer, nematic liquid crystal (LC), and ZnO NPs by the polymerisation induced phase separation (PIPS) process. The morphological and electro-optical properties of PDLC with various diluent contents, liquid crystal contents, and nanomaterial doping were investigated comprehensively, and a morphological study shows that multiple factors jointly affected the droplet size, thereby altering a variety of properties including driving voltage and contrast. Compared with pure PDLC, the NPs doped PDLC reduces the threshold voltage from 39.5 V to 21.1 V and the driving voltage from 71.0 V to 45.0 V, which greatly improves the electro-optical performance of PDLC with a few sacrifices in contrast. This opens new possibilities for achieving a wide range of applications for energy-efficient PDLC.
Two kinds of liquid crystalline monomers with different mesogenic units grafted graphene oxide (M2-g-GO and M4-g-GO) were successfully synthesised to enhance thermal and mechanical properties of nanocomposites, owing to the improvement of dispersion and interface interaction between the fillers (M2-g-GO and M4-g-GO) and the epoxy matrix (DGEBA, E-51). And the structures and comprehensive properties of M2-g-GO, M4-g-GO, and their nanocomposites were analysed using FTIR, SEM, XRD, Raman, DSC, TGA, and POM tests. Compared with the neat epoxy and 3 wt% GO/epoxy composites, thermal properties of M2-g-GO/epoxy and M4-g-GO/epoxy nanocomposites were obviously improved, meanwhile, mechanical property measures prove M2-g-GO/epoxy and M4-g-GO/epoxy nanocomposites that exhibit higher impact strength, flexural strength and modulus. For example, the Td5% of epoxy nanocomposites with 1, 2, 3, and 4 wt% M2-g-GO were around 385°C, which was about 15°C larger than that of the neat epoxy. The Td5% of epoxy nanocomposites with 2 and 3 wt% M4-g-GO were 383°C and 388°C, respectively, which was about 13°C and 18°C larger than that of the neat epoxy. In a word, the M2-g-GO and M4-g-GO as fillers exhibit underlying feasibility to improve thermal and mechanical properties of nanocomposites.
A series of polymer dispersed liquid crystal (PDLC) films with thiol-isocyanate-ene ternary network were successfully prepared by nucleophile-initiated thiol-ene click reaction and thiol-isocyanate coupling reaction. The effects of isocyanate content, reaction temperature and liquid crystal (LC) content on the electro-optical properties of PDLC were investigated. It is found that with the decrease of isocyanate content or reaction temperature, there are lower voltages as well as higher transmittance. When the LC content increases, the polymer structure changes and the phase inversion occurs. The results indicate that PDLC films with good electro-optical properties could be prepared by nucleophile-initiated thiol-ene click reaction and thiol-isocyanate coupling reaction, providing a new idea for PDLC preparation.
Polymer-dispersed liquid crystal (PDLC) films doped with barium titanate (BTO) nanoparticles were prepared by nucleophile-initiated thiol-ene click reaction. The effects of BTO content and reaction temperature on the electro-optical properties of PDLC were explored. It is found that the size of liquid crystal droplets is bigger when the amount of BTO increases. As the BTO content increases, the transmittance first rises and then falls, while the voltage that drives the liquid crystal orientation first drops and then rises. The electro-optical properties of PDLC doped with BTO show a similar changing trend with the change of reaction temperature. However, when the reaction temperature becomes higher, the influence of BTO on the electro-optical properties of PDLC becomes greater. Doping BTO helps to obtain PDLC films with good electro-optical properties.
As a typical class of electrically light-transmittance-switchable (ELTS) composites materials, polymer dispersed liquid crystal (PDLC) films have been widely used in displays, smart windows, light shutters, etc. However, the commercialised PDLC film still requires a comparatively high voltage to maintain its transparent state, leading to huge power consumption and even a potential safety risk. In this regard, we proposed a ‘heat followed UV’ stepwise polymerisation strategy for preparing a kind of ELTS film with a low driving voltage (~20.7 V) through constructing a coexistent system of polymer dispersed and polymer stabilised liquid crystal (PD&SLC). In this new PD&SLC system, vertically orientated polymer networks were formed within LC domains to induce the vertical alignment of LC, thereby reducing the driving voltage. Also, the as-made PD&SLC film exhibited good flexibility due to the high content of polymer. Moreover, the effects of the liquid crystalline polymerisable monomers content on the polymer morphologies as well as the electro-optical properties of the as-made PD&SLC films were elaborately investigated.
In this study, optical diffusers based on epoxy resin/thiol/nematic liquid crystal composites were prepared using different curing temperatures, curing times and curing agents (thiols). Additionally, the effects of the curing temperatures, the curing times and the thiols on the polymer ball microstructures and the optical properties of optical diffusers were investigated systematically. For applications of optical diffusers, the optimized curing temperature has been achieved by combining the high transmittance and high haze. A novel optical diffuser with ultrahigh transmittance (>94.0%), ultrahigh haze (>94.0%) and excellent diffusing ability has been obtained, of which the polymer morphology is single polymer micro-balls without aggregation.
To study effects of the crosslinking agent/diluents/thiol on morphology of the polymer matrix and the electro-optical properties of polymer-dispersed liquid crystal (PDLC) films, samples were prepared by ultraviolet (UV)-initiated polymerisation. Due to the interaction between thiol–acrylate reaction and acrylate monomers polymerisation, the sample compositions were the foremost determinant to the microstructures which in turn played an essential role on the electro-optical properties of the PDLC films. With the increasing content of the crosslinking agent, the LC droplet size decreased, while the thiol had a contrary effect on the LC droplet size. It was demonstrated that the superior properties of the low-driven voltage (37.2 V), the high contrast ratio (148.2), the short response time (14.9 ms) and the high saturation transmittance (86.6%) could benefit from a novel microstructure which had a dense surface and meshes with microspheres attached. It was of great significance for the optimisation and the potential applications of the PDLC films.
Ultrasensitive flexible sensors with multi-sensing functions are required for various applications in flexible electronics era. Here we demonstrate flexible polymer-dispersed liquid crystal (PDLC)-integrated-organic field-effect transistors (OFETs) (PDLC-i-OFETs), which sensitively respond to various stimulations including weak gas (air) flow, direct physical touch, light, and heat. The flexible PDLC-i-OFETs were fabricated by spin-coating the poly(methyl methacrylate) (PMMA)-dispersed 4,4’-pentyl-cyanobiphenyl (5CB) layers on the poly(3-hexylthiophene) (P3HT) channel layers of OFETs with 200 μm-thick poly(ethylene naphthalate) (PEN) substrates. The flexible PDLC-i-OFET devices could sense very weak nitrogen gas flow (0.3 sccm), which cannot be felt by human skins, and stably responded to direct physical touches (0.6~4.8 g load). In addition, the present devices showed very sensitive photoresponses to a visible light and exhibited excellent heat-sensing characteristics at a temperature of 25~70 °C. In particular, the present flexible PDLC-i-OFET devices could sense two different stimulations at the same time, indicative of promising multi-sensing capabilities.
Polymer-dispersed liquid crystals (PDLCs), composed of micro-sized liquid crystal domains embedded in a continuous polymer matrix, are an important class of materials that hold promise for many opto-electronic applications. In this review, based on an analysis of research publications appearing between 2006 and 2012, preparation of conventional PDLCs and nanoparticles-doped PDLCs, their properties, and their application for opto-electronic devices are described.
Polymer dispersed liquid crystal (PDLC) films were prepared by polymerization-induced phase separation method with nematic LC content as low as 40 wt%, and the electro-optical properties were carefully investigated. To accomplish this, the structure of multi-functional curable epoxy monomers with different composition feed ratios and the weight percentages of the two groups were examined in this study. The combined effects of heat-curable monomers’ structure on the conspicuous morphology of polymer network of PDLC films formed small holes and suitably distributed coin-like networks in both groups A and B, respectively. The detailed characteristics and morphology of polymer network of PDLC films were analyzed by employing liquid crystal device parameter tester, UV-Vis-NIR spectrophotometer and scanning electron microscope. Meanwhile, the enhanced curing temperature effects on the alkyl chain length, short flexible chain length, and rigid chain segment containing epoxy monomers structure on the increasing morphology of polymer network as well as electro-optical properties of PDLC films were also studied. It was found that the LC domain size of the polymer network could be regulated by adjusting the structure and composition ratio of curable epoxy monomers, and then the electro-optics of the PDLC films could be optimized, which is beneficial for decreasing the total LC content in PDLC devices.
Polymer network liquid crystals (PNLC) were prepared from UV curable polyurethane acrylate (PUA) and a nematic liquid crystal mixture (E7) at a fixed film composition of 1.5/8.5 (polymer/LC) by weight. The polymer networks were obtained upon curing the reactive mixture of hydroxyethyl acrylate (HEA) terminated polyurethane prepolymer, monoacrylate and triacrylate (4/3/3).The effects of prepolymer molecular structure viz., length of polyurethane (PU) segment, molecular weight (Mn) and functionality (f) of polyol(PPG), and type of hard segment were studied in terms of morphology, voltage–transmittance relationship, off-state transmittance–temperature relationship, and thermal properties of the films. Aromatic diisocyanate (TDI) segment of PU showed greater chemical affinity with aromatic LC molecules and gave smaller domain size resulting in higher threshold (V10) and driving (V90) voltages. The increase in prepolymer molecular weight gave larger polymer–LC phase separation and decreased V10 as well as V90, together with smaller nematic–isotropic transition temperature depression. Increase in functionality and decrease in Mn of PPG gave smaller domain size, smaller decay time, greater V10 and V90, longer rise time, and greater depression of TNI in film.
In this paper, we present a new type of parallax barrier, for switchable 2D/3D display vision, obtained by using a Reverse Mode Polymer Dispersed Liquid Crystal display (RV-PDLC). The parallax barrier was prepared by sandwiching the RV-PDLC film between two ITO conductive glass supports. Strips of ITO were removed from one of the supports, in order to alternate conductive to not conductive strips. In this way the RV-PDLC films could be electrically switched in a parallax barrier able to turn a 2D image in a 3D one. The RV-PDLC barrier was obtained in tree steps: (1) a nematic diacrylate monomer (NDM) was dissolved in a nematic liquid crystal (NLC) in the presence of a small quantity of radical polymerisation initiator; (2) the mixture was homeotropically aligned between the glass conductive supports, previously treated with an aligning chemical; (3) the liquid crystal monomer was polymerised by UV curing. A 2D/3D switchable device was finally obtained by coupling the parallax barrier with a 2D tablet display.
Nucleophile-initiated thiol-ene click reaction is a highly novel and efficient method of preparing polymer-dispersed liquid crystal (PDLC) films. The effects of thiol monomers on the electro-optical properties of PDLC films prepared by nucleophile-initiated thiol-ene click reaction were investigated in this work. The thiol monomers were dithiol, trithiol, tetrathiol or their mixture. It was found that the increase of functionality could lead to the increase of threshold voltage and saturation voltage and the decrease of off-state transmittance. The influence of reaction temperature was also investigated. The results indicated that functionality and reaction temperature had combined effects on the electro-optical properties of PDLC films.
Nanoparticle dispersions in liquid crystalline materials at low concentrations allow both investigating the formation of defects in liquid crystal (LC) and enhancing the light-scattering properties of LC optical devices. Reverse mode LC dispersions are LC devices, which look like transparent in their OFF state, when no electric field is applied, and opaque in their ON state. In this paper, a new reverse mode device, formed by a dispersion of a LC mixture in a silica nanoparticle crosslinked network, is presented. The morphology and the electro-optical properties of these silica nanoparticle/LC composites were investigated for two different LC mixtures with a negative dielectric anisotropy. The observed transmittances and relaxation times were found to depend strongly on the silica amount and chemical–physical properties of LC used in the sample preparation.
This work provides a roll-to-roll processed flexible multi-responsive smart film containing tungsten bronze nanorods and phase-seperated liquid crystals-polymer, which can reversibly control the passage of visible light via temperature, electric...
Dye-doped polymer-dispersed liquid crystal (PDLC) is an attractive material for application in smart windows. We report a novel dye-doped PDLC system with low driving voltage (Vdr) fabricated by nucleophile-initiated thiol-ene click reaction for the first time. The Vdr decreased significantly with the addition of monofunctional monomer. The influences of reaction temperature on the electro-optical properties of PDLC are also investigated in this work.
Morphologies of polymer networks are playing a key role in affecting the electro-optical (E-O) properties of a polymer dispersed liquid crystals (PDLCs) material. In this paper, the relationships between the polymer morphologies and the E-O properties of PDLCs are investigated in an unexplored thermally curing system based on epoxy/low-molar-mass-mercaptan. The polymer morphologies in this curing system can be regulated from polymer beads to porous polymer matrix by varying the liquid crystals (LCs) contents, chemical structures of epoxy monomers, and functionality of thiol hardeners. Interestingly, great improvements of E-O properties occur as the polymer structures change from polymer beads to porous frameworks, even though with an abnormal low LCs contents. Eventually, the mechanism for the formation of different polymer structures are illustrated and the relationships between the E-O properties and polymer structures are established. This work provides new insights in morphology control and optimizing the E-O properties in thermally cured PDLCs films.
Polymer-dispersed liquid crystal (PDLC) and polymer-stabilized liquid crystal (PSLC) systems are the two primary distinct systems in the field of liquid crystal (LC) technology, and they are differentiated by their unique microstructures. Here, we present a novel coexistent system of polymer-dispersed and polymer-stabilized liquid crystals (PD&SLCs), which forms a homeotropically aligned polymer network (HAPN) within the LC droplets after a microphase separation between the LC and polymer matrix and combines the advantages of both the PDLC and PSLC systems. Then, we prepare a novel thermally light-transmittance-controllable (TLTC) film from the PD&SLC system, where the transmittance can be reversibly changed through thermal control from a transparent to a light-scattering state. The film also combines the advantageous features of flexibility and a potential for large-scale manufacturing, and it shows significant promise in future applications from smart windows to temperature sensors.
Holographic photopolymer composites have garnered a great deal of interest in recent decades, not only because of their advantageous light sensitivity, but also due to their attractive capabilities of realizing high capacity three-dimensional (3D) data storage that is long-term stable within two-dimensional (2D) thin films. For achieving high performance holographic photopolymer composites, it is of critical importance to implement precisely spatiotemporal control over the photopolymerization kinetics and gelation during holographic recording. Though a monochromatic blue light photoinitibitor has been demonstrated to be useful for improving the holographic performance, it is impractical to be employed for constructing holograms under green light due to the severe restriction of the First Law of Photochemistry, while holography under green light is highly desirable considering the relatively low cost of laser source and high tolerance to ambient vibration for image reconstruction. Herein, we disclose the concurrent photoinitiation and inhibition functions of the rose bengal (RB)/N-phenylglycine (NPG) system upon green light illumination, which result in significant enhancement of the diffraction efficiency of holographic polymer dispersed liquid crystal (HPDLC) gratings from zero up to 87.6±1.3%, with an augmentation of the RB concentration from 0.06 to 9.41 ×10-3 mol L-1. Interesting, no detectable variation of the (ϕ^(1⁄2) k_p)⁄(k_t^(1⁄2) ), which reflects the initiation efficiency and kinetic constants, is given when increasing the RB concentration. The radical inhibition by RBH• is believed to account for the greatly improved phase separation and enhanced diffraction efficiency, through shortening the weight-average polymer chain length and subsequently delaying the photopolymerization gelation. The reconstructed colored 3D images that are easily identifiable to the naked eye under white light demonstrate great potential to be applied for advanced anticounterfeiting.
Polymer dispersed liquid crystal (PDLC) films with the size gradient of the LC droplets were prepared based on the epoxy/acrylate hybrid polymer matrix. The ultraviolet (UV) intensity gradient was induced by the UV-absorbing dye over the thickness of the samples. Taking advantage of the difference between the epoxy monomers and acrylate monomers in polymerisation rates and the UV intensity gradient, the gradient distribution of the LC droplet size was formed in PDLC films. The effect of the size gradient of the LC droplets on the electro-optical and the light-scattering properties of PDLC films was investigated. The results showed that due to the size gradient distribution of the LC droplets, PDLC films could exhibit the strong light scattering in the UV-visible-near infrared (VIS-NIR) region. Consequently, it provides a potential approach for modulating NIR light transmittance.
Polymer dispersed liquid crystal (PDLC) films can be prepared by the thermal polymerization-induced phase separation method, but the curing time is more than 7 hours, which leads to high energy consumption. In this study, the effects of triethylamine as a catalyst on the curing process and the electro-optical properties of PDLC films have been studied. Furthermore, PDLC films are prepared with different amounts of triethylamine to investigate the best ratio of the catalyst. It was found that using triethylamine as the catalyst could shorten the curing time from 7 hours to 1 hour, and the film with 5 wt% of triethylamine has the best electro-optical properties, and is as good as one prepared by the traditional method. All in all, employing triethylamine as a catalyst has been proved to be an energy efficient way of making PDLC films.
In this paper, polymer dispersed liquid crystals (PDLC) films with LC content as low as 40 wt% were prepared, and the electro-optical properties were carefully investigated. To accomplish this, different (meth)acrylate copolymerizaiton monomers have been used. The electro-optical properties and morphologies of the PDLC films were strongly influenced by the chemical structure of copolymerization monomers (hydroxypropyl methacrylate (HPMA), glycidyl methacrylate, hydroxypropyl acrylate) and their feed ratio. Lower driven voltage and higher contrast ratio were achieved when the PDLC films showed a morphology with suitably LC domain size. At high HPMA content, a thin polymer film was formed on the surface of PDLC samples, which is beneficial to decrease the total LC content in PDLC devices. Copyright © 2013 John Wiley & Sons, Ltd.
Phase-separation behavior and electro-optical properties were investigated for the composite films consisting of the three dimensional polymer network and low molecular weight liquid crystals (LCs) embedded in the network. The composite films were prepared by a solvent cast method from uniform solutions. It was confirmed from solvent-evaporation time dependence of light scattering that the phase-separated structure of the polymer and LC components was formed via the spinodal decomposition. The resultant domain size of LC in the composite film was controlled by regulating the solvent evaporation rate. The electro-optical properties based on the electric field controlled-light scattering of the composite film was strongly dependent on the LC domain size. With increasing the LC domain size in the composite film, the rise and decay response speeds were increased and decreased, respectively, due to a decrease in an interfacial interaction between the LC and polymer phases.
Polymer dispersed liquid crystal (PDLC) films were formed by thermal polymerisation-induced phase separation in epoxy monomer/polyamine/liquid crystal (LC) mixtures. The effects of the epoxy monomer functionality on the morphology of polymer network, electro-optical and peel strength of PDLC films were studied. As the functionality of epoxy monomer increased, the LC domain size of PDLC films decreased resulting in higher threshold voltage and saturation voltage, longer rise time, shorter decay time, meanwhile the peel strength of PDLC films enhanced.
Epoxy polymer dispersed liquid crystal (PDLC) films based on ethyleneglycol diglycidyl ether (EGDE)/nematic liquid crystal (SLC1717) composites were prepared by ultraviolet-initiated cationic polymerisation using triphenylsulfonium hexafluoroantimonate (Ar3SSbF6 ) as the photoinitiator. The effect of the composition of the mixture and curing temperature on the microstructure of the polymer network and the electro-optical properties was investigated. It was found that the liquid crystal (LC) domain size of the polymer network could be regulated by adjusting the LC content and the curing temperature. The electro-optical properties of PDLC films could then be optimised.
A (polymer network/nematic liquid crystal/chiral dopant) composite exhibiting a chiral nematic (N-*) phase at room temperature has been developed. Because the helical twisting power of the chiral dopant increases with increasing temperature and the polymer network affects the molecular rearrangement of the liquid crystal, the bandwidth of the selective reflection spectrum of the N-* phase becomes wider and narrower reversibly with increasing and decreasing temperature, respectively. (C) 2003 American Institute of Physics.
We have investigated the morphology and electro-optical properties of reverse mode polymer dispersed liquid crystals as a function of liquid crystal loading. Reverse mode shutters have been obtained by a polymerization-induced phase separation of mixtures, consisting of a liquid crystalline monomer and a non-reactive nematic liquid crystal, placed between rough conductive surfaces. Such surfaces are able to keep the photopolymerizable mixtures homeotropically aligned without the use of any aligning polymer substrate. OFF state transmittances are always larger than 80% and the switching fields decrease if the non-reactive liquid crystal percentage is increased. Both rise and decay times are always lower than 10 ms. The electro-optical properties have been related to the sample morphology and a simple mode is proposed.
Polymer films containing dispersions of liquid crystal microdroplets have considerable potential for use in displays and other light control devices. These polymer-dispersed liquid crystal (PDLC) films operate by electric field control of light scattering, rather than by polarization control as in the case of twisted nematic systems. The scattering characteristics of the PDLC films are determined by the refractive indices of the polymer and liquid crystal and by the size of the microdroplets. We have found that it is possible to regulate the microdroplet size by controlling the droplet formation rate (i.e. the cure kinetics of the film). Using calorimetry and scanning electron microscopy, we determined the influence of cure kinetics on microdroplet size for epoxy-based PDLCs. We found that droplet size increased with increasing cure time constant. However, the relationship changed as cure temperature was varied, perhaps as a result of competing cure processes. We also determined the phase behaviour of the epoxy-based PDLCs. The liquid crystal acted as a plasticizer, depressing the glass transition temperature of the PDLC samples slightly below that of the pure epoxy. The temperature and enthalpy of the nematic to isotropic transition of the liquid crystal material in the microdroplets were both functions of cure temperature. From the transition enthalpy it was possible to estimate a, the fraction of liquid crystal contained in the droplets; we found that a decreased with increasing cure temperature, presumably as a result of greater liquid crystal solubility in the epoxy matrix at higher temperatures.
This paper deals with the changes on switching fields and response times of a PDLC system when small percentages of the thermoplastic polymer are replaced by the same quantity of monomer. An improving of the electro-optical properties, i.e. a reduction in driving voltages and rise times, is obtained when about 10 weight % of polymer is replaced by monomers with a high solubility in liquid crystal droplets. Morphology changes are observed for substitutions larger than 20 weight %.
This paper describes a new class of light control films consisting of submicron liquid crystal droplets dispersed in ultraviolct-cured polymer matrices. These films, which can respond optically to both applied electric fields and temperature changes, are potentially useful for displays and light shutters. The optical performance of these films depends on a variety of structural, electro-optical and thermal properties. This report describes scanning electron microscope studies of film structure, measurements of voltage dependent film transmittance and light scattering, and calorimetric studies which indicate that microdroplet formation in the films occurs as a result of phase separation which takes place during the cure process.
Polymer-dispersed liquid crystal (PDLC) films were prepared from thermal polymerisation-induced phase separation in heat-curable monomers/nematic liquid crystal (LC) mixtures. For PDLCs with a certain amount of LCs, the microstructure and the refractive index of polymer networks could be influenced by the relative content of epoxy monomers, owing to their different chemical structures. The effect of these factors on the electro-optic properties of films was also investigated.
Nematic emulsions and polymer dispersed liquid crystals (PDLCs) are composite materials formed by micron-sized droplets of mesogenic molecules, which are embedded in fluid or solid matrices, respectively. Both systems can be switched from an opaque to a trasparent state by application of a suitable electric field. The main differences reside in the switching field and relaxation time values. Polymer dispersed liquid crystal films are generally characterized by higher switching fields and faster relaxation times. On the contrary, nematic emulsions show lower switching fields and longer decay times. Recently, we have obtained PDLC films by photopolymerization of nematic emulsions. Such films keep the advantages of conventional PDLCs, but have the opportunity of a finer control of liquid crystal droplet size. In this article we have focused our attention to the polymerization process, which converts nematic emulsions in polymer dispersed liquid crystals. In particular, we have investigated the electro-optical changes induced in nematic emulsions by the polymerization of the fluid matrix as a function of the irradiation time. A progressive increase in the switching fields is observed, while the decay times decrease in a significant way. Such results can be explained by assuming a decrease of liquid crystal droplet viscosity and an increase of boundary restoring forces acting on liquid crystal due to the polymerization process. © 2002 American Institute of Physics.
Polymer‐dispersed liquid crystal (PDLC) films operating in reverse mode are transparent electro‐optical devices, which can be turned into an opaque state by application of a suitable electric field. The effect was investigated of different UV powers, used during the polymerization process, on the electro‐optical and morphology properties of PDLCs, working in reverse mode operation. Films were obtained by UV polymerization of mixtures of a low molecular weight nematic liquid crystal and a photopolymerizable liquid crystal monomer, homeotropically aligned by rough conductive surfaces. The electro‐optical and morphology properties of samples were related to the polymerization conditions. Samples polymerized by lower UV powers exhibited “polymer ball” morphology and an electro‐optical response due to the liquid crystal director reorientation, whereas samples obtained at higher UV powers showed a “Swiss cheese” morphology and an electro‐optical response due to dynamic scattering. In addition, we observed by conductivity and IR measurements that UV exposure induces a degradation of the nematic liquid crystal.
We present evidence that polymer dispersed liquid crystals (PDLC) with high on state transparency can be obtained following two conditions: the solubility of the liquid crystal within the pre-polymerized resin should be low and the resin itself should not be a mixture but a single chemical species. We prepared PDLC's according to such provisions and measured high on state transparencies. Experimental data have also been interpreted in terms of existing light transmission theories. Results indicate that the high transparency is associated with a high degree of homogeneity of the polymer matrix.
The electro-optical dynamics, hysteresis effects, and microscopic structure of polymer-dispersed films of nematic liquid crystal are probed in order to gain insight into the operation of this new class of liquid crystal light valves. In tracking the rise and decay response times of these devices, it appears that there are both ‘fast’ (0·1·l.0ms) and ‘slow’ (10-1000ms) processes that occur in the film. A model is proposed which explains these results, in which the reorientation of the nematic droplets takes place in two stages: a fast reorientation by the nematic within the bulk of the droplet, followed by a slower rotation of the nematic nearer the droplet surface (including the point disclinations). This model agrees with a similar proposal made by Doane et al in a previous study of related films. This model is also used to explain both the behaviour of the films in response to short voltage pulses and hysteresis effects present in the film. The response time of these films can be tailored by adjusting the droplet size within the film, as well as the choice of the drive waveform and voltage. The non-spherical shape of the nematic droplets in the film is proposed to be the most important factor controlling the electro-optic properties of these devices. Data is presented which shows that the more distorted the nematic cavity, the more quickly the film decays, and the higher the field required for reorientation. It is proposed that the minimization of deformation energy of the nematic in a non-sperical cavity is the primary driving force for relaxation in these films, rather than previously postulated ‘surface interactions’
An overview of polymer dispersed liquid crystal (PDLC) materials, their physical properties, and potential applications in the optic and electrooptic industry is presented. These optoelectronic materials have unique properties which are expected to expand liquid crystal technology into new display and light shutter applications. Recent research by small and large industrial and university laboratories on device physics and chemistry has provided substantial progress towards the commercialization of these materials. Work to date on such features as response times, switching voltage, and contrast as well as material preparation procedures and unique optical characteristics will be reviewed. These materials are also of interest to basic physics because of new kinds of physical phenomena brought on by the confinement of a nematic liquid crystal to small submicron-size droplets. Enhanced surface-to-volume ratio, large nematic deformations and problems associated with molecular anchoring at a polymer wall allow for new studies of surface mediated phenomena.
The response times and operating voltages of light shutters formed from polymer dispersed liquid crystals (PDLCs) have been studied experimentally and the results compared with calculations based on non-sperhically shaped nematic droplet models. The experiments were performed on light shutters with elongated and uniformly aligned droplets where the relaxation time and voltage response were measured. It is shown that the droplet shape can be a dominant factor, particularly for the relaxation time, and the data are compared with equations derived in terms of the aspect ratio of the droplet l = a/b, where a and b are the lengths of the semi-major and semi-minor axes, respectively, of the elongated droplet. It is further demonstrated that the electric field inside the droplet can be considerably smaller than the applied field, due to the conductivity and dielectric properties of the polymer and liquid crystal materials. These data are used to obtain values for the ratio of the conductivities of the polymer binder and liquid crystal droplet, as well as the anisotropy of the conductivity in the liquid crystal.
Polymer‐dispersed liquid crystal (PDLC) films were prepared by dual UV and heat curing from UV‐ and heat‐curable monomers. For PDLC films with a certain amount of the UV‐ and heat‐curable monomers, the mesh size (the size of LC domains) of the polymer network could be adjusted by adjusting the relative content of the two types of monomers, enabling the electro‐optical properties of the film to be optimised.
Polymer dispersed liquid crystal (PDLC) films were prepared by ultraviolet (UV) radiation induced polymerization of photopolymerizable monomers in nematic liquid crystal (LC)/monomers composites. The effect of a photopolymerizable monomer 2-hydroxyethylmethacrylate (HEMA) containing a hydrogen bond on the transmittance of the PDLC films in the wavelength region of 300−2500 nm was investigated by UV−visible (VIS)−near-infrared (NIR) spectrophotometers, scanning electron microscopy, and an infrared spectrometer. It was surprisingly found that the addition of a small amount of HEMA monomer containing a hydrogen bond could dramatically decrease the off-state transmittance in the NIR region partially due to the formation of a compact and thick-border polymer network. Then the conceivable mechanism concerning the effect of HEMA monomers containing a hydrogen bond on the microstructure of the polymer network was investigated experimentally and theoretically. This provided a potential approach for the electrically controlled modulation of NIR radiation.
Experimental studies on the morphology and thermal and electrooptical behavior of PDLC films consisting droplets of nematic LC (6CB) up to a 40% weight fraction in a polyester resin matrix crosslinked by diacrylane are presented. The PDLC samples were prepared by LC separation from a solution in a UV-polymerizing oligoester resin between TIO-coated glass plates. The electrooptical and switching results depending on the UV curing time of the polyester matrix are shown. The curing process of the polymer binder leading to phase separation of the LC causes increasing polymer stiffness affected by the boundary condition on the LC droplets. The reorientational dynamics of LC in PDLC, which reflects the character of switching curves for varying voltage and with progressing curing (varying from 40 s to 15 min), is discussed on the basis of the kinetic analysis of light scattering by the system. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 455–463, 1999
The analysis of the chemorheological behavior of an epoxy prepolymer based on a diglycidylether of bisphenol-A (DGEBA) with a liquid aromatic diamine (DETDA 80) as a hardener was performed by combining the data obtained from Differential Scanning Calorimetry (DSC) with rheological measurements. The kinetics of the crosslinking reaction was analyzed at conventional injection temperatures varying from 100 to 150°C as experienced during a Resin Transfer Molding (RTM) process. A phenomenological kinetic model able to describe the cure behavior of the DGEBA/DETDA 80 system during processing is proposed. Rheological properties of this low reactive epoxy system were also measured to follow the cure evolution at the same temperatures as the mold-filling process. An empirical model correlating the resin viscosity with temperature and the extent of reaction was obtained to carry out later a simulation of the RTM process and to prepare advanced composites. Predictions of the viscosity changes were found to be in good agreement with the experimental data at low extents of cure, i.e., in the period of time required for the mold-filling stage in RTM process. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4228–4237, 2006
Polymer dispersed liquid crystal (PDLC) films were prepared by photopolymerization of liquid crystal (LC)/polymerizable monomers/photoinitiator composites. The effects of the structures of the polymerizable monomers on the electro-optical properties of PDLC films were investigated. It was found that the length of the molecular chain and the rigidity and flexibility of molecules influenced the structure of the polymer network in the PDLC films somewhat, and then affected the electro-optical properties of the composites accordingly. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1369–1375, 2008
The electro-optic and thermo-optic properties of polymer dispersed liquid crystal (PDLC) films have been investigated. The effects of applied voltage and temperature on liquid crystal droplet morphology and its transmission characteristics were studied. Threshold voltage (Vth) and optical transmission increases with increasing temperature. It may be due to the reduction in effective voltage drop across the liquid crystal droplets in the composite matrix. The liquid crystal droplet size was found to vary in the range of 5–28 μm.
Optical devices based on polymer-dispersed liquid crystal (PDLC) thin films derive their functional properties from the electric-field-induced reorientation of (sub)micrometer-sized polymer-dispersed liquid crystal (LC) droplets. In these materials, the LC reorientation dynamics are strongly dependent on droplet size and shape, as well as polymer/LC interfacial interactions. The dynamics also vary spatially within individual droplets. This Account describes studies of individual PDLC droplets and their field-induced dynamics by high-resolution near-field scanning optical microscopy (NSOM) and multiphoton-excited fluorescence microscopy (MPEFM). Included are studies of native ("pure") PDLCs and those doped with ionic compounds and dyes; the latter are used in sophisticated photorefractive materials.