Electro-Optical Properties of Reverse-Mode Films of Planar Aligned Polymer-Dispersed Liquid Crystal

We prepared polymer-dispersed liquid crystal (PDLC) by using nematic liquid crystal 5CB and biphenyl methacrylate type monomer with alkoxy terminal group. Reverse mode properties were achieved by rubbing the glass substrates of the PDLC films in parallel directions so that polymer and liquid crystal molecules are plane-parallel aligned in the sample cell. The monomers with various carbon number (n ≤ 4) of alkoxy group were employed for the formation of the PDLC films. The films were cured by varying monomer concentration and UV curing time. The resulting liquid crystal/polymer composites showed significantly improved driving voltage and contrast ratio.     

Preparation and Characterization of Polymer-dispersed Liquid Crystal Films Using Poly(bornyl methacrylate)

Chiral (+)-bornyl methacrylate and achiral (±)-bornyl methacrylate were synthesized from (+)-camphor and (±)-camphor, respectively. To investigate the influence of the steric environment of the chiral polymers on the electro-optical characteristics of polymer-dispersed liquid crystal (PDLC) films, a commercially available positive nematic liquid crystal E7 was dispersed in the chiral and achiral polymer matrices. The electro-optical characteristics and the microstructures of the PDLC films with chiral and achiral racemized polymers were investigated. It was found that PDLC films with chiral polymers have relatively high contrast ratio and fast falling speed, but there exists a transient damping response when a square pulse of 20ms and 60V is applied. Two distinct morphologies were observed: (1) a ‘continual channel’ with the chiral polymer matrix, and (2) an ‘isolated ball’ with the achiral racemized polymer matrix. The reversible turbid and transparent changes with an applied a.c. electric field were also investigated. © of SCI.     

An investigation into the morphology and electro‐optical properties of 2‐hydroxy ethyl methacrylate polymer dispersed liquid crystals

Polymer dispersed liquid crystal (PDLC) films are fabricated using E7 liquid crystals, tetraethylene glycol diacrylate (TeGDA) crosslinking agent, and 0–66.49 mol % 2‐hydroxy ethyl methacrylate (HEMA). The effects of different levels of HEMA addition on the microstructure and electro‐optical properties of the PDLC samples are examined using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and UV‐Vis spectroscopy, respectively. The results show that the refractive index of the PDLC films is insensitive to the level of HEMA addition. However, an increasing HEMA content improves the degree of phase separation during the polymerization process and increases the size and uniformity of the liquid crystal domain. As a result, the electro‐optical properties of the PDLC films are significantly improved as the level of HEMA addition is increased. Overall, the results show that a PDLC comprising 40 wt % E7 liquid crystals, 33.51 mol % TeGDA and 66.49 mol % HEMA has a high contrast ratio (13 : 1) and a low driving voltage (10 V) and is therefore an ideal candidate for a wide variety of intelligent photoelectric applications. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Comparative study on the electrooptical properties of polymer‐dispersed liquid crystal films with different mixtures of monomers and liquid crystals

This article deals with the study of polymer‐dispersed liquid crystal (PDLC) films that consisted of microdroplets of liquid crystals (LCs) dispersed in a polymer matrix. The PDLC films were fabricated by the photoinduced phase separation method under room‐temperature conditions. To determine the extent of the effects of the molecular structures and their physical properties of different mixtures of monomers and LCs on the morphology and electrooptical properties of the PDLC films, various mixtures were used. A detailed discussion of the obtained results is given. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Preparation of polymer‐dispersed liquid crystal films containing a small amount of liquid crystalline polymer and their properties

Polymer‐dispersed liquid crystal (PDLC) films were synthesized by the copolymerization of liquid crystalline polymer (LCP) precursor, urethane acrylate (UA), and mesogenic monomer (AI) at different conditions. The morphology of polymer matrix changed with the weight ratio of polymer/liquid crystal (LC) ratio and curing temperature, resulting in a large change in the droplet size of LC domains in the PDLC film. The components used in the synthesis of polymer matrix, that is, the weight ratio of LCP, AI, and UA, also strongly influenced the morphology of PDLC films. A small amount of LCP was copolymerized with UA and AI in the preparation of polymer matrix to improve the electrooptical properties such as the viewing angle. Added LCP also affected the morphology and the properties of PDLC. The hydrophobicity of LCP caused changes in the droplet size of LC domain in PDLC films and the anchoring energy between matrix polymer and LC droplets. As the hydrophobicity of the matrix increases, the droplet size of LC domain also increases; on the contrary, anchoring energy decreased, leading to the decrease of driving voltage. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 3178–3188, 2000     

聚合物分散液晶彩色电控调光玻璃的试制

简单介绍聚合物分散液晶（PDLC）膜调光玻璃的原理与特性。将油溶性彩色染料添加到PDLC中,试制出彩色电控调光玻璃样品,对样品雾度和透光率进行测试,表明样品在电子窗帘领域具有应用前景。     

Optical property enhancement of dye‐PDLC using active reflector structure

The enhancement of electro‐optical properties in reflective‐type dye polymer dispersed liquid crystal (PDLC) has been achieved by applying an additional white PDLC layer along with dye PDLC layer. This newly modified structure that consists of white PDLC layer and scattering reflector acts as an active reflector. In this practice, an additional arrangement of a polymer barrier layer is made‐up over white PDLC layer, to block the absorption of any solution from dye PDLC. The contrast ratio of this new configuration is almost doubled with low driving voltage and high ON reflectance. Simultaneously, the “off” state has been observed darker than single layer dye PDLC structure. This new configuration can be potentially significant for various display applications such as E‐paper, outdoor billboard, and flexible display. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Studies on electro‐optical properties of polymer matrix/LC/SiO2 nanoparticles composites

Polymer‐dispersed liquid crystal (PDLC) films were prepared by ultraviolet light‐induced polymerization of photopolymerizable monomers in nematic liquid crystal (LC)/monomers/SiO₂ nanoparticles composites, and the effect of SiO₂ nanoparticles on the electro‐optical properties of PDLC films was studied. The observed effect showed that by the adjustment of the SiO₂ nanoparticles content, the refractive index ratio of the LC and polymer could be modulated, and the electro‐optical properties of the polymer matrix/LC/SiO₂ nanoparticles composites could be optimized. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Polymer molecular weight and chain transfer during the photopolymerization of an aliphatic monoacrylate in a smectic liquid crystal

Polymer/liquid crystal (LC) composites offer a unique opportunity to study polymerizations in ordered media, specifically the potential effect mesophase order can have on polymer properties including molecular weight. To develop successful polymer/LC composites for display applications, it is important to understand the effect of mesophase order on polymer molecular weight in order to optimize the electro-optic (EO) properties of the polymer/LC composite. Polymer molecular weight may be influenced in a LC by changes in polymerization rate as LC order is modulated and by chain transfer. This work focuses on the photopolymerization of an aliphatic monoacrylate monomer, decyl acrylate (DA), both in the ordered LC phases of 8CB as well as in isotropic solutions with LC and co-solvent.When DA is polymerized using the LC as the solvent, enhanced polymerization rates and polymer molecular weights are observed in the highly ordered smectic phase compared to the less ordered nematic and isotropic phases. When conducted strictly in an isotropic environment using a co-solvent with increasing 8CB percentages, a dramatic decrease in the polymerization rate and a significant reduction of the polymer molecular weight is observed, implying degradative chain transfer to the LC. NMR results show that this chain transfer is a result of hydrogen abstraction from the liquid crystals, which leads to the reduction in the polymerization rate with increasing 8CB concentration. The most likely site of hydrogen abstraction is from the benzyl hydrogens of the alkyl chain of 8CB. This chain transfer also plays a role for polymerizations performed in the ordered phases of the LC. Chain transfer appears to be less significant when polymerizations are conducted in the smectic phase due to the anti-parallel association of the LC molecules. When polymerizations occur in the less ordered phases, chain transfer dominates leading to a large reduction in polymer molecular weight and polymerization rate.     

Polymer dispersed liquid crystal films for modulating infra-red radiation

The ability of thin films of polymer dispersed liquid crystals to operate as electro-optic devices for modulating infrared radiation between wavelengths of 1 and 5 μm has been investigated. The complex optical properties of the polymer component of the films, and the real component of the refractive indices of the liquid crystals, have been determined using a combination of transmittance spectroscopy and a Kramer's-Krönig dispersion relation utilizing reflectance spectra. The scattering characteristics of single droplets of liquid crystals have been determined and subsequently the contrast ratios of films have been calculated using an anomalous diffraction technique and compared with experimental values. Calculations of film contrast ratios demonstrate good correlation with experimental results for simple film geometries in the absence of multiple scattering. The agreement between experimental and calculated contrast ratios is shown to improve as the light mean free path through the films increases.     

剪切液晶调光玻璃的试制

介绍聚合物分散液晶（PDLC）和应变液晶（SLC）概念，试制出剪切应变液晶调光玻璃样品，样品外观是散射毛玻璃态，对样品施加剪切应力则变成半透明态。给出样品的调光效果照片，半透明态时用样品观察液晶显示器屏幕照片，用样品作屏幕投影效果照片，对样品在散射态的雾度和半透明态的透光率进行测试，对实验现象做出合理解释。剪切液晶调光玻璃是一种新型功能玻璃，在许多领域具有应用前景。     

Polymer/liquid crystal nanocomposites for energy storage applications

High-dielectric constant (high-K) polymer nanocomposites based on nematic liquid crystals and CaCu₃Ti₄O₁₂ (CCTO) nanoparticles have been prepared. The host matrix is polymer dispersed liquid crystals (PDLC) in which LC (E7) droplets are dispersed in different polymer blends ratios of poly vinyl chloride/poly aniline (PVC/PANI). The PDLC (PVC/PANI/E7) in the appropriated ratios; (90/10/5), (75/25/5), and (50/50/5) were composited with 10 wt% CCTO nanoparticles. The IR spectra recorded for the PDLC nanocomposites present a spectrum similar to that of pure PDLC but with a slight shift of the peak positions. The addition of PANI and CCTO to PDLC enhances the thermal stability of the nanocomposites. SEM demonstrates agglomerates of CCTO dispersed in the polymer textures. Moreover, the addition of E7 facilitates the integration of PANI in PDLC matrix. The broadband dielectric spectrum shows high-frequency relaxation in addition to low-frequency interfacial polarization (Maxwell-Wagner type polarization). Besides, ε′ at 50 Hz is in the order of 10⁵ for PDLC/CCTO (50/50/5/10) nanocomposite. In addition, the computed energy density is found to be 74.66 J/cm³. This presumed ratio could be accentuated as a potential candidate for energy storage application with respect to the considerations of device fabrications.     

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

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

Effects of droplet size on photorefractive properties of polymer dispersed liquid crystals

Effects of liquid-crystal droplet size on orientational photorefractive properties of the polymer dispersed liquid crystals are investigated experimentally. The composites consist of the same chemical components, but the liquid crystal droplet size was varied by controlling the fabrication process. Particular attention is given to the observation and qualitative and/or quantitative modeling of the resolution, dependence of the applied dc field, dynamics of grating generation and photocurrents, which is strongly dependent on the liquid crystal droplet size.     

Alignment behaviour of liquid crystals on ethyl cellulose films with banded‐texture structure

Films were formed by casting and shearing ethyl cellulose in chloroform at high coating speed and thin coating thickness. The films were used as alignment layers for liquid crystals. Atomic force microscopy and polarizing optical microscopy (POM) were used to identify the banded‐texture structure of the films. The alignments of nematic liquid crystal 4‐cyano‐4′‐n‐pentylbiphenyl (5CB) droplets on the films were observed by POM. Furthermore, the time‐dependent alignment behaviour of 5CB on the films was recorded and studied. © 2003 Society of Chemical Industry     

Influence of the multi-functional epoxy monomers structure on the electro-optical properties and morphology of polymer-dispersed liquid crystal films

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.     

Photopolymerization behavior and phase separation effects in novel polymer dispersed liquid crystal mixture based on urethane trimethacrylate monomer

Polymer dispersed liquid crystals (PDLCs) are often formed by polymer induced phase separation, based on photopolymerization of multifunctional acrylate monomers. The emerged morphology is controlled by the interplay between polymerization rate and phase separation dynamics, which depends on different parameters such as monomer structure and functionality. In this work, a new PDLC formulation containing urethane trimethacrylate (UTMA) monomer is introduced, which has different molecular weight evolution, polymer gel point, and polymerization kinetics in comparison with some common ester acrylate (such as TMPTA and DPHPA) based PDLC compositions. UTMA is synthesized and characterized by Fourier transform infrared, ¹H‐NMR, and ¹³C‐NMR spectroscopic techniques. Simultaneous examination of polymer evolution and LC phase separation by real‐time infrared spectroscopy shows that the UTMA based PDLC, which contains trifunctional urethane acrylate monomer, has greater amount of bond conversion, polymerization rate, and liquid crystal (LC) phase separation in comparison with TMPTA based PDLC. In spite of the acrylate monomers, which show gel point conversions as low as 1.83–5.72%, UTMA reaches to its maximum rate at 19.5% conversion, which causes higher phase separation and therefore greater LC domain size. The experimental results are explained more precisely by means of SEM and optical microscopy analyses. The results are confirmed by electro‐optics measurements. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Electro‐optical properties of polymer dispersed liquid crystal prepared by successively controlled living radical polymerization

Polymer dispersed liquid crystal (PDLC) films were obtained by successive controlled living radical polymerizations: starting polystyrene (M1) was obtained by reversible addition‐fragmentation polymerization (RAFT), M1 was converted to P‐chloromethylated polystyrene (M2) which was grafted with polystyrene branches by atom transfer radical polymerization (ATRP) to yield RAFT‐initiating graft polymer containing trithiocarbonate moieties in the backbone (M3, RAFT‐active grafted polystyrene), and then PDLC films were prepared by photo‐induced RAFT copolymerization of methyl acrylate with M3 in the presence of a nematic liquid crystal. The electro‐optical properties of the films were investigated for the purpose to apply them to optical devices. Experimental results showed that preferable properties could be acquired by controlling the amount of M3 and the liquid crystal E7 in the polymer matrix of PDLC films. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers     

Temperature and liquid crystal concentration effect on thermal conductivity of poly(styrene) dispersed 5CB liquid crystal

In the present work, we study the thermal behavior of Polymer (Polystyrene) dispersed (4‐cyano‐4′‐pentylbiphenyl) 5CB liquid crystal film composite. A photopyroelectric device was used to study thermal conductivity at homeotropic and planar aligned of 4‐cyano‐4′‐pentylbiphenyl (5CB) liquid crystal. Thermal conductivity of polystyrene (PS) has been determined and calculated from experimental applied data reported in the literature. Thermal conductivity characteristics of the PDLC films were investigated with three prediction models as a function of both temperature and liquid crystal concentration in the polymer matrix. We particularly show the behavior of this thermal conductivity in the ON and OFF state. It was found that the difference in the film thermal conductivity ranges between 3 and 21%, depending on the ON and OFF state and the liquid crystal volume concentration. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 481–486, 2003     

The influence of surfactant in reflective HPDLC gratings

The use of highly functional acrylate monomers (large number of reactive bonds) as the precursor to the polymer-rich phase of holographic polymer-dispersed liquid crystals (HPDLC) is well established as providing the best balance between fast polymerization, gelation, and vitrification. Monomers with >3 double bonds (typically between 4 and 5) yield films with the largest diffraction efficiency, all other things being equal. A consequence of this chemistry is that the HPDLC films formed require very high switching fields to dynamically varying their optical properties. These high switching fields are unappealing for many commercial applications. A potential solution has been to add a non-reactive surfactant-like molecule to the reactive syrup. A substantial lowering of the switching field has been attributed to a modification of the anchoring properties of the LC molecules and the polymer host. The dynamics of switching seem to support this supposition as much longer relaxation times are observed. We investigate differences in the polymerization kinetics using differential photocalorimetry (DPC) and relate these results to differences in HPDLC evolution using real-time scattering experiments. Herein, we present large differences in the final morphology of reflection gratings which are supported by optical measurements. We also indirectly show that the surfactant molecules reside near or at the polymer interface by examining contamination of PDLC droplets using polarized optical microscopy and differential scanning calorimetry.