Enhancement of frequency modulation response time for polymer-dispersed liquid crystal

Through the ferroelectric nanoparticles of BaTiO₃ (BTO) doping, the response time for the frequency modulation of the polymer-dispersed liquid crystal (PDLC) was improved. The BTO-doped PDLC cells were prepared by polymerisation induced phase separation (PIPS) process using UV light. The capacitance of the PDLC composites was measured with an impedance analyzer in the frequency range of 100 Hz–1 MHz at 1 V. The dynamic signal for the response time of the PDLC devices was monitored through a digital oscilloscope. The electro-optical properties of the PDLC were found to strongly depend on the doped BTO concentration. The BTO doping caused a large increase in the capacitance. The dielectric constants were drastically decreased in the samples with rather low BTO doping ratio at a high frequency. No outstanding difference in the rising time of the LC was observed in the BTO-doped PDLC device, but the falling time was significantly decreased from 0.334 to 0.094 s. The present results imply that the nanoparticle-doping technology could improve the electro-optical performance of the PDLC requiring fast response and frequency modulation, such as optical modulators and PDLC-hybrid electroluminescence device for flexible electronic devices.     

松果状纳米氧化铁对液晶电光性能的影响

液晶材料被广泛应用于液晶显示器(LCD)中,但是由于液晶中杂质的存在,导致液晶的应用电压变大,增加了能耗。 为了降低应用中的阈值电压和饱和电压,通常向液晶中添加纳米颗粒来提高电光性能。 本文采用简单的化学沉淀法制备了形貌均一,大小尺寸均匀的松果状氧化铁(P-Fe₂O₃)纳米颗粒。 将其掺杂到向列相液晶4-氰基-4'-戊基联苯(4-cyano-4'-pentylbiphenyl,5CB)中,结果表明,掺杂质量分数为0.5%时,电光性能达到最优,阈值电压和饱和电压分别降低24.8%和45.2%,对比度增大46%,响应时间降低至17.6 ms,此性能优于相同条件下掺杂普通Fe₂O₃纳米颗粒的向列相液晶5CB,其阈值电压和饱和电压分别降低15%和16%。 这归因于松果状Fe₂O₃纳米颗粒可以在向列相液晶5CB中均匀分散,其粗糙的表面吸附了液晶中的杂质离子,减少了杂质离子的屏蔽效应,从而提高了电光性能。     

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.     

Improvement of the electro-optical properties of nematic liquid crystals doped with strontium titanate nanoparticles at various doping concentrations

ABSTRACT

In this study, we doped homogenous aligned nematic liquid crystal (NLC) systems with strontium titanate (SrTiO₃) nanoparticles (NPs), and investigated the impact of doping concentration on the NLC’s electro-optical (EO) properties. SrTiO₃ NP-doped NLC cells maintained a high optical transmittance of 77.51% to 78.41% compared to pure NLC cells (78.09%). At a 0.1 wt.% SrTiO₃ NP doping concentration, twisted-nematic (TN)-LC cells exhibited enhanced EO performance, with a reduced threshold voltage from 1.70V to 1.61V and a shorter response time from 17.03 ms to 10.66 ms without optical defects and degradations. We also observed an improvement in thermal endurance for doping concentrations of 0.05 and 0.1 wt.%.     

Cyano terminated tolane compounds for polymer dispersed liquid crystal application: relationship between cyano terminated tolane based molecular structures and electro-optical properties

ABSTRACT

The structures of the liquid crystal (LC) molecules have a key role in impacting the electro-optical performance of a polymer dispersed liquid crystal (PDLC) film. In this paper, the relationship between the LC molecular structures and the electro-optical properties of PDLC films is investigated based on an unexplored cyano-terminated tolane compounds (CTTCs) doped E8 LCs/UV polymers system. Due to the high polarity of CTTCs, LCs doped with the cyano-terminated tolane (CTT) molecules exhibit high birefringence and large positive dielectric anisotropy. On the whole, PDLC films doped with the CTT molecules exhibit a lower driving voltage than that doped with the pure E8. More excitingly, PDLC films based on CTT molecules with larger length-to-width ratio and longer conjugated system show higher contrast ratio (CR) and faster response time. Eventually, the mechanism of the effects of CTT-based molecular structures and the relationship between the electro-optical performance of PDLC films and CTT molecules are illustrated. This work paves a new way for optimising the electro-optical properties of PDLC films.     

Polyfluorinated cycloalkoxyphenyl ether systems as dopants for liquid crystal display applications

The effects on response times of representative commercially available dielectrically negative (−LC) and positive (+LC) type liquid crystal mixtures doped with small quantities of various tetrafluorinated cycloalkoxyphenyl ether derivatives were investigated. The synthesis of various polyfluoroaryl systems by nucleophilic aromatic substitution processes is described and electro-optical measurements concerning the rise and decay times and voltage holding ratios of each doped LC formulation are reported. The response time of the dielectrically negative LC host was improved upon doping with each of the tetrafluorinated cycloalkoxyphenyl ether derivatives, although no improvement in the performance of the dielectrically positive LC host was achieved.     

Advanced electro-optical smart window based on PSLC using a photoconductive TiOPc electrode

This study demonstrates an electro-optical controllable smart window based on a liquid crystal device. The device integrates a photoconductive electrode based on TiOPc films into a liquid crystal shutter based on polymer-stabilised homeotropic aligned nematic liquid crystals (PSLC) connecting in series. The PSLC light shutter is transparent in electric field-off state; it scatters light as it switches to multi-domain structures in electric field-on states. The TiOPc film has an optically controllable impedance that manipulates the electric field on the PSLC film. The window transits into multi-transmission states according to the intensities of exposure light with an extra applied bias voltage.     

Superior electro-optic properties of liquid crystal system using cobalt oxide nanoparticle dispersion

In this article, we study the electro-optical (EO) properties of the homogeneous aligned nematic liquid crystal (N-LC) doped with cobalt oxide (Co₃O₄) nanoparticles (NPs). The EO characteristics of Co₃O₄ doped N-LC are higher performance, indicating lower threshold voltage (1.33 V), faster rising time (1.479 ms), and faster falling time (9.343 ms) than pure liquid crystal (LC) cells. We have demonstrated these results by investigating the relationship between dielectric constants and LC device properties. Furthermore, we proved NPs doped N-LC cells drive low power operation without capacitance hysteresis. Our experimental results were verified by software simulation based on general physical properties.     

Effects of alkyl chain length of monomer and dye-doped type on the electro-optical properties of polymer-dispersed liquid crystal films prepared by nucleophile-initiated thiol-ene click reaction

ABSTRACT

Polymer-dispersed liquid crystal (PDLC) films containing a series of monomers with different alkyl chain lengths were prepared by nucleophile-initiated thiol-ene click reaction. The effect of alkyl chain length of monomers, dye and temperature on electro-optical properties of PDLC films was investigated. It was found that the alkyl chain length and polymerisation rate of monomers together determine the size of liquid crystal (LC) droplets, thus affecting the electro-optical properties of PDLC. In addition, the type and content of dyes could be optimised to obtain PDLC materials with better comprehensive properties for display.     

Hysteresis-free and fast response polymer-stabilised blue phase liquid crystals

ABSTRACT

The electro-optical properties of γ-Fe₂O₃ nanoparticles doped polymer-stabilized blue phase liquid crystals are investigated. The experimental results show that the hysteresis were effectively suppressed by doping γ-Fe₂O₃. When doped with γ-Fe₂O₃ of 1.0 wt. %, hysteresis-free and fast response were achieved by the coupling effect of polymer and nanoparticle. Our research provides excellent guidance for the development of electro-optical devices with high grey scale accuracy and fast response.     

Thermal and electro-optical properties of cerium-oxide-doped liquid-crystal devices

Doping with cerium (IV) oxide (CeO₂) nanoparticles can significantly enhance the thermal stability and electro-optical (EO) properties of nematic liquid crystal (NLC) systems. Thermal stability was improved without aggregation and reduction of transmittance by adding CeO₂ nanoparticles in a liquid crystal medium. In particular, the EO properties greatly improved in CeO₂-dispersed NLC cells. The threshold voltage reduced from 3.027 to 2.279 V and the response time decreased from 13.097 to 9.970 ms with increased CeO₂ nanoparticles in the NLC cells. The improved properties of liquid crystals doped with CeO₂ nanoparticles depend on the anchoring energy and the electric field of the CeO₂-dispersed liquid crystal displays.     

Influence of SiO2 nanoparticles on electro-optical and thermophysical properties of polyacrylate/liquid crystal composites

ABSTRACT

In this paper, SiO₂ nanoparticle-doped polymer dispersed liquid crystal (PDLC) lenses are made from a mixture of prepolymer, 5CB liquid crystal and SiO₂ nanoparticles by the polymerisation induced phase separation (PIPS) process. The effect of SiO₂ nanoparticles on the electro-optical properties of PDLC ?lms are studied. It is established that SiO₂ nanoparticles affect the microstructure of PDLC ?lms signi?cantly because of the formed agglomerates of SiO₂ nanoparticles. Results show an improvement in the electro-optical properties and a decrease in the response time for doped systems with small amount of SiO₂ nanoparticles. We also observe a shift of nematic–isotropic transition temperature as a function of SiO₂ nanoparticle contents. A good agreement between the electro-optical study and thermophysical properties is reached.     

不同光滑度立方体氧化铁对液晶电光性能的改善

向列相液晶被广泛应用于液晶显示中,但是由于杂质的存在,会导致液晶的驱动电压变大,增加能耗。 为了降低阈值电压和饱和电压,通常向液晶中添加纳米颗粒来提高电光性能。 本文利用水热法制备了表面粗糙和光滑的两种立方体Fe₂O₃纳米颗粒,其形貌均匀,尺寸约550 nm。 将二者分别掺杂到向列相液晶E7中,结果表明,粗糙立方体Fe₂O₃/E7复合体系具有比光滑立方体Fe₂O₃/E7复合体系和向列相液晶E7更优的电光性能,且在掺杂质量分数为0.4%时,其电光性能达到最优,阈值电压和饱和电压分别降低9.9%和11.6%,对比度增大80%,响应时间降低至6.0 ms。 这归因于粗糙立方体Fe₂O₃具有足够的表面积和表面所带电荷更多,所以会更易吸附体系中的杂质离子和减弱杂质离子的屏蔽作用,从而提高了电光性能。     

Investigation of dielectric and electro-optical properties of nematic liquid crystal with the suspension of biowaste-based porous carbon nanoparticles

ABSTRACT

The effect of biowaste porous carbon nanoparticles (PCNPs) on the dielectric and electro-optical properties of nematic liquid crystal (LC) mixture (1823A) of 4-(4-alkyl-cyclohexyl) benzene isothiocyanates and 4-(4-alkyl-cyclohexyl) biphenyl isothiocyanates has been studied. The dielectric permittivity of nematic LC has been increased with the dispersion of carbon NPs. The dielectric anisotropy has been calculated and found to be decreased with the dispersion of PCNPs into the pure nematic LC. The response time and birefringence have been also observed with the variation of temperature, frequency as well as the concentrations of carbon NPs. After the dispersion of PCNPs achieved better birefringence and faster response in the dispersed system, which is the significant application in display devices. Threshold voltage splay elastic coefficient and rotational viscosity have been calculated for both pure and NPs dispersed nematic system. Its value is increased with the dispersion of NPs. Additionally, photoluminescence and figure of merit have investigated as a comparative study of nematic matrix as well the dispersed system. The experimental results have been found to have good agreement with the theoretical data of nematic LC. An effort has been made to explain these experimental results on the basis of interaction between nematic molecules and carbon NPs.     

A new reverse mode light shutter from silica-dispersed liquid crystals

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.     

Enhancement of dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal mixture by dispersing nano-sized copper oxide

ABSTRACT

Here, we present the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal (FLC) mixture, namely W302. The FLC mixture, comprising of pyrimidine compounds, was characterised through dielectric spectroscopy, differential scanning calorimetry (DSC), polarising optical microscopy (POM) and other electro-optical methods. The material parameters such as spontaneous polarisation, rotational viscosity, response time and tilt angle of W302 were found to be 14 nC/cm², 240 mPa.s, 150 µs and 28^?, respectively. The phase transition temperatures of W302 were observed through DSC and further confirmed by the dependence of dielectric loss factor in homogeneously aligned FLC sample with temperature. We also demonstrate the observance of a low-frequency dielectric relaxation mode due to the unwinding of the helix, called as partially unwound helical mode (p-UHM) along with Goldstone mode. The behaviour of p-UHM has been systematically studied with temperature and applied bias field. Further, dispersion of nCuO into host W302 has shown a significant increase in dielectric permittivity. Also, the p-UHM relaxation peak in the dielectric regime has disappeared with the incorporation of nCuO. These studies would be useful to fabricate better electro-optical devices for display, switching and beam steering applications. The formulation and characterization of a ferroelectric liquid crystal (FLC) mixture W302 composed of pyrimidine compounds is presented. Then, we observed the effect of copper (II) oxide nanoparticles (nCuO) on dielectric and electro-optical parameters of a newly prepared and characterized FLC mixture.     

Enhanced electro-optical properties of a nematic liquid crystals in presence of BaTiO3 nanoparticles

Composites of nematic liquid crystals (NLCs) and ferroelectric barium titanate (BaTiO₃) nanoparticles (NPs) have been prepared. The alignments of NPs in the host medium have been demonstrated. Effect of NPs doping on various display parameters of NLCs, namely, threshold voltage, dielectric anisotropy and splay elastic constant has been studied using electro-optical and dielectric studies. The nematic ordering of host supports alignment of NPs parallel to the director which consequently improves electro-optical parameters in the composite system. The dielectric and electro-optic properties of LC–NPs composites have been discussed in frame of conventional theories of NLCs.     

An electrically controlled light-scattering device based on liquid crystal/polymer microsphere composites

ABSTRACT

There is a widely applied prospect of electrical controlled liquid crystal (LC) light-scattering device. Numerous electrical controlled LC light-scattering technologies have been studied, but each technology has its own shortcomings, such as high driving voltage, high hysteresis, complex electrode structure, and serious heating. In this work, the composite of LC and polymer microspheres are used to fabricate light-scattering devices. This device is operated by the vertical electric field and does not require complex preparation process. LC/polymer microsphere composite has the advantages of low driving voltage and zero hysteresis. The role of microspheres in the composites is to change the size and density of a refractive-index-mismatched micro-domain. The effects of the ratio, particle size, and refractive index of microspheres on the optical characteristics of a composite are studied. The normal directional light transmittance at the transparent state and light-scattering state decreases with an increasing weight ratio of microspheres. The particle size of microspheres has negligible influence on the electro-optical properties of composites when the weight ratio of microspheres is small. The LCs doped with Polymethylsilsesquioxane (PMSQ) microspheres or polymethyl methacrylate (PMMA) microspheres are compared, and the mismatched refractive index and density of micro-domain show the influence on the electro-optical properties of the composites.     

Controlling morphological and electro-optical properties via the phase separation in polymer/liquid-crystal composite materials

ABSTRACT

The polymer/liquid-crystal composite materials have been extensively studied for their potential applications. Various optical devices based on this composite material have been proposed and realised. The device performance is highly dependent on the phase separation of this composite material. Here, we investigate the photopolymerisation-induced phase separation in this composite material. Depending on the mass ratios between the polymer and the liquid crystal, the phase separation can be well controlled and subsequently affect the morphological and electro-optical properties. At a fixed ratio, we can realise either phase-separated composite films or conventional polymer-dispersed liquid crystal films with completely different optical properties. By carefully controlling the exposure conditions, the morphologies and electro-optical properties have been studied and optimised in details. With in-depth studies and optimisation, the photopolymerisation-induced phase separation technique could be utilised to realise many different optical functions based on the polymer/liquid-crystal composite materials.     

The effect of polymerizable surfactants on morphology and electro-optical of PDLCs films based on epoxy acrylate system

Polymer dispersed liquid crystals (PDLCs) have been extensively studied for various excellent electro-optical applications. The anchoring interaction of liquid crystals (LCs) molecules on the surface of the polymer cavity surrounding an LCs droplet has a crucial effect on the electro-optical performance of the PDLCs. The effect of polymerizable surfactants on the electro-optical properties of PDLCs films was studied in detail. The active double bonds were polymerized with prepolymer to stabilize the performance of polymer matrix. The experimental results showed that polymerizable surfactants could effectively reduce the driving voltage. The speed of polymerization was monitored by real-time transmittance. The electro-optical properties of PDLC films were measured by Polarimeter (PerkinElmer Model 341). The driving electric field was reduced from 3.9 V/μm to about 2.8 V/μm for doping undec-10-enoic acid at curing temperature 80?°C. The surfactants containing polymerizable functional groups, polarity, and alkyl chain weakened the surface anchoring between LCs droplets and polymer interface. The morphologies of PDLCs films were also investigated by polarizing optical microscopy (POM) and Fourier transform infrared (FTIR) images. The LC droplets were encapsulated by polymerizable surfactant according to FTIR images.