LC Droplet Size Research Articles

Field‐Induced Structural Transitions in Liquid Crystal Microemulsions

AbstractLiquid crystal (LC) droplets have fascinating properties, such as anisotropic properties, in response to external stimuli. As LC droplet size may determine the proper application of soft composites, various results from numerical simulations and experimental observations of LC droplets are reported. Here, detailed topological responses of individual bipolar droplets to electric fields, are shown. The integration of each response influences the entire electro‐optic behavior. Monodispersed LC bipolar droplets are fabricated in a polyvinyl alcohol‐dissolved aqueous solution via a membrane‐emulsification method. The planar anchoring, provided by the polyvinyl alcohol surface, and surface elasticity, guide the entire LC director configuration, associated with topological defects, in the droplets. By simply blade‐coating the solution, bipolar axes of the droplets can be randomly placed on a flat substrate. Two different subsequential stages are found when applying voltages: 1) director reorientation, which is threshold‐less and 2) topological defect movement, which exhibits a threshold‐like behavior. Various initial directions of the bipolar axes with respect to the field direction provide the transition voltage between these two stages. It is believed that this study can provide important clues to handle several fundamental issues in electro‐optics of encapsulated LCs, such as the determination of response times, threshold, and operating voltages.

Label-free optical sensor based on liquid crystal sessile droplet array for penicillin G determination

A sessile droplet array of 4’-pentyl-biphenyl-4-carboxylic acid (PBA) doped 5CB sessile droplets (12 −104 µm) is prepared by inkjet printing and applied for label-free sensing of penicillin G according to the configuration transition from axial to windmill based on the molecular alignment of 5CB at the LC-aqueous interface. Selective determination of penicillin G is achieved according to the target-selective enzymatic hydrolysis catalyzed by penicillinase, exhibiting sensitive detection and fast response with a detection limit of 0.178 ng/mL within 3–5 min. The sensitivity is linear depending on LC droplet size. Under the optimized conditions, the linear detection range is 1.78–178.0 ng/mL. Such a droplet array sensor has also been successfully applied to determine penicillin G in real river and lake water samples with RSD of 102–119%. Such a platform possesses the advantages of simple and cheap construction of an ordered array for multiplex sensing, and easy characterization via a polarizer. The quantification range of this proposed label-free optical penicillin G sensor covers the maximum residue limits in raw milk and river water defined in the European Union Regulation, and thus being applicable for food and environmental monitoring. By engineering LC-based probes, this approach could be extended to sense various molecules and ions in biomedical and chemical scenarios. And the sessile droplets on polymer substrates would also bring the possibility for employing in flexible and wearable devices in the future.

Electro-optical properties and frequency response of polymer-dispersed liquid crystal gratings doped with multi-walled carbon nanotubes

In this paper, a model developed for establishing the lowest possible driving voltage of Holographic polymer-dispersed liquid crystal (H-PDLC) based on the Maxwell–Wagner effect doped with specific nanotubes. The dependences of the turning point frequency on the doping concentration and the distribution of multi-walled carbon nanotubes (MWCNTs) are discussed. The MWCNTs can effectively adjust the relaxation frequency of grating, increase its LC droplet size, and decrease its dielectric permittivity. The diffraction efficiency and threshold voltage of the prepared switchable Bragg grating were found to reach 91% and 0.77 V μm−1, respectively, at 5 kHz. By comparing the results of experiments and simulations, we obtained the mass distribution ratio of MWCNTs in the LC and polymer regions. It was found that the MWCNTs diffuse into the LC regions during the polymerization process, and the use of a suitable driving frequency and doping concentration can minimize the driving voltage. This work provides a reference for studying the dynamic range optical tuning and polymerization kinetics of nanoparticles in switchable gratings.

Effects of multifunctional acrylates and thiols on the morphology and electro-optical properties of polymer-dispersed liquid crystal films

ABSTRACT In the present paper, influence of the functionality of thiol monomers and acrylate crosslinkers, as well as crosslinker chain length on the polymer morphologies and electro-optical properties of the PDLC films, were systematically investigated. Results revealed that increasing the functionality of thiol monomers, both threshold voltage (Vth) and saturation voltage (Vsat) were increased due to the dense microstructures resulting from higher reactivity of thiol monomers. However, with increasing the functionality of acrylate monomers, Vth was increased evidently, but a relatively small rise of Vsat was found with moderate functionality monomer. Additionally, the increase in the chain length of flexible crosslinker resulted in an increase in the average size of LC droplets with a corresponding decrease in Vth and Vsat. On the whole, polymer morphology and electro-optical properties of the PDLC films can be artificially manipulated by adjusting the molecular structure of polymer matrix. A PDLC film exhibiting low driving voltage (24.0 V), moderately high contrast ratio (97.0) and fast response time (<8.0 ms), was found by employing the Pentaerythritol tetrakis (2-mercaptoacetate) (4SH), trimethylolpropane triacrylate (TMPTA) and polyethylene glycol diacrylate (PEGD 600).

The fabrication of novel optical diffusers based on UV-cured polymer dispersed liquid crystals

ABSTRACTPolymer dispersed liquid crystals (PDLCs) with different sizes of the LC droplets are prepared based on the ultraviolet (UV) light curable acrylate monomers/LCs composites to fabricate the optical diffuser films. To acquire light diffusers with high optical performance, the effects of the monomer structure and the UV light intensity on the micro-structure of the PDLC films are studied. Results show that the PDLC films could exhibit a strong light scattering at the premise of maintaining high transmittance in the visible region. As the LC droplets are spherically dispersed in the polymer networks, when the size of LC droplets is about 3.0 μm, the haze can reach 88.5% and the transmittance is nearly 90.0%, which can be used as a bottom diffuser film. While when the size of LC droplets is about 10.0 μm, the haze and transmittance are 39.2% and 90.2%, respectively; hence, it can be a good choice for a top diffuser film. With the advantages of simple preparation, roll-to-roll industrial production and tunable optical properties, it is supported that the films based on UV-cured PDLC films can be applied as outstanding optical diffuser films in the liquid crystal display industry.

Effects of crosslinking agent/diluents/thiol on morphology of the polymer matrix and electro-optical properties of polymer-dispersed liquid crystal

ABSTRACTTo 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.

High performance dye-doped emulsion type PDLC for transmittance variable devices

ABSTRACTPolymer dispersed liquid crystal (PDLC) has been developed for a light shutter due to the advantage of high transmittance. To achieve high transmittance in PDLC's transparent mode and high opacity in shading mode, the black dye-doped emulsion type of PDLC using a membrane filter was investigated with variations in LC droplet size and LC layer thickness. The transmittance difference (ΔT) between the on and off states strongly depended on LC droplet size, and there existed an optimal value to provide higher ΔT, which implies that our method is an effective one for PDLC with a high ΔT.

Analysis of morphological behaviour and electro-optical properties of silica nanoparticles doped polymer dispersed liquid crystal composites

We have prepared silica nanoparticles (SNPs) doped polymer dispersed liquid crystal (PDLC) composites by using varying concentrations of SNPs. The varying concentration of SNPs has a profound impact on the morphology and electro-optical properties of PDLC. The polarizing optical microscopy (POM) studies revealed a substantial non- homogeneity/dual sized droplet morphology in the LC droplet size induced by the SNPs. The relative transmission was enhanced in the SNP-PDLC composites as compared to the undoped PDLC. The threshold voltage (Vth) of the doped composites decreased as compared to the undoped PDLC but increasing SNP concentration had a slight impact on the threshold voltage (Vth) as well as transmission. The effect of SNPs on nematic-isotropic transition temperature (TN-I) and ionic conductivity was also clarified.

Study on the electro-optical properties of polyimide-based polymer-dispersed liquid crystal films

The reflectivity control device, initially developed for attitude control, is utilised to control the solar sail orbit by switching the states between absorption and specular reflection. Actually, the major parts of the device are the polymer-dispersed liquid crystal (PDLC) films. Here, PDLC films based on polyimide (PI) as polymer matrix and a low molecular weight LC can be prepared by the thermally induced phase separation (TIPS) method. The influences of cooling rate and the content of LC on the size and uniformity of LC droplets dispersed in a polymer matrix by a TIPS process were investigated. It was found that a fast cooling rate gave smaller droplet sizes and hence a more uniform distribution as compared to the ones produced under a slow cooling rate. If the LC content was increased, the droplet size would be increased. Furthermore, the effect of LC droplet size on the electro-optical properties of the PI-based PDLC films was discussed, such as transmittance, threshold voltage, driving voltage and contrast ratio (CR).

에멀전 방식의 고분자 분산형 액정의 전기 광학 특성에 미치는 액적 크기 분포의 영향

Department of Display Engineering, Hoseo University, Asan 336-795, Korea(Received December 23, 2014; Revised May 7, 2015; Accepted June 9, 2015)Abstract: We established the emulsion method using membrane filter with precise control of LC droplet distribution in PDLC. PDLC cells with various LC droplet size distributions such as single droplet sizes of 1.0 μm, 1.9 μm and 3.5 μm, the mixture of two different LC droplet sizes and the mixture of three different LC droplet sizes were fabricated and the electro-optical properties of the emulsion type PDLC cells with various droplet size distribution were investigated. In the appropriate droplet size range, the PDLCs with the single droplet sizes distributions have good electro optical properties than those with the mixture of three different LC droplet sizes. In addition, the PDLC cells with the mixture of two different LC droplet sizes have the better electro optical properties than those with single droplet sizes distribution. The PDLC cell with dual droplet size distribution of 1.0+1.9 μm shown the best electro optical properties than the PDLC cells with other size distributions. This method enabled us to find the proper LC droplet size distribution for achieving both high transmittance and contrast ratio.Keywords: PDLC, Emulsion, Membrane filter, Droplet size, Flexible ()*+

Studies on the electro-optical and the light-scattering properties of PDLC films with the size gradient of the LC droplets

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.

Effect of Urethane Acrylate Oligomer and UV Curing Temperature on Electro-Optical Property of Polymer-Dispersed Liquid Crystal

Polymer-dispersed liquid crystal (PDLC) films were prepared by photopolymerization after substituting the reactive diluent monomer 2-ethylhexyl acrylate with an aliphatic urethane acrylate (UA) oligomer in a conventional pre-polymer (PN393). The effects of UA concentration and UV curing temperature on the PDLC films’ physical properties were examined. The average LC droplet size decreased with increasing UA concentration. The driving voltage decreased by 30% and the response time increased by 26%. The PDLC films with 30 wt% of UA showed the lowest driving voltage and the highest contrast ratio when the LC pre-polymer mixture was UV-cured slightly above the nematic-isotropic coexistence temperature.

Droplet Size Dependent Electro-Optical Properties of PDLCs with One Plastic Substrate

We established the accurate LC droplet size control technique based on membrane filtering method, and based on the technique fabricated emulsion type PDLC cells with various LC droplet size distribution configuration such as single LC layer with different droplet sizes, the combination of different droplet sizes both inside layer and layer by layer, and investigated the electro-optical properties of them. In this paper, we discuss in detail on the electro-optical properties of the emulsion PDLC with various droplet size distribution and layer configuration.

Characterization and Morphology of Polymer-Dispersed Liquid Crystal Films

The electro-optical properties and reducing morphologies of the PDLC films were strongly influenced by the chemical structure of curable diglycidyl ether (DGEBA), and tri-functional trimethylol propane triglycidyl ether (TMPTGE) significantly. Higher driven voltage and contrast ratio were achieved when the PDLC films showed a reducing morphology with suitably distributed small holes of LC droplet size. The detailed characterization and morphology of polymer network of PDLC films were analyzed by employing liquid crystal device (LCD) parameters tester, UV-VIS-NIR spectrophotometer, an Abbe Refractometer, and scanning electron microscopy (SEM). Meanwhile, for PDLCs with a certain amount of LC; the microstructure and the refractive index of polymer networks could be influenced by decreasing the composition ratio of monomers ethylene glycol diglycidyl ether (EGDE) and polyethylene glycol diglycidyl ether (PEGDE).

Optically-isotropic nanoencapsulated liquid crystal displays based on Kerr effect

An innovative liquid crystal display (LCD), incorporating in-plane switching mechanism with LC nanoencapsulation, is developed and its electro-optic (EO) performances are characterized. The nanoencapsulated LCDs exhibit electric-field induced transitions from an optically-isotropic state to an anisotropic one based on Kerr effect. Extremely small size of LC droplets embedded in the nanoencapsulated layer, less than a quarter of wavelength of incident light, appear transparent in the voltage-off state and gradually becomes birefringent in response to applied voltage, exhibiting the optical behavior of uniaxial retarder. Due to optically-isotropic nature and a large Kerr effect, they show wide viewing angle characteristics and good EO performances.

Effects of Molar Ratios of Thiol-Ene Monomers on the Morphology and Electro-Optical Properties of Polymer Dispersed Liquid Crystal Films

We synthesized tetrafunctional allyl ether monomer (4-AE) and investigated the effects of the different molar ratios of trimethylolpropane tris-(3-mercaptopropionate) (3-SH) to 4-AE on the photopolymerization behavior, morphology, and electro-optical properties of thiol-ene-based PDLC films. Photo-DSC and DSC analyses revealed that the PDLC sample containing 45 wt% 3-SH and 45 wt% 4-AE gave the highest exotherm, the fastest photocure rate, and the highest Tg due to the matched stoichiometry. Morphological observations and electro-optical measurements showed that the PDLC sample with the matched molar ratio gave the smallest LC droplet size, the highest threshold, driving voltages, and lowest saturation transmittance because the orientation of LC molecules got difficulty in small droplets. The stoichiometric ratios of 3-SH to 4-AE played an important role in controlling the photocure rate, phase separation rate, microstructures of LC droplets, and electro-optical properties of thiol-ene-based PDLC systems.

Reduction of Volume Shrinkage in Holographic Polymer Dispersed Liquid Crystal Based on Epoxy Containing Polymer Matrices

Abstract The grating formation dynamics, SEM images, diffraction efficiency, and electrooptical properties of transmission grating of holographic polymer dispersed liquid crystals (HPDLCs) have been investigated by incorporating three different types of monofunctional reactive diluents into the host polymer matrices. A gradual increase of real time diffraction efficiency, a decrease of LC droplet size, and an increase of diffraction efficiency of the composite film were obtained with the addition of epoxy acrylate monomers due to the increased viscosity of polymer/LC mixtures leading to decreased droplet coalescence. In addition, epoxy acrylate monomers induced decreased volume shrinkage according to their ring structure with bulky group. Driving voltage increased with the addition of epoxy monomers whereas rise time and decay time decreased as a result of the decreased LC droplet size. At an optimum composition of epoxy monomer, a minimum switching voltage of 6 V µm−1 and rise time of 0.15 ms and a decay time of 18.05 ms were respectively obtained.

Sol–gel matrix dispersed liquid crystal composite: Influence of methyltriethoxysilane precursor and solvent concentration

Influence of methyltriethoxysilane (Me-TES) concentration on operating voltage and optical transmittance of ormosil dispersed liquid crystal (ODLC) electro-optical films is investigated. The films are prepared using tetraethoxysilane (TEOS) and Me-TES in molar ratios ranging from 1:5 to 5:1, respectively. Possibility to obtain ODLCs without using solvents in the synthesis is demonstrated for the first time. Although some alcohol (approximately 3–4 moles per 1 mole of alkoxide) is created by chemical reactions during ODLC formation, it is shown that generally avoiding solvents in the process results in wider LC droplet size distribution, increased thickness of films, and larger variation of the transmittance. It is also shown that lower operating voltages can be achieved than reported earlier while also using significantly lower concentrations of Me-TES. In addition, a novel approach to avoid cracking of the ODLC films by using polyvinylpyrrolidone (PVP) is introduced.

Monomer Functionality Effects in the Formation of Thiol−Ene Holographic Polymer Dispersed Liquid Crystals

Holographic polymer dispersed liquid crystals (HPDLCs) are a polymer/liquid crystal (LC) composite with wavelength selective diffraction that can be switched in microseconds with electric field. As such, HPDLCs are widely applicable in optical and photonic devices. Recently, replacing acrylate polymer with thiol−ene polymer as host for HPDLCs has improved the stability and electrooptic performance of these materials. This work examines the electrooptic performance and morphology of thiol−ene-based HPDLC reflection gratings. To further understand the relationships between the formation and performance of thiol−ene HPDLCs, electrooptic performance was characterized as a function of polymerization rate, gel point conversion, and the presence of excess monomer (stoichiometry). To this end, HPDLC formulations were examined to isolate the contribution of ene monomer functionality, thiol monomer functionality, and thiol−ene stoichiometry. In all formulations, the performance of HPDLC reflection gratings is correlated to the generation of morphology typified by well-defined polymer/LC lamellae and small LC droplet size. Increasing the rate of polymerization through increasing laser intensity, increasing ene monomer functionality, or thiol−ene stoichiometry improves the diffraction efficiency (DE) of HPDLCs by reducing LC droplet size. Gel point conversion is also critical to producing well-performing HPDLC reflection gratings. HPDLCs based on thiol−ene polymer with gel points from 40 to 60% monomer conversion show optimal optical behavior. This increase in performance is related to the impact of gel point on morphology, as low gel point conversion leads to the formation of small droplets and poorly defined grating structures while high conversion gel points lead to the formation of large LC droplets. Therefore, optimal HPDLC materials are formed with moderate conversion gel points that allow both well-defined grating structure and small droplet formation.

Polymer-dispersed liquid crystal structure for variable optical attenuator application

To improve variable optical attenuator (VOA) performance, the object of this study is to know the most appropriate structure of a polymer-dispersed liquid crystal (PDLC) composite. In fact, using its electro-optical properties, PDLC can be inserted between two optical fibers for adjusting the coupling efficiency. We utilize here the photopolymerization-induced phase separation (PIPS) method as a fabrication process. Furthermore, we present a theoretical analysis of light scattering by anisotropic spheres based on the anomalous diffraction approach (ADA) and compare this model with some experimental results. The optical attenuation (OA) range is optimized in the case where LC, the droplets nematic directors, are randomly oriented. If the nematic directors have a quasiprivileged direction, we demonstrate that PDLC transmission depends on the incident light polarization. Polarization-dependent loss (PDL) and OA behaviors are then studied at the 1.55-μm wavelength according to several parameters such as PDLC thickness, LC droplet size, and the device driving voltage.