Properties Of Polymer Dispersed Liquid Crystal Research Articles

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

AbstractLiving graft macromolecule has been prepared through reversible addition‐fragmentation chain transfer (RAFT) living radical polymerization in one step. Then, it was used to make polymer‐dispersed liquid crystal (PDLC) by controlling the mole ratio of styrene (St) to 1,6‐hexanediol diacrylate (HDDA) and adjusting the content of prepared graft macromolecule. The results showed that electro‐optical properties of PDLC have been optimized. Different concentration of living graft macromolecule and different mole ratio of St/HDDA led to substantial improvement of driving voltage (threshold voltage and saturation voltage) and memory effect of PDLC simultaneously. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

The improvement of electro-optical properties of polymer-dispersed liquid crystals using copolymer macroinitiator with different glass transition temperature

In this article, copolymer macroinitiators prepared with styrene and iso-octyl acrylate by reversible additional-fragmental chain transfer polymerization were used to prepare polymer-dispersed liquid crystals (PDLCs) with methyl acrylate. The electro-optical properties of the PDLCs were investigated. The results showed that the glass transition temperature (Tg) of the macroinitiator has a great influence on the memory effect of the resulting PDLCs. Low driving voltage and low memory effect PDLCs could easily be obtained with copolymer macroinitiators. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

The influence of crosslinking agents on the morphology and electro‐optical performances of PDLC films

AbstractThe morphology of polymer‐dispersed liquid crystal (PDLC) has a major impact on its electro‐optic properties. The PDLC was prepared by photochemical polymerization from UV‐curable monomers/crosslinking agent/initiator/LC mixture. The influences of crosslinking agents on the PDLC system had been studied. The microstructures of the PDLC were strongly influenced by the insertion of polyethylene glycol (600) diacrylate (PEGDA 600) crosslinker, relatively long and flexible branch, and diethylene glycol diacrylate (DEGDA) crosslinker along the network bone. The relative weight ratio of crosslinking agents and polymerization temperature had a significant effect on the LC domain size, and which resulted in influencing the electro‐optic properties of PDLC. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Preparation and characterization of polymer-dispersed liquid crystal (PDLC) with different cation exchange capacity (CEC) clays

This work describes the morphology and electro-optical properties of polymer-dispersed liquid crystals (PDLCs). These composites consist of the nematic liquid crystal E7, filled with different types of inorganic nanoparticles in Norland optical adhesive (NOA65) polymer matrices. Natural clays CL120 (trade name: CN-C34 with higher cation exchange capacity CEC) and CL42 (trade name: PK802 with lower CEC) are used as various inorganic nanofillers. Wide angle X-ray diffraction was exploited to examine the dispersion of the inorganic nanoparticles in PDLCs. The morphology of liquid crystal droplets in PDLCs is investigated by the field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) as well. Transmittance vs. applied voltage is measured to study the electro-optical properties and the response times of the PDLCs. It is suggested that doping with inorganic nanoparticles in PDLCs effectively reduces the driving voltage and improves the electro-optical characteristics.

Effect of Alignment Layer on Electro-Optic Properties of Polymer-Dispersed Liquid Crystal Displays

The electro-optic properties of polymer-dispersed liquid crystal (PDLC) cell having parallel alignment layers were examined and compared to the control cell without any pre-treatment. Although the liquid crystal droplets of the treated PDLC was observed to be much smaller than that of the control cell, the contrast ratio of both cells are quite similar over a wide temperature range. The control cell performed better in terms of response time and operating voltage. The retardation characteristics of the cells showed sharp contrast, which may bring about inconsistent consequence in determining their electro-optic properties.

Temperature stability of electro-optic properties of polymer dispersed liquid crystal with different crosslinking monomer in PN393 base pre-polymer

The electro-optic properties of polymer-dispersed liquid crystals (PDLCs) fabricated by photo-induced phase separation have been investigated. A mixture of TL205 (80 wt%) liquid crystal and PN393 (20 wt%) pre-polymer having different crosslinking monomers was used for producing various PDLCs to compare its temperature stability. As a reference, a commercialized PN393 formulation was closely examined to compare the effects of replacing the crosslinker. The liquid crystal domain size varied from 3.5mm to 10.3 μm depending on the crosslinker used. The larger domain size of PDLCs consisting of vinyl ether shows higher contrast ratio but faster response time and higher turnon voltage, which is contrary to the prediction. Acrylate-based PDLCs were found to be turned on, although abruptly increasing below 20°C, and also showed heavy variation of other electro-optic over a wide operation temperature range. By substituting a vinyl-ether for the acrylate crosslinker, the temperature behavior was greatly improved.

UV tuning of the electro‐optical and morphology properties in polymer‐dispersed liquid crystals

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.

The effect of molecular weight of polymer matrix on properties of polymer-dispersed liquid crystals

The electro-optical properties and memory effects are important characters of polymer-dispersed liquid crystal (PDLC) devices. Molecular weight of polymer matrix influences the morphology of liquid crystal droplets in PDLC film and the performance of PDLC devices. In this letter, PDLC films were doped with a small amount of chain transfer agent (CTA), in order to control the molecular weight of polymer matrix. It is observed that the addition of CTA induces a reduction of threshold voltage. In addition, the effect of molecular weight on memory effects of PDLC devices was also discussed. It is found that the entanglement between polymer and liquid crystal molecule increased with the molecular weight, which reduces the memory effect.

Photoswitching in polymer-dispersed liquid crystals

While dealing with photonics, we find that user-controllable electro-optical devices are of great importance. In this paper, the optical control of the light scattering properties of polymer-dispersed liquid crystals has been achieved by dissolving a photoconductive material in the polymer matrix. In the dark, an electric field, lower than the threshold one, is applied across the film, which, consequently, appears opaque. After irradiation with an adequate light beam, the change in the electrical properties of the polymer matrix enhances the effective field across the liquid crystal droplets, making the film transparent. The device can be switched off by turning off either the light source or the electric field. The effects of light intensity and photoconductor concentration on the film phototunability are also discussed.

Electro-optical properties of polymer-dispersed liquid crystal prepared by monodisperse poly(methyl methacrylate)/fluorinated liquid crystal microcapsules

A liquid crystal (LC) containing the fluorine group was encapsulated into poly(methyl methacrylate) (PMMA) particles by using the solute co-diffusion method (SCM). The PMMA/LC composites were applied to a polymer-dispersed liquid crystal (PDLC) and their electro-optical properties were investigated. The medium solvency, methylene chloride (MC) content, and the crosslinking density of substrate particles were adjusted for the complete swelling of LC into the substrate particles during the SCM procedure. In comparison with the PDLC film prepared by conventional PIPS method, the PDLC by microcapsules showed a good electro-optical property due to uniform and mononuclear type of LC domains. In addition, the size of LC droplets in the particles increased with increasing the LC content, resulting in the enhancement of electro-optical property. The hysteresis effect of PDLC using LC microcapsules was negligible.

Fine adjustment of conductivity in polymer-dispersed liquid crystals

The electrical properties of polymer-dispersed liquid crystals (PDLCs) are an important characteristic in their electro-optical performance. Conductivity effects can set up depolarization fields in the films reducing the effective field across the liquid crystal droplets. Both theoretical and experimental investigations have confirmed that the electric field across nematic droplets depends on the liquid crystal and polymer conductivities. In this letter, we have found that the doping of a PDLC with low percentages of a conductive polymer allows a fine adjustment of polymer matrix conductivity. In addition, we have found a large reduction in the re-orientation fields and relaxation times as a function of conductive polymer loading. Results are in rather good agreement with a simple phenomenological model.

Thermophysical, dielectric, and electro-optic properties of nematic liquid crystal droplets confined to a thermoplastic polymer matrix.

The thermophysical, dielectric and electro-optic properties of polymer-dispersed liquid crystal (PDLC) films made of monodisperse polystyrene (PS) and 4-n-pentyl-4(')-cyanobiphenyl (5CB) are investigated by polarized optical microscopy, differential scanning calorimetry, ac impedance analysis, and forward transmittance measurement technique. The PS-5CB system exhibits an upper critical solution temperature (UCST) shape phase diagram with a wide isotropic+isotropic (I+I) miscibility gap between the isotropic and nematic+isotropic (N+I) regions. An absorption domain in the dielectric spectrum of PDLC films was clearly observed at low frequency, and unambiguously assigned to the confined liquid crystalline phase in both nematic and isotropic states. The correlation between the dielectric and electro-optical results for PS-5CB (30:70) samples has shown that in the vicinity of the low frequency absorption domain ( approximately 200 Hz at T=25 degrees C), a drastic decrease in the optical transmittance of the film occurs. This phenomenon can be related to an interfacial polarization process resulting from a charge accumulation at the droplet-polymer interface (Maxwell-Wagner-Sillars effect).

Polymer-dispersed liquid-crystal materials fabricated with frontal polymerization

We report on the morphological and thermal properties of polymer-dispersed liquid crystals (PDLCs) fabricated with frontal-polymerization-induced phase separation (FPIPS). Frontal polymerization is characterized by a rapid-conversion, high-temperature, and large-thermal-gradient environment. A comparison is made between the morphological and thermal properties of PDLCs fabricated with FPIPS and traditional thermal-polymerization-induced phase separation. Characterization includes differential scanning calorimetry to probe the glass and nematic-to-isotropic transitions and scanning electron microscopy to evaluate the phase-separated morphology. In addition, the frontal temperatures and velocities are reported for PDLCs fabricated with frontal polymerization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 204–212, 2003

Electro-Optical Properties of Polymer-Dispersed Liquid Crystals

This paper deals with the electro-optic (EO) properties of polymer-dispersed liquid crystals (PDLCs). Several systems are considered to analyze the effects of preparation conditions and film characteristics on the EO response functions. Particular emphasis is put on systems based on mixtures of the commercial compound ASX-95, the difunctional acrylate tripropylene diacrylate and the eutectic mixture of low-molecular-weight liquid crystals E7. Other systems are considered to assess the influence of monomer functionality on EO properties using for example the trifunctional glycerylpropoxytriacrylate. Various modes of preparing PDLCs are considered based on the mechanism of polymerization-induced phase separation using either electron-beam (EB) or UV radiation curing processes. The dose is changed in both techniques to improve film strength and determine which method leads to the best samples in terms of EO response functions. Other important parameters, such as film thickness, composition and applied voltage, are also considered to evaluate the impact on these functions. The article focuses on a comparison of EO performances of films elaborated by exposure to EB and UV radiations. Under similar conditions, one definitively finds a net superiority of the former technique. In addition this technique does not require any photoinitiator and leads to a higher conversion of the monomeric compounds, i.e. higher mechanical strength and less severe aging conditions.

Polymer-dispersed Liquid Crystals: Effect of Partial Matrix Fluorination on Polymer Bead-based Morphology

The morphology and electrooptical properties of polymer-dispersed liquid crystals (PDLCs) with partially fluorinated polymer matrices are investigated. Films were prepared via photo-induced free-radical polymerization of an initially isotropic solution comprised of pentafunctional (5 reactive groups) acrylate monomer and nematic liquid crystal molecules. Phase separation of discrete domains of LC is induced by the photopolymerization. The film morphologies consisted of aggregated polymeric particles dispersed throughout a continuous liquid crystal medium. Changes in morphology and electrooptical properties were observed as trifluoroethyl and hexafluoroisopropyl methacrylate were partially substituted for the multifunctional acrylate monomer. Methyl methacrylate was used in control films due to the chemical similarities to fluorinated monomers. The incorporation of fluorinated monomers resulted in better definition of the polymer bead morphology and improvement in contrast ratio, while control films displayed less pronounced changes in optical properties. Real-time transmittance monitoring and polarized optical microscopy (POM) revealed that the monofunctional monomer addition resulted in a delay of the LC phase appearence with increasing comonomer concentration. However, a LC phase appeared earlier for fluorinated systems as compared to nonfluorinated control films of similar comonomer concentration. This was interpreted as an indication of lower LC solubility in the semifluorinated polymer matrix.

TSC study and electro-optical properties of epoxy/acrylic polymer-dispersed liquid-crystal film in DICY thermal cure

We report on the structures and electro-optical properties of epoxy/acrylic polymer-dispersed liquid-crystal (PDLC) films. A thermal stimulated current (TSC) analysis was used to investigate the physical structures of PDLC. In the TSC spectrum of PDLC, three relaxation peaks were observed: the glass transition of the liquid crystal, the glass transition of the polymer matrix, and the ρ transition. The ρ transition represents the discharge behavior of space charges, and its intensity increased as the curing time and content of the curing agent dicyandiamide (DICY) increased. The pre-UV-cured films with different DICY contents were thermally cured at 130 °C for various periods. The electro-optical properties of PDLC, such as the contrast ratio and switching voltage, increased as the curing time of DICY, the content of DICY, or both increased. As the ambient temperature increased from 10 to 40 °C, the contrast ratio and switching voltage of PDLC gradually decreased. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 507–514, 2001

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

Optical Properties of Polymer-dispersed Liquid Crystals

Electrooptical properties of polymer-dispersed liquid crystals (LCs) are analyzed from the point of view of the order created by the electric field, dynamic changes of the order (orientation) parameter, and optical transmittivity. Optical phase retardation causes an optical phase shift and is related to thermal effects. Nonlinear optical properties are investigated from the point of view of self-transparency, nonlinear gratings, threshold degenerate wave mixing, optical bistability and second harmonic generation.

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

Amplified stain-rate dependence of deformation in polymer-dispersed liquid-crystal materials

The mechanical properties of polymer-dispersed liquid crystals (PDLCs) are investigated to elucidate their fundamental microstructural and mechanistic underpinnings, and overall structural integrity under deformation. High strain-rate loading experiments and quasistatic compression tests on PDLC materials reveal an amplified strain-rate dependence relative to the matrix material. A finite-element micromechanical model of PDLC materials explains this behavior in terms of strain-rate concentrations at the equators of the spherical liquid-crystal droplets. The model also predicts a weak dependence of the aspect ratio of deforming liquid-crystal droplets on their volume fraction.