Electrooptic properties of polymer dispersed liquid crystals

An investigation of the electrooptic properties of polymer dispersed liquid crystals (PDLC) is presented. These materials are light modulating systems. They show a reversible optical response from an opaque state to a highly transmitting state under the action of an appropriate electric field which aligns the liquid crystal director. The switching voltage required to establish such an electric field has been monitored as a function of (i) the starting materials used for the preparation of the PDLCs, (ii) the ageing (curing time) of the PDLC cells. Other physical properties, such as the electrical resistivity and the dielectric constant of the materials, have been measured. The correlations between these properties have been studied. The PDLC switching voltage appears to be strongly correlated with the resistivity. Our data suggest that ionic impurities play a dominant role with respect to the electrooptic response of PDLC films.     

Effects of a chemically modified multiwall carbon nanotubes on electro-optical properties of PDLC films

Polymer dispersed liquid crystal (PDLC) films are fabricated by well-known polymerization-induced phase separation method. In this paper, the dispersion of multi-walled carbon nanotubes (MWCNT) in liquid crystals has been enhanced by chemical modification and we have investigated their effects on the morphology, electro-optical properties and conductivity of the PDLC films. Results indicated that the threshold voltage and the saturation voltage of PDLC films decreased with the increase of the doping concentration of MWCNT or chemically modified MWCNT, because carbon nanotubes can enhance the electric field by reducing the resistivity of the medium and increasing the capacitance of the cells. It can be viewed obviously that the contrast ratio of the PDLC films doped with the chemically modified MWCNT is higher than that of the MWCNT.     

Photopolymerisation kinetics and electro-optical properties in mixtures of dichroic dye-doped nematic liquid crystal and photocurable polymer

This work reports the effect of dye on the photopolymerisation and electro-optical properties of polymer dispersed liquid crystal (PDLC) composite films. Dichroic PDLC (DPDLC) films based on a photocurable polymer and nematic liquid crystal (LC) with an azo dichroic dye were prepared by photopolymerisation-induced phase separation method. Polarising optical microscopy has been used for monitoring the phase separation kinetics and two-phase morphology evolution in the DPDLC system. LC domains with radial structures during initial period of phase separation adopted a resultant morphology of bipolar configuration over the course of polymerisation. The phase separation and morphology of LC domains was found to be dependent on the amount of dye used. Moreover, the addition of small amount of dye reduced the switching voltage, and enhanced the contrast ratio with improved switching time in the PDLC films. It was shown that, under the application of an electric field, the molecular orientation and absorbance of dichroic dye can be controlled in DPDLC to induce non-linearity and colour contrast without the use of polarisers.     

Advantageous voltage-holding ratio characteristics induced by in-plane electric fields, and the optimization concept of liquid crystals for an in-plane switching electro-optical effect

In-plane switching (IPS) of liquid crystals showed advantageous voltage-holding ratio (VHR) characteristics so that liquid crystals with low resistivity could provide higher VHRs compared with the twisted nematic effect. This experimental result was obtained when electric fields were applied approximately parallel to the substrate plane using the IPS electro-optical effect. We found that the in-plane electric field generates supplementary capacities which support retention of an externally applied voltage over the liquid crystal layer during non-selected periods of the active matrix driving scheme, because the liquid crystal layer can be connected with an insulating layer, an orientation layer and even a substrate in parallel. Based on these advantageous VHR characteristics, liquid crystal materials suitable for the IPS effect were appropriately optimized. We propose evaluation parameters, derived from the physical switching principles of the liquid crystals, to obtain lower driving voltage and faster response speeds. These parameters are effective in optimizing the physical properties of liquid crystals without variation of the cell gap. We use the proposed evaluation parameters and the advantageous VHR characteristics to demonstrate the optimization approach and we suggest a novel possible use of liquid crystal materials with low resistivity which cannot be implemented conventionally. Finally, we prove that liquid crystals with low resistivity generate the Ir internal potential by the drift of ionic species.     

Electro-Optical Properties of Inorganic Oxide/Liquid Crystal Composite Film by the Sol-Gel Process

The Electro-optical properties of a composite ceramic/liquid crystal membrane, with the dispersed liquid crystals (LCs) in the high dielectric constant inorganic oxides, were investigated. The composite membranes having bicontinuous micron structure of matrices and LCs were made by the sol-gel process via the syntheses of multicomponent sols using tetra-n-butyltitanate, barium diethoxyethoxide, methyltriethoxsilane and polyethylene glycol (PEG) for phase separation. The switching behaviors of the composite films have been observed by applying AC voltages at 1 kHz. It was also confirmed that the driving voltage of the composite film depended on the composition of the inorganic oxide matrix. The composite films of high dielectric constant were driven at an electric field under 10 kV/cm, much less than that of existing PDLC.     

Mechanism of electrooptic switching time enhancement in ferroelectric liquid crystal/gold nanoparticles dispersion

It is well established that incorporation of nanoparticles (NPs) in the structure of ferroelectric liquid crystals (FLCs) leads to a decrease in their electrooptic response time. Several approaches have been suggested to explain this effect (decrease in rotational viscosity of FLCs, ions enhanced localised electric field, dipole–dipole interaction among NPs and FLC molecules, FLC ordering). In this article, we will report the role of the voltage divider formed by the structural elements of a FLC cell based on ferroelectric liquid crystal/gold nanospheres (FLC/GNSs) dispersion in enhancement of the switching time. Using the impedance spectroscopic measurements, it was demonstrated that the dispersing of GNSs leads to the increase in the voltage drop on FLC/GNSs layer in comparison with the pristine FLC one. Consequently, the electrooptic response time of the FLC/GNSs cell is faster than that of the pristine one. However, the rotational viscosity of the FLC does not depend on the presence of the GNSs.     

The effect of nanographites dispersed in Undec-10-enic acid on optical property and morphology of PDLC films

The electric conductivity is an important factor for reducing the switching voltage of polymer-dispersed liquid crystals (PDLC) films. The electric conductivity of polymer matrix is changed by doped nanographite which is uniform dispersed in polymer matrix in acid condition. The influence of doped nanographite to switching electric field is studied. With increasing of doped nanographite, the switching voltage is dramatically reduced. The effect of nanographite on the polymerization and electro-optic are discussed. The kinetic polymerization of the PDLCs is monitored in lights scattering by UV/VIS spectrometer. The polymerization speed is compared by the max scattering point in different samples which doped by nanographite. The electro-optic of PDLCs films is measured by Polarimeter (PerkinElmer Model 341) to determine the threshold voltage. Information gained from polarizing optical microscope and Fourier transform infrared image depict the morphology of the liquid crystal droplets dispersed in polymer matrix.     

The electro-optical properties of 'charged' PDLCs

The application of high intensity electric fields to polymer dispersed liquid crystal (PDLC) films can induce changes in their electro-optical properties and morphology. In particular, a quasilinear electro-optical response to an external electric field can be achieved if an internal built-in d.c. field is induced. In this work, we show how the liquid crystal/polymer weight ratio influences the electro-optical response of 'charged' PDLCs, i.e. of PDLC films after the application of a high intensity electric field. We observed that a quasilinear electro-optical response can be achieved in a well determined range of composition. Larger liquid crystal concentrations are unable to maintain the built-in field, while PDLCs with lower liquid crystal loadings do not allow the onset of a built-in d.c. field.     

Complex study of reorientational dynamics of the liquid crystal in PDLC films

Morphological, electro-optical and switching properties of polyester resin/nematic liquid crystal composite films have been studied for varying composition (10-40 wt% of LC), temperature (20-50 C), film thickness (10-75 mum) and UV curing time of the matrix (0.25-12 min). The PDLC films were formed by LC separation in a UV polymerization process of the thin layer of oligoester resin (liquid crystal mixture) between ITO coated glass plates. The electro-optical and response behaviour based on the electric field controlled light scattering of the composite films was observed. The results were interpreted in terms of effective anchoring strength at the interface of the polymer and liquid crystal depending either on the area fraction of the interface in the composite film (dependent on the size and shape of the liquid crystal droplets) or the stiffness and resistivity of the polyester resin changing in the course of the crosslinking polymerization.     

Switchable columnar metallomesogens

Chiral columnar liquid crystals have recently appeared as a promising new type of ferroelectric materials. To date, all the columnar liquid crystals that have been reported to show ferroelectric switching consist of organic compounds. However, metal-containing liquid crystals open this field to a significant number of new structures and offer the possibility of adding to the ferroelectric behavior other properties inherent to the presence of metals in the structure, such as magnetism, as well as the use of new methods of characterization (EPR, synchrotron radiation, etc.). The potential of columnar metal-containing liquid crystals as ferroelectric materials has been demonstrated even though only a few organic columnar ferroelectric liquid crystals have been described. As a first approach to this type of material, this concepts article describes the results obtained with chiral metal beta-diketonates that show ferroelectric switching in the columnar mesophase. It has been shown that these materials have a helical columnar arrangement in the mesomorphic state, and a chiral superstructure has been proposed from circular dichroism studies. This type of supramolecular structure plays a fundamental role in the ferroelectric properties of these compounds. The discussion is mainly focused on the strategy employed for the molecular design, and on the interpretation of the mesophase structure and the electrooptic effect. The use of a diverse range of techniques, both those commonly used in the field of liquid crystals and those that are more specific will be highlighted, and the principles of these specific techniques are summarized together with a justification of their applicability to this study.     

Elaboration and electro-optic study of a new type of open porosity PDLC

A new polymer dispersed liquid crystal (PDLC) system characterized by easy processing and an open porosity has been elaborated. This PDLC is based on a pre-formed, porous, thin polymer film of a commercially available PVDF-HFP copolymer wetted by the eutectic mixture of cyano bi- and ter-phenyls known as E7 (Merck Ltd, UK). This new process is of interest because of its simplicity, and the fact that there is no risk of intermixing between the liquid crystal and the polymer matrix as occurs in a conventional PDLC. An electric field applied across the thin film results in a change in its transmission, due to the reorientation of the liquid crystal director, as already known for closed porosity PDLCs. The electro-optic properties of this PDLC have been studied and semi-quantitatively interpreted on the basis of the response theory of conventional closed porosity PDLCs.     

The electro‐optical properties of ‘charged’ PDLCs

The application of high intensity electric fields to polymer dispersed liquid crystal (PDLC) films can induce changes in their electro‐optical properties and morphology. In particular, a quasilinear electro‐optical response to an external electric field can be achieved if an internal built‐in d.c. field is induced. In this work, we show how the liquid crystal/polymer weight ratio influences the electro‐optical response of ‘charged’ PDLCs, i.e. of PDLC films after the application of a high intensity electric field. We observed that a quasilinear electro‐optical response can be achieved in a well determined range of composition. Larger liquid crystal concentrations are unable to maintain the built‐in field, while PDLCs with lower liquid crystal loadings do not allow the onset of a built‐in d.c. field.     

An investigation on a novel PDLC film’s fabrication and its electro-optical properties

A polymer dispersed liquid crystal (PDLC) film that has good electro-optical properties is produced by the method of polymerized-induced phase separation. Based on the application foreground, its capability parameters, such as contrast ratio, work voltage, and visual angle, are characterized for the first time by a white light but not a fixed wavelength light. The results show the PDLC film has a low work-voltage of 20 V, more than 150° visual angle, high stability, and long lifetime. The differences between plastic and glass ITO-coated substrates of PDLC films are also studied in this paper. The plastic substrate has better property and will have a wider perspective especially in the portable, tender and folded display devices. Due to adjustable properties of film by electric field, PDLC has the potential application for display device, sensor, switch, grating, and new generation analytical apparatus.     

Reverse mode operation polymer dispersed liquid crystal with a positive dielectric anisotropy liquid crystal

The development of electrically activated chromogenic materials is important for their potential applications in smart windows. Several previous works have reported on reverse mode operation polymer dispersed liquid crystals (PDLCs) based on negative dielectric anisotropy liquid crystals. They have a transparent OFF state, which turns opaque after the application of a suitable external electric field. Nevertheless, these devices have some limitations such as the use of large amount of expensive liquid crystals with peculiar physical‐chemical properties. In addition, a good matching between the refractive index of liquid crystal and the polymer matrix one is required. The main result of this work is the achievement of reverse mode operation devices prepared with a positive dielectric anisotropy liquid crystal and characterized by a high OFF state transmittance obtained by the onset of high intensity built‐in DC electric fields in a direct mode operation PDLC, which allows the OFF state homeotropic alignment of liquid crystal directors. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Highly transparent memory states in polymer dispersed liquid crystal films

Electro-optical properties of polymer dispersed liquid crystal (PDLC) films which have reversed morphology are investigated. Highly transparent memory states, for which transmittances exceed more than 80 per cent, are observed in these PDLC films. The saturation voltage V₉₀ can be decreased by a PTF (a phase transition with a field) operation and becomes 10 times lower than that without PTF operation. A contrast ratio of more than 600 is obtained in the memory state of a thick PDLC film.     

The effect of voltage controlled orientation order of liquid crystals in non-acrylic polymer dispersed liquid crystals films

The nematic liquid crystals (LCs) are randomly dispersed material with random orientation order in polymer dispersed liquid crystal (PDLC) films. The LCs change their orientation from random to vertical as electric field is applied. This transformation of orientation order of nematic liquid crystals in the PDLC films is controlled by many factors operating simultaneously. For instance, some factors like the internal forces of attractions among the neighboring LC molecules, anchoring with polymeric matrix, ITO glass boundaries, and chemical structures of the materials are less studied. The learning of extent of vertical orientation of liquid crystal droplets in an electric field is essential to attain optimum electro optical properties of PDLCs. In this finding, bipolar and radial LCs droplets with random orientation have been observed in non-acrylic polymeric media. It is learned that with small increase of contents of external material, the extent of vertical orientation has been varied intensely. The extent of vertical orientation of LCs molecules increases as the contents of external non-acrylic polymeric material decreased. For this study, the orientations of LCs with respect to material type/contents, external applied force, and restoration of electric filed as hysteresis have been studied in details.     

The effect of mono-sized liquid crystal domains on electro-optical properties in a polymer dispersed liquid crystal prepared by using monodisperse poly(methylmethacrylate)/liquid crystal microcapsules

Highly mono-sized poly(methyl methacrylate) (PMMA)/liquid crystal (LC) microcapsules having a mono-sized single LC domain were prepared by the solute codiffusion method and solvent evaporation. The size of the LC domain in the microcapsules could be controlled by the amount of LC introduced during the swelling stage. The electro-optical properties of the polymer dispersed liquid crystal (PDLC) prepared by using the microcapsules was highly improved. In particular, the threshold voltage was lowered and the switching behaviour with an applied electric field was sharpened drastically compared with PDLC prepared simply by solvent evaporation-induced phase separation.     

The effect of mono-sized liquid crystal domains on electro-optical properties in a polymer dispersed liquid crystal prepared by using monodisperse poly(methylmethacrylate)/liquid crystal microcapsules

Highly mono-sized poly(methyl methacrylate) (PMMA)/liquid crystal (LC) microcapsules having a mono-sized single LC domain were prepared by the solute codiffusion method and solvent evaporation. The size of the LC domain in the microcapsules could be controlled by the amount of LC introduced during the swelling stage. The electro-optical properties of the polymer dispersed liquid crystal (PDLC) prepared by using the microcapsules was highly improved. In particular, the threshold voltage was lowered and the switching behaviour with an applied electric field was sharpened drastically compared with PDLC prepared simply by solvent evaporation-induced phase separation.     

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     

Measurement of the rotational viscosity of ferroelectric liquid crystals based on a simple dynamical model

Rotational viscosity and spontaneous polarization are the most important properties of a ferroelectric liquid crystal with regard to its switching time in surface stabilized or a.c. field stabilized displays. Whereas there is an abundant literature about spontaneous polarization, only a few attempts have been made to determine the rotational viscosity. We set up a model for the electric response of a ferroelectric liquid crystal cell on application of an electric field. For the application of a triangular wave voltage we derive a relation between the rotational viscosity, the spontaneous polarization, the tilt angle, the maximum induced polarization current and the electric field strength. Experiments are carried out on several ferroelectric liquid crystals and the derived relation was used to determine the rotational viscosity. The relation between the rotational viscosity and the polarization on the one hand and the optical switching time on the other hand is discussed in some detail.