Mark Goulding

Lateral fluoro substituted 4-alkyl-4″-chloro-1,1′:4′,1″-terphenyls and derivatives. Useful high birefringence, high stability liquid crystals

Abstract By modification of the basic 4-alkyl-4″-chloro-1,1′:4′,1″-terphenyl structure with lateral fluoro substituents and ethyl linking groups, a wide range of mesogens has been synthesized. Control over the phase behaviour, especially optimization of nematic properties, can be achieved by the degree and position of lateral fluoro substitution and/or the presence and position of an ethyl linking group. The effect of these changes on the physical properties such as dielectric anisotropy (Δe) and birefringence (Δn) has also been studied. Compounds of low melting point, wide nematic range and high birefringence (>0·2) have been made. These materials are suitable for active matrix display applications where high chemical and photostability are necessary.

Transport of charged Aerosol OT inverse micelles in nonpolar liquids.

Surfactants such as Aerosol OT (AOT) are commonly used to stabilize and electrically charge nonpolar colloids in devices such as electronic ink displays. The electrical behavior of such devices is strongly influenced by the presence of charged inverse micelles, formed by excess surfactant that does not cover the particles. The presence of charged inverse micelles results in increased conductivity of the solution, affecting both the energy consumption of the device and its switching characteristics. In this work, we use transient current measurements to investigate the electrical properties of suspensions of the surfactant Aerosol OT in dodecane. No particles are added, to isolate the effect of excess surfactant. The measured currents upon application of a voltage step are found to be exponentially decaying, and can be described by an analytical model based on an equivalent electric circuit. This behavior is physically interpreted, first by the high generation rate of charged inverse micelles giving the suspension resistor like properties, and second by the buildup of layers of charged inverse micelles at both electrodes, acting as capacitors. The model explains the measurements over a large range of surfactant concentrations, applied voltages, and device thicknesses.

Banana‐shaped dopants for flexoelectric nematic mixtures

The synthesis is reported of new meta‐substituted quaterphenyls and several related dimers as dopants for nematic mixtures with flexoelectric properties. The number and nature of the terminal substituents is varied from apolar with a small dipole moment to polar with a strong dipole moment. The number of methylene units in the flexible aliphatic spacer between the polyfluorinated aromatic groups of the dimers is also varied. Dimers capable of banana‐shaped conformers induce an increase in the flexoelectric coefficients of mixtures containing them and reduce the clearing point to a small extent.

Liquid Crystals with a Thiomethyl End Group: Lateral Fluoro Substituted 4-(trans-4-(n-Propyl) Cyclohexylethyl-4′-Thiomethylbiphenyls and 4-n-Alkyl-4″-Thiomethylterphenyls

Abstract The synthesis and mesogenic properties of some lateral fluoro substituted 4-(trans-4-(n-propyl)cyclohexylethyl)-4′-thiomethylbiphenyls and 4-n-alkyl-4″-thiomethylterphenyls are described. Birefringence (Δn) and dielectric anisotropy (Δϵ) measurements for these compounds are also given and compared with analogous terminal chloro, cyano and methoxy compounds. It is found that the use of a terminal thiomethyl group produces compounds of intermediate Δn, Δϵ and clearing point than analogous terminal chloro, methoxy and cyano compounds. The terminal thiomethyl group is also more smectogenic than any of the other three terminal endgroups. In most cases, however, the melting points of the compounds are too high to be of practical use.

Recent developments in materials for TFT/PDLC devices

Liquid crystal mixtures for use over an active matrix substrate must have a high resistivity and, to maintain high value during the lifetime of the display, the materials must be very stable. Many such liquid crystal mixtures are known and used in twisted nematic displays but they are of low birefringence; very few high birefringence liquid crystals of this type are known. Several classes of liquid crystal which fulfill these criteria have been developed and formulated into low melting point mixtures suitable for use with UV curing prepolymers. The properties of PDLC films containing these new mixtures show that the high birefringence values now possible can lead to substantially increased contrast ratios.

Poly(dimethylsiloxane)-Stabilized Polymer Particles from Radical Dispersion Polymerization in Nonpolar Solvent: Influence of Stabilizer Properties and Monomer Type

Particles used in electrophoretic display applications (EPD) must possess a number of specific properties ranging from stability in a nonaqueous solvent, high reflectivity, low polydispersity, and high charge density to name but a few. The manufacture of such particles is best carried out in the solvent of choice for the EPD. This opens up new interests in the study of nonaqueous dispersion polymerization methods, which deliver polymer particles suspended in low dielectric constant solvents. We explore in this article the use of a poly(dimethylsiloxane) macromonomer for the stabilization of poly(methyl methacrylate) polymer particles in dodecane, a typical solvent of choice for EPDs. The use of this stabilizer is significant for this method as it is directly soluble in the reaction medium as opposed to traditionally used poly(12-hydroxystearic acid)-based stabilizers. Additionally, the present study serves as a baseline for subsequent work, where nonaqueous dispersion polymerization will be used to create polymer particles encapsulating liquid droplets and solid pigment particles. In this article, the influence of the macromonomer molecular weight and concentration on the properties of the synthesized particles is studied. In addition, we investigate the possibility of synthesizing polymer particles from other monomers both as a comonomer for methyl methacrylate and as the only monomer in the process. The influence of macromonomer concentration is also studied throughout all experiments.

Rod-shaped dopants for flexoelectric nematic mixtures

We report the synthesis and liquid crystalline behaviour of two series of para-substituted terphenyls as dopants with a rigid rod-like shape, rather than a wedge-, pear- or banana-shape, for guest–host nematic mixtures with flexoelectric properties. One series of liquid crystalline dopants is of low-to-strongly negative dielectric anisotropy and the other is of low-to-strongly positive dielectric anisotropy. The usefulness of apolar and polar rod-like dopants as components of flexoelectric nematic mixtures of positive dielectric anisotropy for use in LCDs is investigated in general and the dependence of the flexoelectric properties of the doped nematic mixtures on the polarity of the dopants is studied in particular. The correlation between the concentration of the dopant and the magnitude of the flexoelastic ratio of several guest–host nematic mixtures is investigated.

26.3: Low Voltage Zenithal Bistable Devices with Wide Operating Windows

Key parameters for any bistable device are operating voltage and temperature range. The development of liquid crystal mixtures for use with grating aligned Zenithal Bistable Displays (ZBD™) is described for the first time. Latching at voltages less than 5V, with 20V addressing faster than 80μs per line and operation over a temperature range from −20°C to 80°C are all demonstrated in the same 4μm device.

Nematogenic laterally fluorinated biphenyls with polar terminal groups

Abstract The physical properties are reported for a series of positive dielectric anisotropy (δe) alkylbiphenyls possessing a halogen-containing terminal group and incorporating varying degrees of lateral fluorination. The nematic range and dielectric and optical anisotropies can be varied by the appropriate choice of terminal group and extent of lateral substitution. These materials possess greater stability than the corresponding cyano compounds and are suitable for use in applications that require high stability.

Electrokinetic pixels with biprimary inks for color displays and color-temperature-tunable smart windows.

We report on the advanced implementation of the biprimary color system in applications where subtractive color is performed inside a single pixel to alter the magnitude and color of reflection (electronic paper displays) or the optical transmission and color temperature (smart windows). A novel device structure can switch between four states: clear, black, either of two complementary colors from RGB and CMY sets, and also mixed states between one of these four states. The device structure utilizes an electrokinetic pixel structure, which combines the spectral performance of in-plane electrophoretic devices with the improved switching speeds of vertical electrophoresis. The electrophoretic dispersions are dual-particle dual-colored and are controlled using two traditional planar electrokinetic electrodes on the front and back substrates, along with a third electrode conveniently located at the perimeter of each unit cell. Demonstrated performance includes contrast ratios reaching ~10∶1, reflectance of ~62%, and transparency of ~75%. For electronic paper displays, these results provide a pathway to double the reflective performance compared to the traditional RGBW color-filter approach. For smart windows, the technology provides not only control of shade (transmission) but also provides complete control over color temperature. Furthermore, this three-electrode device can be roll-to-roll fabricated without need for any alignment steps, requiring only a single micro-replication step followed by self-aligned contact printing of the third electrode.