J. David Margerum

Liquid crystal dyes with high solubility and large dielectric anisotropy

Several new liquid crystal dyes were synthesized and their physical properties characterized. These dyes exhibit an excellent solubility in nematic liquid crystals, high birefringence, and huge dielectric anisotropy. High solubility results from their small enthalpy of fusion and relatively low melting point, high birefringence from their long molecular conjugation, and huge dielectric anisotropy from their large dipole moment. Potential use of these dyes in infrared and millimeter wave liquid crystal modulators are emphasized.

Near Ultraviolet Photostability of Liquid Crystal Mixtures

Abstract Liquid crystal material photostability was investigated for application in high intensity projectors operating with both near ultraviolet (UV) and visible light. Commercially available mixtures were studied in transparent test cells and their lifetimes were correlated with their chemical structures. Substantial improvements were observed in the photostability of hybrid field effect cells by using pure nematic mixtures whose major components had one or two cyclohexyl groups in place of phenyl groups. However. overriding the importance of these structural effects, polar impurities present in some of the nematic mixtures with cyclohexane groups caused a large decrease in photostability. Results were obtained by examining the photostability of eight different commercial liquid crystal mixtures, with highly positive dielectric anisotropy, whose components include a variety of structural types. Although the resistivity of each liquid crystal mixture decreased steadily with exposure, the reported end of...

Improved dc dynamic scattering with redox dopants in ester liquid crystals

Redox dopants are used to improve the dc dynamic scattering (DS) characteristics of phenyl benzoate liquid crystals as compared to the use of salt‐type conductive dopants. The redox dopants are selected by their electrochemical properties so that they react preferentially and reversibly at the electrodes, thus providing the current carriers for the dc‐DS effect. Low‐threshold voltages (∼2 V), high scattering efficiency (99% at 20 V), and long dc‐DS lifetimes (up to 18 000 h) are obtained with an ester liquid crystal containing a redox dopant pair consisting of di‐n‐butyl‐ferrocene and (2,4,7‐trinitro‐9‐fluorenylidene) malononitrile.

Effects of Molecular Length on Nematic Mixtures

The properties of nematic mixtures of 4-alkoxyphenyl 4-alkylbenzoates are studied as a function of their average molecular length (L). The Ls of the mixtures vary between 20.46 and 27.14 A, but they all have clearpoints in the 51 to 58° C range. The flow viscosity increases linearly with L. The dielectric constant, refractive index, birefringence, and density decrease linearly with L, and the dielectric anisotropy becomes more negative. With tetrabutylammonium tetraphenylboride added as a salt dopant, the conductivity anisotropy (σ∥/στ) decreases with increasing L, and the dynamic-scattering (DS) threshold voltage increases correspondingly. The effect of L on the DS decay time (τD) is highly dependent on the surface alignment. In surface ⊥ cells, τD increases strongly with L, while in surface ∥ cells, τD decreases slightly with increasing L. The temperature dependence of σ∥/στ indicates that the longer L mixtures, with about ten or more total alkyl carbons from both end groups, have cybotactic n...

Electro-optical applications of liquid crystals

Abstract The unique properties of thermotropic liquid crystals have led to the development of many new electro-optical devices, particularly for display applications. Basic properties and surface alignments of liquid crystals are reviewed with regard to these applications. Electro-optical effects based on conductivity and field effect alignment are described for both nematic and cholesteric materials. Three applications of nematics are selected for more detailed discussion: a flat panel television display using dynamic scattering activated by a semiconductor matrix, a watch display using polarization modulation of twisted nematic cells on transparent segment electrodes, and a large screen projection system using tunable birefringence in a photoactivated light valve.

Addressing factors for polymer-dispersed liquid-crystal displays

Suitable addressing methods for polymer dispersed liquid crystal (PDLC) displays are dependent upon both the type of display and the PDLC properties. Video displays require fast frame rates, high resolution, and gray scale. The addressing method depends upon PDLC properties such as threshold voltage, operating voltages, response times, resistivity, and hysteresis effects. the properties of PDLC films which are formed by ultraviolet (UV) polymerization of LC/monomer solutions depend upon their formation conditions. The threshold and operating voltages, contrast ratio and response times are affected by formation factors including the UV intensity, temperature, LC concentration, LC composition, field pre- alignment conditions, cell thickness, and droplet shape. The PDLC resistivity and hysteresis properties are more difficult to control, and can strongly affect the light throughput, response times, and gray scale of video displays. Apparent limitations from these properties can be overcome by utilizing pulse mode addressing factors pertinent for active matrix and photoactivated light valve PDLC displays.

Effects of Molecular Length on Nematic Mixtures: II Anisotropic and Dynamic Scattering Properties of 4-Alkoxyphenyl 4-Alkylcyclohexanecarboxylate Mixtures

The properties of nematic liquid-crystal mixtures of 4-alkoxyphenyl trans-4-alkylcyclohexane-carboxylates are studied as a function of temperature and their average molecular length (L). Mixtures are prepared with clearpoints near 72° C (±3° C) and with [Lbar] varying between 21.20 and 26.15 A. At 25° C, their flow viscosity increases exponentially (from 16.3 to 50.6 cP) as [Lbar] increases. The conductivity anisotropy (σ∥/στ) also varies tremendously, decreasing from 1.62 to 0.49 as L increases, when compared at 25° C with tetrabutylammonium tetraphenylboride as dopant. The low values of σ∥/στ and its temperature dependence indicate that cybotatic nematic characteristics occur when the average total number of alkyl carbons from both end groups is 8.5 or more. Short range smectic effects are dominant in the longer mixtures. All of the mixtures have a negative dielectric anisotropy, which linearly becomes less negative with increasing L. At 25° C, dynamic scattering (DS) is observed only in the sh...Abstract The properties of nematic liquid-crystal mixtures of 4-alkoxyphenyl trans-4-alkylcyclohexane-carboxylates are studied as a function of temperature and their average molecular length (L). Mixtures are prepared with clearpoints near 72° C (±3° C) and with [Lbar] varying between 21.20 and 26.15 A. At 25° C, their flow viscosity increases exponentially (from 16.3 to 50.6 cP) as [Lbar] increases. The conductivity anisotropy (σ∥/στ) also varies tremendously, decreasing from 1.62 to 0.49 as L increases, when compared at 25° C with tetrabutylammonium tetraphenylboride as dopant. The low values of σ∥/στ and its temperature dependence indicate that cybotatic nematic characteristics occur when the average total number of alkyl carbons from both end groups is 8.5 or more. Short range smectic effects are dominant in the longer mixtures. All of the mixtures have a negative dielectric anisotropy, which linearly becomes less negative with increasing L. At 25° C, dynamic scattering (DS) is observed only in the sh...

Ester Liquid Crystal Mixtures for Dynamic Scattering at Elevated Temperatures

Abstract Studies are made on the effect of eievated temperature on the properties, thermal stability, and dynamic scattering (DS) of liquid crystal, ester mixtures. Six eutectic nematic mixtures are formulated with clearpoints in the range of 72[ddot] to 81[ddot]C; structural effects are evaluated by using different combinations of components form eight classes of esters, including phenyl benzoates, phynyl thiobenzoates, phenyl cyclohexanecarboxylates, phenyl acyloxybenzoates, as well as a phenyl benzoyloxybenzoate and a phenyl biphenylcarboxylate structure. The mixtures are charactersized at 25[ddot]C, and their viscosity, dielectric anisortropy, conductivity anisotropy, and dc-activated DS effects are studied as a function of temperature. Thermal instabilities are studied at 100[ddot]C, With and without redox conductivity dopants. The DS stability with 20 V dc is studied at 55[ddot]C for three of the more stable mixtures, both in transparent cells and in cells with reflective metal electrodes.

Effect of Dopants on the Conductivity Anisotropy and AC Dynamic Scattering of Liquid Crystals

Abstract The dynamic scattering (DS) characteristics and the conductivity anisotropy ratio (Rσ = σ∥/σ1) are shown to depend upon the structure and concentration of conductivity dopants as well as upon the structure of two nematic liquid crystals. For both a phenyl benzoate liquid crystal and for MBBA the ac threshold voltage (V th) decreases with increasing Rσ, and the optical density of scattering at 30 V rms is directly proportional to Rσ. In each liquid crystal there is a linear relationship between V −2 th and R −1σ, in agreement with the Carr-Helfrich theory.

Effects of Molecular Length on Nematic Mixtures. III. Anisotropic Properties of 4-Alkylphenyl 4-Alkoxybenzoate Mixtures

Abstract Properties of multicomponent nematic mixtures of 4-alkylphenyl 4-alkoxybenzoates are studied as a function of temperature and their average molecular length (L). The Ls vary between 21.47 and 27.67 A for mixtures with clearpoints in the 43 to 60°C range. The refractive indices, density, and the e1 component of dielectric permitivity all decrease approximately linearly as L increases. There are much larger changes in flow viscosity (η), conductivity anisotropy ([sgrave]|/[sgrave]|), and dielectric anisotropy (Δe). At 25°C as L increases: The is relatively high and increases sharply (from 47 to 78 cP), the [sgrave]|/[sgrave]| decreases and drops below unity (from 1.35 to 0.58), and the Δe changes from positive to negative (from +0.27 to -0.26). The temperature dependence of [sgrave]|/[sgrave]|, with tetrabutyl-ammonium tetraphenylboride as a dopant, indicates cybotactic nematic characteristics are present for the longer L mixtures in which there are an average of about nine or more alkyl carbons f...