Camille I. Van Ast

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...

Experimental Methods for Determining the Eutectic Composition of a Multi-Component Liquid Crystal Mixture

Abstract Experimental techniques are described for determining the eutectic composition of multicomponent mixtures of nematic liquid crystals. These techniques were applied to the formulation of nematic esters, when the idealized eutectic composition calculated by the Schroeder-Van Laar relationship did not give a good melting point minimum for the mixture. One experimental procedure was based on the assumption that the composition of the initial nematic melt from a frozen, crystallized mixture is closer to a true eutectic than is the calculated value from which the frozen mixture was formulated. Another experimental procedure was based on the assumption that a new eutectic could be obtained by adding small amounts of a new component to an existing multicomponent eutectic mixture, using the a plot of the melting point versus the percentage of the new component added to determine a new eutectic mixture corresponding to a new melting point minimum. High performance liquid chromatography was used to estimate...

Conductive Polymer-Based Transducers as Vapor-Phase Detectors

Publisher Summary This chapter discusses the development of conductive polymer-based transducers for the detection of volatile organic compounds (VOCs) and other gaseous pollutants for application in environmental monitoring. In the experiment described in the chapter, highly sensitive sensor elements derived from polyaniline (PANI) are demonstrated for NO2 and other VOCs. A figure of merit is defined to allow the comparison of the response of different sensors to particular challenge vapors at various concentrations. The use of excess silane coupling agents to modify the surface of the sensor substrates in combination with excess of a sulfonic acid used to convert PANI to its conductive state results in highly sensitive, stable, and reversible VOC sensors, along with some polythiophene derivatives, for the detection of ketones, esters, and aromatic hydrocarbons. Individual sensors show different sensitivities to each class of VOC used in the study so that an array of these sensors generated a characteristic signature for each class demonstrating selectivity and classification.

Poly(methacrylic anhydride) positive electron beam resist

Poly(methacrylic anhydride) (PMAH) is a positive resist for use in electron beam (e‐beam) lithography. It is derived by heating coatings of the precursor polymer, poly(tert‐butyl methacrylate) or PtBMA, on the wafer. Crosslinks are introduced during the thermal conversion. The sensitivity and line profiles of PMAH e‐beam images are significantly affected by the synthesis route, the pre‐exposure processing, and the development method. Development with organic solvents can swell and soften the resist, especially in areas that received some exposure to radiation. This causes the walls of the image to expand or flow into the image cavity, thereby limiting the sensitivity at which high resolution can be achieved. However, the use of a new, nonswelling, basic developer gives PMAH a sensitivity of 2.5 μC/cm2 with better than 0.5 μm resolution.

Effects of Molecular Length on Nematic Mixtures — IV: Structure Effects on Viscosity of Ester Mixtures

The flow viscosity (η) of nematic ester mixtures is studied as a function of average molecular length (\(\bar L\)), chemical structure, and temperature. When \(\bar L\) is increased by use of longer alkyl end groups, the η of less polar mixtures increases while the η of more polar mixtures changes slightly or even decreases. However, when cybotactic nematic characteristics occur with increased \(\bar L\), then η increases sharply. Studies of η are made for eighteen different classes of ester structures used as additives in 4-alkoxyphenyl 4-alkylbenzoate (R0-R’) mixtures at fixed values of \(\bar L\) for both additive components and mixtures. Many interesting effects of structure on η are observed, and approximate class viscosities (ηclass) at 25°C are assigned to each of the 18 classes. The η25° of other ester mixtures is estimated by summing ηclass times the mole fraction of that class present in the mixture. These ηcalc values are generally within 10% of the actual η25° for multicomponent ester mixtures containing some RO-R’ components. Temperature variations often result in a non-linear plot for log η vs T-1 of ester mixtures. The apparent activation energy between 25° and 40°C generally increases strongly with the η of the mixture, ranging from 6.7 to 12.0 kcal/mole between η25° values of 16 and 188 cP.