Use of Liquid Crystals as Vapor Detectors

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Abstract
The visual detection between crossed polarizers of organic vapors on various liquid crystal films is described. The liquid crystal is utilized as a smear on glass or as an impregnant in microporous polypropylene. The organic vapor induces a change of the liquid crystal from a birefringent mesophase to an isotropic liquid. The effect is generally reversible, i.e., removal of the film from the contaminated atmosphere restores the original state of the film. Non-selective detection is achieved easily in the ppm range. Of the liquid crystals examined, the most sensitive detector was found to be N-(p-methoxybenzilidine)-p-n-butylaniline (MBBA). Optical reflection from cholesteric materials, has been utilized previously for detection purposes. However, dependence on temperature and the need for instrumental monitoring to achieve high sensitivity do not currently allow the achievement of simple detector devices. In contrast, temperature effects and the need for instrumental monitoring are not as important with the systems described in the present paper. Also, opportunities exist for bettering sensitivity and selectivity.