Planar Orientation of Nematic Liquid Crystals by Chemisorbed Polyvinyl Alcohol Surface Layers

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Abstract
Boundary surface films of monomolecular thickness of polyvinyl alcohol PVA deposited in a specific way onto transparent ITO or TAO electrodes induced parallel alignment of most available nematic liquid crystals having positive dielectric anisotropy. Chemisorption of PVA is improved by a previous ethyl silicate immersion bath. Stable surface layers are obtained by condensation reaction during curing at 180°C. Further, parallel orientation is achieved mechanically by unidirectional buffing of the PVA film. By changing the duration of solution adsorption, one can vary the tilt angle. From concentration measurements of PVA on ITO surfaces by means of a C14 marker technique it has been found that the molecules of PVA are lying not flat on the surface; those segments of the polymer chain which are not directly attached to the surface form loops. These polymer loops become asymmetric after buffing and curing and thus generate tilted profiles in the adjacent liquid crystal layer. The influence of these various treatments on the static and transient optical transmission characteristics of two particular liquid crystal mixtures is discussed. The change in sharpness of transition is correlatable to the anchoring characteristics of the PVA film through chemical interactions, while the rise and decay characteristics are more related to the mechanical orientation of the surfactant film. These silica-PVA surfactant films are very convenient for the mass production of durable, large-area liquid crystal displays.