Orientation of Ferroelectric Liquid Crystals by Self-Assembled Bilayer Molecular Assembly
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Orientation of Ferroelectric Liquid Crystals by Self-Assembled Bilayer Molecular Assembly Sukhmal C. Jain, Vivechana Dixit, Vinod K. Tanwar and S. M. Shivaprasad Polymeric and Soft Materials Division, National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi-110012, India ABSTRACT Molecular films of amino propyl triethoxy silane (APTES) attached with cinnamoyl moieties were deposited on indium tin oxide (ITO) coated glass and quartz substrates by selfassembly technique. The deposition of the monolayer films resulted in a large change in the wettability of the substrates. X-ray photoelectron spectroscopy (XPS) and UV-spectroscopy were used to study the structure and properties of the grown films. The molecular assembly was photodimerized by irradiating it with a linearly polarized UV-light. Thus created polymer-solid interface has been shown to produce good, stable planar orientation of ferroelectric liquid crystal mixtures. The electro-optical and switching properties of the ferroelectric liquid crystal cells have been investigated and found to be quite similar to those prepared in conventional manner. INTRODUCTION The field of self-assembled (SA) monolayers and multilayers of various organic surfactants have witnessed a phenomenal growth in last one decade due to increasing level of sophistication in their synthesis and characterization [1-8]. The SA technique provides for the needed designed flexibility, both at the individual molecular level and the material level and thus presents unique opportunities to create interfaces with desired structure and properties at different substrates. In this technique, the molecules are directly chemisorbed on a hydroxylated substrate under controlled environmental conditions leading to the formation of a two-dimensional ordered molecular assembly. The monolayers of alkoxy-alkyl-amine-silanes have been used for wide ranging applications including [1-3] surface modification of silica and alumina, chromatography, adhesion promoters between polymer and silica or glass surfaces, immobilization of the enzymes etc. Recently, a new technique based on photodimerization of the self-assembled bilayer molecular assembly to orient nematic liquid crystals has been reported [11-14]. Photo-alignment techniques [11-20] are finding great favor, as they do not require any mechanical rubbing of the substrates, which can be a source of dust, and electrostatic charge buildup that can be detrimental to the life and quality of the liquid crystal displays. In the present work, we wish to report on the uniform planar orientation of ferroelectric liquid crystals utilizing anisotropic polymer-solid interfaces created by photodimerization of the molecular films of APTES attached with cinnamoyl moieties. The growth of the molecular films has been monitored by i) contact angle measurement, ii) XPS and iii) UV-spectroscopy. The optical polarizing microscopy and electrooptical measurements were used to study the alignment and switching behavior of photo-aligned liquid crystal cells.
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