Properties of Organosiloxane liquid crystals for dye guest host ferroelectric display devices

Abstract

The purpose of this thesis is to investigate a new series of organosiloxane liquid crystals and the effects of dye addition by mixing and synthetic addition. The organosiloxane compounds are the result of a building block process that seeks to combine the high tilt and wide phase range properties of laterally substituted biphenyl benzoate mesogens with the polymer ruggedness of siloxane head groups. Initially the biphenyl benzoate mesogenic precursors are investigated with bromine, chlorine and fluorine substituents. The materials yield 20-45° tilt angles with 70-130 nCcm"^ spontaneous polarisation. When combined with a siloxane head group, the only liquid crystal phase seen is a 50°C wide SmC* phase in each of the three compounds. The siloxane moieties micro-segregate and form a virtual backbone. As a consequence of the new molecular arrangement, temperature independent, near 45° or 45° tilt angles are measured for all three compounds. Switching time measurements reveal sub-millisecond switching down to 40°C below the I—>SmC* transition. The attachment of a biphenyl benzoate molecule at either end of the siloxane core results in a series of bi-mesogenic compounds. These compounds yield 60°C wide antiferroelectric SmC*A phases. The antiferroelectric nature is proposed to arise due to a bent conformation of the bi-mesogenic molecules. The spontaneous polarisation is further enhanced to 100-140 nCcm'^ whilst the other electro optic properties broadly follow similar trends to the mono-siloxane compounds. The synthesis of 45° tilt ferroelectric compounds opens up the possibility of many new device applications. The feasibility of transferring the benefits offered by ferroelectrics to the field of dyed and single polariser devices is investigated. Three commercial dyes are mixed with the laterally halogen substituted mono mesogenic organosiloxanes. The phase sequence of the host is maintained along with the near 45° tilt angles of the host material. The magnitude of the spontaneous polarisation is reduced in line with the dilution effect of the non-liquid crystalline dye compounds. The switching times, although marginally increased with dye addition, still remained in the hundred microsecond regime demonstrated by the hosts. With the addition of one of the commercial dyes to an antiferroelectric host, three optically distinct states can be observed in a suitably oriented cell. This single polariser tri-state dye guest host device is only made possible by the near 45° tilt angle of the bi-mesogenic organosiloxane compounds. In order to introduce more dye into the organosiloxane hosts, a nitrostilbene dye molecule is grafted onto a siloxane core via an eleven carbon length alkyl chain. Following the pattern of the host organosiloxanes, monomeric and dimeric nitrostilbene compounds are studied, and the effect that grafting has on the dye's absorption properties is examined. This attempt to mimic the host structure proved highly successful, with the resultant organosiloxane dyes exhibiting liquid crystalline phases and showing complete miscibility with the organosiloxane hosts over the entire concentration range. Of particular note is the presence of antiferroelectricity in a mono-mesogenic ferroelectric host on addition of a dimeric non-ferroelectric dye. The results of this work show that the virtual backbone effect in low molar mass organosiloxane liquid crystals can be used to increase the solubility of a dye guest moiety and influence the stability and form of the mesophases. The ferroelectric / antiferroelectric characteristics of the liquid-crystal host can be retained and room temperature ferroelectric phases are created with broad temperature invariant properties. In this thesis, as well as the study of the physical and chemical properties of these new materials, some of the potential devices that now become possible are considered.

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