Electro-Optic Effects in Liquid Crystal Side Chain Polymers

Abstract

The recent work of Krigbaum1 and Blumstein2 on main chain polymer liquid crystals and that of Finkelmann3 and Ringsdorf4 on side chain systems has generated considerable interest in the potential of these materials for use in electro-optic devices. Indeed over the last three years or so papers have started appearing reporting on electro-optic effects in a variety of polymer liquid crystal systems5,6. The main thrust of this initial work has been to examine nematic and cholesteric polymer liquid crystals presumably in the hope of producing electro-optic effects and displays analogous to those observed with monomeric mesophases. However, as observed by Ringsdorf and Zentel6 and Finkelmann et al5, for such polymer materials, the operating parameters are not as convenient as for monomeric systems, i.e. the threshold voltages are higher, the response times are two to ten times slower and the operating temperatures are not usually convenient. Whilst the promise of these new polymer materials would seem to be in combining the polymer specific with the monomeric specific liquid crystal properties the resultant high viscosity associated with the polymer chain would seem to be a major drawback to these systems. The obvious conclusion is therefore that such polymers, in their pure state, will always have inferior performance to equivalent monomers. However as Finkelmann pointed out7 the operating voltages for side chain systems are not impractical and it is possible to use the mesophase to glass transition (at Tg) to store information written electro-optically in the nematic or cholesteric polymer phases. This storage property is fundamental to these polymer liquid crystal materials and appears to be their most important feature. For such a storage device to be useful Tg must be above ambient temperatures. This implies very slow response times (minutes to hours) if the material is heated just above Tg due to the inherent high viscosity of the polymer or very high operating temperatures (~200°C) if response times of less than a second are to be achieved in these nematic or cholesteric polymer liquid crystals. These features would not seem to be desirable for practical storage devices requiring contrast between written and unwritten regions.