Electro-optic studies of the flexoelectric effect in chiral nematic liquid crystals

Musgrave, Bronje (2000) Electro-optic studies of the flexoelectric effect in chiral nematic liquid crystals. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

With the advent of global telecommunications networks and the Internet, the development of portable display technology has gained a new impetus. Liquid crystal devices have played a major role in this area, most conspicuously as displays in laptop computers. To date, these liquid crystalline devices have been generally based on the rather slow (~30ms) dielectric response of the achiral nematic liquid crystal phase, although more expensive devices based on the faster (< 100 μs) ferroelectric switching of the smectic C* phase are in production.

The research presented in this thesis relates to a new switching effect recently discovered in the chiral nematic phase. The flexoelectrically-driven rotation of the chiral nematic phase's optic axis is fast - of the order 10 μs to 1ms - proportional to the applied field amplitude and completely in-plane. The optic axis has been deflected by over 30^o from the equilibrium position in some materials. These electro-optic properties make the 'flexoelectro-optic' effect a potential contender in the liquid crystal device market.

The present thesis contains the first studies of the effect of molecular structure on flexoelectric coupling in the chiral nematic phase. Several homologous series of estradiol-cyanobiphenyl bimesogenic materials synthesized for this work have been characterized and their electro-optic properties investigated. The chiral nematic phases of these materials have unusually strong flexoelectro-optic effects and respond to a sub-millisecond timescale. The ratios of the effective flexoelectric coefficient to the mean splay-bend elastic constant, e/K, in the present materials lie in the range 0.3 to 0.6 C N^-1 m^-1, and are the highest measured to date: the highest value previously published is 0.12 C N^-1 m^-1, measured for the commercial mixture TM216. In order to interpret the effect of the bimesogens' molecular structure, achiral nematic monomesogens and bimesogens have been doped with chiral additives and the resultant mixtures' flexoelectro-optic properties have been analysed. From this work it has been possible to determine that the polar cyanobiphenyl group is the key to the strong response to the estradiol-cyanobiphenyl materials. In conclusion, a recommendation is made, for the first time, for a general molecular structure likely to exhibit a strong flexoelectro-optic response: namely, bimesogenic materials composed of highly polar end groups separated by a flexible spacer.