Thomas Geelhaar

Ferroelectric mixtures and their physico-chemical properties

Abstract The requirements for ferroelectric liquid crystals (FLC) for an application in fast-switching electro-optic devices are summarized. Typical molecular structures for FLC-mixtures are discussed. New non-chiral SmC-substances as well as some chiral dopants are shown. Important material parameters which are presented include spontaneous polarization, tilt angle, helical pitch in the smectic and cholesteric phase, rotational viscosity, response time, optical anisotropy and dielectric anisotropy. New developed broad range FLC-mixtures with SmC*-phases, SmI*-phases or SmA*-phases at room temperature are introduced.

Liquid-crystalline reference compounds

Abstract A polar (PCH-5) and a non-polar (152) compound are introduced as liquid-crystalline bench-mark compounds. The materials chosen are affordable, stable, easy to handle and possess nematic mesophases between 30–54·9 and 24–103·4°C. All relevant macroscopic physical properties, such as refractive indices n o n e, dielectric permittivities ∥⊊, elastic constants K 11, K 22 and K 33, and bulk and rotational viscosities are given as a function of temperature, wavelength and frequency, when applicable. The reference compounds are available from E. Merck, Darmstadt.

Liquid crystals for active matrix displays

Abstract Active matrix liquid crystal displays belong to one of the fastest growing fields in display research and development. Several pocket TVs using this technology are already available commercially. High quality displays for TV and instrumentation require an understanding and optimization of liquid crystal material parameters. We have, therefore, investigated the influence of the elastic, dielectric and optic properties on the electrical resistance of the liquid crystal as well as on the response times and the viewing angle dependence of active matrix displays. Based on correlations derived from our investigations, we have developed new liquid crystals which have suitable properties and a very high electrical resistivity.

The existence of four dielectric modes in the planar oriented S*c phase of a fluorinated substance

Abstract Soft mode and Goldstone mode properties have been studied for a fluorinated substance. The dielectric spectra have been measured on the planar oriented samples, in the frequency range from 10 Hz to 10 MHz. The thickness of the sample was 10 μm and two kinds of capacitors were used: (i) a low resistance EHC cell and (ii) gold coated electrodes. A bias field up to 10kV cm−1 has been applied to show the existence of both the soft mode and domain mode in the S*c phase below T c. In the S*c phase a strong Goldstone mode has been observed with a low critical frequency (v G C = 15 Hz). The high frequency side at the Goldstone mode spectrum is accompanied by a shoulder which consists of the soft mode and domain mode as well. In the vicinity of the S*A-S*C transition the dielectric parameters of the soft mode obey a Curie-Weiss law. The slope ratio is equal to −1·62 for the inverse of dielectric increments (Δ∈−1) and −1·73 for the critical frequencies (v c) obtained by using gold electrodes. The respectiv...

New fluorinated heterocyclic compounds for improved FLC mixtures

New suitable homologues of SmC host materials including different classes of fluorinated and non-fluorinated alkyl-alkyloxy-phenylpyrimidines were developed. Some materials were chosen to develop a new SmC host mixture. Several new chiral dopants have been synthesized. We found an influence of the different ring-systems and various side chains on the physical properties. Chiral dopants with more than one chiral center exhibit interesting properties. One dopant with a fluorine atom at the chiral center and a biphenyl-cyclohexane core showed the best properties and was chosen to introduce, together with the new SmC host mixture, new FLC-mixtures with improved properties. By using low-volt multiplex driving schemes and also high-volt multiplex driving schemes it is shown that these mixtures with an adjustable Ps fulfill the requirements for both methods.