Dielektrische und elektro-optische Untersuchungen an ferroelektrischen, antiferroelektrischen und nematischen Fluessigkristallen sowie Polymer-Fluessigkristall-Systemen

The properties of liquid crystalline phases are forecasted hardly from the molecular properties of the molecules of the phase. Therefore, the author investigates ferroelectric and antiferroelectric liquid systems. The influence of diverse doping agents on the chirality conditioned properties are investigated using a mixture of P6O8 and BP56 as an achiral host material with a phase sequence of SmC 62 C SmA 67 C N 93 C Iso. Different terphenyls of two homologous series and with two chirality centers per molecule are used as chiral doping agents. The liquid crystalline parameters such as tilt angle and viscosity only depend on the chemical composition of the mixtures and not from the chemical composition of the doping agents. The spontaneous polarization and the switching time are linear to the concentration of the chiral components. The encapsulation of ferroelectric liquid crystalls in a polymer lowers the spontaneous polarization and effective tilt angle. The relaxation intensity decreases by one order of magnitude while the relaxation frequence increases by one order of magnitude. A network with SmC"* symmetry induces a local SmC"* order in an orthogonal SmA"* phase of polymer stabilized ferroelectric liquid crystals. A Goldstone-like relaxation is observed at the phase transition temperature. The relaxation intensity and relaxation frequency of this mode increases with increasing polymer concentration till to a saturation value of 5 wt.% polymer. A polymeric network in the SmA"* phase induces a local orthogonal arrangement in the SmC"* phase and a depression of the Goldstone mode in the range of the SmC"* phase near the phase transition temperature. In the SmC"*_A of newly synthesized antiferroelectric liquid crystals a ''non-cancellation'' mode is observed. A second mode with lower relaxation frequency has a very low relaxation intensity and is characterized by a molecular rotation around a short axis. In the middle frequency range of the dielectric investigation of MHPBC (MHPBC = 4-(1-methyl heptyloxycarbonyl)phenylene-4'-octyl-biphenylene-4-carboxylate) the author observes a relaxation process in the SmC"*_A phase. The relaxation process is characterized by an extremely high relaxation frequency and a low relaxation intensity in comparison to the Goldstone mode of the SmC"* phase. The ferrielectric SmC"* phase has three relaxation processes in the antiferroelectric phases of MHPBC. The temperature dependence of the relaxation frequency is not influenced by the different phases. The dielectric high-frequency investigations of dielectric negative nemates supply nearly the same relaxation frequencies such as cyano compounds.Available from: http://deposit.ddb.de/cgi-bin/dokserv?idn=967375932 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman