Physical Properties of Novel Ferroelectric and Antiferroelectric Liquid Crystals

Electro-optic studies of a group of novel low molar mass liquid crystals which contain ferroelectric, ferrielectric and antiferroelectric phases are reported. In addition an account of some preliminary work on the effects of high pressures on liquid crystalline phases is given. Two series of liquid crystalline materials are described and characterised. One group contains alkyl sulfanyl substituted phenyl propiolates and alkyoxy substituted phenyl propiolates. It is found that sulfanyl substitution removes any frustrated phases (TGBA, BPs ) that were present in the material, and enhances the SmA phase. Sulfanyl substitution also slightly reduces the saturated values of the Ps and tilt angle. Moving the chiral centre further from the rigid core of the molecule and substituting this chiral centre with a more electro-negative atom results in a decrease in the Ps from 90nC/cm2 to 55nC/cm2 at large reduced temperatures. The tilt angle is unchanged. The second group of materials contains phenyl and biphenyl carboxylates, most of which have antiferroelectric phases. The Ps of these materials is high ( 120nC/cm2), as is the tilt angle ( 28°). The appearance of a wide SmC*alpha, phase results in a much smaller Ps (90nC/cm2) and tilt angle (18°) at large reduced temperatures compared to other materials without this phase. This indicates that the SmC*alpha phase appears in materials which have a reduced desire to form strong antiferroelectric and ferroelectric phases. The properties of the SmC*alpha, phase have been found to change from antiferroelectric to ferrielectric on cooling. The electroclinic coefficient is found to initially decrease and then increase again in this phase. In planar aligned cells the SmC*A phase is found to form a distinctive texture which is filled with a large number of focal conic defects. Distinctive textures for the SmC*alpha phase can only be found in homeotropically aligned cells. It is proposed that the appearance of the SmC*alpha phase is highly dependant on surface anchoring conditions. The field induced SmC*A-SmC phase transition has been studied. It is found that this transition is not accompanied by a change from chevron to bookshelf states. This change does occur, but at much higher fields than those required for an antiferroelectric to ferroelectric transition. One of the materials displayed a monotropic phase below the SmC*A phase. A study of the properties of this phase revealed it to be a higher order antiferroelectric phase, with a larger layer spacing than that observed in the SmC*A phase. The exact structure of this monotropic phase is not yet clear. The electro-optic characteristics of the SmC* phase of certain materials displayed an unusual voltage dependence. At low voltages, when a helix was still present in the material, apparently saturated hysteresis loops, and double current pulses could be observed. At high voltages the usual ferroelectric switching was seen. This effect was studied using stroboscopic microscopy and compared to the switching observed in the SmC* and SmI* phases of CE8 (Merck). It is found that the switching in CE8 does not change significantly on increasing the voltage. The high voltage switching in the new materials occurs in the same way as that observed in CE8, with characteristic boat shaped domains being observed. At low voltages the switching in the new materials occurs via the nucleation and growth of stripe shaped domains, and a metastable third state appears. This switching mechanism has been attributed either to the existence of stable azimuthal angles in the material, or the existence of a new helical ferrielectric phase. In the latter case the threshold field for unwinding the helix is greater than the threshold field required for a ferrielectric to ferroelectric phase transition. High pressure studies of the liquid crystal phase are also reported. The construction of a piece of apparatus which allows the direct observation of liquid crystal phases under high pressure is described. This instrument can be used for a range of electrical and optical measurements. Pressure vs. temperature phase diagrams of both known and novel materials have been plotted. The PS as a function of pressure has also been measured.