Flexoelectricity in Liquid Crystals

• Main Author: Jackson, Daniel
• Published: University of Southampton 2007
• Subjects: 541.37
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.487116

A rich variety of symmetric and non-symmetric liquid crystal dimers have been synthesised . and studied in order to investigate their structure property relations; in particular their flexoelectric properties. The project is introduced in Chapters 1 and 2 where Chapter 1 gives a brief general background to liquid crystals and liquid crystal synthe~is, and Chapter 2 gives a more detailed background to the flexoelectric effect. The research can be broadly divided into two parts. Chapters 3 and 4 focus on seven conventional non-symmetric liquid crystal dimer series containing two mesogenic groups ether linked through a flexible spacer. These non-symmetric dimers all contain a ( . cyanobiphenyl mesogenic group at one end and either a poly-fluorinated biphenyl or a phenyl-cyclohexyl-alkane mesogenic group at the other. Almost all these materials possess nematic phases (and in one series a smectic phase) and show strong flexoelectric coupling to an applied field with some experiments yielding very large values for the flexoelastic ratio. Chapter 5 focuses on two series of bent core liquid crystal dimers, wher~ the long chain spacer is divided by a catechol or resorcinol based disruptor group. The disruptor is located in the centre of the molecule and symmetric cyanobiphenyl mesogenic groups are at each end. These materials show a curious odd-even effect in the nematic-isotropic transition temperatures which is further investigated in Chapter 6 by reducing the symmetry of the compounds and studying the changes in the transitional ·properties. 1.ills was achieved in one series by changing the relative position of the catechol linking group along the spac~r chain or, in a different series, by altering one of the mesogenic end groups to a di-fluorinated biphenyl (a mesogenic moiety studied in Chapter 3). The liquid crystal properties were investigated by·optical microscopy, differential scanning calorimetry and deuterium NMR spectroscopy. The flexoelastic ratios were determined from thed~pendence of the tilt in the optic axis on an applied electric field