Experimental studies of director dynamics and distributions in liquid crystals

Abstract

This Thesis reports the results of investigations into the director dynamics and the director distributions in several thermotropic, calamitic liquid crystal systems, made using EPR spectroscopy. Chapter 1 gives an introduction to liquid crystals, including a description of the principal phase types and a review of basic structure-property relationships. The fundamental properties of liquid crystals are described, and a link is made with the techniques that are used to study them and how they are used. In Chapter 2, the EPR experiment and its applicability to the study of liquid crystal systems is described. In Chapter 3, the experiment that is used to measure the rate of magnetic field-induced director realignment in the nematic phase of a typical low molar mass liquid crystal is described. The EPR experiment shows very clearly that the macroscopic alignment of the director is maintained during relaxation through 45° but is lost upon relaxation through -80°. In Chapter 4, the technique developed in Chapter 3 is used to study the different director reorientation responses in the nematic phases formed by an odd and even dimer from a homologous series and in a monomer that may be considered to be the basic building block of these two dimers; the most striking result is that the monomer shows a much faster director reorientation than either of the dimers and that there is very little to differentiate between the rates of the response of the odd and the even dimer. The director distribution in a rapidly spinning nematic sample situated in a magnetic field is studied in Chapter 5 using field gradient EPR spectroscopy which gives added information about the spatial distribution of the director that is not available in conventional EPR studies. A simple model distribution function can be parameterised to give a good fit to all of the experimental spectra. In Chapter 6, the director distribution in an unusual smectic phase formed by several odd dimers is studied using EPR. Simultaneous fitting of several of the angular-dependent EPR spectra suggest that the experimental spectra are well fitted using a simple analytical function for the director distribution.

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