Structural investigations of novel liquid crystals

This thesis is primarily concerned with the characterization and study of novel liquid crystal compounds using X-ray diffraction techniques. Chapter 1 introduces the many different classes of thermotropic liquid crystals and discusses their different structures and properties. The theory for X-ray diffraction is presented in chapter 2, where the relationship between the molecular structure and the X-ray diffraction pattern is explained. The many forms of the X-ray diffraction pattern for thermotropic liquid crystal phases are described in chapter 3, where an account of the methodology used for the identification of these phases is also given. In chapter 4, the measurement of tilt angles and pretransitional effects are investigated and a comparison with previous studies is made for the compounds 4,4'-di-n-heptyloxyazoxybenzylidene-4'-n-pentylaniline. The nature of the smectic A to smectic C phase transition is also examined for the ninth and tenth homologues of terphthalidene-bis-(4-n-alkylaniline). Chapter 5 investigates the asymmetric dimer series, α-(4-butylamiline benzylidene-4'-oxy)-ω-(4-nitroaxobenzene-4'-oxy)alkanes. The properties of three homologous series of the asymmetric dimer α-(4-cyanobiphenyl-4'-oxy)-ω-(4-n-alkylanilinebenzylidene-4'-oxy)alkanes are also presented. The dependence of the thermal stability of the smectic phase on the length of the alkyl spacer chain is unusual. The stability does not simply increase with increasing chain length but passes through a minimum. This behaviour is rationalised whereby the structure of the smectic phase changes from an interdigitated to an intercalated one on increasing chain length. Finally, chapter 6 discusses the possibility of producing a biaxial nematic phase by combining the properties of discotic and calamitic systems. The behaviour of 2,3,4-trihexyloxycinnamic acid and the tenth and eleventh homologues of α,ω-bis[penta(4-pentylphenylethynyl)phenoxy]alkanes are investigated.