Oriented molecules theoretical studies on mixed liquid crystals of opposite crystal

Abstract

The thesis consists of two parts. Part I describes theoretical studies in mixed liquid crystals of opposite diamagnetic anisotropies. Part II deals with experimental NMR studies in single and mixed liquid crystals of opposite diamagnetic anisotropies.

Basic principles connected with the work are given in Chapter 1. Chapters 2 and 3 constitute Part I of the thesis. A theoretical treatment, based on Landau theory, of the behaviour of the order parameter in mixed liquid crystals of opposite diamagnetic anisotropies is developed and reported in Chapter 2. These results satisfactorily explain not only the concentration-dependent switching transition but also the observed variation of the order parameter as a function of relative concentrations of the two types of liquid crystals (temperature being fixed).

The experimental results in the following systems have been theoretically interpreted:

N-(p-ethoxy benzylidene)-p-n-butylaniline (EBBA) and a ternary eutectic mixture of propyl, pentyl, and heptyl bicyclohexyl carbonitrile (ZLI-1167), with benzene as probe molecule.

Trans-p-pentyl-4-(4-cyanophenyl)-cyclohexane (S-1114) and ZLI-1167, with acetonitrile as probe molecule.

EBBA and ZLI-1167, with acetonitrile as probe molecule.

A similar theory to explain the temperature-dependent switching transition in mixed liquid crystals of opposite diamagnetic anisotropies is developed and reported in Chapter 3. This also explains the behaviour of the order parameter as a function of temperature (relative concentration being fixed). In addition, the theory has been extended to explain the nematic–isotropic transition observed in mixtures of liquid crystals.

Part II consists of Chapters 4 and 5. Chapter 4 deals with experimental NMR studies on two benzo-bis-isothiazoles, namely benzo(1,2-d:4,3-d’)- and benzo(2,1-d:3,4-d’)bis-isothiazoles (referred to as I and II, respectively) in the nematic phase of EBBA. The proton NMR spectra, including the Q-satellites, have been studied. The HH and OH direct dipolar couplings determined from the spectra have been corrected for harmonic vibrations, and the relative internuclear distances (r-structures) determined. The results show considerable bond fixation in compound I. The absolute sign of the orientation parameter in compound I is determined to be positive.

Chapter 5 deals with experimental NMR studies on trimethyl phosphate oriented in the mixture of EBBA and ZLI-1167. Both the ¹H and ³¹P NMR spectra have been investigated in mixed liquid crystals of opposite diamagnetic anisotropies. A combination of NMR results and potential energy calculations shows that the gauche and trans conformers are preferred, with the gauche conformer having lower energy. The conformer ratios have been computed using known bond lengths and bond angles, assuming that the order parameters of the two preferred conformers are similar. The H and P chemical shift anisotropies have been determined from the spectra coexisting near the critical point in the mixture of liquid crystals of opposite diamagnetic anisotropies.

The last chapter (Chapter 6) summarizes the results reported in the thesis.