Molecular Organisation in “de Vries” Smectic Liquid Crystals: Characterisation and Theory

Abstract

The study of anomalous temperature dependence of smectic layer thickness began fifty years ago. Liquid crystals exhibiting such properties were later classified as de Vries smectic liquid crystals. In this thesis I have carried out a number of experimental and theoretical approaches to understand the nature of de Vries type liquid crystals. A detailed study based on chiral doping in achiral de Vries smectic liquid crystals is presented. The chiral dopant is mixed with the achiral host in varying weight percentages and studied using layer thickness and electro-optic measurements. We show that adding chiral doping induces ferroelectricity in the mixtures and exhibit chiral de Vries smectic properties. Followed by that, we present a number of experimental studies on novel chiral de Vries smectics. High resolution experiment reveals that, the measured layer thickness and birefringence as a function of temperature exhibit a characteristic trend reversal within the smectic A* phase of de Vries type liquid crystals. Such behaviour is consistently observed in all the materials studied in this thesis. The electric field dependent birefringence and the apparent tilt angle exhibit a large field induced response with a characteristic sigmoidal shaped curve in de Vries smectic A* phase. Such electro-optic response is fitted to different models. We present a series of modifications in the electro-optic modelling to understand the microscopic origin of the large electroclinic effect in de Vries smectic liquid crystals. Finally we show that the molecular tilt angle with respect to layer normal emerges in smectic A* phase and increases with a reduction in temperature as expected from the high resolution experimental results. The emerged molecular tilt angle shows a volcano shaped wide orientational distribution function. This implies that the maximum probable molecular tilt angle is finite in de Vries smectic A* phase.