Elastic Properties and Phases of Bent Core Liquid Crystal

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Abstract

The recent interest in bent core liquid crystal has shown many unique physical properties, such the anomalous behaviour of the elastic constants (SplayK1, Twist K2, and BendK3). In bent core liquid crystals it is observed that K3<K1 contrary to what is observed in conventional calamitic liquid crystals. In this thesis we aim to understand how including an intrinsic bend affect liquid crystalline systems. Mixtures of oxadiazole based bent core and calamitic liquid crystals were studied. Generally a stable nematic phase at room temperature can be achieved, however one unique phase was observed in one mixture which is studied in detail later. The mixture all show similar effects with increases in bent core dopant significantly lower values of K3 and K2. These results are expected as the lowering of K3 is seen in many other studies, however generalising K2 is more difficult due lack of studies of the K2. The physical properties of four oxadiazole based bent-core liquid crystals were also examined. The materials are chosen to study the effects of terminal chain length, substitution with fluorine, and swapping alkoxy to alkyl. Changes in the terminal chain length and fluoro-substitution have been found to have little influence on the elastic constants at equivalent reduced temperatures. However, the material with an alkyl terminal chain exhibits much reduced elastic constants than the alkoxy terminal chain compounds. Importantly, using molecular field theory and atomistic modelling, the calculated elastic constants are in excellent agreement with the experimental values pointing to the importance of the bend angle. The elasticity in the nematic phase formed from bent-core molecules is not strongly influenced by changes to the terminal chains or the presence of fluorine.The elastic properties of a bent-core thiadiazole nematic liquid crystal are also investigated. The elastic constants across the whole nematic range were studied. It was found that the thaidiazole bent core materials exhibited elastic properties similar to calamitic liquid crystals (K3>K1). Such behaviour is analogous to calamitic liquid crystals but is in contrast to all other bent-core nematic materials reported to date. Such a result questions some of the current explanations for the elastic behaviour of bent-core materials. Using molecular field theory and atomistic modelling the different elastic behaviour predicted is again in excellent agreement with experimental results. The bend angle is again shown to be an important part in determining the physical properties of bent-core nematic liquid crystals.In a mixture from an oxadiazole dopant and calamitic host liquid crystal, it was found that a filament structure appears in the nematic phase. The filaments appear to interfere with the measurements for elastic constants. In order to understand the filament structure many methods were used including SAXS, dielectric permittivity, and DSC. It was found that the mixture had formed a gel – like phase. The gel is composed of a liquid crystal network and a liquid crystal background, not seen before in any gel system. Due to the liquid crystalline properties both the network and the background can be aligned and manipulated. The new gel phase can possess many new unique properties which warrant further studies understand further into how fundamentally the phase is forming.

Keyword(s)

liquid crystal, nematic, elastic, flexoelectricity, bent core, gel, double liquid crystal gel

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