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The University of Southampton

Research project: Luckhurst: Flexoelectric Behaviour of Liquid Crystal Dimers

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A uniformly aligned nematic with its director parallel to a unique direction has a centre of symmetry and so is non-polar.

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However, as Robert Meyer has shown if the director distribution is deformed either with a splay or bend deformation the center of symmetry is lost and a polarity results. This intriguing behaviour is known as flexoelectricity after the piezoelectric behaviour exhibited by certain crystals. The inverse of this flexoelectric effect is the deformation of the director distribution induced when an electric field is applied to a nematic and which results in a change of its optical properties. This inverse flexoelectric effect has the potential to be exploited in a display device with a fast switching rate. However, the development of suitable materials for use in such a display is a problem because the relationship between the molecular structure and the magnitude of the flexoelectric response is not clear which makes their rational design difficult. However, experiment suggests that liquid crystal dimers in which two mesogenic groups are linked by a flexible spacer have some desirable flexoelectric properties. To help improve the quantitative design of these materials we have used a theory developed by Alberta Ferrarini to predict the flexoelectric behaviour of the dimers allowing for the many conformations adopted by the spacer. This theory first calculates the geometry of the different conformers with their different charge distributions at a quantum mechanical level and then at the continuum level introduces the director deformation. The results of these calculations show that it is the bent conformations formed by introducing gauche links in the spacer that make the dominant contributions to the flexoelectric behaviour. In addition, this behaviour is predicted to be in agreement with that observed. We are now extending these challenging calculations to other liquid crystal dimers as well as including the important effect of conformational averaging.



Related research groups

Computational Systems Chemistry
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