Research at CMMPEMaterials — Liquid crystal synthesis

Jump directly to the following sections: Bimesogenic LCs - Organosilxane LCs


Bimesogenic liquid crystals

Figure 1. Generic bimesogen structure. Research has been carried out on varying the spacer length and composition, as well as the substituents that are located at the lateral and terminal positions Y and X.

Selected Publications:

Structure-flexoelastic properties of bimesogenic liquid crystals
Morris, S.M., Clarke M.J. , Blatch A.E., Coles H.J.
Phys. Rev. E, 75, 041701, (2007)

Strong Flexoelectric Behaviour in Bimesogenic Liquid Crystals
Coles H.J., Clarke M.J., Morris, S.M., Broughton B.J. and Blatch A.E.
J. Appl. Phys. 99(3), 034104, (2006)

The effect of the molecular structure on flexoelectric coupling on the chiral nematic phase
Coles, H.J., Musgrave, B.M., Coles, M.J. and Willmott,
J. J.Mater.Chem., 11, 2709-16, (2001)

 

 

The research team, led by Prof. Coles, has developed a range of liquid crystal compounds, referred to as bimesogens, to enhance the switching properties of the flexoelectro-optic effect. This electro-optic effect, which is usually swamped by dielectric coupling in conventional liquid crystal compounds, is a fast in-plane deflection of the optic axis when an electric field is applied perpendicular to the helical axis of a chiral nematic. The motivation was to create compounds that possessed large flexoelectric coefficients but at the same time had a low dielectric anisotropy so as to minimize dielectric coupling to an electric field. Through synthesis and characterisation, these bimesogenic structures have been shown to exhibit larger tilt angles at lower electric field strengths than that observed previously with conventional monomesogenic structures. In addition to the flexoelectro-optic effect, mixtures of these compounds showed other remarkable properties in the form of wide temperature range blue phases.

An example of the generic bimesogenic structure is shown in Figure 1, where it can be seen that it consists of two terminal mesogenic units that are separated by a flexible alkyl spacer. These compounds show the well-known odd-even effect in the transition temperatures, dielectric and optical properties, flexoelectric properties, and also the excitation threshold and slope efficiency when used in LC lasers, when the length of the spacer is varied. A range of compounds have been synthesised and characterised to gain insight into the structure-property relationships such as changing the lateral and terminal substituents and changing the nature of the chain (e.g. methylene, ether or ester). Research has shown that the structures with an odd-number of units in the spacer result in enhanced flexoelectro-optic properties.

Organosiloxane liquid crystals

For some years the research team under Prof. Coles has been interested in the properties of siloxane and organosiloxane containing liquid crystals. Original work focussed on the development of siloxane containing side chain liquid crystal polymers; other investigations have concentrated on low molar mass systems. In almost all cases, such low molar mass compounds exhibit the Smectic liquid crystal phase; the action of the siloxane component is to stabilise the Smectic (layered) phase over the nematic (non-layered) phase.

The layered arrangement confers unusual properties on Smectic systems and allows observation of electro-optic effects which can be exploited in devices. For example, in Smectic A systems, application of a low frequency electric field causes a highly scattering texture to form while application of a higher frequency field induces a clear state. These two effects can be combined to give a device which possesses two stable states at zero field. Other work has focussed on the development of Smectic C organosiloxane liquid crystals for application in fast switching, rugged ferroelectric devices.

We have carried out extensive research on synthetic routes and the physical properties of these low molar mass organosiloxane liquid crystals e.g. in the anisotropic properties of the conductivity and in the electro-optic properties for potential device applications. These materials are found to exhibit multi-stable behaviour because any state induced in the device is retained after the removal of an external voltage, and, furthermore, they do not require polarisers.

Extensive further information is available in the publications section.

 

Figure 2. Low molar mass organosiloxane generic structure
generating the Smectic A phase.

Figure 3. Example of layered arrangement
in the Smectic A liquid crystal phase.