Ferroelectric and antiferroelectric low molar mass liquid crystalline organosiloxanes

A detailed study has been made of the properties of a series of low molar mass organosiloxane liquid crystalline materials. These materials consist of one or more mesogenic moieties attached via alkyl-chain spacers to a siloxane group. Previous studies have shown that in these systems microphase separation of the three constituent moieties occurs. The agglomeration of the siloxane units results in an effective polymer backbone and the alkyl spacer serves to decouple the mesogenic moiety from the 'backbone'. The resulting molecular order gives such systems the electro-optic properties of low molar mass systems combined with some of the robust mechanical properties of polymers. In this thesis systems with both one (monomesogenic) and two (bimesogenic) mesogenic groups linked to the siloxane moiety have been investigated. Optical microscopy and differential scanning calorimetry were used to probe the phase behaviour of the materials. A range of experimental techniques were used to facilitate a full characterisation of the ferroelectric and antiferoelectric properties of the materials. Mixtures of two low molar mass monomesogenic organosiloxane liquid crystal materials were studied. The two materials differed only in the incorporation of an azo, rather than ester based, linkage group into the miesogenic moiety in one. All the mixtures exhibited a room temperature chiral smectic C phase. The ferroelectric properties of the mixtures were studied as a function of the concentration of the azo material. Both the tilt angle and the spontaneous polarisation were found to be largely temperature independent with values for the mixtures of the order of 34° and 15nCcm-2 at 25°C respectively. Fast electro-optic response times of the order of ~250muS at 25°C were measured. Increasing the concentration of the azo material served to reduce the spontaneous polarisation, increase the tilt angle and increase the response time. Also the effect of UV light on the mixtures was investigated. For 2mWcm-2 UV the conformational change in the molecules on illumination resulted in the formation of an isotropic phase. A range of symmetrical bimesogenic organosiloxane low molar mass materials were studied. These materials exhibited a single antiferroelectric phase, at high temperatures (>~50°C), of some 60°C range. The measured tilt angles were high, close to 45° and largely temperature independent across the phase. The electrooptic response times were fast, in the region of 200muS at 80°C. The effect of varying the number of siloxane units in the material was established. All the materials exhibited ferroelectric phases above room temperature with high temperature independent tilt angles and spontaneous polarisation in the region of 41° and 75nCcm-2 respectively at 90°C. Response times were fast, of the order of 100muS at 90°C. A small decrease in the tilt angle and spontaneous polarisation was observed with an increasing number of siloxane units. One of these materials has been found to show an antiferroelectric phase in addition to the ferroelectric phase. This series of materials was of significant interest because of the ability of the length of the siloxane spacer to influence the order of the system.