The investigation of some novel polysiloxane liquid crystal systems

A range of liquid crystal units based on mainly phenyl benzoate esters have been synthesised and characterised. These have been attached to polysiloxane systems to form ring, linear and elastomeric polymer liquid crystals. The effect of system purity on the phase transitions of the ring materials was found to be crucial, and this has resulted in a phase diagram for a 'doped' ring system where the doped material exhibits liquid crystalline behaviour while the pure material does not. Properties of linear material were found generally to be consistent with data in the literature. Extremely low glass transition temperatures were not observed even though pure poly(hydrogenmethylsiloxane) is considerably more flexible than its hydrocarbon equivalent. Effects of degree of polymerisation and spacer length were as expected. Linear polymers were cross-linked to produce equivalent elastomeric materials.We have characterised a range of these systems and investigated the effect of deformation on the LC order within the elastomers through the use of FTIR. Phase transitions for the cross-linked systems were generally slightly lower than the analogous linear materials which is in agreement with data found in the literature. Partial substitution by cyano-terminated molecules enabled us to determine the peak ratios of Si-H/CN stretching frequencies on deformation. Small enhancements, approx 7%, of LC order parallel to the direction of stretching were observed. In addition, a novel 'ring' elastomer incorporating liquid crystal units has been produced. Cyclic materials were substituted with liquid crystal units and cross-linked to yield elastomers with 25% and 37% mesogen density. Although elastomers were produced, no liquid crystal behaviour was observed. Glass transition temperatures in the region of -100 °C were observed. It is believed that by increasing mesogen density in these materials, elastomers with extremely low glass transition temperatures and liquid crystal properties could be achieved.