Selected physical properties of liquid grystallime polysiloxanes

Abstract

The static and dynamic electrical behaviour, the electrical conductivity, the macroscopic viscosity and the optical transmission characteristics of side chain polysiloxane liquid crystal polymers (LCPs) have been studied. Particular emphasis has been placed on the study of the molecular dynamics using dielectric relaxation spectroscopy and the low field (AC) and high field (DC) conductivity, with the intention of relating structural variations to the behaviour of these materials, and of identifying the most appropriate LCPs for use in practical optical storage devices. The effect on the molecular dynamics of the polymer backbone and the core, spacer and end groups of the mesogenic units was investigated. The LCP responses were shown to be broadly similar to those of low molar mass liquid crystals, though with longer relaxation times. The dynamic responses of structurally different LCPs were shown to be related when these were observed at a temperature referenced to the glass transition temperature. Values of the static permittivity of several LCPs have also been measured and tabulated. Low field electrical conductivity measurements were used to show that the magnitude of the conductivity was comparable to that of commercial liquid crystals. A method was identified whereby the conductivity of different LCPs could be compared. The texture of the measuring electrode surface was found to play an important role in determining the impedance of electrical double layers formed near the surface of the sample, with smooth polymer based surfactants reducing the formation of space charge. High field DC conductivity measurements showed that Schottky-type charge emission occurred at the electrodes at elevated temperatures, except when the smooth polymer based surfactants were present. This was probably a result of the reduction in the local field near the electrode due to a better physical match at the LCP/surfactant interface. The bulk viscosity of mixtures of an LCP and commercial liquid crystal were also measured and the results extrapolated to the viscosity of the LCP. The relationship between electrical conductivity and bulk viscosity was examined. The optical absorption spectra of blue pleochroic dyes have been measured to identify appropriate dyes for use with LCPs in laser written optical memories. The stability of the alignment of selected LCPs with time and temperature was also observed optically.