Viscosity coefficients of nematic liquid crystals: II. Measurements of some nematic liquid crystals^†

Abstract

Oscillating plate and rotational viscosity measurements are reported for a series of liquid crystals and include n-p-cyano-p-hexylbiphenyl (K18), 4-n-hepthyl-4′-cyanobiphenyl (K21), ethyl-cyclohexyl-ethyl-6-fluoro – n-propyl-biphenyl (I32), n-propyl-cyclohexyl-ethyl-6-fluoro – n-butyl-biphenyl (I43) and a n-pentyl-cyclohexyl-cyanophenyl : n-heptyl-cyclohexyl-cyanophenyl mixture. Rotational viscosity measurements were carried out over a temperature range from ambient to ∼90°C. Comparison of the values at a temperature of 5 K above the below the clearing point indicate an odd–even effect as the chain length of the hydrocarbon tail is altered. The principle viscosities η₁, η₂, η₃ and η₄₅were measured using an oscillating plate viscometer and the temperature dependences used to calculate the activation energies for flow in the various directions. The magnitude of the activation energy is shown to change with the length of the hydrocarbon chain. The incorporation of the cyclohexyl group imparts flexibility and reduces the activation energy flow, whilst the presence of the fluoro group increases the interactions between molecules, and this is reflected in higher values of the viscosity. The change of viscosity with alignment angle is explored for two of the systems studied and the fit to theory investigated. The Leslie–Ericksen coefficients are calculated for these systems and discussed in terms of changes in the molecular interactions.

Keywords:

• Leslie–Ericksen coefficients
• liquid crystals
• nematic
• oscillating plate viscometer
• rotational viscosity

Acknowledgements

The authors wish to acknowledge the support of EPSRC for the funding of the magnet, without which this study could not have been undertaken. One of us (RO) wishes to acknowledge the support of an EPSRC CASE award with Merck and additional financial support from RSRE.

Notes

^†This paper is dedicated to the memory of Professor F.M. Leslie who was a co-investigator of the research described here.