A SAXS study of flow alignment of thermotropic liquid crystal mixtures

Abstract

We report on the capillary flow behaviour of thermotropic liquid crystal mixtures containing 4-n-octyl-4′-cyanobiphenyl (8CB) and 4-n-pentyl-4′-cyanobiphenyl (5CB). The liquid crystal mixtures are studied in the Nematic (N) and Smectic (S_A) phases at room temperature. Polarised optical microscopy (POM), rheology and simultaneous X-ray diffraction (XRD)/capillary flow experiments are performed to characterise the system.

Polarised optical microscopy reveals a dramatic change in optical texture when the 5CB content is increased from 20 to 30% in the mixtures. X-ray diffraction results show that the system goes through a S_A–N phase transition, such that the mixtures are smectic for 10–20% 5CB and nematic for 30–90% 5CB. Smectic mixtures flow with the layers aligned along the flow direction (mesogens perpendicular to flow) while nematic mixtures flow with the mesogens aligned in the flow direction. Simultaneous XRD/shear flow experiments show that the S_A–N transition is independent of the flow rate in the range 1–6 ml min⁻¹.

The correlation length of the liquid crystal order decreases with increasing 5CB content. Rheology is used to prove that the correlation length behaviour is related to a reduction in the viscosity of the mixture.

Keywords:

• X-ray diffraction
• nematic
• smectic, mixtures
• flow alignment

Acknowledgements

VC thanks Mr. Nick Spencer (Biocentre, University of Reading) for assistance with XRD experiments, and Gemma Newby for assistance during rheology experiments.