Title:
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Adsorption and confinement effects in liquid crystals
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Three different types of liquid crystal have been studied under confinement within two novel
sample environments. The thermotropic cyanobiphenyls 5CB and 8CB were also doped with
both a core deuterated, fluorinated version of 110CB (F17) and the fully deuterated terphenyl
d-5CT; their adsorption was measured using neutron reflectometry (NR). At the air/5CB
interface, the adsorbed amount ofF17 decreased with increasing temperature within the nematic
phase, it then remained constant in the isotropic with a small increase owing to nematic wetting
at the surface. At the solid / nematic interface there was strong adsorption of F17 (4.9 ±0.9 mg
m-2 ) with a fluorinated silane (PFDS) coating. There was negligible adsorption (0.4±0.2 mg
m-2) for the hydrogenous (OTS) coated surface. In 8CB the F17 enhanced the surface smectic
wetting at the fluorinated surface.
NR was also used with a novel confinement cell. Both 8CB and 10CB were confined but
showed no change in their structures and it appeared the 8CB nematic phase was squeezed
out of the contact area. A surfactant, alcohol and D20 lyotropic system was also squeezed
out under 1 bar of confinement for 80% water content, however, with 75% water the lyotropic
remained in place with 1 bar of confinement. Phosphatidyl choline lipid bilayer stacks were
the third liquid crystalline system to be confined. Despite the repeat distance, d, increasing for
bilayers of DSPC and DPPC confined from the fluid (Lα phase and decreasing overall from
the gel (Lβ1) phase, the same d was reached at 5 bar of confinement in both cases. Model fitting
showed; for all lipid chain lengths, the lipid component increased in thickness and the water
decreased, suggesting that the phase had changed.
A confinement cell for use with X-ray reflection (XRR) and small angle X-ray scattering
(SAXS) was designed and commissioned, through experiments on lipids and cyanobiphenyls.
The reflectivity results were consistent with the NR results. SAXS was applied to a pair of
planar aligned surfaces with a twist between the alignment directions. The decrease in thickness
with applied confinement induced the twist grain boundary phase for angles between 68°and
90°. Hybrid alignment conditions showed a conversion from non-scattering to planar alignment
with confinement
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