Dielectric properties of the nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl in porous membranes

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Abstract
Broadband dielectric spectroscopy (up to 109 Hz) is employed to study the molecular dynamics of the liquid crystal 4-n-pentyl-4′-cyanobiphenyl (5CB) in the free bulk phase and confined in cylindrical channels of Anopore membranes having a diameter of 0.2 μm and length of about 60 μm. The bulk samples of 5CB orient almost homeotropically between the untreated metal electrodes of the measurement set-up, and two relaxation processes are observed: the slower δ-relaxation is assigned to hindered rotation (180° flips) of the molecules around their molecular short axis, and a faster second process is attributed to the tumbling of the molecules about this axis. In the confined 5CB samples, the membrane pores align the nematic director axially or radially depending upon their surface preparation. Planar (axial) alignment is always found in untreated membranes, whereas radial alignment was achieved by treatment with decanoic acid. Consequently the director field is fixed perpendicular or parallel to the electric field and we are able to study each of the two relaxation processes separately by appropriate surface treatment of the pores. The frequencies of both processes are found to be unchanged with respect to the bulk phase. We extract the frequency dependence of the dielectric anisotropy δε from the dispersion curves of ε∥ and ε⊥. Two changes of sign of δε = (ε∥–ε⊥) are detected as predicted in the literature.