DIELECTRIC-PROPERTIES OF THE NEMATIC LIQUID-CRYSTAL 4-N-PENTYL-4'-CYANOBIPHENYL IN POROUS MEMBRANES

Broadband dielectric spectroscopy (up to 10(9) Hz) is employed to stud y the molecular dynamics of the liquid crystal 4-n-pentyl-4'-cyanobiph enyl (5CB) in the free bulk phase and confined in cylindrical channels of Anopore membranes having a diameter of 0.2 mu m and length of abou t 60 mu m The bulk samples of 5CB orient almost homeotropically betwee n the untreated metal electrodes of the measurement set-up, and two re laxation processes are observed: the slower delta-relaxation is assign ed to hindered rotation (180 degrees flips) of the molecules around th eir molecular short axis, and a faster second process is attributed to the tumbling of the molecules about this axis. In the confined 5CB sa mples, the membrane pores align the nematic director axially or radial ly depending upon their surface preparation. Planar (axial) alignment is always found in untreated membranes, whereas radial alignment was a chieved by treatment with decanoic acid. Consequently the director fie ld is fixed perpendicular or parallel to the electric held and we are able to study each of the two relaxation processes separately by appro priate surface treatment of the pores. The frequencies of both process es are found to be unchanged with respect to the bulk phase. We extrac t the frequency dependence of the dielectric anisotropy delta epsilon from the dispersion curves of epsilon(parallel to) and epsilon(perpend icular to). Two changes of sign of delta epsilon = (epsilon(parallel t o) - epsilon(perpendicular to)) are detected as predicted in the liter ature.