Dielectric studies of some re-entrant nematogens

Abstract

The static dielectric constants (ε ll and ει) as well as the dispersion of εII in the radio frequency range have been investigated in the nematic, smectic A and re-entrant nematic phases of two pure compounds and a mixture, viz., 4-cyano- phenyl-3'-methyl-4(4'-n-undecylbenzoyloxy)benzoate (llCPMBB), 4-cyanophenyl-3'-methyl-4(4'-n-dodecylbenzoyloxy)benzoate (12CPMBB) and 35.8:64.2 weight mixture of 6 OCB and 8 OCB. In all three cases, the 'average dielectric constant (ε) evaluated from the mesured ε ll and ει values shows an unusual behaviour. AS the temperature is decreased from the isotropic phase ε, which is less than the extrapolated isotropic value (ε is ) owing to the antiparallel ordering of the is molecules, decreases throughout the nematic range and over a part of the smectic range. On further decrease of temperature 'starts incrbasine. This reversal in the trend is evidently related in some way with the reversal of the sign of the thermal expansion of the layer spacing. This increasing trend in E continues right through up to the lowest temperature of the re-entrant nematic, where it practically merges with ε is

The frequency of relaxation (fR) of ε II was determined as a function of I I temperature in all the three mesophases. A plot of fR vs 1/T exhibits several interesting features: (i) the normal nematic phase has a higher activation energy (W) than the smectic phase, (ii) in the smetic phase the plot consists of two distinct straight lines with different slopes (or different W) indicating a kind of structural rearrangement in the system, (iii) the re-entrant nematic phase has a muchhigher W than the normal nematic phase. This is the first experimental evidence of the mol6cular associations being different in the two nematic phases.