Temperature Dependence of the Static Relative Permittivity of Octylcyanobiphenyl (8CB)

Abstract

Measurements of the relative static permittivity ε of octylcyanobiphenyl (8CB) in the smectic A, the nematic and the isotropic phases between 22°C and 140°C are reported. The resolution of the measurements was better than 0.01% and the temperature stability was better than 0.01 °C. Detailed measurements have been carried out near the AN and NI transitions. For the AN transition. no discontinuity in ε_|| or ε_| was observed, which is consistent with the second-order nature of this transition in 8CB. From the dielectric anisotropy in the N-phase, a critical exponent β ≃ 0.25 was obtained for the temperature dependence of the order parameter. This value of β is in support of the hypothesis of quasi-tricriticality for this transition. In the isotropic phase of 8CB, a pretransitional anomalous maximum in ε was observed near T _N1- The same effect was also measured for two other materials with cyano-end groups (undecylcyanobiphenyl and pentylcyanophenylcyclohexane). For two compounds without a cyano-end group (p, p'-di-n-heptylazobenzene and trans-p-methoxy-p'-heptyloxy-α-cyano-stilbene) this pretransitional effect was not observed. An explanation for this effect may be found in local antiparallel orientation of the molecular dipoles of the cyano-compounds. The possible role of nematic fluctuations in the isotropic phase is also discussed.