Orientational Order Parameters in the Smectic C and Nematic Phase of Heptyloxyazoxybenzene (HAB)

Polarity sensitive electric responses in a twisted smectic-C liquid crystal

The observation of two polarity-sensitive electrical responses found in the low-frequency (<1 Hz) regime of a square wave field is reported for an achiral rodlike smectic-C liquid crystal with negative dielectric and conductivity anisotropies and in the 90°-twisted configuration. The first involves a transient director modulation appearing at each polarity reversal and vanishing under steady field conditions. The instability is polarity sensitive, with the maximum distortion localized near the negative electrode instead of the sample midplane. This is inferred from the wave-vector orientation alternating in the two halves of the driving cycle between the alignment directions at the two substrates. Various electro-optic characteristics of this temporal phenomenon are also described. Following a similar observation in nematic liquid crystals, we associate the transient periodic order with the Carr-Helfrich mechanism assisted by quadrupolar flexoelectric polarization obtaining under electric field gradients. The second polarity-sensitive effect manifests in the relative shift of the periodic Fréedericksz pattern upon field reversal. The shift, which is linear in field for low fields, tends to saturate for large fields. It is interpreted as due to flexoelectric polarization associated primarily with the c director twist about the layer normal. A model involving a periodic wedgelike band, which has the twist localized within it and is flanked by two uniformly and transversely aligned regions, accounts for the flexoelectric shift of the optical pattern.

Optical birefringence studies of a binary mixture with the nematic-smectic Ad-re-entrant nematic phase sequence

We report the measurements of birefringence as a function of temperature of a binary system 4-cyanophenyl [4'(4''-n-heptylphenyl)]benzoate (7CPB) + 4-cyanophenyl 4-nonylbenzoate (9.CN) showing a nematic-smectic A(d)-re-entrant nematic phase sequence by means of the optical transmission method. The temperature dependence of the birefringence has been determined from the transmitted intensity data and the orientational order parameters have been calculated. These observations indicate that re-entrant nematic to induced smectic A(d) and induced smectic A(d) to nematic phase transitions for all the mixtures are of second order. There is a continuous change in the Δn values at the nematic-smectic A(d) and smectic A(d)-re-entrant nematic phase transitions. However, for some mixtures a slight increase in birefringence on cooling in the vicinity of the smectic A(d)-re-entrant nematic transition has been observed. We have also fitted our experimental results with those calculated from the modified McMillan theory as proposed by Luckhurst and Timimi.