Optical birefringence studies of a binary mixture with the nematic–smecticAd-re-entrant nematic phase sequence

Abstract

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–smecticAd-re-entrant nematic phase sequence by means of the optical transmission method.The temperature dependence of the birefringence has been determined from thetransmitted intensity data and the orientational order parameters have beencalculated. These observations indicate that re-entrant nematic to induced smecticAd and inducedsmectic Ad to nematic phase transitions for all the mixtures are of second order. There is a continuous change in theΔn values at thenematic–smectic Ad and smectic Ad-re-entrant nematic phase transitions. However, for some mixtures aslight increase in birefringence on cooling in the vicinity of the smecticAd-re-entrant nematic transition has been observed. We have also fitted our experimentalresults with those calculated from the modified McMillan theory as proposed by Luckhurstand Timimi.