Pretransitional Optical Activity in Chiral Smectic Liquid Crystals

Pretransitional optical activity measurements in smectic liquid crystals of both law and high chirality are reported. In the low chirality sample, p-(n-(decyloxybenzylidene)-p-amino-(2-methylbutyl)cin-namate (DOBAMBC), the results indicate that the optical activity in both the isotropic phase just above the smectic A phase and in the smectic A phase just above the smectic C* phase can be explained in terms of a Landau-deGennes free energy with no coupling between the chiral modes. In the high chirality sample, 1-methylheptyl 4′-[(4″-tetradecyloxyphenyl)propioloyloxy]biphenyl-4-carboxylate (14PlM7), the data in the isotropic phase above the recently discovered smectic A* phase cannot be explained in this way, even when coupling between the chiral modes is considered. Whether this is due to some feature of the smectic A* phase or is simply the result of strong coupling between the orientational and positional order parameters is a question which remains unanswered.