DETERMINATION OF THE DIELECTRIC BIAXIALITY IN A CHIRAL SMECTIC-C PHASE

A method is presented allowing the determination of the three principal values (epsilon-1, epsilon-2, epsilon-3) of the dielectric tensor for a chiral smectic-C liquid crystal in the case that the material has an uncompensated helix. Hence it is applicable for essentially all single substances and therefore suitable for developing correlations between molecular structure and dielectric properties of the smectic-C* materials. The method requires two samples and uses three different measurement geometries. The first sample has the smectic layers parallel to glass plates and is also used for the determination of the tilt angle-theta needed in the evaluation of (epsilon-1, epsilon-2, epsilon-3). The two other measurements are made on a sample with the smectic layers essentially perpendicular to the glass plates, allowing chevron or uniformly tilted layer structure in the sample. One measurement is taken in the presence of the helix and another in the nonhelical (unwound) state obtained by applying a bias field. Measurements have been performed on one ester compound exhibiting A* and C* phases. The dielectric tensor components have been calculated and are presented as functions of temperature and frequency.