Optical rotatory power of different phases of an antiferroelectric liquid crystal and implications for models of structure

The antiferroelectric liquid crystal (AFLC) under investigation possesses different helical smectic phases. The various phases have been identified through a texture under cross-polarizers with a homeotropic alignment of the AFLC. Measurements of the optical rotatory power (ORP) of these phases have elucidated the ability of this method for finding phase transitions between several phases and for investigating the helical structure of the antiferroelectric phases. The optical rotatory power as a function of temperature at a fixed wavelength of light was measured for different phases of the investigated AFLC material. The values of the pitch for some of the phases have been calculated from the ORP data. The results of the ORP rule out the simple ``clock'' model or a clock model with a long pitch superimposed on to it. The results can be explained only in terms of biaxial models, either Ising-type models or a highly distorted ``clock'' model. It is also found that in the SmC^*_A phase the sense of the helix in the investigated material is left handed, and is opposite to that observed in the SmC^* phase. The reversal of the helix from left to right handed occurs during the phase transition from the SmC^*_FI1(SmC^*_γ) to SmC^*_FI2(AF) phase. This fact also allows for SmC^*_A and AF phases to be distinguished from each other.