Dielectric and optical rotatory power investigations of an antiferroelectric liquid crystal 12OF1M7 in a homeotropic cell: implications for models of the structure of ferrielectric phases

Abstract

The antiferroelectric liquid crystal (AFLC) under investigation possesses different helical smectic phases. Measurements of the optical rotatory power (ORP) of these phases have elucidated the quality of this method for finding the phase transitions between several phases and for investigating their helical structure. The optical rotatory power as a function of temperature for seven wavelengths of light in the range 441 to 665 nm was measured for different phases of the AFLC material investigated, (R)-1-methylheptyl 4-(4'-n-dodecyloxybiphenyl-4-yl-carbonyloxy)-3-fluorobenzoate, with acronym 12OF1M7. The values of the pitch and the optical anisotropy in the plane of the smectic layers for the FiLC (SmC*FI3) phase and SmC*Aphase have been calculated from the ORP data. The results of the ORP rule out the simple clock model for describing the structure of the SmC* (SmC*I1) and AF (SmC*FI2) phases. The results for these phases can be explained either in terms of the modified Ising model or the highly distorted clock model. The ORP measurements establish the existence of SmC*FI3 found already from dielectric, polarization and polarized IR spectroscopy.