Optical and X-ray studies on the twisted smectic C and twisted nematic phases: evidence for a skew-cybotactic type of cholesteric structure

Abstract

The optical rotation p and the pitch P of mixtures of p-n-heptyloxyazoxybenzenea nd cholesteryl benzoate have been measured as function of temperature in the twisted smectic C and the twisted nematic phases for two compositions (10% and 15% of cholesteryl benzoate by weight). Both p and P change discontinuously at the twisted smectic C-twisted nematic point, the values being higher in the lower-temperature phase. The birefringence δn of pure HOAB has also been determined in the nematic and smectic C phases. The observed p in the twisted nematic phase cannot be accounted for in terms of the measured values of δn and P. The calculated value is too high indicating that, in contrast to the classical cholesteric structure, the molecules in this phase are not normal but inclined with respect to the helical axis. This has been confirmed directly by X-ray studies; the diffraction pattern has an inner ring characteristic of a skew-cybotactic structure with a tilt angle of 39° with respect to the layer normal. However the calculated rotatory power for such a structure turns out to be only about 0.5 of the observed value. The discrepancy decreases slightly with increasing sample thickness suggesting that it may be due to a boundary layer in which the molecules tend to align themselves parallel to the glass surface.