The Temperature Sensitivity of the Selective Reflection by Cholesteric Mesophases and Its Possible Limitations

The selective reflectance of S-cholesteryl 14-phenyltetradecanethioate was determined as a function of the wavelength and temperature at several constant values of temperature and wavelength respectively. The dependence of wavelength λp measured at maximum selective reflectance on temperature T could be curve fitted within the measurement uncertanty of ±0.2m°C to Keating's formula and to the empirical expression λp= λp + A (T - T 0)−2. Since T0 is the temperature at which the cholesteric helix assumes an infinite pitch, we conclude that the region of extremely high temperature dependence of the selective reflectance of a cholesteric liquid crystal may be linked with a pretransitional effect at the cholesteric-smectic phase transition. We measured a maximum temperature coefficient of selective reflectance of about 50,000% per °C and found that the spontaneous temperature fluctuations of the sample must be smaller than 2×10−5 °C. A theoretical estimate, using the generalized Nyquist theorem, indicates that for a bandwidth of 1 Hz (centered at about 1 Hz) the spontaneous temperature fluctuations caused by the statistical nature of the heat transfer between sample and environment cannot be smaller than 10−8 °C.