Strömungsorientierung und Vorumwandlungserscheinungen in der isotropen Phase thermotroper Flüssigkristalle

An apparatus was constructed for the measurement of the flow birefringence in liquids. In the construction good control of the temperature and accuracy in the determination of the rate of flow were treated with priority. The temperature can be adjusted to ±0,01 K. The optical anisotropy can be measured down to ∆n = 10 −9 . To simplify the measurements an computer aided control system has been implemented. A homologous series of six liquid crystals has been measured, as well as three other liquid crystals. A divergence of the Maxwell Constant is observed as the phase transition is approached. In addition to the flow birefringence, the density, kinematic viscosity and refraction index have been measured. The values of the flow birefringence and the Maxwell constant of the liquid crystals are presented in tables. Appropriate functions have been fitted to the values of the auxiliary measurements, the functions and parameters are given in tables as well. Two homologous series of nonmesogenic compounds have been measured: the n-alcanoic acids from C 6 to C 12 , and the corresponding ethyl esters. The measurements in the isotropic phase of the liquid crystals have been evaluated in the framework of the Landau-de Gennes theory, using a simple mean-field approach with a critical exponent of 1 and an extended equation with an adjusted critical exponent. The results have been compared. The mean-field approach describes the results insufficently. A better description is achieved with a critical exponent < 1.