Orientational order parameter-II - A birefringence study

Different techniques are available for the estimation of order parameter in the nematic phase of liquid crystals. However, most of these methods are not applicable to Cholesteric and smectic phases and therefore, the experimental data regarding the order parameter in these phases are hardly available. Measurement results of birefringence and refractive index of liquid-crystalline N-(p-n-pentyloxybenzylidene)-p-n-pentylaniline, 5O.5, N-(p-n-pentylbenzylidene)- p-n-pentylaniline, 5.5, N-(p-n-pentylbenzylidene)-p-n-pentyloxyaniline, 5.O5, and N-(p-n-hexyloxybenzylidene)-p-n-pentyloxyaniline, 6O.O5, were presented. Using these data, the polarizability anisotropy was calculated for both Vuks and Neugebauer local field models. Polarizabilities calculated in this way were applied to determine δα(polarizability anisotropy in the case of perfect orientation) using either Haller's and/or Subramhanyam's normalization procedures. Further, the polarisability anisotropy in the perfect order was calculated theoretically using the θ-function model developed by Lippincott et al. These procedures enabled the calculation of the orientation order parameter. Polarizabilities calculated using both local field models and from θ-function model developed by Lippincott et al. have been compared. The values of polarizability anisotropy for both local electric field models differ significantly. No criterion is known to decide which value is correct. To avoid the determination of uncertain x_i and δαvalues considering different local field models, a simple procedure developed by Kuczynski et al. was used for evaluation of S, based solely on birefringence measurement. This procedure gave very reasonable results.