Polarized Raman scattering studies of orientational order in uniaxial liquid crystalline phases

Abstract

The measurement of vibrational Raman depolarization ratios has been used to study molecular orientational order in uniaxial single domain nematic and smectic liquid crystal samples. This technique is demonstrated to obtain the same microscopic order parameter 〈P₂〉=1/2 〈3 cos²ϑ−1〉, where ϑ is the angle between a molecular long axis and the uniaxial direction, as other existing methods. In addition, the next higher moment of the orientational distribution function 〈P₄〉=1/8 〈35 cos⁴ ϑ−30 cos²ϑ+3〉 has been measured for the first time. The physical basis, theoretical apparatus, and experimental methods necessary for the application of this technique are thoroughly detailed in this paper. Measurements are presented of the temperature dependence of 〈P₂〉 and 〈P₄〉 of N‐ (p′‐butoxybenzylidene) ‐p‐cyanoaniline (BBCA) dissolved in N‐ (p′‐methoxybenzylidene) ‐p‐cyanoaniline (MBBA) and of pure MBBA in the isotropic and nematic phases, and in the isotropic, nematic, smectic A, and smectic B phases of N‐ (p′‐butoxybenzylidene) ‐p‐n‐octylaniline (40.8). In the nematic phases the new quantitative information marks significant discrepancies with existing theories of nematic ordering. In the smectic phases the results show unambiguously the anticipated high degree of molecular orientational order.