X-Ray Photographic Studies of Liquid Crystals: II. Apparent Molecular Length and Thickness in Three Phases of Ethyl-p-ethoxybenzal-p-aminobenzoate

Abstract

A general theory is developed for the relation between the molecular length and the position of a diffraction maximum found at diffraction angles of the order of 4°, for nematic and isotropic phases containing elongated molecules. Using this theory and a previously developed one for the relation between the intermolecular distance and the position of the major diffraction maximum at larger diffraction angles, molecular parameters are determined for three phases of ethyl-p-sthoxybenzal-p-aminobenzoate. These parameters are: the thickness of the smectic layers (19.941 Å), the length of the molecules in the isotropic phase (21.4 Å), and the intermolecular distances in the smectic (4.894 Å), nematic (4.932–4.950 Å), and isotropic phases (4.950–5.182 Å), all as a function of temperature. From these data, densities and volume expansion coefficients are calculated which agree very well with literature data on similar compounds. There appears to be no room for rotation of the molecules around their long axes, not even in the isotropic phase. In the isotropic liquid the experimentally determined molecular length is very close to the calculated length of the stretched molecule (21.6 Å), but the thickness of the smectic layers is significantly less than this. The angle between the long molecular axes and the smectic planes (in the smectic A phase) is found to be very close to 90°. The nematic phase appears to be a classical nematic phase: there is no significant ordering of the molecules apart from the near parallelism of the long axes of neighboring molecules. The transition temperatures foud are: C[sbnd]L = 93.4 °C; N[sbnd]L = 87.6 °C; S[sbnd]N = 80.4 °C.