No abstract

Conoscopic observations have been made on a mixture of potassium laurate, 1-decanol, and D2O in the discotic-uniaxial, biaxial, and cylindric-uniaxial nematic phases. The differences between the principal refractive indices have been measured, leading to a complete determination of the tensor order parameter in all the three phases. In the biaxial phase, the symmetric invariants of the order parameter are found to have a linear dependence on temperature, in good agreement with the prediction of the mean-field model.

A new homologous series of achiral banana-shaped mesogens ('Dn') has been synthesized and studied by the classical techniques (optical microscopy, diOE erential scanning calorimetry, X-ray diOE raction, miscibility studies and electro-optic investigations). The short homologues (D6-D8) exhibit a two-dimensional phase 'B1 x ' diOE erent from a B1 phase with a rectangular lattice. The longer homologues (D9-D14) present a mesophase which displays the defects of the B7 phase of the PIMB-NO 2 compounds. Nevertheless the D9-D14 mesophase is not miscible with the B7 phase, and contrary to B7, exhibits a bistable behaviour ('ferroelectric' type) suggesting at least a B7 variant.