CRYSTALLINE POLYMORPHISM AND PHASE-TRANSITION BEHAVIOR OF MESOGENIC 4-HEPTYLOXY-4'-CYANOBIPHENYL (7OCB)

Four different solid phases were found for the title compound: square- plate and needle crystals, which are metastable, a solid found in a co llapsed plate crystal and the most stable crystalline phase found in a commercially available powder specimen. The phase transition behavior of each solid to the nematic phase was investigated by DSC. The phase relations are proposed in terms of Gibbs energy and temperature diagr am. The structure of the plate crystal has been determined at 243 K. T he crystal has a distinct smectic-like structure composed of bilayers. The CN groups are closely arranged between antiparallel molecules wit hin a bilayer, resulting in infinite networks with the C-N distances; 3.41-3.64 Angstrom.