Homeotropic and Planar Alignment of Discotic Liquid Crystals: The Role of the Columnar Mesophase

Blends of two metal-free phthalocyanine mesogens exhibiting different mesophases (Col r phase from room temperature to isotropization for the first one, and Col r at room temperature then Col h from around 60 °C to isotropization for the second one) have been studied in order to determine the relationship between the type of mesophase and the alignment behavior. The phase diagram of this system has been built and evidence of full solid-state miscibility of the two pure constituents in all proportions and temperatures is presented. Investigation of phase alternation at room temperature as a function of composition revealed that border compositions exhibit Col r phases similar to the pure constituents, whereas Col h mesophase was stabilized for intermediate compositions. Combined polarized optical microscopy observations and X-ray diffraction measurements showed that homeotropic alignment is adopted only for mixed samples exhibiting Col h mesophase, thus demonstrating that the presence of a Col h mesophase is a necessary condition for homeotropic alignment.