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

TLDR: In this paper, a phase diagram of two metal-free phthalocyanine mesogens exhibiting different mesophases (Colr phase from room temperature to isotropization for the first one, and Colr at room temperature then Colh from around 60 °C to the isotropisation for the second one) was built and evidence of full solid-state miscibility of the two pure constituents in all proportions and temperatures is presented.

Abstract: Blends of two metal-free phthalocyanine mesogens exhibiting different mesophases (Colr phase from room temperature to isotropization for the first one, and Colr at room temperature then Colh 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 Colr phases similar to the pure constituents, whereas Colh 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 Colh mesophase, thus demonstrating that the presence of a Colh mesophase is a ne...