Polycatenar mesogens with a perfluorinated moiety showing liquid crystalline polymorphism and, microscopically, a continuous smectic A to structured, fluid, optically isotropic phase transition

Abstract

A polycatenar mesogen consisting of a stiff, four ring aromatic core with three long alkoxy chains and a bulky, perfluorinated substituent has been synthesized and the phase behavior investigated. Due to strong incompatibility among the constitutional moieties, the material can be expected to self-organize into nano-segregated structures. It has been found that the material shows the unique liquid crystalline polymorphism of a cubic phase, a columnar phase and a smectic A phase as a function of temperature. By analyzing small-angle reflection patterns, the cubic phase is assigned as Im3m symmetry with a unit cell parameter of a ≈ 28 nm, the columnar phase as tetragonal symmetry with a ≈ 11 nm and the smectic A phase as an interdigitated one with a layer separation of 8.2 nm. Interestingly a small-angle reflection is clearly observed far above the smectic A to isotropic phase transition temperature, which reveals that the isotropic phase is not a normal liquid phase, but rather a structured, fluid, optically isotropic (Iso) phase. Moreover the d-spacing and the half-width of the small-angle reflection are observed to continuously change at the smectic A to Iso phase transition, which indicates that the phase transition takes place continuously at the microscopic level.