Phase behaviors and molecular and supramolecular structural identifications of a liquid crystalline second generation monodendron

A second generation monodendron (abbreviated as G₂NB) based on the AB₂ mesogenic monomer, 13-hydroxy-1-(4-hydroxyphenyl)-2-(4-hydroxy-4″-p-terphenylyl)tridecane building blocks, exhibits multiple mesophase transformations with varying temperatures. On the basis of thermal transition behaviors studied by differential scanning calorimetry, structural analyses carried out by wide-angle and small-angle X-ray scatterings and electron diffraction experiments, and morphologies observed in polarized light microscope, the equilibrium phase transition sequence and phase structures have been identified. From the glass transition temperature at 28 °C, the most ordered phase is a highly ordered liquid crystalline (LC) smectic B (S_h^B) phase. This phase is defined on the basis of the relationship between the LC mesogenic orientation direction and the layer normal. Within the layers, a molecular lateral hexagonal packing exists. It is followed by a low ordered LC smectic C (S_c) phase appeared in a narrow temperature range of ∼2°C and quickly entered a nematic phase. At a high temperature of 92 °C, G₂NB reaches the isotropic melt. An interesting observation is that the layer spacing of these S_h^B and S_c phases are much larger than the chemical repeat unit of the LC monomer; rather, it is a representative of the whole size of the G₂NB monodendron. Therefore, the S_h^B phase is a combined supramolecular layer structure and a lateral molecular hexagonal packing, while the S_c phase is a pure supramolecular phase. A surprising finding is that the supramolecular S_c phase formation possesses a stronger cooling rate dependence than that of the S_h^B phase formation. This provides an opportunity to control the appearance of the Sc phase formation by only varying the cooling rate. Possible packing structural model and formation mechanisms of this phenomenon are discussed.